Deleterious effects of increased heart rate on infarct size in the conscious dog

Ddeterious Effects of hxeased Heart Rate on Infarct Size in theConsciousDog

WILLIAM E. SHELL, MD BURTON E. SOBEL, MD, FACC La Jolla, California

From the Department of Medicine, School of Medicine, University of California, San Diego, La Jolla, Calif. This study was supported in part by U. S. Public Health Service MIRU Contract PH 43-68 NHLI-1332, U. S. Public Health Service NHLI Special Fellowship 1 F03 HL4650501 (Dr. Shell), and U. S. Public Health Service Research and Career Development Award 1 -K4-HL-50, 179-01 (Dr. Sobel). Manuscript received August 30, 1972, accepted November 1, 1972. Address for reprints: Burton E. Sobel, MD, University Hospital, 225 W. Dickinson St., San Diego, Calif. 92103.

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This study was designed to determine whether augmentation of heart rate influences infarct size after coronary occlusion in unanesthetized dogs, some of which had experimentally induced atrioventricular (A-V) block. Coronary occlusion was produced in 27 conscious dogs by constriction of an externalized coronary arterial snare placed 3 to 5 days earlier. Heart rate was augmented at selected intervals after coronary occlusion by ventricular pacing or administration of isoproterenol or atropine, and left atrial pressure was monitored in selected animals in each group. Infarct size was determined from analysis of serial serum creatine phosphokinase (CPK) changes and verified by myocardial CPK analysis. After coronary occlusion alone, release of CPK from myocardial tissue ceased within approximately 14 hours, and calculated infarct size therefore became constant. Augmentation of heart rate at that time by ventricular pacing or administration of isoproterenol or atropine led to extension of infarction with additional myocardial necrosis averaging 40, 72 and 40 percent, respectively, of the initial infarct size in the 3 groups. Stepwise increments of heart rate led to progressively smaller increments in infarct size, and even modest increases in heart rate (from 60 to 90 beats/min) in dogs with A-V block led to marked increases in infarct size averaging 73 percent f 17 (SE, no. = 4). Augmentation of heart rate with isoproterenol as late as 72 hours after coronary occlusion led to marked extension of infarction. Thus, acceleration of heart rate after coronary occlusion consistently and markedly increased the extent of myocardial necrosis.

Prognosis after acute myocardial infarction appears to depend conby the ratio of siderably on infarct size,1,2 which is influenced myocardial oxygen supply to demand.3 The present study was designed to determine the effects of imposed alterations of heart rate, a major determinant of myocardial oxygen consumption, on infarct size in unanesth%tized dogs subjected to coronary occlusion. Infarct size was assessed by means of serial serum creatine phosphokinase (CPK) determinations in accordance with a method we have recently developed that serves to quantify infarct size in conscious animals. Methods Coronary artery occlusion was produced in 27 conscious dogs by constricof the left anterior descending coronary artery with an externalized snare placed 3 to 5 days earlier as previously described.4 Lidocaine (1 mg/kg body weight intravenously) was administered immediately before occlusion. Occlusion was not performed until serum CPK activity had re-

tion

turned to normal after the initial operative

procedure.

Jugular

venous cath-

eters were used to obtain blood samples at 90 minute intervals after coronary occlusion for serial serum CPK analysis. Infarct size was determined

from analysis of serial serum CPK changes as described later (see section on analysis of data). Heart rate, monitored hourly, was altered beginning 14

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to 16 hours after coronary artery occlusion by right ventricular pacing and administration of atropine, or isoproterenol in dogs with sinus rhythm and in other animals with experimentally produced atrioventricular (A-V) block. In selected dogs subjected to each intervention, mean left atria1 pressure was monitored periodically with the use of a chronically implanted left atria1 catheter. At the conclusion of each experiment the dog was anesthetized with pentobarbital, the heart excised and infarct size verified by myocardial CPK analysis.4*5 Ventricular pacing was accomplished with multistranded wire sutured to the right ventricular myocardium and attached to a Medtronic fixed-rate pacemaker. The pacemaker was not activated until 14 hours after coronary occlusion. Isoproterenol (0.15 mg/kg in oil) was administered subcutaneously, and atropine (1 mg) was given intravenously. Corresponding injections did not influence serum CPK activity in normal dogs. In some dogs complete A-V block was produced before coronary artery occlusion by direct injection of 40 percent formalin into the A-V node at the time the snare was initially placed. In these animals the heart was exposed through a right thoracotomy, the left anterior descending coronary artery was exposed by traction on the right ventricle, and a coronary arterial snare placed loosely around the vessel. Pacemaker wires were sutured to the right ventricle. Lidocaine, 1 mg/kg intravenously, was administered to prevent ventricular fibrillation at the time of production of A-V block. The coronary sinus was then palpated through the right atrium, and 0.3 to 1.0 ml of formalin was injected approximately 1 cm anterior and 1 cm inferior to the sinus. The area selected for injection was the site that induced second degree A-V block when palpated. Injection of formalin produced prompt, complete A-V block associated with a spontaneous heart rate of 40 to 50 beats/min. During the 5 day surgical recovery period, heart rate was maintained at 80 beats/min to minimize postoperative mortality. Coronary occlusion was produced by constriction of the snare 5 days after the initial operation at a time when serum CPK activity had returned to normal. Heart rate was then immediately decreased to 60 beats/min for 14 hours. Fourteen hours after occlusion heart rate was increased to 90 beats/min to determine the effect of this modest increase (from 60 to 90 beats/min) on infarct size. Twenty-four hours after coronary artery occlusion each animal was sacrificed so that infarct size could be assessed directly by myocardial CPK analysis. Myocardial biopsy specimens were obtained, and homogenates of the specimens and of the whole left ventricular muscle prepared as previously described425 in 0.25 M sucrose, 0.001 M neutralized sodium ethylenediaminetetraacetic acid (EDTA) and 0.001 M mercaptoethanol (pH 7.4). Protein content of the homogenates and of 16,000 X g supernatant fractions were determined by the biuret procedures and CPK in serum and tissue fractions was assayed spectrophotometrically by the Rosalki method as previously described.597 Results were expressed as international units (IU)/ml of plasma, IU/mg of protein or IV/g of myocardium. Analysis of Data Infarct sire was calculated by two methods: one based on serial changes in serum CPK and the other based on direct determination of depletion of myocardial CPK.4 The principle underlying the calculation from serum changes is that the instantaneous rate of change of serum enzyme activity (dE/dt) (E = enzyme concentration; t =

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is due to 2 competing phenomena: (1) release of CPK into the circulation as a function of time (f[t]) and (2) disappearance of CPK conforming to first order kinetics with a constant fractional disappearance rate (kd). Thus, dE/dt = f(t) + k,E. CPK, (the amount of CPK released into the CPK distribution space) can be calculated by rearrangement and integration of the terms of the equation (Jot f(t)dt = [dE/dt. - k,E] dt) and multiplication of Jot f(t)dt by the distribution space for CPK (9 percent body weight).4 In the dog, 30 percent of the CPK activity lost from myocardium is released into the serum; the remainder is inactivated locally.4,5 Thus, CPK depleted from myocardium (CPKJ = CPKJ0.3. We have shown4 that infarct size is linearly related and closely correlated (r = 0.98) to CPK, when infarct size is expressed in CPK gram equivalents (CPK-g-eq) (1 CPK-g-eq = that quantity of myocardium from which CPK depletion is equal in magnitude to CPK depletion in 1 g of myocardium exhibiting homogenous necrosis). The second method used to determine the infarct size is based on myocardial CPK analysis since CPK, can be obtained directly from tissue measurements.4 The ratio of CPK, to CPK, is an important factor used to determine of infarct size from serum values. This ratio was verified and found to be consistent in our experiments despite the introduction of interventions. The value for Ji f(t)dt was obtained with the use of a Fortran program on a Sigma 3 computer. The computer program was utilized to plot serial CPK changes and to calculate f(t) and J,” f(t)dt as a function of time.r time)

myocardial

Results Typical changes in serum CPK actiuit?, after coronary occlusion in the conscious dog are illustrated in Figure 1. Beginning 3 hours after occlusion serum

CPK activity rises rapidly to a peak (time to peak 720 min f 30, mean f SE, no. = 11) and decays monoexponentially with an average fractional disappearance rate of 0.45 percent/min as previously reported.4 When cumulated CPK, is calculated from serial serum CPK changes, it is apparent that CPK release becomes negligible by 14 hours after occlusion (Fig. 1). This phenomenon has been confirmed in 27 dogs. Thus, calculated infarct size is generally constant 14 hours after coronary occlusion in the conscious dog and by this time from which effects of interventions can be judged readily.

a base line exists on infarct size

Ventricular pacing: The influence of heart rate on infarct size was examined by augmenting heart rate 14 hours after occlusion. Right ventricular pacing was used in 6 dogs to increase heart rate from 90-110 to 180 beats/min. In each instance a new increase in serum CPK activity occurred in association with the increase in cumulated CPK (Fig. 2). Infarct size was calculated from both the initial increases in CPK (before ventricular pacing) and secondary increases (after pacing) (Table I). There was a consistent augmentation of infarct size with (average increase 40 percent f 5.4, [SE] over a range of initial infarct size from 1.1 to 30 CPK-g-eq). Isoproterenol and atropine: The influence of heart rate augmentation produced by administration of isoproterenol or atropine was examined in a simi-

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1 I

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FIGURE 1. Serial changes in serum CPK activity after coronary artery occlusion in the conscious dog (A) and cumulated CPK released calculated from serial serum changes (S). A, serum CPK activity is plotted on a log scale on the ordinate and time expressed as minutes after coronary occlusion is plotted on the abscissa. Cumulated CPK released was calculated as described in the text from the observed serum CPK values plotted in A. These calculated cumulated CPK released values are shown in B. Cumulated CPK released is plotted on the ordinate, and time after coronary occlusion, in minutes, is plotted on the abscissa.

FIGURE 2. Changes in serum CPK activity in a conscious dog after coronary occlusion alone and following subsequent augmentation of heart rate by right ventricular pacing. A, CPK activity increases after coronary occlusion alone and begins to decrease as in Figure 1. At the time indicated by the arrow heart rate was increased and maintained at 180 beats/min. This resuited in a subsequent increase in serum CPK activity accounting for the second peak. B, cumulated CPK released calculated from the raw data in A is plotted on the ordinate. Again the arrow indicates the onset of ventricular pacing. Calculated cumulated CPK released approaches a constant before ventricular pacing. However, after ventricular pacing, calculated cumulated CPK released increases before again leveling off approximately 10 hours later.

lar fashion. In each of 11 dogs given isoproterenol, augmentation of infarct size occurred (Table I). The mean augmentation was the greatest seen with any intervention employed in our study (average increase 77 percent f 23, over a range of initial infarct size from 0.5 to 39 CPK-g-eq). In 5 of 6 dogs, atropine augmented infarct size; in the sixth there was no change despite an increase in heart rate from 110 to 180 beats/min. In these 6 dogs infarct size increased by an average of 40 percent f 28 over a range of initial infarct size from 1 to 43 CPK-g-eq. Sequential increases in heart rate: To determine whether smaller sequential increases in heart rate in-

fluence infarct size less profoundly than an initial large increase we performed additional experiments. Heart rate was increased to 120 beats/min by ventricular pacing from the control spontaneous rate of 90 to 110 beats/min until CPK release again became negligible. Heart rate was then increased to 140, 160, and 180 beats/min stepwise, in each case only after CPK release had become negligible. Results of a typical experiment of this type are depicted in Figure 3. In each of 3 experiments the initial augmentation of heart rate from 100 to 120 beats/min produced a major augmentation of infarct size. With each sequential increase in heart rate, a progressively small-

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TABLE Effect

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I of Increasing

Heart Rate on Infarct

Size

Extension of Infarction Infarct Size After After Coronary Occlusion Augmentation of Heart Rate Alone Experiment no. and Intervention Ventricular pacing (rate increased to 180/min) 1 2 3 4 5 6 Mean + SE

(CPK-g-e@

15 1.7 1.1 30 16 14 13.2 i. 4.8

Increase in Infarct Size Produced by Increasing Heart Rate

(CPK-g-e@

4.5 3.2 0.5 7.4 5.9 8.8 5.0 f

(%)

30 45 45 24 36 61 1.2

l2-

04

0

40 zt 5.4

lsoproterenol administration (rate increased to 180/min) 1 2 3 4 5 6 7 8 9 10 11 Mean f SE

2.0 19 18 1.0 5.0 2.0 0.5 0.7 39 3.0 12 91 3.9

150 3.0 13 68 46 8.5 30 0.3 140 7.1 15 0.3 1.2 240 43 1.0 17 56 1.2 40 48 5.8 5.3 zt 1.7 77 f

23

Atropine administration (rate increased to 180/min) 1 2 3 4 5 6 Mean + SE

1.5 34 37 42 0.7 6.0 20 f 7.9

0.5 5.6 23 9.1 0 2.0 6.7 St 3.5

35 16 61 21 0 33 40 f

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A-V block before occlusion, ventricular pacing after occlusion (rate increased to 90/min) 1 2 3 4 Mean f SE

12 39 40 10 25 + 8.1

8.6 13 20 10 13 i 3.0

71 72 50 100 73 f

12

SE = standard

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FIGURE 3. The effect of stepwise augmentation of heart rate on cumulated CPK released in conscious dogs with coronary occlusion. Cumulated CPK released is plotted on the ordinate, and time after coronary artery occlusion is plotted on the abscissa. Numbers in parentheses indicate the calculated magnitude of myocardial necrosis, expressed as CPK-g-eq, associated with each heart rate. As can be seen, after coronary artery occlusion during an interval when heart rate was between 95 and 105 beats/min infarction of 14 CPK-g-eq of myocardium occurred as indicated by cumulated CPK released derived from serial serum CPK changes. When heart rate was increased and maintained at 120 beats/min, an additional 4.5 CPK-g-eq of myocardium underwent infarction. Subsequent increments in heart rate to 140. 160 and 180 beats/min were associated with progressively smaller augmentations of infarct size.

error. TIME (min) AFTER CORONARY ARTERYOCCLUSION

er extension of infarction resulted (Fig. 3). Thus, it appears that a major augmentation of infarct size occurs with relatively modest increases in heart rate. To examine the influence on infarct size of augmentation of heart rate from slower initial heart rates, 4 conscious animals with A-V block were studied. Coronary occlusion was produced and serum CPK sampled until cumulated CPK, had become negligible (approximately 14 hours after occlusion).

FIGURE 4. Serial serum CPK changes in a conscious dog after coronary artery occlusion alone and after augmentation and maintenance of heart rate at ldO/min 72 hours later with isoproterenol. The time when isoproterenol administration was initiated is indicated by the vertical arrow. During the time indicated by the break in the abscissa, serum CPK activity remained between 10 and 24 mlU/ml. Coronary artery occlusion alone was associated with a characteristic peak of serum CPK activity and subsequent return to normal. Administration of isoproterenol 72 hours later led to extension of infarction associated with additional release of CPK into the circulation and a secondary peak of serum CPK activity.

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Heart rate was then increased from 60 to 90 beats/ min. This resulted in a consistent augmentation of infarct size similar in magnitude to that induced by isoproterenol. The average increase was 73 percent f 17 over a range of initial infarct size of 10 to 40 CPK-g-eq. Thus, in the presence of A-V block, the increase of heart rate from 60 to 90 beats/min resulted in a major extension of infarction. Additional experiments were performed to determine the length of time during which myocardium remains susceptible to potentially deleterious interventions. CPK, was calculated from serum values, and heart rate increased from base-line value to 180 beats/min 24, 48 or 72 hours later with isoproterenol. In each case, heart rate was increased only after CPK, was no longer increasing. In each of 3 experiments of this type, administration of isoproterenol led to extension of infarction (Fig. 4). Verification of infarct size: In all experiments in this investigation infarct size was verified directly by analysis of myocardial CPK. The ratio of CPK,, calculated from serum values, to CPK. depleted from myocardium, measured directly, remained constant (0.3 f 0.02, mean f SE, no. = 27). Thus, results from serum calculations were not spuriously influenced by disproportionate losses of myocardial CPK due to variations in heart rate. In each of 10 dogs without coronary occlusion, administration of isoproterenol or atropine or maintenance of heart rate at 180 beats/min by ventricular pacing for 24 hours led to no discernible increase in serum CPK activity. Results of left atria1 pressure measurements in selected conscious dogs exposed to each intervention showed that congestive heart failure did not occur as a consequence of coronary occlusion alone or with imposed tachycardia. Discussion

The results of our investigation demonstrate that increases in heart rate augment the extent of myocardial ischemic injury after coronary artery occlusion in the conscious dog. When the initial heart rate is low, the most striking augmentation of infarct size is associated with modest increases in heart rate. Regardless of the intervention used to accelerate heart rate (ventricular pacing or administration of atropine or isoproterenol), tachycardia increased infarct size. Susceptibility of the heart to additional ischemit injury associated with acceleration of heart rate

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persisted for at least 72 hours after coronary artery occlusion. Electrocardiographic changes similar to those associated with myocardial ischemia are commonly recognized clinically when tachycardia occurs spontaneous1y.s Furthermore, experimentally induced tachycardia in open chest dogs with coronary occlusion results in increased S-T segment elevation in epicardial electrocardiographic recordings3 However, it has been difficult to assess the influence of heart rate on myocardial damage per se, rather than on electrocardiographic manifestations of ischemia. The method we utilized permits quantitative assessment of the extent of ischemic myocardial injury during evolution of infarction in the conscious dog. Results indicate that in conscious dogs with coronary occlusion but without left ventricular failure the extent of myocardial necrosis increases when heart rate is increased, even modestly, above 60 beats/min. Augmentation of infarct size by tachycardia is probably related to several factors. Acceleration of heart rate increases myocardial oxygen demand by increasing oxygen consumption per beat and total beats per minute.g Although coronary blood flow increases with tachycardia in the normal coronary bed, collateral coronary flow, which occurs primarily during diastole, is impaired by tachycardia.lO Thus, after coronary occlusion tachycardia probably increases oxygen demand of the marginally perfused myocardium but decreases collateral flow to it. Ventricular pacing and administration of atropine or isoproterenol augmented infarct size to different degrees although they produced comparable increases in heart rate. Since isoproterenol increases oxygen demand substantially, in part by altering contractility, it is not surprising that this agent increased infarct size most profoundly. Increased heart rate led to augmentation of infarct size even when it was imposed as long as 72 hours after coronary occlusion. Accordingly, increases in heart rate induced by routine administration of atropine to prevent ventricular arrhythmias may be hazardous in the presence of myocardial ischemia. The extent of myocardial necrosis influences the mortality and morbidity after acute myocardial infarction in man.132 Results of our investigation suggest that infarct size can be minimized by maintenance of a relatively slow heart rate as long as cardiac output is not compromised and ventricular arrhythmias are not precipitated.

References Sobel BE, Bresnahan GF, Shell WE, et al: Estimation of infarct size in man and its relation to prognosis. Circulation 46:640-646,1972 Harnarayan C, Bennett MA, Pentecost BL, et al: Quantitative study of infarcted myocardium in cardiogenic shock. Srit Heart J 32:726-732, 1970 Maroko PR, Kjekshus JK, Sobel BE, et al: Factors influencing infarct size following experimental coronary artery oc-

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ciusions. Circulation 43:67-62, 1971 Shell WE, Kjekshus JK, Sobel BE: Quantitative assessment of the extent of myocardial infarction in the conscious dog. by means of analysis of serial changes in serum creatine phosphokinase activity. J Clin Invest 50:2614-2625, 1971 5 Kjekshus JK, Sobel BE: Depressed myocardial creatine phosphokinase activity following experimental myocardial infarction in rabbit. Circ Res 27:403-414, 1970

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6. Gornell AG, Bardawlll CJ, David serum proteins by means of the Chem 177:751-766,1949

MM: Determination of biuret reaction. J Biol

7. Rosalki SB: An improved procedure for serum creatine phosphokinase determination. J Lab Clin Med 69:696-705, 1967 6. Helfant RH, Forrester JS, Hampton JR, et al: Coronary heart disease: differential hemodynamic, metabolic and electro-

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cardiographic effects in subjects with and without angina pectoris during atrial pacing. Circulation 42:601-610, 1970 9. Braunwald E: Control of myocardial oxygen consumption. Physiologic and clinical considerations. Amer J Cardiol 27:416-432,197l 10. Brown BG, Gundel WD, Gott VL, et al: Hemodynamic determinants of retrograde arterial coronary flow following acute coronary occlusion, (abstr.). Circulation 46:suppl ll:lOO, 1972

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