Distribution of Myocardial Blood Flow Measured by Hydrogen Polarography RICHARD A. MOGGIO, M.D., GRAEME L. HAMMOND, M.D.
Distribution of myocardial blood flow was studied by polarographic recording of hydrogen desaturation in open chest dogs. Flow was measured during normal cardiac activity, reactive hyperemia following 60 seconds of coronary artery occlusion, and left ventricular hypertension produced by either partial supravalvular aortic occlusion or subvalvular outflow constriction. During normal cardiac function, blood flows in the subepicardium and subendocardium were approximately equal. Reactive hyperemia increased flow to both the subepicardium and the subendocardium. Left ventricular hypertension decreased subendocardial flow relative to subepicardial flow in proportion to the degree of hypertension. Marked supravalvular obstruction with ventricular hypertension reduced subendocardial flow to two-thirds that of subepicardial flow. This decrease was further accentuated when the left ventricular end diastolic pressure was elevated.
C LINICAL, histological, and experimental evidence has indicated that the subendocardial layer of the left ventricle is vulnerable to underperfusion.3'4'6'9"2'14'15 Conclusions about the mechanisms leading to ischemia and the extent to which the subendocardium is underperfused are subject to debate and, to a degree, may represent differences in methods and techniques of investigation.11 The vasodilatory capacity of the subendocardium has been variously described11 and the contribution of the left ventricular lumen to subendocardial perfusion through the myocardial sinusoids has been
studied.5'10 The present study was designed to evaluate subendocardial and subepicardial flow under several conditions of ventricular pressure and coronary perfusion. The technique of polarographic recording of local hydrogen gas desaturation by intramyocardial platinum electrodes, a method not previously applied to measurment of transSubmitted for publication August 25, 1975. Reprint requests: Richard A. Moggio, M.D., Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510. Supported by a grant from The John A. Hartford Foundation.
From the Surgical Cardiovascular Research Laboratory, Yale Medical School, 333 Cedar Street, New Haven, Connecticut
myocardial hemodynamics, provided repeated observations in the intact experimental canine preparation. Materials and Methods
Adult mongrel dogs weighing between 20 and 40 kg were anesthetized with 25 mg/kg of sodium penthothal and respirations controlled mechanically. The chest was opened through a median sternotomy and the heart stabilized with pericardial sutures. The ascending aorta and the proximal anterior descending coronary artery were encircled with tapes. In experiments producing left ventricular hypertension with systemic normotension, a large balloon catheter was advanced through the posterior myocardium into a subvalvular position in the left ventricular outflow tract. Left ventricular pressure (LVP) was monitored by a transducer catheter placed into the cavity directly through a stab wound in the posterior myocardium. Systemic pressure was similarly recorded in the descending aorta via femoral artery cannulation. Two unipolar pacemaker electrodes, 2 mm in diameter, with bared platinum tips 1 mm in length, were placed through the epicardium into the inner and outer 3 mm of anterior left ventricular myocardium and held in place by epicardial sutures. These probes were connected to a high gain, low resistance differential amplifier. Two standard KCI calomel reference electrodes from the amplifier were attached to the dog's skin to complete the physiologic circuit. A positive potential of 0.3 volts applied to the reference electrode, as suggested by Aukland, et al.1'2 minimized the oxygen reduction at the
282
Vol. 183
*
283
HYDROGEN POLAROGRAPHY
No. 3
-Systemic Pressure
Subendocardium
chamber V/NA
Subepicardiui
FIG. 1. Schematic diagram of experimental preparation with platinum electrodes placed in the layers of myocardium.
Left Coronary Tourniquet
Subaauvular Balloon
Amplifier
2 Channel Recorder
References LV Pressure
platinum electrode. Current produced in the circuit was recorded on a Brush multichannel recorder (Fig. 1). The preparation was maintained on room air ventilation. At the beginning of an experimental run, 2.5% hydrogen, balanced with room air, was provided for ventilation until myocardial tissue saturation was obtained. Saturation was recognized by the recording of stable plateaus of currents produced at the electrodes. The hydrogen gas was then discontinued with resultant tissue desaturation of hydrogen. The desaturation currents were similarly recorded and followed until the prehydrogen infusion, baseline currents had been reached. This procedure was carried out under several conditions. In one group of animals, reactive hyperemia was produced by 60 sec occlusion of the anterior descending coronary artery. In another group of animals, left ventricular hypertension was produced either by supra-
valvular tourniquet constriction of the aorta, or by subvalvular balloon obstruction of the left ventricular outflow tract. For all animals in each group, curves were also obtained during states of normal cardiac activity. Curves obtained during normal activity were randomly alternated with reactive hyperemia or ventricular hypertension recordings, and sufficient time was allowed between runs to allow cardiac activity to return to baseline levels. Local myocardial flow was derived from the slope of the desaturation curve obtained from each electrode. The recorded saturation curves were plotted onto semilogarithmic paper with units of current deflection and time as the coordinates. The formula of each curve was calculated according to the method of least squares, and the half-time (T1/2) of the desaturation curve obtained. Flow, in ml/min/100 gm of tissue was deter-
TABLE 1. Reactive Hyperemia: Data from 12 Experimental Animals
Flow (ml/min/l00 gin) Flow Distribution
Blood Pressure
Heart Rate
subendocardium
subepicardium
subendo/subepi
Normal Activity
127 28 95 ± 23
168 ± 21
92 ± 35
88 ± 36
1.04 ± 0.28
Reactive Hyperemia
122 ± 25
167 ± 17
547 ± 110*
560 ± 121*
0.98 ± 0.27
*
92 ± 17
P < 0.002 significant difference from normals.
MOGGIO AND HAMMOND
284
Ann.
Surg. March 1976
TABLE 2. Mild Supravalvular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gm)
Blood Pressure
Heart Rate
Normal Activity
148 ±27* 97 ±24
173
SupravalvularAortic Stenosis
174 ± 27t 103 ± 20t 2± 2
175 ± 13
* Systemic pressure. t Ventricular pressure. t Aortic root diastolic pressure.
16
subendocardium 66
14
64 ± 16
subepicardium 64
14
77 ± 1611
Flow Distribution subendo/subepi 1.02
0.17
0.83 + 0.14§
§ P < 0.002. P < 0.01 significant differences from normals.
mined according to the formula: 69.3 Flow = 69.3 T112 derived from the Fick equation. '2 Curves deviating from linearity (r < 0.90) were discarded. Placement of the electrodes was verified by myocardial examination at the conclusion of each experiment. Results For each group of experimental animals, the mean blood pressures, heart rates, subendocardial and subepicardial flows were determined. The subendocardial to subepicardial ratios reflected the distribution of blood flow across the myocardium. Statistical comparisons for significance between normal and experimentally altered conditions used the Student's t distribution. Mean flow values and mean flow ratio values + SD were used to calculate significant differences between means with N = number of experimental values and N - 1 = degrees of freedom. Normal activity. Mean normal flows for each group of dogs are shown in Tables 1 through 5. Values are recorded as means + SD. Although absolute calculated
flow varied considerably, the subendocardial/subepicardial ratio was not significantly different from 1.00 in any group. That is, the subendocardium was perfused equally as well as the subepicardium during normal cardiac states. An example of tissue saturation curves obtained during normal activity in a single preparation is shown in Fig. 2a. The desaturation curve between points B and C is plotted on semilogarithmic paper and the resultant slopes and half-times calculated (Fig. 2b). Reactive hyperemia. After 60 seconds of coronary artery occlusion desaturation curves recorded the reactive hyperemic phase of coronary flow. Peak flows were obtained for about 30 sec following release of the tourniquet, after which flows gradually returned to normal. Peak flows increased 5 to 6 times normal, both in the epicardial and endocardial layers (Table 1), with a nonsignificant relative underperfusion of the inner layer. Mild left ventricular hypertension. Elevations of left ventricular pressure to 25 to 45 mmHg above normals were obtained by subvalvular and supravalvular outflow constrictions. Flow rates obtained during supravalvular hypertension (Table 2) showed an increase in epicardial flow but a decrease in endocardial flow. The relative
TABLE 3. Mild Subvalv'ular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gm) Flow Distribution
Blood Pressure Normal Activity
121 18* 86 ±18
Heart Rate 165
21
subendocardium 61
20
subepicardium 62
19
subendo/subepi 1.01
0.19
162 ± 16t 2± 2
157 ± 17
Subvalvular Outflow Stenosis 108 ± 14* 80± 10 * Systemic pressure. t Ventricular pressure. t P < 0.01 significant differences from normals.
52 ± 15t
61 ± 21
0.86 ± 0.15t
HYDROGEN POLAROGRAPHY
Vol. 183.o No. 3
285
TABLE 4. Marked Supravalvular Left Ventricular Hypertension: Data From 9 Experimental Animals
Flow (ml/min/100 gm) Blood Pressure 132
Normal Activity
SupravalvularAortic Stenosis * Systemic pressure. t Ventricular pressure. t Aortic root diastolic pressure.
25*
90 ±20 209 ± 19t 128 ± 17t 12 ± 8
Heart Rate 169
subendocardium 88
19
173 ± 13
29
66 ± 25§
subepicardium 87 ±32
101 ± 14"1
Flow Distribution subendo/subepi 1.01
0.24
0.65 ± 0.19§
§P < 0.002. IP < 0.01 significant differences from normals.
underperfusion of the endocardium, as shown by the decrease in flow distribution from 1.02 to 0.83, was significant. When flow was measured during subvalvular outflow obstruction (Table 3), no increase in epicardial flow, but a decrease in endocardial flow, was observed. The ratio of 0.86 was slightly greater than that of 0.83 obtained during supravalvular hypertension. Desaturation curves during ventricular hypertension in a single preparation are shown in Fig. 3a, and the calculated flows and ratio are illustrated in Fig. 3b. Marked left ventricular hypertension. More severe constriction of ventricular outflow produced ventricularsystemic gradients of 70 to 100 mmHg. Flow changes in this group were similar to those of mild LV hypertension, but more marked. During supravalvular hypertension (Table 4), epicardial flow increased 28% while subendocardial decreased 25%. The flow ratio of 0.65 indicated a significant relative underperfusion of the subendocardium. Flows during marked subvalvular obstruction showed parallel changes (Table 5), but the underperfusion of the endocardial layer, though still significantly different from normal, was less marked (0.79 compared to 0.65) than that of supravalvular constriction.
The average flow distribution of 0.65 for all runs increased slightly to 0.67 when the LVEDP was less than 12 mmHg, but decreased even more from a normal of 1.01 to 0.63 when the LVEDP was elevated above 12 mmHg. These differences were not statistically significant among themselves. Discussion
The method of polarographic recording of hydrogen gas desaturation has been shown to provide a good estimate of local myocardial blood flow.2 Its principle is that current produced by oxidation of molecular hydrogen to hydrogen ions at the surface of the platinum electrode is proportional to the partial pressure of hydrogen at the electrode. An electrode saturated with hydrogen will produce a steady current, and when the hydrogen is washed out by unsaturated blood, the rate of decrease in the current reflects the rate of desaturation, i.e., the rate of blood flow through the tissue. Application of a +0.3v potential against the KCI calomel reference electrode effectively reduces molecular oxygen at the platinum electrodes by
99o. Local disturbances to flow around the electrode tip
TABLE 5. Marked Subvalvular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gin) Flow Distribution Blood Pressure
Normal Activity
140 ± 18* 96 ± 17 211 ± 10t 5± 3
Subvalvular Outflow Stenosis 110 ± 23* 70 ± 22 * Systemic pressure. t Ventricular pressure. t P < 0.002 significant difference from normals.
Heart Rate
subendocardium
subepicardium
subendo/subepi
9
54
10
56± 11
0.98 ±0.13
160 ± 12
44
11*
56
0.79 ± 0.19*
164
10
MOGGIO AND HAMMOND
286
Ann. Surg. * March 1976
FIG. 2a. Recordings of saturation and desaturation currents obtained during normal cardiac activity. Each darker graph line represents twelve seconds on the time scale.
CURVES OBTAINED FROM MYOCARDIUM DURING NORMAL ACTIVITY Point A: Beginning of hydrogen saturation B: Beginning of desaturation (hydrogen saturation) C: Complete woshout of hydrogen, return to baseline are minimal and, though a layer of uncirculated fluid the curve.2 In no case was hematoma formation evimay surround the electrode, its effect is to reduce the dent, and the probes consistently remained stable in the current and to produce a delay in the initial slope of myocardium throughout the course of the experiment. Verification that electrodes were localized 3-4 mm the curve, but does not influence the overall slope of from the epicardial and endocardial surfaces assured that NORMAL CARDIAC ACTIVITY diffusion loss of hydrogen through the surfaces of the 50 heart did not occur. F- 69.3 The formula for calculating blood flow is derived from the Fick equation and assumes that the tissue-blood partition co-efficient is 1.0, and that arterial recirculation is negligible during desaturation recordings.' The 20 low blood solubility of H2 provided minimal arterial circulation after a single passage through the lungs8 and recordings obtained from electrodes placed directly z in the left ventricular chamber showed less than 10%o M 10 arterial recirculation. Subendocordium Although the absolute flow measurements obtained by F*1I5 ml /min this technique may be subject to errors of accuracy, U. the advantages of the method are distinct. Simultaneous recordings from two electrodes permit comparison of Subepicordium inner and outer flows in the intact working heart. The Fa18 mI/mnin continued function of the electrodes over 3-5 hours permitted repeated determinations under different conditions in the same animal. Moreover, because repeated measSubendocardium 2 _ 0.98 Subepicordium urements could be obtained, each preparation served as its own control, and allowed direct comparison of results obtained during normal and experimentally altered cardiac activities. This is clearly an advantage over single 48 24 study techniques, such as tissue uptake of diffusible TIME (sec) isotopes and vascular uptake of radioactive microspheres. That the subendocardium is equally perfused during FIG. 2b. Desaturation curves plotted on semilogarithmic axes, normal activity is in general agreement with other tissue resultant flow rates and flow ratio during normal cardiac activity. C,)
z
0
w
5
0
.
Vol. 183
o
.,' .. +,.4.;',-.Tt4 I
287
HYDROGEN POLAROGRAPHY
No. 3
........
..........
C ....
FIG 3a. Recordings of saturation and desaturation currents obtained during left ventricular hypertension. Each darker graph line represents twelve seconds on the time scale.
...
......
m
.
I-
4
i--
*i-
--r' +--,~4=~'l04-
r-
A
TIME
B
q-
i^ A . - 4 - f --t r- _
----
P
;..
..
. .
-i-
,-f-
..-i.
r,I
--
A-
-,W-
-
a
-1.1-0000
17
CURVES OBTAINED FROM MYOCARDIUM DURING SUPRAVALVULAR HYPERTENSION Point A: B: C:
Beginning of hydrogen saturation Beginning of desaturation (hydrogen saturation) Complete washout of hydrogen, return to baseline
uptake studies, such as the clearance studies of Moir and DeBra12 and the metabolic study of Griggs.4 In contrast, Winbury15 and Harley6 reported underperfusion of the subendocardium. These latter studies used externally monitored clearance techniques and tissue Po2 determinations, the possible shortcomings of which have been suggested by Moir.1" The ability of the subendocardium to vasodilate in response to ischemia produced by coronary artery occlusion contrasts with earlier studies which indicated that these vessels are incapable of further vasodilation.7'16 The large vasodilatory capacity demonstrated in this experiment agrees with the findings of Buckberg, et al.3 obtained in dogs with normal coronary arteries. Since reactive hyperemia is not a true steady-state condition, the hydrogen desaturation technique does not provide an accurate flow value, but approximations of flows over short periods provide an estimate of myocardial vasodilation. Left ventricular hypertension, both subvalvular and supravalvular, produced relative underperfusion of the subendocardium. This decrease in blood flow occurred without a significant increase in heart rate. It is likely, however, that with increased aortic outflow resistance, the systolic ejection time increased, and the diastolic fraction, during which time most coronary perfusion occurs, decreased proportionately. Concurrently extravascular resistance rose with increasing ventricular pressure, and it is likely that both the shortened diastole and increased extravascular pressure contributed to subendocardial underperfusion. These speculations corroborate the conclusions of Buckberg3 who correlated subendocardial underperfusion with both supply (diastolic pressure time index) and demand (tension time index) factors. Contrary to their observations of increased heart rates (which contributed to decreased diastolic times) no increased rate was ob-
served in this study. Tachycardia certainly plays a role in decreasing endocardial flow, as shown by Neill, et al.13 but the present study further supports the need to consider the extravascular pressure effects on subendocardial blood flow. This conclusion is substantiated by the observations that the normal vasodilation ability of the subendocardium is inhibited by elevated ventricular pressures, and that elevated aortic root diastolic presLEFT VENTRICULAR HYPERTENSION 50
20 CD
z 10
z 0
w
2
2
24
48
72
96
120
144
TIME (seC) FIG. 3b. Desaturation curves plotted on semilogarithmic axes with resultant flow rates and flow ratio during left ventricular hypertension.
288
MOGGIO AND HAMMOND
sures also fail to change the transmyocardial flow gradient. At a given level of hypertension, however, the production of a ventricular to coronary pressure (or systemic pressure) gradient by subvalvular outflow obstruction lessened the underperfusion of the subendocardium, compared to that during supravalvular constriction. Based on previous studies in this laboratory5'10 which demonstrated transport of left ventricular luminal blood directly into the myocardium in conditions of reduced coronary pressure, it is suggested that the myocardial sinusoids may help supply the endocardium during reduced coronary pressure (or elevated subvalvular ventricular pressure) and provide a protecting source of oxygenated blood. Although the amount of blood supplied by the coronary-luminal pathways is inadequate to compensate fully for the decrease in myocardial flow, their contribution (an average of 3 to 5% of total myocardial flow) might be sufficient, under certain conditions, to prevent significant subendocardial ischemia. In conclusion, the technique of intramyocardial recording of hydrogen as desaturation has demonstrated the equality of subendocardial and subepicardial blood flow under normal conditions, and has shown the large reserve vasodilatory capacity of the subendocardium. It has been shown that left ventricular hypertension alters the distribution of myocardial blood flow to produce subendocardial underperfusion, and it is suggested that, under certain conditions, the myocardial sinusoids may provide a protective collateral source of blood to the potentially ischemic inner myocardium.
References 1. Aukland, K., Bower, B. F.,and Berliner, R.W.: Measurement of
Local Blood Flow with Hydrogen Gas. Circ. Res., 14:164, 1964.
Ann. Surg. * March 1976
2. Aukland, K., Kiil, F., Kjekshus, J., and Semb, G.: Local Myocardial Blood Flow Measured by Hydrogen Polarography; Distribution and Effect of Hypoxia. Acta Physiol Scand., 70:99, 1967. 3. Buckberg, G. D., Fixier, D. E., Archie, J. P., and Hoffman, J. I. E.: Experimental Subendocardial Ischemia in Dogs With Normal Coronary Arteries. Circ. Res., 30:67, 1972. 4. Griggs, D. M., Tchokoev, V. V., and Chen, C. C.: Transmural Differences in Ventricular Tissue Substrate Levels Due to Coronary Constriction. Am. J. Physiol., 222:705, 1972. 5. Hammond, G. L. and Moggio, R. A.: Function of Microvascular Pathways in Coronary Circulation. Am. J. Physiol., 220:1463, 1971. 6. Harley, A., Harper, J. R., and Estes, E. H.: Distribution of Blood Flow in the Myocardium Measured by Clearance of Xenonl33. Circulation, 33-34: 111-122, 1966. 7. Kirk, E. S. and Honig, C. R.: Non-uniform Distribution of Blood Flow and Gradients of Oxygen Tension Within the Heart. Am. J. Physiol., 207:661, 1964. 8. Klocke, F. and Wittenberg, S. M.: Heterogeneity of Coronary Blood Flow in Human Coronary Artery Disease and Experimental Myocardial Infarction. Am. J. Cardiol., 24:782, 1969. 9. Levine, H. D. and Fort, R. V.: Subendocardial Infarction: Report of Six Cases and Critical Survey of the Literature. Circulation, 1:246, 1950. 10. Moggio, R. A., Kabemba, J. M., and Hammond, G. L.: Coronary Ventricular Lumen Blood Exchange Demonstrated by 51Cr-Labeled Erythrocytes. Am. J. Physiol., 221:955, 1971. 1 1. Moir, T. W.: Subendocardial Distribution of Coronary Blood Flow and the Effect of Antiangina Drugs. Circ. Res., 30:621, 1972. 12. Moir, T. and DeBra, D. W.: Effect of Left Ventricular Hypertension, Ischemia, and Vasoactive Drugs on the Myocardial Distribution of Coronary Flow. Circ. Res., 21:65, 1967. 13. Neill, W. A., Phelps, N. C., Oxendine, J. M., et al.: Effect of Heart Rate on Coronary Blood Flow Distribution in Dogs. Am. J. Cardiol., 32:306, 1973. 14. Roberts, W. C. and Morrow, A. G.: Causes of Early Postoperative Death Following Cardiac Valve Replacement. J. Thorac. Cardiovasc., Surg., 54:422, 1967. 15. Winbury, M. M.: Redistribution of Left Ventricular Blood Flow Produced by Nitroglycerine. Circ. Res., 28,29:I-140, 1971. 16. Winbury, M. M., Howe, B. B., and Weiss, H.: Effect of Nitroglycerine and Dipyridamole on Epicardial and Endocardial Oxygen Tension. J. Pharmacol. Exp. Ther., 176:184, 1971.
Distribution of myocardial blood flow was studied by polarographic recording of hydrogen desaturation in open chest dogs. Flow was measured during normal cardiac activity, reactive hyperemia following 60 seconds of coronary artery occlusion, and left ventricular hypertension produced by either partial supravalvular aortic occlusion or subvalvular outflow constriction. During normal cardiac function, blood flows in the subepicardium and subendocardium were approximately equal. Reactive hyperemia increased flow to both the subepicardium and the subendocardium. Left ventricular hypertension decreased subendocardial flow relative to subepicardial flow in proportion to the degree of hypertension. Marked supravalvular obstruction with ventricular hypertension reduced subendocardial flow to two-thirds that of subepicardial flow. This decrease was further accentuated when the left ventricular end diastolic pressure was elevated.
C LINICAL, histological, and experimental evidence has indicated that the subendocardial layer of the left ventricle is vulnerable to underperfusion.3'4'6'9"2'14'15 Conclusions about the mechanisms leading to ischemia and the extent to which the subendocardium is underperfused are subject to debate and, to a degree, may represent differences in methods and techniques of investigation.11 The vasodilatory capacity of the subendocardium has been variously described11 and the contribution of the left ventricular lumen to subendocardial perfusion through the myocardial sinusoids has been
studied.5'10 The present study was designed to evaluate subendocardial and subepicardial flow under several conditions of ventricular pressure and coronary perfusion. The technique of polarographic recording of local hydrogen gas desaturation by intramyocardial platinum electrodes, a method not previously applied to measurment of transSubmitted for publication August 25, 1975. Reprint requests: Richard A. Moggio, M.D., Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510. Supported by a grant from The John A. Hartford Foundation.
From the Surgical Cardiovascular Research Laboratory, Yale Medical School, 333 Cedar Street, New Haven, Connecticut
myocardial hemodynamics, provided repeated observations in the intact experimental canine preparation. Materials and Methods
Adult mongrel dogs weighing between 20 and 40 kg were anesthetized with 25 mg/kg of sodium penthothal and respirations controlled mechanically. The chest was opened through a median sternotomy and the heart stabilized with pericardial sutures. The ascending aorta and the proximal anterior descending coronary artery were encircled with tapes. In experiments producing left ventricular hypertension with systemic normotension, a large balloon catheter was advanced through the posterior myocardium into a subvalvular position in the left ventricular outflow tract. Left ventricular pressure (LVP) was monitored by a transducer catheter placed into the cavity directly through a stab wound in the posterior myocardium. Systemic pressure was similarly recorded in the descending aorta via femoral artery cannulation. Two unipolar pacemaker electrodes, 2 mm in diameter, with bared platinum tips 1 mm in length, were placed through the epicardium into the inner and outer 3 mm of anterior left ventricular myocardium and held in place by epicardial sutures. These probes were connected to a high gain, low resistance differential amplifier. Two standard KCI calomel reference electrodes from the amplifier were attached to the dog's skin to complete the physiologic circuit. A positive potential of 0.3 volts applied to the reference electrode, as suggested by Aukland, et al.1'2 minimized the oxygen reduction at the
282
Vol. 183
*
283
HYDROGEN POLAROGRAPHY
No. 3
-Systemic Pressure
Subendocardium
chamber V/NA
Subepicardiui
FIG. 1. Schematic diagram of experimental preparation with platinum electrodes placed in the layers of myocardium.
Left Coronary Tourniquet
Subaauvular Balloon
Amplifier
2 Channel Recorder
References LV Pressure
platinum electrode. Current produced in the circuit was recorded on a Brush multichannel recorder (Fig. 1). The preparation was maintained on room air ventilation. At the beginning of an experimental run, 2.5% hydrogen, balanced with room air, was provided for ventilation until myocardial tissue saturation was obtained. Saturation was recognized by the recording of stable plateaus of currents produced at the electrodes. The hydrogen gas was then discontinued with resultant tissue desaturation of hydrogen. The desaturation currents were similarly recorded and followed until the prehydrogen infusion, baseline currents had been reached. This procedure was carried out under several conditions. In one group of animals, reactive hyperemia was produced by 60 sec occlusion of the anterior descending coronary artery. In another group of animals, left ventricular hypertension was produced either by supra-
valvular tourniquet constriction of the aorta, or by subvalvular balloon obstruction of the left ventricular outflow tract. For all animals in each group, curves were also obtained during states of normal cardiac activity. Curves obtained during normal activity were randomly alternated with reactive hyperemia or ventricular hypertension recordings, and sufficient time was allowed between runs to allow cardiac activity to return to baseline levels. Local myocardial flow was derived from the slope of the desaturation curve obtained from each electrode. The recorded saturation curves were plotted onto semilogarithmic paper with units of current deflection and time as the coordinates. The formula of each curve was calculated according to the method of least squares, and the half-time (T1/2) of the desaturation curve obtained. Flow, in ml/min/100 gm of tissue was deter-
TABLE 1. Reactive Hyperemia: Data from 12 Experimental Animals
Flow (ml/min/l00 gin) Flow Distribution
Blood Pressure
Heart Rate
subendocardium
subepicardium
subendo/subepi
Normal Activity
127 28 95 ± 23
168 ± 21
92 ± 35
88 ± 36
1.04 ± 0.28
Reactive Hyperemia
122 ± 25
167 ± 17
547 ± 110*
560 ± 121*
0.98 ± 0.27
*
92 ± 17
P < 0.002 significant difference from normals.
MOGGIO AND HAMMOND
284
Ann.
Surg. March 1976
TABLE 2. Mild Supravalvular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gm)
Blood Pressure
Heart Rate
Normal Activity
148 ±27* 97 ±24
173
SupravalvularAortic Stenosis
174 ± 27t 103 ± 20t 2± 2
175 ± 13
* Systemic pressure. t Ventricular pressure. t Aortic root diastolic pressure.
16
subendocardium 66
14
64 ± 16
subepicardium 64
14
77 ± 1611
Flow Distribution subendo/subepi 1.02
0.17
0.83 + 0.14§
§ P < 0.002. P < 0.01 significant differences from normals.
mined according to the formula: 69.3 Flow = 69.3 T112 derived from the Fick equation. '2 Curves deviating from linearity (r < 0.90) were discarded. Placement of the electrodes was verified by myocardial examination at the conclusion of each experiment. Results For each group of experimental animals, the mean blood pressures, heart rates, subendocardial and subepicardial flows were determined. The subendocardial to subepicardial ratios reflected the distribution of blood flow across the myocardium. Statistical comparisons for significance between normal and experimentally altered conditions used the Student's t distribution. Mean flow values and mean flow ratio values + SD were used to calculate significant differences between means with N = number of experimental values and N - 1 = degrees of freedom. Normal activity. Mean normal flows for each group of dogs are shown in Tables 1 through 5. Values are recorded as means + SD. Although absolute calculated
flow varied considerably, the subendocardial/subepicardial ratio was not significantly different from 1.00 in any group. That is, the subendocardium was perfused equally as well as the subepicardium during normal cardiac states. An example of tissue saturation curves obtained during normal activity in a single preparation is shown in Fig. 2a. The desaturation curve between points B and C is plotted on semilogarithmic paper and the resultant slopes and half-times calculated (Fig. 2b). Reactive hyperemia. After 60 seconds of coronary artery occlusion desaturation curves recorded the reactive hyperemic phase of coronary flow. Peak flows were obtained for about 30 sec following release of the tourniquet, after which flows gradually returned to normal. Peak flows increased 5 to 6 times normal, both in the epicardial and endocardial layers (Table 1), with a nonsignificant relative underperfusion of the inner layer. Mild left ventricular hypertension. Elevations of left ventricular pressure to 25 to 45 mmHg above normals were obtained by subvalvular and supravalvular outflow constrictions. Flow rates obtained during supravalvular hypertension (Table 2) showed an increase in epicardial flow but a decrease in endocardial flow. The relative
TABLE 3. Mild Subvalv'ular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gm) Flow Distribution
Blood Pressure Normal Activity
121 18* 86 ±18
Heart Rate 165
21
subendocardium 61
20
subepicardium 62
19
subendo/subepi 1.01
0.19
162 ± 16t 2± 2
157 ± 17
Subvalvular Outflow Stenosis 108 ± 14* 80± 10 * Systemic pressure. t Ventricular pressure. t P < 0.01 significant differences from normals.
52 ± 15t
61 ± 21
0.86 ± 0.15t
HYDROGEN POLAROGRAPHY
Vol. 183.o No. 3
285
TABLE 4. Marked Supravalvular Left Ventricular Hypertension: Data From 9 Experimental Animals
Flow (ml/min/100 gm) Blood Pressure 132
Normal Activity
SupravalvularAortic Stenosis * Systemic pressure. t Ventricular pressure. t Aortic root diastolic pressure.
25*
90 ±20 209 ± 19t 128 ± 17t 12 ± 8
Heart Rate 169
subendocardium 88
19
173 ± 13
29
66 ± 25§
subepicardium 87 ±32
101 ± 14"1
Flow Distribution subendo/subepi 1.01
0.24
0.65 ± 0.19§
§P < 0.002. IP < 0.01 significant differences from normals.
underperfusion of the endocardium, as shown by the decrease in flow distribution from 1.02 to 0.83, was significant. When flow was measured during subvalvular outflow obstruction (Table 3), no increase in epicardial flow, but a decrease in endocardial flow, was observed. The ratio of 0.86 was slightly greater than that of 0.83 obtained during supravalvular hypertension. Desaturation curves during ventricular hypertension in a single preparation are shown in Fig. 3a, and the calculated flows and ratio are illustrated in Fig. 3b. Marked left ventricular hypertension. More severe constriction of ventricular outflow produced ventricularsystemic gradients of 70 to 100 mmHg. Flow changes in this group were similar to those of mild LV hypertension, but more marked. During supravalvular hypertension (Table 4), epicardial flow increased 28% while subendocardial decreased 25%. The flow ratio of 0.65 indicated a significant relative underperfusion of the subendocardium. Flows during marked subvalvular obstruction showed parallel changes (Table 5), but the underperfusion of the endocardial layer, though still significantly different from normal, was less marked (0.79 compared to 0.65) than that of supravalvular constriction.
The average flow distribution of 0.65 for all runs increased slightly to 0.67 when the LVEDP was less than 12 mmHg, but decreased even more from a normal of 1.01 to 0.63 when the LVEDP was elevated above 12 mmHg. These differences were not statistically significant among themselves. Discussion
The method of polarographic recording of hydrogen gas desaturation has been shown to provide a good estimate of local myocardial blood flow.2 Its principle is that current produced by oxidation of molecular hydrogen to hydrogen ions at the surface of the platinum electrode is proportional to the partial pressure of hydrogen at the electrode. An electrode saturated with hydrogen will produce a steady current, and when the hydrogen is washed out by unsaturated blood, the rate of decrease in the current reflects the rate of desaturation, i.e., the rate of blood flow through the tissue. Application of a +0.3v potential against the KCI calomel reference electrode effectively reduces molecular oxygen at the platinum electrodes by
99o. Local disturbances to flow around the electrode tip
TABLE 5. Marked Subvalvular Left Ventricular Hypertension: Data From 10 Experimental Animals
Flow (ml/min/100 gin) Flow Distribution Blood Pressure
Normal Activity
140 ± 18* 96 ± 17 211 ± 10t 5± 3
Subvalvular Outflow Stenosis 110 ± 23* 70 ± 22 * Systemic pressure. t Ventricular pressure. t P < 0.002 significant difference from normals.
Heart Rate
subendocardium
subepicardium
subendo/subepi
9
54
10
56± 11
0.98 ±0.13
160 ± 12
44
11*
56
0.79 ± 0.19*
164
10
MOGGIO AND HAMMOND
286
Ann. Surg. * March 1976
FIG. 2a. Recordings of saturation and desaturation currents obtained during normal cardiac activity. Each darker graph line represents twelve seconds on the time scale.
CURVES OBTAINED FROM MYOCARDIUM DURING NORMAL ACTIVITY Point A: Beginning of hydrogen saturation B: Beginning of desaturation (hydrogen saturation) C: Complete woshout of hydrogen, return to baseline are minimal and, though a layer of uncirculated fluid the curve.2 In no case was hematoma formation evimay surround the electrode, its effect is to reduce the dent, and the probes consistently remained stable in the current and to produce a delay in the initial slope of myocardium throughout the course of the experiment. Verification that electrodes were localized 3-4 mm the curve, but does not influence the overall slope of from the epicardial and endocardial surfaces assured that NORMAL CARDIAC ACTIVITY diffusion loss of hydrogen through the surfaces of the 50 heart did not occur. F- 69.3 The formula for calculating blood flow is derived from the Fick equation and assumes that the tissue-blood partition co-efficient is 1.0, and that arterial recirculation is negligible during desaturation recordings.' The 20 low blood solubility of H2 provided minimal arterial circulation after a single passage through the lungs8 and recordings obtained from electrodes placed directly z in the left ventricular chamber showed less than 10%o M 10 arterial recirculation. Subendocordium Although the absolute flow measurements obtained by F*1I5 ml /min this technique may be subject to errors of accuracy, U. the advantages of the method are distinct. Simultaneous recordings from two electrodes permit comparison of Subepicordium inner and outer flows in the intact working heart. The Fa18 mI/mnin continued function of the electrodes over 3-5 hours permitted repeated determinations under different conditions in the same animal. Moreover, because repeated measSubendocardium 2 _ 0.98 Subepicordium urements could be obtained, each preparation served as its own control, and allowed direct comparison of results obtained during normal and experimentally altered cardiac activities. This is clearly an advantage over single 48 24 study techniques, such as tissue uptake of diffusible TIME (sec) isotopes and vascular uptake of radioactive microspheres. That the subendocardium is equally perfused during FIG. 2b. Desaturation curves plotted on semilogarithmic axes, normal activity is in general agreement with other tissue resultant flow rates and flow ratio during normal cardiac activity. C,)
z
0
w
5
0
.
Vol. 183
o
.,' .. +,.4.;',-.Tt4 I
287
HYDROGEN POLAROGRAPHY
No. 3
........
..........
C ....
FIG 3a. Recordings of saturation and desaturation currents obtained during left ventricular hypertension. Each darker graph line represents twelve seconds on the time scale.
...
......
m
.
I-
4
i--
*i-
--r' +--,~4=~'l04-
r-
A
TIME
B
q-
i^ A . - 4 - f --t r- _
----
P
;..
..
. .
-i-
,-f-
..-i.
r,I
--
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-,W-
-
a
-1.1-0000
17
CURVES OBTAINED FROM MYOCARDIUM DURING SUPRAVALVULAR HYPERTENSION Point A: B: C:
Beginning of hydrogen saturation Beginning of desaturation (hydrogen saturation) Complete washout of hydrogen, return to baseline
uptake studies, such as the clearance studies of Moir and DeBra12 and the metabolic study of Griggs.4 In contrast, Winbury15 and Harley6 reported underperfusion of the subendocardium. These latter studies used externally monitored clearance techniques and tissue Po2 determinations, the possible shortcomings of which have been suggested by Moir.1" The ability of the subendocardium to vasodilate in response to ischemia produced by coronary artery occlusion contrasts with earlier studies which indicated that these vessels are incapable of further vasodilation.7'16 The large vasodilatory capacity demonstrated in this experiment agrees with the findings of Buckberg, et al.3 obtained in dogs with normal coronary arteries. Since reactive hyperemia is not a true steady-state condition, the hydrogen desaturation technique does not provide an accurate flow value, but approximations of flows over short periods provide an estimate of myocardial vasodilation. Left ventricular hypertension, both subvalvular and supravalvular, produced relative underperfusion of the subendocardium. This decrease in blood flow occurred without a significant increase in heart rate. It is likely, however, that with increased aortic outflow resistance, the systolic ejection time increased, and the diastolic fraction, during which time most coronary perfusion occurs, decreased proportionately. Concurrently extravascular resistance rose with increasing ventricular pressure, and it is likely that both the shortened diastole and increased extravascular pressure contributed to subendocardial underperfusion. These speculations corroborate the conclusions of Buckberg3 who correlated subendocardial underperfusion with both supply (diastolic pressure time index) and demand (tension time index) factors. Contrary to their observations of increased heart rates (which contributed to decreased diastolic times) no increased rate was ob-
served in this study. Tachycardia certainly plays a role in decreasing endocardial flow, as shown by Neill, et al.13 but the present study further supports the need to consider the extravascular pressure effects on subendocardial blood flow. This conclusion is substantiated by the observations that the normal vasodilation ability of the subendocardium is inhibited by elevated ventricular pressures, and that elevated aortic root diastolic presLEFT VENTRICULAR HYPERTENSION 50
20 CD
z 10
z 0
w
2
2
24
48
72
96
120
144
TIME (seC) FIG. 3b. Desaturation curves plotted on semilogarithmic axes with resultant flow rates and flow ratio during left ventricular hypertension.
288
MOGGIO AND HAMMOND
sures also fail to change the transmyocardial flow gradient. At a given level of hypertension, however, the production of a ventricular to coronary pressure (or systemic pressure) gradient by subvalvular outflow obstruction lessened the underperfusion of the subendocardium, compared to that during supravalvular constriction. Based on previous studies in this laboratory5'10 which demonstrated transport of left ventricular luminal blood directly into the myocardium in conditions of reduced coronary pressure, it is suggested that the myocardial sinusoids may help supply the endocardium during reduced coronary pressure (or elevated subvalvular ventricular pressure) and provide a protecting source of oxygenated blood. Although the amount of blood supplied by the coronary-luminal pathways is inadequate to compensate fully for the decrease in myocardial flow, their contribution (an average of 3 to 5% of total myocardial flow) might be sufficient, under certain conditions, to prevent significant subendocardial ischemia. In conclusion, the technique of intramyocardial recording of hydrogen as desaturation has demonstrated the equality of subendocardial and subepicardial blood flow under normal conditions, and has shown the large reserve vasodilatory capacity of the subendocardium. It has been shown that left ventricular hypertension alters the distribution of myocardial blood flow to produce subendocardial underperfusion, and it is suggested that, under certain conditions, the myocardial sinusoids may provide a protective collateral source of blood to the potentially ischemic inner myocardium.
References 1. Aukland, K., Bower, B. F.,and Berliner, R.W.: Measurement of
Local Blood Flow with Hydrogen Gas. Circ. Res., 14:164, 1964.
Ann. Surg. * March 1976
2. Aukland, K., Kiil, F., Kjekshus, J., and Semb, G.: Local Myocardial Blood Flow Measured by Hydrogen Polarography; Distribution and Effect of Hypoxia. Acta Physiol Scand., 70:99, 1967. 3. Buckberg, G. D., Fixier, D. E., Archie, J. P., and Hoffman, J. I. E.: Experimental Subendocardial Ischemia in Dogs With Normal Coronary Arteries. Circ. Res., 30:67, 1972. 4. Griggs, D. M., Tchokoev, V. V., and Chen, C. C.: Transmural Differences in Ventricular Tissue Substrate Levels Due to Coronary Constriction. Am. J. Physiol., 222:705, 1972. 5. Hammond, G. L. and Moggio, R. A.: Function of Microvascular Pathways in Coronary Circulation. Am. J. Physiol., 220:1463, 1971. 6. Harley, A., Harper, J. R., and Estes, E. H.: Distribution of Blood Flow in the Myocardium Measured by Clearance of Xenonl33. Circulation, 33-34: 111-122, 1966. 7. Kirk, E. S. and Honig, C. R.: Non-uniform Distribution of Blood Flow and Gradients of Oxygen Tension Within the Heart. Am. J. Physiol., 207:661, 1964. 8. Klocke, F. and Wittenberg, S. M.: Heterogeneity of Coronary Blood Flow in Human Coronary Artery Disease and Experimental Myocardial Infarction. Am. J. Cardiol., 24:782, 1969. 9. Levine, H. D. and Fort, R. V.: Subendocardial Infarction: Report of Six Cases and Critical Survey of the Literature. Circulation, 1:246, 1950. 10. Moggio, R. A., Kabemba, J. M., and Hammond, G. L.: Coronary Ventricular Lumen Blood Exchange Demonstrated by 51Cr-Labeled Erythrocytes. Am. J. Physiol., 221:955, 1971. 1 1. Moir, T. W.: Subendocardial Distribution of Coronary Blood Flow and the Effect of Antiangina Drugs. Circ. Res., 30:621, 1972. 12. Moir, T. and DeBra, D. W.: Effect of Left Ventricular Hypertension, Ischemia, and Vasoactive Drugs on the Myocardial Distribution of Coronary Flow. Circ. Res., 21:65, 1967. 13. Neill, W. A., Phelps, N. C., Oxendine, J. M., et al.: Effect of Heart Rate on Coronary Blood Flow Distribution in Dogs. Am. J. Cardiol., 32:306, 1973. 14. Roberts, W. C. and Morrow, A. G.: Causes of Early Postoperative Death Following Cardiac Valve Replacement. J. Thorac. Cardiovasc., Surg., 54:422, 1967. 15. Winbury, M. M.: Redistribution of Left Ventricular Blood Flow Produced by Nitroglycerine. Circ. Res., 28,29:I-140, 1971. 16. Winbury, M. M., Howe, B. B., and Weiss, H.: Effect of Nitroglycerine and Dipyridamole on Epicardial and Endocardial Oxygen Tension. J. Pharmacol. Exp. Ther., 176:184, 1971.
