Electrocardiographic changes after bilateral carotid sinus denervation in the rat HANS R. BAUR AND CLAUS A. PIERACH University of Minnesota Unit for Teaching and Research in Internal Medicine, Division of Cardiology, Northwestern Hospital, Minneapolis, Minnesota 554~
BAWR, HANS R., AND CLAUS A. PIERACH. Electrocardiugraphic changes after bi&ateral carotid sinus denervation in the rat. Am. J. Physiol. 237(4): H475-H480, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(4): H475-H480, 1979.-The effect of bilateral carotid sinus denervation (CSD) on the ECG and on the monophasic ventricular action potential (MAP) was studied in 28 and 4 rats, respectively. After CSD, T wave changes, sin&r to those seen in man after carotid endarterectomy, were observed and mean Q-T prolongations of 19 ms were recorded (P < 0.001). Mean MAP increased by 10 ms (P < 0.02). P-QRS and heart rate remained stable. Propranolol, 10 mg/kg iv, before CSD and 10 rnga kg-’ day-’ iv for 2 days prevented ah ECG abnormalities. Atropine, 1 mg/kg iv, before CSD and 2.5 mg=kg-’ day-’ iv for 2 days had no effect. Isoproterenol, 0.02-0.06 r-18iv, after CSD produced further lengthening of the MAP and Q-T interval. Blood gases and electrolytes remained unchanged, and cardiac histology was unremarkable. These results suggest that CSD produces alterations in cardiac sympathetic activity in the rat leading to MAP and Q-T prolongations and to changes in the T wave form. Similar mechanisms may be operative after carotid endarterectomy in man.
and
This report concerns an attempt to elucidate the mechanism of these ECG changes in an animal model by studying the ECG of rats having a sequential carotid sinus denervation. It was recognized that species differences in respect to the importance of the carotid sinus depressor nerves relative to other baroreceptors, the initiation and distribution of sympathetic activity, and the action of catecholamines on the rat Purkinje fibers and myocardium could vitiate any easy transfer of the observations to the human.
n
l
autonomic imbalance; pathetic activity
ventricular
repolarization;
cardiac
sym-
SINUS and carotid body in the regulation of the circulation and respiration have been part of the classic physiological contributions of the past half-century (12). With the advent of endarter&omy for obstruction at the bifurcation of the common carotid artery, investigations revealed alterations in the anoxic drive of respiration and modifications in blood pressure in accord with the findings in the dog (2, 13,21). We have described gross T wave changes in five patients after a staged bilateral carotid endarterectomy (5). These abnormalities were similar to those attributed to neurogenic causes (sympathetic imbalance) except that the Q-T interval was only minimally prolonged. The mechanism of these changes was not established. If they were neurogenic, they could have been initiated by cerebral lesions related to the operation, e.g., by emboli or by local trauma to the sympathetic chain or increased sympathetic traffic consequent to loss of the baroreceptor reflexes from the carotid sinus, Neither the degree of hypertension postoperatively nor the nature of the repolarization abnormality supported a hypertensive etiology, and there were no supporting factors for a subendocardial ischemic lesion. THE ROLES OF THE CAROTID
0363~6135/79/0000-0000$01.25
Copyright
0
1979
the
American Physiological
METHODS
Twelve male rats (Sprague-Dawley), weighing 200-240 g, were anesthetized with pentobarbital sodium 4 mg/lOO g intraperitoneally. The right or left carotid bifurcation was exposed, and the nerve plexus innervating the carotid sinus and carotid body was removed. The anatomy of this area has been well illustrated by Greene (10) and DeKock (7). The vagus nerve was identified and carefully preserved during this procedure, but some injury to the superior cervical ganglion was sometimes inevitable due to its close proximity to the carotid bifurcation. However, the major cardiac sympathetic fibers coming from the middle and inferior cervical ganglia were not traumatized. Twelve-lead electrocardiograms were obtained before and after surgery, with a Health Tech electrocardiograph. This equipment has a virtually flat response to 360 cycles/s. The ECG signals were simultaneously transmitted to a Sanborn oscilloscope where any single ECG lead could be recorded at any desired instant with a Polaroid camera (usually V3 and V5 were selected). The site of each precordial electrode was marked by a steel clip in order to get comparable electrocardiograms. The second denervation procedure was done 24 h later. Preliminary investigations had shown that bilateral denervation at the same operation regularly resulted in respiratory arrest and/or asystole, probably due to mechanical carotid sinus stimulation. During the second procedure, a preand postoperative ECG were again recorded. Twentyfour hours later another ECG was obtained, and the rats were killed. Their hearts were fixed in 10% Formalin for histological examination. The animals tolerated the sequential denervation procedures well, and within 0.5 h they had recovered from the anesthesia and showed a normal behavior in their cages. After the second denervation their respiration became slower and deeper, and initially it was irregular. Society
H475
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H476
Before the first, and after the last ECG, arterial blood was drawn from a carotid artery in three animals for measurements of pH, POT, and PCO~ and of sodium, potassium, and calcium concentration. The effectiveness of the carotid sinus denervation was assessedin three additional animals given the same protocol in which intra-arterial blood pressures were recorded before the first and after the second denervation. For this purpose a femoral artery was cannulated with a small polyethylene catheter connected to a HewlettPackard transducer, and recordings were obtained on a Sanborn 301 blood pressure recorder. The same protocol was followed in an additional 20 rats, subdivided into 5 groups of 4 animals. In group I, a smaIl polyethylene catheter was introduced into the left internal jugular vein after the second operation, and during the recording of the ECG, propranolol, 10 mg/kg (2 mg/ml), was injected rapidly. The ECG was obtained before, and 5 min after the injection. Group II received propranolol, 10 mg/kg intravenously before the first and before the second surgery, followed by 10 mg/k g Per day by a continuous intravenous infusion with a Harvard pump (5 ml/day), starting after the first surgery until the animals were killed. Group III received atropine 1 mg/ kg intravenously before the first and before the second surgery, followed by 2.5 mg/kg per day by continuous infusion, similar to group II. In group IV the carotid sinus area was left intact, and only the superior cervical ganglia were resected in a staged manner. In group V simulated monophasic ventricular action potentials (MAP) were recorded from the anterior ventricular wall, toget .her with a conventional surface ECG at the same time intervals as previously described. A 25-gauge needle was advanced transthoracically through the left fourth or fifth intercostal space perpendicular to the chest wall until MAPS were recorded. The site of chest entry was marked to aid in the obtaining of comparable tracings. Twenty-four hours after the second surgery, isoproterenol, 0.02-0.06 ,ug (0.2 pg/ml) was injected intravenously under continuous ECG monitoring. Twelve-lead ECGs and MAPS were obtained before and at 1 and 5 min after termination of the injection. In two additional animals isoproterenol injections were made without carotid sinus denervation. Q-T intervals were measured in V5, because measurements in this lead were easy and most reproducible. Preliminary studies had shown that there was very little variation from lead to lead. The MAP duration, reflected by the monophasic injury potential, was measured as described by Autenrieth et al. (3), Hoffman et al. (14), and Kuo and Surawicz (16). The variations of measurements were within 5 ms. The records of the MAP were acceptable if 10 or more similar potentials were recorded with a smooth repolarization, and the tracing had an amplitude exceeding 3 mV.
H. K. BAUR
AND
AVR
C. A. I’IERACH
AVL
AVF
V5 . . .
. . .
. . ., *...
~ ,
.
.
. .
. . .
.
.
. . .
the first (0.
. .
.
. .
,
,
. . .
.
. . . ,. .
. .
.
,
.
.
.
I... . .
,
,
.
. .
.
L .
.
.
FIG. 1. Twelve-lead KG before and 24 h after the second denervatlon
..,. 1::: .
.
.
H
. * .
. . . . . . ..a, . . . . . ..L..
.
(A), after
,
the second
(B),
were present, and these became more apparent after the second denervation. Changes in cardiac repolarization were pronounced in the records 24 h after the second procedure, at which time broad rounded T waves and mean Q-T prolongations of 19 ms were present (Table 1). There was very little variation in the Q-T interval or MAP before surgery (Table l), and no rate-related changes were observed. P-R intervals and QRS complexes were not changed. Changes in heart rate were transient and of minor degree, averaging -3 beats/min after the first, and +26 beats/min after the second procedure. Propranolol, administered 24 h after the second denervation, produced slowing of the heart rate of 80-100 beats/min, but did not reverse the T wave changes. RESULTS However, when the drug had been given preoperatively A typical ECG sequence is depicted in Figs. 1 and 2. and then as continuous infusion, no ECG abnormalities c The first denervation had no immediate effects. After 24 occurred (Fig. 3). No preventive effect was demonstrated h, before the second surgery, slight T wave alterations with atropine (Fig. 4). Resection of the superior cervical
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ECG
CHANGES
AFTER
CAKOTID
SINUS
H477
DENEIWATION
2. ECG lead V5 before the first th e second (B), and 24 h after the second denervation (C). (For details, see text.) FIG.
(A), after
TABLE
animals (light microscopy). There were no subendocardial hemorrhages.
1. Heart rates, Q-T intervals,
beforeand
and MAPS after carotid sinus denervation Heart Rate, beats/min
Q-T Interval, ms
MAP,
ms
DISCUSSION
The observed electrocardiographic changes are comparable with those noticed in man (5), although no inversion of T wave polarity was noted in the rat. Although 426 t 18.0 64 t 1.63 34 2 2.50 heart rates were not controlled in this study, the Q-T and MAP alterations occurred at similar mean heart rates 452 t 14.3 (before surgery 434 beats/min; 24 h after surgery 431 431 ?I 16.8 82 t 2.16 45 k 1.79 beats/min) making the possibility of rate-related changes unlikely. The QRS complex remained unchanged after Values are means t SE in 12 rats. Prolongations of Q-T and MAY carotid sinus denervation (CSD), implying that the obdurations are significant (unpaired f test, P < 0.001 and P < 0.02, served T wave alterations represent primary abnormalirespectively). ties in ventricular recovery. Denervation of the baro- and chemoreceptor has proganglion had no effect on heart rate or the ECG (Fig. 5). found cardiovascular and respiratory effects that are The mean MAP recorded from the anterior ventricular mediated through reciprocal changes in sympathetic and wall, 24 h after the second denervation, was prolonged parasympathetic nervous system activity (12). This view by 10 ms (Table 1 and, Fig. 6). had been confirmed (20) and challenged (6, 9) in recent cIsoproterenol injected at the time of the maximal ECG years, and especially the role of the efferent vagal activity changes did not change the MAP or reverse the T wave in the carotid sinus reflex remains controversial. Howchanges when small doses of 0.02 ,ug were used. Doses of ever, it is generally agreed that cutting the carotid sinus 0.04 and 0.06 pg regularly lengthened the MAP and nerves, as we did in our rats, leads to increased cardiac produced S-T depressions and Q-T interval prolonga- sympathetic activity due to loss of tonic inhibition of the tions on the surface ECG. The same changes were ob- sympathetic medullary centers (6,8) and perhaps also to served in the two control animals. a decrease of the efferent vagal tone (20). The prevention The mean arterial blood pressure rose 30-50 mmHg in of all ECG abnormalities with propranolol, but not with all three rats tested, demonstrating the effectiveness of atropl?-e, in our study, is in agreement with this view and suggests that the observed changes in the T wave and the procedure. No difference was found between pre- and postopera- the MAP are brought about by a changed sympathetic tive values for blood gasesand electrolytes in the three discharge pattern to the heart. The postoperative inanimals tested. The cardiac histology was normal in all crease in heart rate and bl.ood pressure after the second Preoperative Immediately after 1st denervation 24 h after 1st denervation Immediately after 2nd denervation 24 h after 2nd denervation
434 t 12.2 431 -+ 16.0
63 t 1.35
35 t 2.55
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H. Ii.
lervat
iorl (13). Cmt inuous
propranolol
HALJR
AND
C. A. PIIWACH
infusion.
(Fw
det ails, see text.)
see text.)
IW;. 6. -MAP
befort
the first
(A) and 24 h after
the second
denervation
(B).
(For
details,
see text.)
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
ECG
CHANGES
AFTEK
CAKOTII)
SINUS
H479
DENEKVATION
procedure would be in accord with this hypothesis, although a nonspecific stimulus due to the surgery cannot be ruled out, The concomitant loss of chemoreceptor activi .ty produced respiratory changes COmparab le to those observed in man (15). It is not clea.r if carotid body destruction added to the changes caused bv the carotid sinus denervation. The cardiovascular infiuence of the carotid body is primarily mediated t,hrough the vagus nerve in the cat and in the dog (8, 19), and its loss might produce a decrease in cardiac vagal tone. The repolarization changes and the MAP prolongation observed in our study closely resemble the changes seen after right stellate ganglion transsection or left stimulation in the dog (16, 23). Although strong restraints are mandatory in comparing different species, the similarities suggest that CSD may not initiate a uniform distribution of increased sympathetic neural traffic to the heart, but possiblv produce regional differences in the duration of individual action potentials, resulting in an altered temporal and spatial recoverv process of the myocardium, However, this remains speculative and is &t based on actual evidence. We only documented prolonged MAPS on the anterior wall, and if shortening of the MAP occurred in other areas of the ventricle, it was not accessible by our technique. The injury potentials recorded with the needle electrodes may not be exactly comparable to membrane potentials. However, they were reproducible, and their duration did not depend on the magnitude of the small epicardial injury produced by the tip of the needle. At autopsy, the needle marks were found within l-2 mm on the anterior surface of the heart. The superior sympathetic ganglion was occasionally damaged during CSD, and the possibility had to be considered that bilateral injury to the sympathetic nerve fibers rather than CSD could have been responsible for the observed ECG changes. However, bilateral transsec-
tion of the sympathetic trunk below the superior cervical ganglion did not have any effect on the ECG. Other factors known to influence the T wave form, like electrolyte disorders or drugs, did not play a role in our experiment. In contrast to some other experimental studies (I I), cardiac histology was unremarkable in our rats indicating that the neurogenic T wave changes are noi necessarily associated with myocardial necrosis or hemodmge. Isoproterenoi, even in small doses, lengthened the MAP, both in the control animal and in the rat subjected to CSD. This is consistent with the observed effect of norepinephrine on the membrane potentials of the rat atria (22) and in contrast to changes observed in the canine heart (3, IS), where rate effects may dominate the situation. It is possible that the changes we have previously described in man (5) are also responsible for the observed repolarization changes in the experimental animal, as we have experimentally produced a more pronounced variant of a situation that might occur after surgical handling of the carotid arteries in man (2, 13, 21). Mechanisms similar to those discussed in the present study have been put forward to explain repolarization changes seen with cerebral vascular disorders (1) after experimental stimulation of certain brain areas with known sympathetic connections (11, 17, 18) or after intracoronary injections of epinephrine (4). We thank I3r. H. B. Burchell for advice in designing this study and preparing the manuscript and Dr. J. H. Keed for technical assistance. This work was supported in part by a grant from the Swiss National Foundation for Kesearch. Address reprint requesh to C. A. Pierach, M. D., Abbott-Northwestern Hospital, 810 East 27th St., Minneapolis MN 55407 Keceived
23 March
1979; accepted
in final
form
21 June
1979.
KE=F’EKENCES I.
2.
3.
4.
5.
6.
7.
t3.
9.
ARII~DSKOV, J. A., K. MILLAH, M. J. BURGESS, AND W. VINCENT. The electrocardiogram and the central nervous system. Yrog. Cardio~vz.w. Dis. 13: 210-2 16, 1970. ANCELL-JAMES, J. E., AND J. S!. I'. LUMIXY. The effects of carotid endarterectomy on the mechanical propert,ies of the carotid sinus and carotid sinus nerve activity in atherosclerotic patients. HF. J. Siug. 61: 805-810, 1974. AUTENRIETH, G., B. SURAWICZ, C. S. Krro, ANY M. ARITA. l’rimary T-wave abnormalities caused by uniform and regional shortening of ventricular monophasic action potential in dug. Cirddort 51: 668-676, 1975. LARGER, A. C., J. A. HERD, AND M. R. LIEHOWITZ. Chronic catheterization of the coronary artery: induction of GCG pattern of myocardial ischemia by in tracoronary epinephrine. I“rr~ Sou. Es’xp. Biol. Med. 107: 474-477, 1961. RAUR, H. R., AND C. A. PIERACH. Electrocardiographic changes after bilateral carotid endarterectomy. N. EngL .I. Med. 291: 11211122, 1974. BEHKOWITZ, W. L)., R. J. SCHERLAG, E. STEIN, AND A. N. DAMATO. Kelative roles of sympathetic and parasympathetic nervous systems in the carotid sinus reflex in dogs. Circ. Res. 24: 447-455, 1969. DEKOCK, L. I,. The carotid body system of the higher vertebrat,es. At*tn i4nat. 37: 265-279, 1959. I~OWNING, S. E., AND J. H. SIE:(;EI,. Baroreceptor and chemoreceptor influences on sympathetic discharge to the heart, Am. J. Yhysiol. 204: 471-479, 1963. GIJCK, G., ANI) E. BRAUNWAIJX Relative roles of the sympathetic and parasympathetic nervous systems in the reflex control of heart
rate. Circ. Hes. 16: 363475, 1965. 10. GREENE, E. C. Anatomy of the Rat. Philadelphia: The American Philosophical Society, 1935. 11. GREENHOOT, J. H., AND U. I). REICHENBACH. Cardiac injury and subarachnoid hemorrhage. J. Neurosurg. 30: 52 1-531, 1969. 12. HEIMANS, C., AND E. NEILS. Reflexogenic Areas of the Cardiovasc&r S&stem. Roston: Little, Brown, 1958. 13. HICKEY, R. F., W. K. EHRENFELD, F. N. HAMILTON, AND C. I-? LARSON. Bilateral carotid endarterectomy with attempted preservation of the carotid body function. Ann. Surg. 175: 268-273, 1972. 14. HOFFMAN, B. F., 1’. F. CRANEFIELD, E. LEPESCHKIN, B. SURAWICZ, AND H. C. HERRLICH. Comparison of cardiac monophasic action potentials recorded by intracellular and suction electrodes. Am. J. Physiol. 196: 1297-1301, 1959. 15. HOI,TON, P., AND J. B. WOOD. The effects of bilateral removal of the carotid bodies and denervation of the carotid sinuses in two human subjects. J. Physiol. London 181: 365-378, 1965. 16. Kuo, C. S., AND B. SURAWICZ. Ventricular monophasic action potential changes associated with neurogenic T-wave abnormalities and isoproterenol administration in dogs. An. J. CardioZ. 38: 170177, 1976. 17. MELVILLE, K. I., B. BLUM, H. E. SHISTER, AND M. D. SILVER. Cardiac ischemic changes and arrhythmias induced by hypothalamic stimulation. Am. J. Cardiol. 12: 781-791, 1963. 18. PORTER, K. W., K. KAMIKAWA, AND J. H. GREENHOOT. Persistent electrocardiographic abnormalities experimentally induced by stimulation of the brain. Am. Heart J. 64: 815-819, 1962. 19. PRZBYLA, A. C., G. A. BOBB, S. H. LAU, AND A. N. DAMATO.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
H480 Intracardiac chemoreflex.
conduction disturbances produced via the carotid Am. J. Physiol. 222: 959-966, 1972. 20. THAMES, M. I)., AND H. A. KONTOS. Mechanisms of baroreceptorinduced changes in heart rate. Am. J. Phpiol. 218: 2X-256, 1970. 21. WADE, J. G., C, P. LARSON, R. F. HICKEY, W. K. EHRENFEI~D, AND J. W. SEVERINGHAWS. Effect of carotid endarterectomy on carotid chemoreceptor and baroreceptor function in man. N. Eq$ J. Med. 282: 823-829, 1970.
H. R. BAUR
AND
C. A. PIERACH
22. WEBB, J. L., AND P. B. HOLLANDER. The action of acetylcholine and epinephrine on the cellular membrane potentials and contractility of rat atrium. Circ. Res. 4: 332-336, 1956. 23. YANOWITZ, F., J. B. PRESTON, AND J. A. ABILD~K~V. Functional distribution of right and left stellate innervation to the ventricles: production of neurogenic electrocardiographic changes by unilateral alteration of sympathetic tone. Circ. Res. 18: 416-428, 1966.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
BAWR, HANS R., AND CLAUS A. PIERACH. Electrocardiugraphic changes after bi&ateral carotid sinus denervation in the rat. Am. J. Physiol. 237(4): H475-H480, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(4): H475-H480, 1979.-The effect of bilateral carotid sinus denervation (CSD) on the ECG and on the monophasic ventricular action potential (MAP) was studied in 28 and 4 rats, respectively. After CSD, T wave changes, sin&r to those seen in man after carotid endarterectomy, were observed and mean Q-T prolongations of 19 ms were recorded (P < 0.001). Mean MAP increased by 10 ms (P < 0.02). P-QRS and heart rate remained stable. Propranolol, 10 mg/kg iv, before CSD and 10 rnga kg-’ day-’ iv for 2 days prevented ah ECG abnormalities. Atropine, 1 mg/kg iv, before CSD and 2.5 mg=kg-’ day-’ iv for 2 days had no effect. Isoproterenol, 0.02-0.06 r-18iv, after CSD produced further lengthening of the MAP and Q-T interval. Blood gases and electrolytes remained unchanged, and cardiac histology was unremarkable. These results suggest that CSD produces alterations in cardiac sympathetic activity in the rat leading to MAP and Q-T prolongations and to changes in the T wave form. Similar mechanisms may be operative after carotid endarterectomy in man.
and
This report concerns an attempt to elucidate the mechanism of these ECG changes in an animal model by studying the ECG of rats having a sequential carotid sinus denervation. It was recognized that species differences in respect to the importance of the carotid sinus depressor nerves relative to other baroreceptors, the initiation and distribution of sympathetic activity, and the action of catecholamines on the rat Purkinje fibers and myocardium could vitiate any easy transfer of the observations to the human.
n
l
autonomic imbalance; pathetic activity
ventricular
repolarization;
cardiac
sym-
SINUS and carotid body in the regulation of the circulation and respiration have been part of the classic physiological contributions of the past half-century (12). With the advent of endarter&omy for obstruction at the bifurcation of the common carotid artery, investigations revealed alterations in the anoxic drive of respiration and modifications in blood pressure in accord with the findings in the dog (2, 13,21). We have described gross T wave changes in five patients after a staged bilateral carotid endarterectomy (5). These abnormalities were similar to those attributed to neurogenic causes (sympathetic imbalance) except that the Q-T interval was only minimally prolonged. The mechanism of these changes was not established. If they were neurogenic, they could have been initiated by cerebral lesions related to the operation, e.g., by emboli or by local trauma to the sympathetic chain or increased sympathetic traffic consequent to loss of the baroreceptor reflexes from the carotid sinus, Neither the degree of hypertension postoperatively nor the nature of the repolarization abnormality supported a hypertensive etiology, and there were no supporting factors for a subendocardial ischemic lesion. THE ROLES OF THE CAROTID
0363~6135/79/0000-0000$01.25
Copyright
0
1979
the
American Physiological
METHODS
Twelve male rats (Sprague-Dawley), weighing 200-240 g, were anesthetized with pentobarbital sodium 4 mg/lOO g intraperitoneally. The right or left carotid bifurcation was exposed, and the nerve plexus innervating the carotid sinus and carotid body was removed. The anatomy of this area has been well illustrated by Greene (10) and DeKock (7). The vagus nerve was identified and carefully preserved during this procedure, but some injury to the superior cervical ganglion was sometimes inevitable due to its close proximity to the carotid bifurcation. However, the major cardiac sympathetic fibers coming from the middle and inferior cervical ganglia were not traumatized. Twelve-lead electrocardiograms were obtained before and after surgery, with a Health Tech electrocardiograph. This equipment has a virtually flat response to 360 cycles/s. The ECG signals were simultaneously transmitted to a Sanborn oscilloscope where any single ECG lead could be recorded at any desired instant with a Polaroid camera (usually V3 and V5 were selected). The site of each precordial electrode was marked by a steel clip in order to get comparable electrocardiograms. The second denervation procedure was done 24 h later. Preliminary investigations had shown that bilateral denervation at the same operation regularly resulted in respiratory arrest and/or asystole, probably due to mechanical carotid sinus stimulation. During the second procedure, a preand postoperative ECG were again recorded. Twentyfour hours later another ECG was obtained, and the rats were killed. Their hearts were fixed in 10% Formalin for histological examination. The animals tolerated the sequential denervation procedures well, and within 0.5 h they had recovered from the anesthesia and showed a normal behavior in their cages. After the second denervation their respiration became slower and deeper, and initially it was irregular. Society
H475
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
H476
Before the first, and after the last ECG, arterial blood was drawn from a carotid artery in three animals for measurements of pH, POT, and PCO~ and of sodium, potassium, and calcium concentration. The effectiveness of the carotid sinus denervation was assessedin three additional animals given the same protocol in which intra-arterial blood pressures were recorded before the first and after the second denervation. For this purpose a femoral artery was cannulated with a small polyethylene catheter connected to a HewlettPackard transducer, and recordings were obtained on a Sanborn 301 blood pressure recorder. The same protocol was followed in an additional 20 rats, subdivided into 5 groups of 4 animals. In group I, a smaIl polyethylene catheter was introduced into the left internal jugular vein after the second operation, and during the recording of the ECG, propranolol, 10 mg/kg (2 mg/ml), was injected rapidly. The ECG was obtained before, and 5 min after the injection. Group II received propranolol, 10 mg/kg intravenously before the first and before the second surgery, followed by 10 mg/k g Per day by a continuous intravenous infusion with a Harvard pump (5 ml/day), starting after the first surgery until the animals were killed. Group III received atropine 1 mg/ kg intravenously before the first and before the second surgery, followed by 2.5 mg/kg per day by continuous infusion, similar to group II. In group IV the carotid sinus area was left intact, and only the superior cervical ganglia were resected in a staged manner. In group V simulated monophasic ventricular action potentials (MAP) were recorded from the anterior ventricular wall, toget .her with a conventional surface ECG at the same time intervals as previously described. A 25-gauge needle was advanced transthoracically through the left fourth or fifth intercostal space perpendicular to the chest wall until MAPS were recorded. The site of chest entry was marked to aid in the obtaining of comparable tracings. Twenty-four hours after the second surgery, isoproterenol, 0.02-0.06 ,ug (0.2 pg/ml) was injected intravenously under continuous ECG monitoring. Twelve-lead ECGs and MAPS were obtained before and at 1 and 5 min after termination of the injection. In two additional animals isoproterenol injections were made without carotid sinus denervation. Q-T intervals were measured in V5, because measurements in this lead were easy and most reproducible. Preliminary studies had shown that there was very little variation from lead to lead. The MAP duration, reflected by the monophasic injury potential, was measured as described by Autenrieth et al. (3), Hoffman et al. (14), and Kuo and Surawicz (16). The variations of measurements were within 5 ms. The records of the MAP were acceptable if 10 or more similar potentials were recorded with a smooth repolarization, and the tracing had an amplitude exceeding 3 mV.
H. K. BAUR
AND
AVR
C. A. I’IERACH
AVL
AVF
V5 . . .
. . .
. . ., *...
~ ,
.
.
. .
. . .
.
.
. . .
the first (0.
. .
.
. .
,
,
. . .
.
. . . ,. .
. .
.
,
.
.
.
I... . .
,
,
.
. .
.
L .
.
.
FIG. 1. Twelve-lead KG before and 24 h after the second denervatlon
..,. 1::: .
.
.
H
. * .
. . . . . . ..a, . . . . . ..L..
.
(A), after
,
the second
(B),
were present, and these became more apparent after the second denervation. Changes in cardiac repolarization were pronounced in the records 24 h after the second procedure, at which time broad rounded T waves and mean Q-T prolongations of 19 ms were present (Table 1). There was very little variation in the Q-T interval or MAP before surgery (Table l), and no rate-related changes were observed. P-R intervals and QRS complexes were not changed. Changes in heart rate were transient and of minor degree, averaging -3 beats/min after the first, and +26 beats/min after the second procedure. Propranolol, administered 24 h after the second denervation, produced slowing of the heart rate of 80-100 beats/min, but did not reverse the T wave changes. RESULTS However, when the drug had been given preoperatively A typical ECG sequence is depicted in Figs. 1 and 2. and then as continuous infusion, no ECG abnormalities c The first denervation had no immediate effects. After 24 occurred (Fig. 3). No preventive effect was demonstrated h, before the second surgery, slight T wave alterations with atropine (Fig. 4). Resection of the superior cervical
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
ECG
CHANGES
AFTER
CAKOTID
SINUS
H477
DENEIWATION
2. ECG lead V5 before the first th e second (B), and 24 h after the second denervation (C). (For details, see text.) FIG.
(A), after
TABLE
animals (light microscopy). There were no subendocardial hemorrhages.
1. Heart rates, Q-T intervals,
beforeand
and MAPS after carotid sinus denervation Heart Rate, beats/min
Q-T Interval, ms
MAP,
ms
DISCUSSION
The observed electrocardiographic changes are comparable with those noticed in man (5), although no inversion of T wave polarity was noted in the rat. Although 426 t 18.0 64 t 1.63 34 2 2.50 heart rates were not controlled in this study, the Q-T and MAP alterations occurred at similar mean heart rates 452 t 14.3 (before surgery 434 beats/min; 24 h after surgery 431 431 ?I 16.8 82 t 2.16 45 k 1.79 beats/min) making the possibility of rate-related changes unlikely. The QRS complex remained unchanged after Values are means t SE in 12 rats. Prolongations of Q-T and MAY carotid sinus denervation (CSD), implying that the obdurations are significant (unpaired f test, P < 0.001 and P < 0.02, served T wave alterations represent primary abnormalirespectively). ties in ventricular recovery. Denervation of the baro- and chemoreceptor has proganglion had no effect on heart rate or the ECG (Fig. 5). found cardiovascular and respiratory effects that are The mean MAP recorded from the anterior ventricular mediated through reciprocal changes in sympathetic and wall, 24 h after the second denervation, was prolonged parasympathetic nervous system activity (12). This view by 10 ms (Table 1 and, Fig. 6). had been confirmed (20) and challenged (6, 9) in recent cIsoproterenol injected at the time of the maximal ECG years, and especially the role of the efferent vagal activity changes did not change the MAP or reverse the T wave in the carotid sinus reflex remains controversial. Howchanges when small doses of 0.02 ,ug were used. Doses of ever, it is generally agreed that cutting the carotid sinus 0.04 and 0.06 pg regularly lengthened the MAP and nerves, as we did in our rats, leads to increased cardiac produced S-T depressions and Q-T interval prolonga- sympathetic activity due to loss of tonic inhibition of the tions on the surface ECG. The same changes were ob- sympathetic medullary centers (6,8) and perhaps also to served in the two control animals. a decrease of the efferent vagal tone (20). The prevention The mean arterial blood pressure rose 30-50 mmHg in of all ECG abnormalities with propranolol, but not with all three rats tested, demonstrating the effectiveness of atropl?-e, in our study, is in agreement with this view and suggests that the observed changes in the T wave and the procedure. No difference was found between pre- and postopera- the MAP are brought about by a changed sympathetic tive values for blood gasesand electrolytes in the three discharge pattern to the heart. The postoperative inanimals tested. The cardiac histology was normal in all crease in heart rate and bl.ood pressure after the second Preoperative Immediately after 1st denervation 24 h after 1st denervation Immediately after 2nd denervation 24 h after 2nd denervation
434 t 12.2 431 -+ 16.0
63 t 1.35
35 t 2.55
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H. Ii.
lervat
iorl (13). Cmt inuous
propranolol
HALJR
AND
C. A. PIIWACH
infusion.
(Fw
det ails, see text.)
see text.)
IW;. 6. -MAP
befort
the first
(A) and 24 h after
the second
denervation
(B).
(For
details,
see text.)
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ECG
CHANGES
AFTEK
CAKOTII)
SINUS
H479
DENEKVATION
procedure would be in accord with this hypothesis, although a nonspecific stimulus due to the surgery cannot be ruled out, The concomitant loss of chemoreceptor activi .ty produced respiratory changes COmparab le to those observed in man (15). It is not clea.r if carotid body destruction added to the changes caused bv the carotid sinus denervation. The cardiovascular infiuence of the carotid body is primarily mediated t,hrough the vagus nerve in the cat and in the dog (8, 19), and its loss might produce a decrease in cardiac vagal tone. The repolarization changes and the MAP prolongation observed in our study closely resemble the changes seen after right stellate ganglion transsection or left stimulation in the dog (16, 23). Although strong restraints are mandatory in comparing different species, the similarities suggest that CSD may not initiate a uniform distribution of increased sympathetic neural traffic to the heart, but possiblv produce regional differences in the duration of individual action potentials, resulting in an altered temporal and spatial recoverv process of the myocardium, However, this remains speculative and is &t based on actual evidence. We only documented prolonged MAPS on the anterior wall, and if shortening of the MAP occurred in other areas of the ventricle, it was not accessible by our technique. The injury potentials recorded with the needle electrodes may not be exactly comparable to membrane potentials. However, they were reproducible, and their duration did not depend on the magnitude of the small epicardial injury produced by the tip of the needle. At autopsy, the needle marks were found within l-2 mm on the anterior surface of the heart. The superior sympathetic ganglion was occasionally damaged during CSD, and the possibility had to be considered that bilateral injury to the sympathetic nerve fibers rather than CSD could have been responsible for the observed ECG changes. However, bilateral transsec-
tion of the sympathetic trunk below the superior cervical ganglion did not have any effect on the ECG. Other factors known to influence the T wave form, like electrolyte disorders or drugs, did not play a role in our experiment. In contrast to some other experimental studies (I I), cardiac histology was unremarkable in our rats indicating that the neurogenic T wave changes are noi necessarily associated with myocardial necrosis or hemodmge. Isoproterenoi, even in small doses, lengthened the MAP, both in the control animal and in the rat subjected to CSD. This is consistent with the observed effect of norepinephrine on the membrane potentials of the rat atria (22) and in contrast to changes observed in the canine heart (3, IS), where rate effects may dominate the situation. It is possible that the changes we have previously described in man (5) are also responsible for the observed repolarization changes in the experimental animal, as we have experimentally produced a more pronounced variant of a situation that might occur after surgical handling of the carotid arteries in man (2, 13, 21). Mechanisms similar to those discussed in the present study have been put forward to explain repolarization changes seen with cerebral vascular disorders (1) after experimental stimulation of certain brain areas with known sympathetic connections (11, 17, 18) or after intracoronary injections of epinephrine (4). We thank I3r. H. B. Burchell for advice in designing this study and preparing the manuscript and Dr. J. H. Keed for technical assistance. This work was supported in part by a grant from the Swiss National Foundation for Kesearch. Address reprint requesh to C. A. Pierach, M. D., Abbott-Northwestern Hospital, 810 East 27th St., Minneapolis MN 55407 Keceived
23 March
1979; accepted
in final
form
21 June
1979.
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H480 Intracardiac chemoreflex.
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