Etiology of the Negative Chronotropic Responses to Transient Coronary Artery Occlusion in the Anesthetized Rhesus Monkey WILLIAM A. ALTER, III, PH.D., ROBERT N. HAWKINS, PH.D., AND DELBERT E. EVANS, PH.D. SUMMARY Etiology of the negative chronotropic response to corartery occlusion was studied in chloralose-anesthetized monkeys. One-minute occlusion of the circumflex (CIRC) coronary artery resulted in marked negative chronotropic responses and consistent alterations in atrial electrograms. These responses were dependent on interruption of flow to a small proximal CIRC branch, and postmortem examination revealed that it perfused the sinus node region. The negative chronotropic response was not dependent on any
apparent neural reflexes because it was not affected by autonomic blockade. Coronary artery occlusion in anesthetized monkeys can result in significant decreases in heart rate and changes in atrial electrical activity when flow to the pacemaker region is interrupted. We suggest that (1) rhesus monkeys may be suitable for study of the sick sinus syndrome, and (2) atropine-resistant bradycardia and atrial arrhythmias observed in postinfarction patients may be due to sinus node artery blockade.
CHANGES IN HEART RATE after either myocardial infarction in man or experimental coronary artery occlusion in other species have been reported. In the cat, occlusion of any of the three main coronary arteries results in marked bradycardia.1 In the dog, on the other hand, several investigators have reported that coronary artery occlusion resulted in a tachycardia,2 bradycardia,3 or no heart rate change.4 Webb et al.5 reported that patients, examined within hours after suffering a myocardial infarction, frequently demonstrated either a tachycardia or a bradycardia. They further associated the direction of the heart rate change with the site of infarction, i.e., tachycardia was seen most often after anterior infarction, whereas bradycardia usually oc-
recording aortic blood pressure and for drug administration, respectively. Animals were paralyzed with pancuronium (0.1 mg/kg) and artifically respired. Blood samples were drawn every 30 minutes to insure that blood gases and pH were maintained within normal limits. To analyze overall cardiac function, the blood pressure, lead II electrocardiogram, and heart rate were recorded. The latter was detected by a cardiotachometer triggered by the systolic blood pressure peaks. To study regional cardiac function, a transsternal thoracotomy was performed, and the heart was stabilized in a pericardial cradle. Using a dissecting microscope, the
onary
curred after posterior infarction. Lippestad and Marton6
also reported that blockade of flow in the sinus node artery lead to sinus arrest and A-V nodal rhythm in patients after posterior infarctions. Bradycardia after myocardial infarction may further exacerbate myocardial hypoperfusion due to a reduced cardiac output. In contrast, postinfarction tachycardia may increase the size of the infarct by further comprising ischemic tissue in the border zones and may also increase the incidence of ventricular fibrillation.7 The cause for these chronotropic responses and their effect on cardiac function are of major importance in post heart attack therapy. This study was undertaken to determine the etiology of the chronotropic responses to coronary artery occlusion in the nonhuman primate (Macaca mulatta). In addition, the effect of occlusion-induced heart rate changes on regional blood flow and contractile force was also studied to determine if these changes affect the magnitude of the ischemic insult. can
Methods Adult rhesus monkeys weighing 3-5 kg, unselected as to sex, were sedated with phencyclidine HC1 (1-2 mg/kg, i.m.) and then anesthetized with alpha-chloralose (75 mg/kg, i.v.). The femoral artery and vein were catheterized for From the Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Maryland. Address for reprints: Dr. William A. Alter, III, Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Maryland 20014. Received September 19, 1977; revision accepted October 24, 1977.
bifurcation of the left coronary artery was exposed. Particular care was taken to avoid any visible pericoronary nerves in this region. Regional mechanical activity was measured by miniature Walton-Brodie strain gauges attached parallel to the epicardial fibers. One strain gauge was placed on the anterior apical surface in the region normally perfused by the anterior descending (LAD) branch of the left coronary artery, while the second was secured to the epicardium near the base of the left ventricle in the region normally perfused by the circumflex (CIRC). The hydrogen polarograph technique was used to determine regional myocardial blood flow. Two platinum wire electrodes (tip diameter 0.2 mm) were secured within the epicardium near each foot of the strain gauges. This method for measuring myocardial blood flow was first reported by Aukland in 1964.1 Recent reports by Gross and Winbury9 and Kjekshus15 demonstrated that flows determined by this method are in agreement with those determined by either an electromagnetic flow probe or microspheres, respectively. Briefly, the hydrogen polarograph technique measures blood flow by the Fick principle. Hydrogen gas is added to the inhaled air (maximum concentration less than 5%) until the tissues are saturated. Concentrations of hydrogen gas in tissues are detected by the platinum electrodes which are each polarized to a +0.65V DC potential relative to an indifferent electrode on the sternum. Inhalation of hydrogen is then discontinued and subsequently washed from the tissues. The slope of the hydrogen washout curve is calculated according to a first-order reaction with a rate constant k = (0.693/T½/2) X 100, where T1/2 is the time in minutes for hydrogen levels to decrease by one-half. Myocardial blood
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757
ministered and occlusions were then repeated. Postmortem analysis of the distribution of the coronary arteries was conducted after filling them with colored latex solutions (Cementex). It was found that manual injections of the latex consistently filled vessels with diameters as small as 80 to 100 microns. Results are reported as mean values ± standard error of the mean. Statistical significance between data groups was determined by Student's t-test for paired data and was considered significant when P < 0.05.
flow is expressed in ml/min/l00 g tissue and is calculated by the equation MBF = K/Xw, where w is the weight of the tissue and X is the blood-tissue partition coefficient for hydrogen which is approximately equal to 1.8 To insure that the platinum electrodes did not damage the surrounding myocardium, the output of each polarograph channel was AC-coupled to a storage oscilloscope and the resulting local epicardial electrograms were examined. Any significant ST-segment deviation was taken as evidence of myocardial damage, and blood flows were not calculated from these electrodes. In a few animals, right atrial electrical activity was detected by silver electrodes secured to the epicardial surface of the right atrium near its junction with the superior vena cava and attached to a standard ECG preamplifier. During the course of the experiment, each of the major branches of the left coronary artery (left anterior descending and circumflex) were occluded several times. To permit repeated occlusions of these arteries, a modified Heifetz aneurysm clip was utilized. Application of the clip to a coronary artery effectively eliminated any flow through the vessel but did not appear to damage nearby pericoronary nerves. Support for the latter stems from studies in anesthetized cats in which a similar experimental technique is being utilized to study the vagal cardio-cardiac reflexes during coronary artery occlusion. In this study, repeated occlusions with the same occlusion clip fail to alter the magnitude of this neurally mediated reflex (Alter, unpublished observations). A one-hour control period was allotted prior to studying the effects of coronary artery occlusion. Several occlusions of both the LAD and circumflex arteries were then accomplished with 10-minute control periods between occlusions. Preliminary studies revealed that most of the chronotropic responses reached their maximum within the first minute after coronary artery occlusion. In addition it was found that the incidence of ventricular fibrillation was markedly reduced for these brief occlusions. Therefore, one minute was selected as the occlusion period in these experiments. To determine the role of autonomic reflexes in any chronotropic responses to occlusion, atropine sulfate (0.5 mg/kg) and propranolol (0.5-1.0 mg/kg) were ad-
Results Physiological data obtained from 13 chloralose-anesthetized rhesus monkeys will be discussed in this report. Baseline heart rate, prior to LAD occlusion, averaged 186 ± 7 beats per minute (BPM) while aortic pressure was 134 ± 4/73 i 7 mm Hg. Control myocardial blood flow (MBF) near the apex of the anterior left ventricular wall averaged 118 i 10 ml/min/l00 g tissue, while that near the base of the left ventricle was 122 ± 7 ml/min/100 g. There were no significant differences in the baseline parameters prior to occlusions of other coronary arteries (table 1). Occlusion of the left anterior descending coronary artery (LAD) resulted in a 12 ± 3% decline in systolic aortic pressure which was accompanied by a small increase in heart rate (6 ± I beats/min) in nine of 13 animals. An example of these changes is shown in figure 1. In this animal, evidence of regional ischemia and dysfunction was obtained near the apex of the anterior left ventricular wall where myocardial blood flow declined 74% below baseline values. This ischemia resulted in a 75% decrease in contractile force. After one minute, flow was restored to the LAD, resulting in a return of heart rate and aortic pressure to preocclusion values. A marked reactive hyperemia was recorded in the previously ischemic region. In this animal (fig. 1), myocardial blood flow rose 67% above baseline; contractile force also underwent a significant postocclusion overshoot of 50%. Results from all animals indicate that LAD occlusion failed to elicit any significant changes in myocardial blood flow and contractile force in the basal region perfused by the circumflex artery. The data are summarized in table 1. Occlusion of the circumflex branch of the left coronary
TABLE 1. Cardiac Responses to Coronary Artery Occlusion Heart rate (beats/min)
Contractile force (% change from preocclusion values) ALVA LATLVB
Aortic pressure (mm Hg)
Myocardial blood flow (ml/min/100 g) ALVA LATLVB
Anterior descending artery occlusion Control
During
During Post
1*
§
Post
Control
7
186 +6 191 -45 +4
7
6*t 1*
134 ' 4/73 117 = 6/63 138 - 5/72
7
7t 9
-74 4t 7 +557=
+88 +4
136 108 137
Circumflex artery occlusion 5/74 & 7 -12 9 -68 6/54 6t +77 4 +25 6/75 7
131 130 129
9/81 9/77 9/75
7 3
118 25 267
10
121
9
4t
491
122 95 150
7 11
7
7t 7t
99= 11 132 - 7
127 31 243
2 3
134 137 132
18 25 22
119 103 123
10
6t
24T
Sinus node artery occlusion Control
During
Post
192 -48 +3
10
6*t 1*
10 10 16
-10
+55
4 2
-10
-55
*Change in heart rate. IDifference from pre-occlusion value is significant at the 0.05 level. fReturned to preocclusion values.
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1
11 14 14
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CIRCULATION
758 CLIP ON
CLIP ON
CLIP OFF
0
B..E
CLIP OFF
15 0-
100_ 50oLI
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100
H.R.
MUF 114
a c
150_ 100
190
30
MBF 94 u
0
LU
agALVIA
0
U..
uJ -I
10
I.-
u
sec
MBF 156 145
:
158
62
AL'.V,
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10
sec
278
O-
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RHESUS 10-26-76 FIGURE 1. Cardiovascular responses to a one-minute occlusion of the left anterior descending coronary artery (LAD) in an anesthetized rhesus monkey. The area of ischemia and reduced contractile force is restricted to the anterior wall of the left ventricle (ALVA). Note that LAD occlusion led to a small (10 beats/min) tachycardia and moderate hypotension. B. P. = aortic blood pressure; H. R. = heart rate; MBF = mean myocardial bloodflow obtained from two electrodes below each strain gauge (MBF in ml/min/100 g); AL VA = contractile force from strain gauge on apical region of left ventricle; LA TL VB = contractile force from strain gauge on basal region of left ventricle.
LATL
RHESUS 10-26-76 FIGURE 2. Cardiovascular responses to a one-minute occlusion of the circumflex coronary artery (CIRC) in an anesthetized rhesus monkey. The area of marked ischemia is restricted to the lateral wall of the left ventricle (LA TL VB). The severity of this ischemia is reflected in the appearance of systolic expansion during CIRC occlusion. In this animal, moderate ischemia and reduced contractile force were also noted at the apex of the anterior wall of the left ventricle (AL VA). Note that CIRC occlusion resulted in a marked decline in heart rate and moderate hypotension.
artery also resulted in a decrease in systolic aortic pressure. In contrast to the small tachycardia observed during LAD
The most notable difference between the responses to the two occlusions was the marked decrease in heart rate which
occlusion, circumflex occlusion resulted in a marked slowing of heart rate (45 ± 6 beats/min) in 11 of 13 animals. Figure 2 shows an example of the responses to circumflex occlusion obtained in the present study. For this animal, occlusion resulted in a 46% decline in systolic aortic blood pressure and a 45% decrease in heart rate. Within the ischemic region at the base of the left ventricle, MBF decreased 85% and the contractile force trace indicated systolic expansion during the occlusion period. Restoration of flow to the circumflex resulted in a return in both heart rate and blood pressure. Within the previously ischemic region, a 109% increase in MBF was recorded and a postocclusion overshoot of contractile force (49%) was also observed. Circumflex occlusion in this particular animal resulted in a 33% decline in MBF and a 40% decrease in contractile force in the apical region. Restoration of flow resulted in a 68% increase in MBF which was accompanied by a 17% overshoot in contractile force. The results obtained from all animals, however, indicated that restoration of flow in the circumflex did not result in significant alterations in myocardial blood flow and contractile force in the apical region. Results from circumflex occlusion are also summarized in table 1.
occurred during circumflex occlusion. This bradycardia may have affected regional cardiac function and blood flow in either the ischemic or nonischemic regions. Table 1 shows, however, that there were no significant differences in the average MBF and contractile force data from the ischemic regions for either occlusion except for the postocclusion overshoot in contractile force in the basal region after circumflex occlusion. This was significantly less than that obtained from the apical region after LAD occlusion. To determine if the autonomic nervous system was involved in any of these changes resulting from coronary artery occlusion, atropine and propranolol were administered in three animals and the occlusions were repeated. Autonomic blockade resulted in a significant decline in baseline heart rate and contractile force for the left ventricle. These results are summarized in table 2. LAD occlusion after autonomic blockade elicited a 13 ± 4% decrease in systolic aortic pressure without any significant change in heart rate. In contrast, circumflex occlusion also elicited a decrease in systolic aortic pressure and a marked slowing of heart rate (32 ± 7 beats/min). Changes in regional contractile force and myocardial blood flow during these occlusions were similar to those observed prior to autonomic blockade. Due to the fact that only three animals
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CHRONOTROPIC RESPONSES TO CAO/Alter, Hawkins, Evans TABLE 2. Cardiac Responses to Coronary Artery Occlusion after Autonomic Blockade¶ Heart rate (beats/min)
Control
During
139 +1
Post
Control
During
Post
12 2*
t 145 7 -32 7*
Post
Control During
=1=
t 147
9 6*
-29
t
Aortic pressure (mm Hg)
Contractile force (% change from preocclusion values) ALVA LATLVB
Anterior descending artery occlusion 118 13/85 * 25 103 17/68 28 -64 10 +11 14 120 18/88 28 +73 -23 +14 5
Circumflex artery occlusion 115* 8/70 10 101 -36 11 -80 20 8/57 9 118 10/75 10 +23 10 +57 17 Sinus node artery occlusiont 118 8/78 * 13 115 * 8/71 * 13 121 * 10/78 i 13
Myocardial blood flow (ml/min/100 g) ALVA LATLVB
119 22 31 11 412 162
106 * 21 101 37 125 28
111 17 6 65 172 18
99 18 22 10 194 35
*Changes in heart rate. tReturned to preocclusion values. tNo significant changes were observed in myocardial blood flow and contractile force during sinus node artery occlusion. lThree animals received atropine sulfate (0.5 mg/kg) and propranolol (0.5-1.0 mg/kg).
were studied after autonomic blockade, no statistical analysis of these data was performed. Another possible mechanism for the bradycardia observed during circumflex occlusion may be ischemia to the sinoatrial node. To evaluate this hypothesis, proximal branches of the circumflex artery were exposed and individually occluded. This revealed that occlusion of the first lateral branch (diameter < 1 mm) of the circumflex resulted in a bradycardia whose magnitude (48 ± 6 beats/min) was not significantly different from that recorded during occlusion of the proximal circumflex artery. Figure 3 depicts an example of these responses. Occlusion of the circumflex branch resulted in a marked bradycardia (70 beats/min) with no consistent changes in aortic pressure or regional contractile force. Myocardial blood flow was down somewhat during this occlusion, but the average response for all animals (table 1) failed to show any significant difference in regional myocardial blood flow during occlusion of this branch of the circumflex artery. After autonomic blockade, occlusion of this circumflex branch still elicited a decrease in heart rate (29 ± 6 beats/min) without any significant change in other cardiac parameters. Since occlusion of this branch resulted in a bradycardia which was not mediated by any apparent neural pathway, this artery was tentatively designated as the sinus node artery (SNA). To further analyze the effects of sinus node artery occlusion on cardiac pacemaker function, surface electrodes were secured to the lateral wall of the right atrium near its junction with the superior vena cava. Occlusion of either the proximal circumflex artery or the sinus node artery resulted in negative chronotropic responses which were accompanied by changes in the ECG and the right atrial electrogram (AEG). Examples of these changes are shown in figures 4 and 5. The upper traces are aortic pressure, and indicate when electrical activity resulted in mechanical systole. In figure 4, occlusion of the circumflex artery proximal to the origin of the sinus node artery resulted in a 55 beats/min decrease in heart rate with an onset latency of 16 seconds. After 30 seconds of occlusion, there was an atrial depolarization which failed to conduct to the A-V node. This was then followed by a premature ventricular contraction with retrograde conduction to the atrium. For the remaining
period of occlusion, the atrial electrogram indicated a sustained shift in the atrial depolarization wave vector, as was evident by the decrease in magnitude of the P wave of the AEG. This was algo reflected in a smaller P wave in the lead II ECG. In addition, circumflex occlusion resulted in a marked ST elevation and hypotension. In contrast, sinus node artery occlusion (fig. 5) failed to significantly alter systolic aortic pressure despite an 88 beats/min decrease in heart rate. Once again, the onset latency for the heart rate decline (13 sec) was approximately one-half that for the change in the P wave of the AEG. In this example, sinus CLIP ON
CLIP OFF
I
1-
150r-
B..
100
|
E 50O E
H.R.:E
l5O
go100
MBTF 130
120
109
mu = 0
ALVfA
I" -I
LI.
10 sec
MBF 100
I.-
Z LATLY
0 UI
RHESUS 9-9-76 FIGURE 3. Cardiovascular responses to a one-minute occlusion of the sinus node artery in an anesthetized rhesus monkey. Interruption of flow to the sinus node region failed to cause any major changes in regional myocardial bloodflow or contractileforce in the left ventricle, but did result in a marked decline in heart rate.
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760
CIRCULATION + 30 sEc
CONTROL B.P.
0150 50 l0
H.R. 8PM
176
231
RIGHT ATRIAL ELECTROGRAM
L-11
ECG
RHESUS 8914 9-22-76 FIGURE 4. Effect of a one-minute circumflex coironary artery occlusion on cardiac electrical activity in an anest,ietized rhesus monkey. Thirty seconds after the onset of occlusicon, heart rate (HR) and blood pressure (BP) had declined significan tly. The atrial electrogram (AEG) revealed an atrial depolarization which was not conducted to the ventricles. This was followed by a piremature ventricular contraction with apparent retrograde condfuction to the atrium. Following this episode, the AEG revealed an ialtered atrial depolarization vector suggesting a change in the pacennaker site during the period of sinus node ischemia. Note also the dramatic change in the electrocardiogram (L-II ECG) duri ng circumflex occlusion.
node artery occlusion resulted in both a reduiced P wave magnitude and decreased PR interval, which sulggests a shift in the pacemaker site. Since the left ventricular muscle was not ischemic, the QRS complex of the ECG ri emained unchanged. Right atrial electrograms were record[ed in a total of four animals, and in all cases, sinus node arte ry occlusion resulted in a significant alteration in the atrial deIrpolarization --- - ---
CONTROL
c 150 _ B.P. E 100E So H.R. BPM
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wave, the latter having an onset latency significantly longer than that for the bradycardia. Postmortem examination of the latex-filled coronary arteries was conducted on 11 animals. In four of these, the sinus node region was perfused by a single artery which originated from the circumflex branch of the main left coronary artery. After arising as the first lateral branch of the circumflex, the sinus node artery gave off branches to the medial surface of the left atrium. It then traversed within the epicardium of the interatrial septum to the superior medial surface of the right atrium, reaching the area of the junction of the right atrial appendage and the superior vena cava. In five animals (fig. 6), the sinoatrial region appeared to be perfused by this circumflex branch as well as one or more smaller branches of the right coronary artery. On most occasions, the sinus node artery (origin from circumflex) also continued onto the lateral surface of the superior vena cava, forming a loop around it. In this small number of animals, there did not appear to be a correlation between the magnitude of the negative chronotropic response to sinus node artery occlusion and the presence of additional sinus node perfusion from a nonoccluded right coronary artery branch. In the remaining two animals, the first lateral branch of the CIRC terminated near the interatrial septum. In these animals, right coronary branches reached the sinus node region; and in one, the right coronary branch looped around the superior vena cava. Physiological evidence was obtained from one of these animals that the right coronary artery might occasionally serve as the sole source of sinus node perfusion. Circumflex artery occlusion failed to elicit a bradycardia, but right coronary artery occlusion resulted in a 20 beats/min bradycardia which was not eliminated by autonomic blockade. Therefore, in 85% of rhesus studied, the CIRC is the origin of the sinus node artery, whereas the latter originates from the proximal right coronary in the remaining 15%. In either case, interruption of flow in the sinus node artery results in an ischemia-induced bradycardia.
+ 50 sQc
+ 2 min
143
143
fKrKfi 231
RIGHT ATRIAL ELECTROGRAM
L-11 ECG
tP
-
1-
-AI%1-IJ
RHESUS 8914 9-22-76 FIGURE 5. Effect of a two-minute sinus node artery occlusion on cardiac electrical activity in an anesthetized rhesus monkey. Fifty seconds after onset of occlusion, the atrial electrogram reveals an alteration in the atrial depolarization vector as well as a decrease in the PR interval, which suggests a shift in pacemaker site. Despite a marked decline in heart rate (88 beats/min), systolic aortic blood pressure was not decreased. Downloaded from http://circ.ahajournals.org/ at University of Hawaii--Manoa on June 22, 2015
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FIGURE 6. Coronary artery perfusion of the sinus node region in a rhesus monkey. Sinus node region in this monkey is primarily perfused by a proximal circumflex branch (arrows), but additional perfusion may be supplied by branches of the right coronary artery.
Discussion The results of this study indicate that chronotropic responses to coronary artery occlusion in the rhesus may be mediated by at least two mechanisms. During LAD occlusion, a small cardioacceleration was observed which was eliminated by autonomic blockade. It was concluded that this positive chronotropic response results from a baroreceptor reflex to the hypotension which occurred during LAD occlusion. It was surprising that the size of the response was small, considering that systolic aortic pressure fell significantly. A possible explanation for this finding comes from the recent report by Feola et al.,3 who demonstrated that despite hypotension, cardiac sympathetic efferent discharge decreased during the first 30 minutes of circumflex artery occlusion. Cardiac pacemaker ischemia is another mechanism which may be responsible for heart rate changes during coronary artery occlusion in the rhesus. In the present study, interruption of blood flow in the parent CIRC or its sinus node branch resulted in a marked bradycardia accompanied by an alteration in the P wave of the right atrial electrogram. Atropine (vagolytic) and propranolol (sympatholytic) administration failed to alter these responses, thus indicating that the changes were not dependent on any autonomic neural reflex. A suitable model for the study of the effect of sinus node ischemia on cardiac function has not been previously reported. Early studies in the dog" contended that bradycardia was produced by sinus node artery occlusion, but James and Reemtsma12 were unable to confirm these findings. This difference was finally resolved by Billette et al.,'3 who were able to obtain bradycardia in the dog by occluding the sinus node artery as well as all collaterals to the pacemaker region. This necessity for removing collateral blood flow before inducing any disruption in pacemaker function does not appear to be necessary in the rhesus. In the present study, five animals appeared to receive sinus node perfusion from branches of both the circumflex and right
761
coronary arteries, and yet occlusion of just the circumflex branch was capable of inducing marked slowing of the heart rate. It is possible that occlusion of both blood supplies to the sinus node region may have induced even more dramatic pacemaker changes. However, the fact that occlusion of just the sinus node artery leads to pacemaker dysfunction may make this animal particularly useful in the study of the sick sinus syndrome. A recent report on patients with coronary artery disease by Jordan et al.14 has demonstrated a relationship between sinus node dysfunction and obstruction of the sinus node artery or its parent vessel. The rhesus monkey may be a useful animal model for the study of sinus node function after permanent interruption of sinus node artery blood flow. In the rhesus, occlusion of the sinus node artery produced both bradycardia and alterations in the atrial electrical activity, with the latter having a latency approximately twice that of the former. The prolonged latency for change in the P wave of the AEG may be explained on the basis of the spread in ischemia in this region. Interruption of flow in the SNA first leads to sufficient ischemia in pacemaker tissue to decrease spontaneous rate, but the region of ischemia must further enlarge before any noticeable change in the atrial depolarization vector can be detected. Atrial arrhythmias, however, were not observed in the present study. The production of altered- atrial electrical activity certainly suggests that occlusions for longer than one minute may lead to these arrhythmias. Studies are currently underway to determine if prolonged sinus node ischemia will lead to atrial arrhythmias. In addition, it is possible that simultaneous ischemia in both ventricular myocardium and the sinus node region may predispose to an increased incidence of ventricular arrhythmias as was suggested by Webb et al.5 The distribution of sinus node arteries in experimental animals shows a significant difference among species. In the dog, the sinus node artery originates as a distal branch of the right coronary artery in 90% of the cases,'5 while arising from the proximal circumflex in the remaining animals. On the other hand, sinus node perfusion in the cat is usually from a proximal branch of the right coronary artery (90%) with the remainder coming from a CIRC branch.' In man, James'5 has shown that the sinus node is perfused by a proximal branch of the right coronary artery in 60% of the hearts examined, whereas the CIRC was the origin in 40% of the cases. In the present study, physiological and anatomical data indicate that the CIRC was the source of sinus node perfusion in approximately 85% of rhesus monkeys. In the other two monkeys, it appears that the sinus node received its blood supply from branches which arose near the origin of the right coronary artery. Anatomically, the course of the sinus node artery in the rhesus was quite similar to that reported in man;'5 that is, the artery passed to the sinus node region and then formed a loop around the superior vena cava. Based on the rather dramatic changes in heart rate and P waveform of the AEG, during just a one-minute interruption of flow in the sinus node artery, it appears that the rhesus monkey will serve as a useful model in studying the long-term effects of sinus node ischemia. It will be interesting to see if prolonged ischemia will lead to important atrial arrhythmias or if sufficient collateral flow exists to permit the sinus node region to continue in its role as the cardiac pacemaker.
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CIRCULATION
It was somewhat surprising that coronary artery occlusion in the rhesus failed to initiate a neurally mediated bradycardia. This cardio-cardiac reflex has been demonstrated in cats,' and appears to be responsible for most incidents of bradycardia seen in post-myocardial infarction patients.5 In both man and cat, atropine usually eliminated or markedly reduced the bradycardia. Other studies in nonhuman primates'6-18 have also failed to observe any vagally mediated bradycardia during coronary artery occlusion. It is possible that these studies in baboons and rhesus monkeys did not report any ischemia-induced bradycardia during occlusion because their experiments did not involve proximal CIRC occlusion. It is also possible that in the present study, the afferent pathways involved in this cardio-cardiac reflex were inadvertently interrupted during exposure of the coronary arteries. This is not likely, however, because studies are currently underway utilizing the same preparation in which we are studying the negative chronotropic response to coronary artery occlusion in the cat. This preparation consistently provides an experimental model in which occlusion results in a neurally mediated bradycardia (Alter, unpublished observations). Another possible cause for the lack of vagally mediated bradycardia during occlusion is the selection of anesthetic. However, the present study utilized the same anesthetic (chloralose) as in cat studies.' Therefore, at this time, we must conclude that in the rhesus monkey, coronary artery occlusion may lead to either a small cardioacceleration mediated by the autonomic nervous system or a bradycardia due to pacemaker ischemia, the latter occurring when either the CIRC or its sinus node branch is occluded. Brief occlusions of coronary arteries resulted in marked changes in myocardial blood flow and contractile force within the ischemic region. In contrast, nonischemic regions failed to demonstrate any significant changes in contractile force. These results differ from those reported by Pashkow'5 who showed that nonischemic myocardium demonstrated an increased contractile force during brief occlusions of the LAD in anesthetized pigs. This may be related to a species and/or anesthetic difference in the two studies. In addition, increased myocardial blood flow in regions outside an ischemic region have been reported in anesthetized dogs;20 however, once again, coronary artery occlusions failed to elicit such changes in the anesthetized rhesus. Removal of the occlusion clip resulted in a marked reactive hyperemia and postocclusion overshoot in contractile force (table 1) in the previously ischemic region whereas no changes were seen in these parameters in the nonischemic region. The only significant difference in response seen after coronary artery occlusion was a decreased overshoot in contractile force at the base of the left ventricle (25 ± 7%) after CIRC occlusion as opposed to that observed at the apex after LAD occlusion (55 ± 7%). Several possible causes for this decrease can be proposed. The decreased response may reflect a decreased magnitude of ischemia in the basal myocardium during CIRC occlusion when compared to that seen for LAD occlusion. This is not likely, however, because myocardial blood flow and contractile force decrease were comparable for both occlusion sites. An alternative explanation might be the lesser involvement of sympathetic inotropic reflexes after CIRC occlusions. This does not appear
VOL 57, No 4, APRIL 1978
to be a realistic hypothesis because postocclusion overshoot in contractile force persisted after autonomic blockade as has been reported by Pagani et al." Indeed, the overshoots in contractile force still demonstrated a greater magnitude after LAD occlusion. Another possible mechanism may lie in the decrease in total ischemic insult during CIRC occlusion. Due to the presence of a bradycardia, the total energy requirements during CIRC occlusion might be expected to be less than those experienced during LAD occlusion where heart rate was unchanged or slightly increased. If the magnitude of the overshoot in contractile force is somewhat dependent on this ischemic insult, then as a consequence of the bradycardia, it would be expected that the overshoot in contractile force would be less. Support for this last possibility awaits additional experiments. Acknowledgment The authors express their appreciation to L. J. Parkhurst and D. E. Padgett for their technical assitance and to C. Anagnostelis for the atrial electrodes.
References 1. Corr PB, Pearle EL, Hinton JR, Roberts WC, Gillis RA: Site of myocardial infarction: A determinant of the cardiovascular changes induced by coronary occlusion. Circ Res 39: 840, 1976 2. Theroux P, Ross J, Franklin D, Kemper WS, Sasayama S: Regional myocardial function in the conscious dog during acute coronary occlusion and responses to morphine, propranolol, nitroglycerin, and lidocaine. Circulation 53: 302, 1976 3. Feola M, Arbel ER, Glick G: Attenuation of cardiac sympathetic drive in experimental myocardial ischemia in dogs. Am Heart J 93: 82, 1977 4. Weiner JM, Apstein CS, Arthur JH, Pirzada FA, Hood WB: Persistence of myocardial injury following brief periods of coronary occlusion. Cardiovasc Res 10: 678, 1976 5. Webb SW, Adgey AAJ, Pantridge JF: Autonomic disturbance at onset of acute myocardial infarction. Br Med J 3: 89, 1972 6. Lippestad CT, Marton PF: Sinus arrest in proximal right coronary artery occlusion. Am Heart J 74: 551, 1974 7. James RG, Arnold JM, Allen JD, Partridge JF, Shanks RG: The effects of heart rate, myocardial ischemia and vagal stimulation on the threshold for ventricular fibrillation. Circulation 55: 311, 1977 8. Aukland K, Bower BF, Berliner RW: Measurement of local blood flow with hydrogen gas. Circ Res 14: 164, 1964 9. Gross GF, Winbury MM: Beta-adrenergic blockade on intramyocardial distribution of coronary blood flow. J Pharmacol Exp Ther 187: 451, 1973 10. Kjekshus JK: Mechanism for flow distribution in normal and ischemic myocardium during increased ventricular preload in the dog. Circ Res 3: 489, 1973 11. Botti G, Visioli 0, Uleri G, Contorni L: Effetti immediati della legatura della "arteria del nodo del sono" su la funzione ritmica sinusale nel cane. Rass Fisiop Cl Ter 29: 149, 1957 12. James TN, Reemtsma K: The response of sinus node function to ligation of the sinus node artery. Henry Ford Hosp Med Bull 8: 129, 1960 13. Billette J, Elharrar V, Porlier G, Nadeau RA: Sinus slowing produced by experimental ischemia of the sinus node in dogs. Am J Cardiol 31: 331, 1973 14. Jordan J, Yamaguchi I, Mandel JW: Characteristics of sinoatrial conduction with coronary artery disease. Circulation 55: 569, 1977 15. James TN: Anatomy of the Coronary Arteries. New York, B. Hoeber Inc., 1961 16. Hill JD, Malinow MR, McNulty WP, Ochsner AJ III: Experimental myocardial infarction in unanesthetized monkeys. Am Heart J 84: 82, 1972 17. Bruyneel KJJ, Opie LH: The value of warning arrhythmias in the prediction of ventricular fibrillation within one hour of coronary occlusion: Experimental studies in the baboon. Am Heart J 86: 373, 1973 18. Weisse AB, Kearney K, Narang RM, Regan TJ: Comparison of the coronary collateral circulation in dogs and baboons after coronary occlusion. Am Heart J 92: 193, 1976 19. Pashkow F, Holland R, Brooks H: Dynamic responsiveness of distant myocardium during transient anterior wall ischemia. (abstr) Am J Cardiol 33: 161, 1974 20. Timogiannakis G, Amende I, Martinez E, Thomas M: ST-segment deviation and regional myocardial blood flow during experimental partial coronary artery occlusion. Cardiovasc Res 8: 469, 1974 21. Pagani M, Vatner SF, Baig H, Braunwald E: Functional rebound on recovery from transient myocardial ischemia in conscious dogs. (abstr) Circulation 53: 425, 1976
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Etiology of the negative chronotropic responses to transient coronary artery occlusion in the anesthetized rhesus monkey. W A Alter, 3rd, R N Hawkins and D E Evans Circulation. 1978;57:756-762 doi: 10.1161/01.CIR.57.4.756 Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1978 American Heart Association, Inc. All rights reserved. Print ISSN: 0009-7322. Online ISSN: 1524-4539
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apparent neural reflexes because it was not affected by autonomic blockade. Coronary artery occlusion in anesthetized monkeys can result in significant decreases in heart rate and changes in atrial electrical activity when flow to the pacemaker region is interrupted. We suggest that (1) rhesus monkeys may be suitable for study of the sick sinus syndrome, and (2) atropine-resistant bradycardia and atrial arrhythmias observed in postinfarction patients may be due to sinus node artery blockade.
CHANGES IN HEART RATE after either myocardial infarction in man or experimental coronary artery occlusion in other species have been reported. In the cat, occlusion of any of the three main coronary arteries results in marked bradycardia.1 In the dog, on the other hand, several investigators have reported that coronary artery occlusion resulted in a tachycardia,2 bradycardia,3 or no heart rate change.4 Webb et al.5 reported that patients, examined within hours after suffering a myocardial infarction, frequently demonstrated either a tachycardia or a bradycardia. They further associated the direction of the heart rate change with the site of infarction, i.e., tachycardia was seen most often after anterior infarction, whereas bradycardia usually oc-
recording aortic blood pressure and for drug administration, respectively. Animals were paralyzed with pancuronium (0.1 mg/kg) and artifically respired. Blood samples were drawn every 30 minutes to insure that blood gases and pH were maintained within normal limits. To analyze overall cardiac function, the blood pressure, lead II electrocardiogram, and heart rate were recorded. The latter was detected by a cardiotachometer triggered by the systolic blood pressure peaks. To study regional cardiac function, a transsternal thoracotomy was performed, and the heart was stabilized in a pericardial cradle. Using a dissecting microscope, the
onary
curred after posterior infarction. Lippestad and Marton6
also reported that blockade of flow in the sinus node artery lead to sinus arrest and A-V nodal rhythm in patients after posterior infarctions. Bradycardia after myocardial infarction may further exacerbate myocardial hypoperfusion due to a reduced cardiac output. In contrast, postinfarction tachycardia may increase the size of the infarct by further comprising ischemic tissue in the border zones and may also increase the incidence of ventricular fibrillation.7 The cause for these chronotropic responses and their effect on cardiac function are of major importance in post heart attack therapy. This study was undertaken to determine the etiology of the chronotropic responses to coronary artery occlusion in the nonhuman primate (Macaca mulatta). In addition, the effect of occlusion-induced heart rate changes on regional blood flow and contractile force was also studied to determine if these changes affect the magnitude of the ischemic insult. can
Methods Adult rhesus monkeys weighing 3-5 kg, unselected as to sex, were sedated with phencyclidine HC1 (1-2 mg/kg, i.m.) and then anesthetized with alpha-chloralose (75 mg/kg, i.v.). The femoral artery and vein were catheterized for From the Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Maryland. Address for reprints: Dr. William A. Alter, III, Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Maryland 20014. Received September 19, 1977; revision accepted October 24, 1977.
bifurcation of the left coronary artery was exposed. Particular care was taken to avoid any visible pericoronary nerves in this region. Regional mechanical activity was measured by miniature Walton-Brodie strain gauges attached parallel to the epicardial fibers. One strain gauge was placed on the anterior apical surface in the region normally perfused by the anterior descending (LAD) branch of the left coronary artery, while the second was secured to the epicardium near the base of the left ventricle in the region normally perfused by the circumflex (CIRC). The hydrogen polarograph technique was used to determine regional myocardial blood flow. Two platinum wire electrodes (tip diameter 0.2 mm) were secured within the epicardium near each foot of the strain gauges. This method for measuring myocardial blood flow was first reported by Aukland in 1964.1 Recent reports by Gross and Winbury9 and Kjekshus15 demonstrated that flows determined by this method are in agreement with those determined by either an electromagnetic flow probe or microspheres, respectively. Briefly, the hydrogen polarograph technique measures blood flow by the Fick principle. Hydrogen gas is added to the inhaled air (maximum concentration less than 5%) until the tissues are saturated. Concentrations of hydrogen gas in tissues are detected by the platinum electrodes which are each polarized to a +0.65V DC potential relative to an indifferent electrode on the sternum. Inhalation of hydrogen is then discontinued and subsequently washed from the tissues. The slope of the hydrogen washout curve is calculated according to a first-order reaction with a rate constant k = (0.693/T½/2) X 100, where T1/2 is the time in minutes for hydrogen levels to decrease by one-half. Myocardial blood
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CHRONOTROPIC RESPONSES TO CAO/Alter, Hawkins, Evans
757
ministered and occlusions were then repeated. Postmortem analysis of the distribution of the coronary arteries was conducted after filling them with colored latex solutions (Cementex). It was found that manual injections of the latex consistently filled vessels with diameters as small as 80 to 100 microns. Results are reported as mean values ± standard error of the mean. Statistical significance between data groups was determined by Student's t-test for paired data and was considered significant when P < 0.05.
flow is expressed in ml/min/l00 g tissue and is calculated by the equation MBF = K/Xw, where w is the weight of the tissue and X is the blood-tissue partition coefficient for hydrogen which is approximately equal to 1.8 To insure that the platinum electrodes did not damage the surrounding myocardium, the output of each polarograph channel was AC-coupled to a storage oscilloscope and the resulting local epicardial electrograms were examined. Any significant ST-segment deviation was taken as evidence of myocardial damage, and blood flows were not calculated from these electrodes. In a few animals, right atrial electrical activity was detected by silver electrodes secured to the epicardial surface of the right atrium near its junction with the superior vena cava and attached to a standard ECG preamplifier. During the course of the experiment, each of the major branches of the left coronary artery (left anterior descending and circumflex) were occluded several times. To permit repeated occlusions of these arteries, a modified Heifetz aneurysm clip was utilized. Application of the clip to a coronary artery effectively eliminated any flow through the vessel but did not appear to damage nearby pericoronary nerves. Support for the latter stems from studies in anesthetized cats in which a similar experimental technique is being utilized to study the vagal cardio-cardiac reflexes during coronary artery occlusion. In this study, repeated occlusions with the same occlusion clip fail to alter the magnitude of this neurally mediated reflex (Alter, unpublished observations). A one-hour control period was allotted prior to studying the effects of coronary artery occlusion. Several occlusions of both the LAD and circumflex arteries were then accomplished with 10-minute control periods between occlusions. Preliminary studies revealed that most of the chronotropic responses reached their maximum within the first minute after coronary artery occlusion. In addition it was found that the incidence of ventricular fibrillation was markedly reduced for these brief occlusions. Therefore, one minute was selected as the occlusion period in these experiments. To determine the role of autonomic reflexes in any chronotropic responses to occlusion, atropine sulfate (0.5 mg/kg) and propranolol (0.5-1.0 mg/kg) were ad-
Results Physiological data obtained from 13 chloralose-anesthetized rhesus monkeys will be discussed in this report. Baseline heart rate, prior to LAD occlusion, averaged 186 ± 7 beats per minute (BPM) while aortic pressure was 134 ± 4/73 i 7 mm Hg. Control myocardial blood flow (MBF) near the apex of the anterior left ventricular wall averaged 118 i 10 ml/min/l00 g tissue, while that near the base of the left ventricle was 122 ± 7 ml/min/100 g. There were no significant differences in the baseline parameters prior to occlusions of other coronary arteries (table 1). Occlusion of the left anterior descending coronary artery (LAD) resulted in a 12 ± 3% decline in systolic aortic pressure which was accompanied by a small increase in heart rate (6 ± I beats/min) in nine of 13 animals. An example of these changes is shown in figure 1. In this animal, evidence of regional ischemia and dysfunction was obtained near the apex of the anterior left ventricular wall where myocardial blood flow declined 74% below baseline values. This ischemia resulted in a 75% decrease in contractile force. After one minute, flow was restored to the LAD, resulting in a return of heart rate and aortic pressure to preocclusion values. A marked reactive hyperemia was recorded in the previously ischemic region. In this animal (fig. 1), myocardial blood flow rose 67% above baseline; contractile force also underwent a significant postocclusion overshoot of 50%. Results from all animals indicate that LAD occlusion failed to elicit any significant changes in myocardial blood flow and contractile force in the basal region perfused by the circumflex artery. The data are summarized in table 1. Occlusion of the circumflex branch of the left coronary
TABLE 1. Cardiac Responses to Coronary Artery Occlusion Heart rate (beats/min)
Contractile force (% change from preocclusion values) ALVA LATLVB
Aortic pressure (mm Hg)
Myocardial blood flow (ml/min/100 g) ALVA LATLVB
Anterior descending artery occlusion Control
During
During Post
1*
§
Post
Control
7
186 +6 191 -45 +4
7
6*t 1*
134 ' 4/73 117 = 6/63 138 - 5/72
7
7t 9
-74 4t 7 +557=
+88 +4
136 108 137
Circumflex artery occlusion 5/74 & 7 -12 9 -68 6/54 6t +77 4 +25 6/75 7
131 130 129
9/81 9/77 9/75
7 3
118 25 267
10
121
9
4t
491
122 95 150
7 11
7
7t 7t
99= 11 132 - 7
127 31 243
2 3
134 137 132
18 25 22
119 103 123
10
6t
24T
Sinus node artery occlusion Control
During
Post
192 -48 +3
10
6*t 1*
10 10 16
-10
+55
4 2
-10
-55
*Change in heart rate. IDifference from pre-occlusion value is significant at the 0.05 level. fReturned to preocclusion values.
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1
11 14 14
VOL 57, No 4, APRIL 1978
CIRCULATION
758 CLIP ON
CLIP ON
CLIP OFF
0
B..E
CLIP OFF
15 0-
100_ 50oLI
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100
H.R.
MUF 114
a c
150_ 100
190
30
MBF 94 u
0
LU
agALVIA
0
U..
uJ -I
10
I.-
u
sec
MBF 156 145
:
158
62
AL'.V,
1-aU -
10
sec
278
O-
IL-
1--
LATL%
O 0
RHESUS 10-26-76 FIGURE 1. Cardiovascular responses to a one-minute occlusion of the left anterior descending coronary artery (LAD) in an anesthetized rhesus monkey. The area of ischemia and reduced contractile force is restricted to the anterior wall of the left ventricle (ALVA). Note that LAD occlusion led to a small (10 beats/min) tachycardia and moderate hypotension. B. P. = aortic blood pressure; H. R. = heart rate; MBF = mean myocardial bloodflow obtained from two electrodes below each strain gauge (MBF in ml/min/100 g); AL VA = contractile force from strain gauge on apical region of left ventricle; LA TL VB = contractile force from strain gauge on basal region of left ventricle.
LATL
RHESUS 10-26-76 FIGURE 2. Cardiovascular responses to a one-minute occlusion of the circumflex coronary artery (CIRC) in an anesthetized rhesus monkey. The area of marked ischemia is restricted to the lateral wall of the left ventricle (LA TL VB). The severity of this ischemia is reflected in the appearance of systolic expansion during CIRC occlusion. In this animal, moderate ischemia and reduced contractile force were also noted at the apex of the anterior wall of the left ventricle (AL VA). Note that CIRC occlusion resulted in a marked decline in heart rate and moderate hypotension.
artery also resulted in a decrease in systolic aortic pressure. In contrast to the small tachycardia observed during LAD
The most notable difference between the responses to the two occlusions was the marked decrease in heart rate which
occlusion, circumflex occlusion resulted in a marked slowing of heart rate (45 ± 6 beats/min) in 11 of 13 animals. Figure 2 shows an example of the responses to circumflex occlusion obtained in the present study. For this animal, occlusion resulted in a 46% decline in systolic aortic blood pressure and a 45% decrease in heart rate. Within the ischemic region at the base of the left ventricle, MBF decreased 85% and the contractile force trace indicated systolic expansion during the occlusion period. Restoration of flow to the circumflex resulted in a return in both heart rate and blood pressure. Within the previously ischemic region, a 109% increase in MBF was recorded and a postocclusion overshoot of contractile force (49%) was also observed. Circumflex occlusion in this particular animal resulted in a 33% decline in MBF and a 40% decrease in contractile force in the apical region. Restoration of flow resulted in a 68% increase in MBF which was accompanied by a 17% overshoot in contractile force. The results obtained from all animals, however, indicated that restoration of flow in the circumflex did not result in significant alterations in myocardial blood flow and contractile force in the apical region. Results from circumflex occlusion are also summarized in table 1.
occurred during circumflex occlusion. This bradycardia may have affected regional cardiac function and blood flow in either the ischemic or nonischemic regions. Table 1 shows, however, that there were no significant differences in the average MBF and contractile force data from the ischemic regions for either occlusion except for the postocclusion overshoot in contractile force in the basal region after circumflex occlusion. This was significantly less than that obtained from the apical region after LAD occlusion. To determine if the autonomic nervous system was involved in any of these changes resulting from coronary artery occlusion, atropine and propranolol were administered in three animals and the occlusions were repeated. Autonomic blockade resulted in a significant decline in baseline heart rate and contractile force for the left ventricle. These results are summarized in table 2. LAD occlusion after autonomic blockade elicited a 13 ± 4% decrease in systolic aortic pressure without any significant change in heart rate. In contrast, circumflex occlusion also elicited a decrease in systolic aortic pressure and a marked slowing of heart rate (32 ± 7 beats/min). Changes in regional contractile force and myocardial blood flow during these occlusions were similar to those observed prior to autonomic blockade. Due to the fact that only three animals
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759
CHRONOTROPIC RESPONSES TO CAO/Alter, Hawkins, Evans TABLE 2. Cardiac Responses to Coronary Artery Occlusion after Autonomic Blockade¶ Heart rate (beats/min)
Control
During
139 +1
Post
Control
During
Post
12 2*
t 145 7 -32 7*
Post
Control During
=1=
t 147
9 6*
-29
t
Aortic pressure (mm Hg)
Contractile force (% change from preocclusion values) ALVA LATLVB
Anterior descending artery occlusion 118 13/85 * 25 103 17/68 28 -64 10 +11 14 120 18/88 28 +73 -23 +14 5
Circumflex artery occlusion 115* 8/70 10 101 -36 11 -80 20 8/57 9 118 10/75 10 +23 10 +57 17 Sinus node artery occlusiont 118 8/78 * 13 115 * 8/71 * 13 121 * 10/78 i 13
Myocardial blood flow (ml/min/100 g) ALVA LATLVB
119 22 31 11 412 162
106 * 21 101 37 125 28
111 17 6 65 172 18
99 18 22 10 194 35
*Changes in heart rate. tReturned to preocclusion values. tNo significant changes were observed in myocardial blood flow and contractile force during sinus node artery occlusion. lThree animals received atropine sulfate (0.5 mg/kg) and propranolol (0.5-1.0 mg/kg).
were studied after autonomic blockade, no statistical analysis of these data was performed. Another possible mechanism for the bradycardia observed during circumflex occlusion may be ischemia to the sinoatrial node. To evaluate this hypothesis, proximal branches of the circumflex artery were exposed and individually occluded. This revealed that occlusion of the first lateral branch (diameter < 1 mm) of the circumflex resulted in a bradycardia whose magnitude (48 ± 6 beats/min) was not significantly different from that recorded during occlusion of the proximal circumflex artery. Figure 3 depicts an example of these responses. Occlusion of the circumflex branch resulted in a marked bradycardia (70 beats/min) with no consistent changes in aortic pressure or regional contractile force. Myocardial blood flow was down somewhat during this occlusion, but the average response for all animals (table 1) failed to show any significant difference in regional myocardial blood flow during occlusion of this branch of the circumflex artery. After autonomic blockade, occlusion of this circumflex branch still elicited a decrease in heart rate (29 ± 6 beats/min) without any significant change in other cardiac parameters. Since occlusion of this branch resulted in a bradycardia which was not mediated by any apparent neural pathway, this artery was tentatively designated as the sinus node artery (SNA). To further analyze the effects of sinus node artery occlusion on cardiac pacemaker function, surface electrodes were secured to the lateral wall of the right atrium near its junction with the superior vena cava. Occlusion of either the proximal circumflex artery or the sinus node artery resulted in negative chronotropic responses which were accompanied by changes in the ECG and the right atrial electrogram (AEG). Examples of these changes are shown in figures 4 and 5. The upper traces are aortic pressure, and indicate when electrical activity resulted in mechanical systole. In figure 4, occlusion of the circumflex artery proximal to the origin of the sinus node artery resulted in a 55 beats/min decrease in heart rate with an onset latency of 16 seconds. After 30 seconds of occlusion, there was an atrial depolarization which failed to conduct to the A-V node. This was then followed by a premature ventricular contraction with retrograde conduction to the atrium. For the remaining
period of occlusion, the atrial electrogram indicated a sustained shift in the atrial depolarization wave vector, as was evident by the decrease in magnitude of the P wave of the AEG. This was algo reflected in a smaller P wave in the lead II ECG. In addition, circumflex occlusion resulted in a marked ST elevation and hypotension. In contrast, sinus node artery occlusion (fig. 5) failed to significantly alter systolic aortic pressure despite an 88 beats/min decrease in heart rate. Once again, the onset latency for the heart rate decline (13 sec) was approximately one-half that for the change in the P wave of the AEG. In this example, sinus CLIP ON
CLIP OFF
I
1-
150r-
B..
100
|
E 50O E
H.R.:E
l5O
go100
MBTF 130
120
109
mu = 0
ALVfA
I" -I
LI.
10 sec
MBF 100
I.-
Z LATLY
0 UI
RHESUS 9-9-76 FIGURE 3. Cardiovascular responses to a one-minute occlusion of the sinus node artery in an anesthetized rhesus monkey. Interruption of flow to the sinus node region failed to cause any major changes in regional myocardial bloodflow or contractileforce in the left ventricle, but did result in a marked decline in heart rate.
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760
CIRCULATION + 30 sEc
CONTROL B.P.
0150 50 l0
H.R. 8PM
176
231
RIGHT ATRIAL ELECTROGRAM
L-11
ECG
RHESUS 8914 9-22-76 FIGURE 4. Effect of a one-minute circumflex coironary artery occlusion on cardiac electrical activity in an anest,ietized rhesus monkey. Thirty seconds after the onset of occlusicon, heart rate (HR) and blood pressure (BP) had declined significan tly. The atrial electrogram (AEG) revealed an atrial depolarization which was not conducted to the ventricles. This was followed by a piremature ventricular contraction with apparent retrograde condfuction to the atrium. Following this episode, the AEG revealed an ialtered atrial depolarization vector suggesting a change in the pacennaker site during the period of sinus node ischemia. Note also the dramatic change in the electrocardiogram (L-II ECG) duri ng circumflex occlusion.
node artery occlusion resulted in both a reduiced P wave magnitude and decreased PR interval, which sulggests a shift in the pacemaker site. Since the left ventricular muscle was not ischemic, the QRS complex of the ECG ri emained unchanged. Right atrial electrograms were record[ed in a total of four animals, and in all cases, sinus node arte ry occlusion resulted in a significant alteration in the atrial deIrpolarization --- - ---
CONTROL
c 150 _ B.P. E 100E So H.R. BPM
VOL 57, No 4, APRIL 1978
wave, the latter having an onset latency significantly longer than that for the bradycardia. Postmortem examination of the latex-filled coronary arteries was conducted on 11 animals. In four of these, the sinus node region was perfused by a single artery which originated from the circumflex branch of the main left coronary artery. After arising as the first lateral branch of the circumflex, the sinus node artery gave off branches to the medial surface of the left atrium. It then traversed within the epicardium of the interatrial septum to the superior medial surface of the right atrium, reaching the area of the junction of the right atrial appendage and the superior vena cava. In five animals (fig. 6), the sinoatrial region appeared to be perfused by this circumflex branch as well as one or more smaller branches of the right coronary artery. On most occasions, the sinus node artery (origin from circumflex) also continued onto the lateral surface of the superior vena cava, forming a loop around it. In this small number of animals, there did not appear to be a correlation between the magnitude of the negative chronotropic response to sinus node artery occlusion and the presence of additional sinus node perfusion from a nonoccluded right coronary artery branch. In the remaining two animals, the first lateral branch of the CIRC terminated near the interatrial septum. In these animals, right coronary branches reached the sinus node region; and in one, the right coronary branch looped around the superior vena cava. Physiological evidence was obtained from one of these animals that the right coronary artery might occasionally serve as the sole source of sinus node perfusion. Circumflex artery occlusion failed to elicit a bradycardia, but right coronary artery occlusion resulted in a 20 beats/min bradycardia which was not eliminated by autonomic blockade. Therefore, in 85% of rhesus studied, the CIRC is the origin of the sinus node artery, whereas the latter originates from the proximal right coronary in the remaining 15%. In either case, interruption of flow in the sinus node artery results in an ischemia-induced bradycardia.
+ 50 sQc
+ 2 min
143
143
fKrKfi 231
RIGHT ATRIAL ELECTROGRAM
L-11 ECG
tP
-
1-
-AI%1-IJ
RHESUS 8914 9-22-76 FIGURE 5. Effect of a two-minute sinus node artery occlusion on cardiac electrical activity in an anesthetized rhesus monkey. Fifty seconds after onset of occlusion, the atrial electrogram reveals an alteration in the atrial depolarization vector as well as a decrease in the PR interval, which suggests a shift in pacemaker site. Despite a marked decline in heart rate (88 beats/min), systolic aortic blood pressure was not decreased. Downloaded from http://circ.ahajournals.org/ at University of Hawaii--Manoa on June 22, 2015
CHRONOTROPIC RESPONSES TO CAO/Alter, Hawkins, Evans
FIGURE 6. Coronary artery perfusion of the sinus node region in a rhesus monkey. Sinus node region in this monkey is primarily perfused by a proximal circumflex branch (arrows), but additional perfusion may be supplied by branches of the right coronary artery.
Discussion The results of this study indicate that chronotropic responses to coronary artery occlusion in the rhesus may be mediated by at least two mechanisms. During LAD occlusion, a small cardioacceleration was observed which was eliminated by autonomic blockade. It was concluded that this positive chronotropic response results from a baroreceptor reflex to the hypotension which occurred during LAD occlusion. It was surprising that the size of the response was small, considering that systolic aortic pressure fell significantly. A possible explanation for this finding comes from the recent report by Feola et al.,3 who demonstrated that despite hypotension, cardiac sympathetic efferent discharge decreased during the first 30 minutes of circumflex artery occlusion. Cardiac pacemaker ischemia is another mechanism which may be responsible for heart rate changes during coronary artery occlusion in the rhesus. In the present study, interruption of blood flow in the parent CIRC or its sinus node branch resulted in a marked bradycardia accompanied by an alteration in the P wave of the right atrial electrogram. Atropine (vagolytic) and propranolol (sympatholytic) administration failed to alter these responses, thus indicating that the changes were not dependent on any autonomic neural reflex. A suitable model for the study of the effect of sinus node ischemia on cardiac function has not been previously reported. Early studies in the dog" contended that bradycardia was produced by sinus node artery occlusion, but James and Reemtsma12 were unable to confirm these findings. This difference was finally resolved by Billette et al.,'3 who were able to obtain bradycardia in the dog by occluding the sinus node artery as well as all collaterals to the pacemaker region. This necessity for removing collateral blood flow before inducing any disruption in pacemaker function does not appear to be necessary in the rhesus. In the present study, five animals appeared to receive sinus node perfusion from branches of both the circumflex and right
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coronary arteries, and yet occlusion of just the circumflex branch was capable of inducing marked slowing of the heart rate. It is possible that occlusion of both blood supplies to the sinus node region may have induced even more dramatic pacemaker changes. However, the fact that occlusion of just the sinus node artery leads to pacemaker dysfunction may make this animal particularly useful in the study of the sick sinus syndrome. A recent report on patients with coronary artery disease by Jordan et al.14 has demonstrated a relationship between sinus node dysfunction and obstruction of the sinus node artery or its parent vessel. The rhesus monkey may be a useful animal model for the study of sinus node function after permanent interruption of sinus node artery blood flow. In the rhesus, occlusion of the sinus node artery produced both bradycardia and alterations in the atrial electrical activity, with the latter having a latency approximately twice that of the former. The prolonged latency for change in the P wave of the AEG may be explained on the basis of the spread in ischemia in this region. Interruption of flow in the SNA first leads to sufficient ischemia in pacemaker tissue to decrease spontaneous rate, but the region of ischemia must further enlarge before any noticeable change in the atrial depolarization vector can be detected. Atrial arrhythmias, however, were not observed in the present study. The production of altered- atrial electrical activity certainly suggests that occlusions for longer than one minute may lead to these arrhythmias. Studies are currently underway to determine if prolonged sinus node ischemia will lead to atrial arrhythmias. In addition, it is possible that simultaneous ischemia in both ventricular myocardium and the sinus node region may predispose to an increased incidence of ventricular arrhythmias as was suggested by Webb et al.5 The distribution of sinus node arteries in experimental animals shows a significant difference among species. In the dog, the sinus node artery originates as a distal branch of the right coronary artery in 90% of the cases,'5 while arising from the proximal circumflex in the remaining animals. On the other hand, sinus node perfusion in the cat is usually from a proximal branch of the right coronary artery (90%) with the remainder coming from a CIRC branch.' In man, James'5 has shown that the sinus node is perfused by a proximal branch of the right coronary artery in 60% of the hearts examined, whereas the CIRC was the origin in 40% of the cases. In the present study, physiological and anatomical data indicate that the CIRC was the source of sinus node perfusion in approximately 85% of rhesus monkeys. In the other two monkeys, it appears that the sinus node received its blood supply from branches which arose near the origin of the right coronary artery. Anatomically, the course of the sinus node artery in the rhesus was quite similar to that reported in man;'5 that is, the artery passed to the sinus node region and then formed a loop around the superior vena cava. Based on the rather dramatic changes in heart rate and P waveform of the AEG, during just a one-minute interruption of flow in the sinus node artery, it appears that the rhesus monkey will serve as a useful model in studying the long-term effects of sinus node ischemia. It will be interesting to see if prolonged ischemia will lead to important atrial arrhythmias or if sufficient collateral flow exists to permit the sinus node region to continue in its role as the cardiac pacemaker.
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It was somewhat surprising that coronary artery occlusion in the rhesus failed to initiate a neurally mediated bradycardia. This cardio-cardiac reflex has been demonstrated in cats,' and appears to be responsible for most incidents of bradycardia seen in post-myocardial infarction patients.5 In both man and cat, atropine usually eliminated or markedly reduced the bradycardia. Other studies in nonhuman primates'6-18 have also failed to observe any vagally mediated bradycardia during coronary artery occlusion. It is possible that these studies in baboons and rhesus monkeys did not report any ischemia-induced bradycardia during occlusion because their experiments did not involve proximal CIRC occlusion. It is also possible that in the present study, the afferent pathways involved in this cardio-cardiac reflex were inadvertently interrupted during exposure of the coronary arteries. This is not likely, however, because studies are currently underway utilizing the same preparation in which we are studying the negative chronotropic response to coronary artery occlusion in the cat. This preparation consistently provides an experimental model in which occlusion results in a neurally mediated bradycardia (Alter, unpublished observations). Another possible cause for the lack of vagally mediated bradycardia during occlusion is the selection of anesthetic. However, the present study utilized the same anesthetic (chloralose) as in cat studies.' Therefore, at this time, we must conclude that in the rhesus monkey, coronary artery occlusion may lead to either a small cardioacceleration mediated by the autonomic nervous system or a bradycardia due to pacemaker ischemia, the latter occurring when either the CIRC or its sinus node branch is occluded. Brief occlusions of coronary arteries resulted in marked changes in myocardial blood flow and contractile force within the ischemic region. In contrast, nonischemic regions failed to demonstrate any significant changes in contractile force. These results differ from those reported by Pashkow'5 who showed that nonischemic myocardium demonstrated an increased contractile force during brief occlusions of the LAD in anesthetized pigs. This may be related to a species and/or anesthetic difference in the two studies. In addition, increased myocardial blood flow in regions outside an ischemic region have been reported in anesthetized dogs;20 however, once again, coronary artery occlusions failed to elicit such changes in the anesthetized rhesus. Removal of the occlusion clip resulted in a marked reactive hyperemia and postocclusion overshoot in contractile force (table 1) in the previously ischemic region whereas no changes were seen in these parameters in the nonischemic region. The only significant difference in response seen after coronary artery occlusion was a decreased overshoot in contractile force at the base of the left ventricle (25 ± 7%) after CIRC occlusion as opposed to that observed at the apex after LAD occlusion (55 ± 7%). Several possible causes for this decrease can be proposed. The decreased response may reflect a decreased magnitude of ischemia in the basal myocardium during CIRC occlusion when compared to that seen for LAD occlusion. This is not likely, however, because myocardial blood flow and contractile force decrease were comparable for both occlusion sites. An alternative explanation might be the lesser involvement of sympathetic inotropic reflexes after CIRC occlusions. This does not appear
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to be a realistic hypothesis because postocclusion overshoot in contractile force persisted after autonomic blockade as has been reported by Pagani et al." Indeed, the overshoots in contractile force still demonstrated a greater magnitude after LAD occlusion. Another possible mechanism may lie in the decrease in total ischemic insult during CIRC occlusion. Due to the presence of a bradycardia, the total energy requirements during CIRC occlusion might be expected to be less than those experienced during LAD occlusion where heart rate was unchanged or slightly increased. If the magnitude of the overshoot in contractile force is somewhat dependent on this ischemic insult, then as a consequence of the bradycardia, it would be expected that the overshoot in contractile force would be less. Support for this last possibility awaits additional experiments. Acknowledgment The authors express their appreciation to L. J. Parkhurst and D. E. Padgett for their technical assitance and to C. Anagnostelis for the atrial electrodes.
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Etiology of the negative chronotropic responses to transient coronary artery occlusion in the anesthetized rhesus monkey. W A Alter, 3rd, R N Hawkins and D E Evans Circulation. 1978;57:756-762 doi: 10.1161/01.CIR.57.4.756 Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1978 American Heart Association, Inc. All rights reserved. Print ISSN: 0009-7322. Online ISSN: 1524-4539
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