Myocardial oxygen consumption in acute experimental cardiac depression

Functional and antiischemic effects of luteolin-7-glucoside in isolated rabbit hearts

1. The functional effects of the flavonoid luteolin-7-glucoside (LUT) were investigated in Langendorff-rabbit hearts perfused at constant pressure. Repetitive myocardial ischemia was induced by coronary artery ligature and quantified from NADH-fluorescence photography. 2. LUT significantly enhanced left ventricular pressure and the global and relative coronary flow (= global coronary flow/pressure-rate product). 3. LUT significantly diminished epicardial NADH-fluorescence area and intensity. 4. LUT is an inodilator possessing cardioprotective properties. These might be related to an improvement of myocardial perfusion and/or to free radical scavenging properties.

Functional and antiischaemic effects of the phosphodiesterase inhibitor levosimendan in isolated rabbit hearts

Several phosphodiesterase inhibitors have been reported to possess antiischaemic properties by improving myocardial oxygen demand/supply balance. Levosimendan is a new phosphodiesterase inhibitor with calcium-sensitizing properties. We investigated the functional and antiischaemic properties of levosimendan in isolated electrically-driven rabbit hearts (Langendorff, constant perfusion pressure: 70 cm H2O, Tyrode solution, Ca++ 1.8 mmol/l, 37 degrees, 200 beats/min.). Acute regional myocardial ischaemia was induced by ligature of a circumflex artery branch and quantified from epicardial NADH-fluorescence photography.

RESULTS

The left ventricular pressure was similarly enhanced by levosimendan 10(-7) M or 5 x 10(-6) M (+15%) (P < 0.05). The global coronary flow was increased more markedly by levosimendan 5 x 10(-6) M (+35-50%) compared to 10(-7) M (+25%) (P < 0.05). The relative coronary flow (= global coronary flow/pressure-rate-product) was significantly enhanced only by levosimendan 5 x 10(-6) M (+20-35%) (P < 0.05), but not by levosimendan 10(-7) M (+10-15%) (P > 0.05). Epicardial NADH-fluorescence area and intensity were significantly diminished by levosimendan (-20%) (P < 0.05), and there was no significant difference between myocardial ischaemia reduction by levosimendan 10(-7) M and 5 x 10(-6) M (P > 0.05).

CONCLUSION

Levosimendan is an inotrope with coronary dilator activity, showing antiischaemic effects in isolated rabbit hearts. These may be caused by (1) phosphodiesterase-inhibition and improvement of myocardial perfusion and/or (2) by oxygen-sparing effects, related to calcium sensitization of myofilaments, particularly at lower concentrations associated with a lower coronary dilator activity.

Investigation of therapeutic mechanisms of atenolol and diltiazem in patients with variable-threshold angina

The effects of beta-adrenoreceptor and calcium channel-blocking agents on the balance of myocardial oxygen supply and demand were studied in 15 patients (2 women and 13 men), aged 46 to 69 (mean 62) years, with variable-threshold angina. An exercise test was performed before therapy was begun, and 24-hour ambulatory ECG monitoring was performed after 1 week of long-acting nitrate therapy, in the third week after random assignment to either atenolol or diltiazem, and in the third week after crossover. All exercise tests were positive except in one patient taking diltiazem. The exercise time to 0.1 mV ST change was 4.2 +/- 1.7 minutes with no therapy, 5.1 +/- 1.6 minutes with nitrates, 6.6 +/- 0.8 minutes with diltiazem, and 6.5 +/- 2.1 minutes with atenolol. The rate-pressure product at 0.1 mV ST change was 20.9 +/- 4.6 with no therapy and 21.2 +/- 5.7 and 22 +/- 4.6 with nitrates and diltiazem, respectively, but fell to 15 +/- 3.9 beats.min.-1 mm Hg 10(-3) after atenolol (p < 0.01). There was no significant difference in the number of anginal attacks or in nitroglycerin consumption per week. The heart rate at the onset of ST-segment change during the exercise test and during ambulatory ECG monitoring was significantly lower during atenolol than during diltiazem treatment. Both atenolol and diltiazem were of similar efficacy in increasing nonischemic exercise duration in patients with variable-threshold angina and acted primarily by slowing the resting heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Beneficial effect of amrinone on the size of acute regional ischemia in isolated rabbit hearts

The effect of inotropes on myocardial ischemia is difficult to predict because they may influence the determinants of myocardial O2 demand and O2 supply differently. Several PDE-inhibitors have been reported to possess antiischemic properties related to their hemodynamic and O2-sparing effects. To assess whether PDE-inhibitors also possess direct cardioprotective properties, the effects of amrinone (2.5 x 10(-5) mol/L) in comparison to isoproterenol (5 x 10(-9) mol/L) and ouabain (1.5 x 10(-7) mol/L) were studied in isolated rabbit hearts perfused according to Langendorff at a constant pressure (70 cmH2O) and electrically driven at a constant pacing rate. Regional ischemia was induced by coronary artery ligation and quantified by epicardial NADH fluorescence. All substances significantly increased the actively developed left ventricular pressure to a similar extent (+20%) (P < 0.05). Coronary flow was significantly decreased by ouabain (-15%) and significantly increased by isoproterenol (+25%) and particularly by amrinone (+50%) (P < 0.05). Neither ouabain nor isoproterenol significantly changed the intensity or the distribution pattern of NADH fluorescence, whereas the size of the ischemic zone was significantly reduced by amrinone (-25%) (P < 0.05). The PDE-inhibitor amrinone was shown to possess a direct cardioprotective effect by improving myocardial perfusion and O2 supply in isolated rabbit hearts.

Influence of (-)-S-Bay K 8644, (+/-)-Bay W 5035 and (+/-)-Bay T 5006 on hemodynamics and FITC-dextran 3 elution kinetics in isolated rat hearts

1. We investigated the effects of the new calcium-agonists (+/-)-Bay W 5035 and (+/-)-Bay T 5006 in comparison to (-)-S-Bay K 8644 on hemodynamics and epimyocardial perfusion in Langendorff rat hearts. 2. At equieffective inotropic concentration, vasoconstriction of coronary resistance vessels was significantly less after (+/-)-Bay W 5035 or (+/-)-Bay T 5006 than after (-)-S-Bay K 8644 application. 3. FITC-Dextran 3 elution kinetics indicated that the epimyocardial vascular volume was significantly reduced only by (-)-S-Bay K 8644. 4. Moreover, (-)-S-Bay K 8644 enhanced transcoronary exchange more markedly than (+/-)-Bay W 5035 or (+/-)-Bay T 5006, reflecting the differences in coronary constrictor activity. 5. We conclude that in comparison to (-)-S-Bay K 8644 the relation between inotropy and vasoconstriction is more favorable for (+/-)-Bay W 5035 or (+/-)-Bay T 5006.

Influence of phosphodiesterase inhibition on myocardial energetics in dilative cardiomyopathy

The effects of inhibition of phosphodiesterase by enoximone on left ventricular haemodynamics and myocardial energetics were investigated in 10 patients with idiopathic dilative cardiomyopathy. After intravenous administration of enoximone, there was a significant reduction of left ventricular systolic pressure from 126 +/- 21 to 93 +/- 16 mm Hg, of left ventricular end-diastolic pressure from 16 +/- 8 to 5 +/- 3 mm Hg and of left ventricular end-diastolic volume from 287 +/- 54 to 215 +/- 69 ml. Left ventricular pressure-volume work decreased significantly from 12.1 +/- 3.6 to 7.6 +/- 2.8 mm Hg.l. Heart rate was 87 +/- 17 before and 103 +/- 18 min-1 after administration of enoximone (p less than 0.01). Left ventricular systolic stress-time integral, a major determinant of myocardial oxygen consumption, decreased by 49% from 91 +/- 32 to 46 +/- 15 10(3) dyn.s/cm2 (p less than 0.01). In contrast myocardial oxygen consumption per beat was reduced by only 18%, from 138 +/- 28 to 113 +/- 17 microliters O2/100 g (p less than 0.01). The economy of myocardial contraction as calculated by the ratio of systolic stress-time integral to myocardial oxygen consumption per beat was 675 +/- 192 before and 370 +/- 128 dyn.s.100 g/cm2.microliter O2 after administration of enoximone. In conclusion, the phosphodiesterase inhibitor enoximone exhibits vascular and myocardial effects. The myocardial effects result in decreased economy of myocardial contraction. The possible molecular mechanisms of these energetic changes are discussed.

Hemodynamic effects of angiotensin and renin inhibition in dogs with acute left ventricular failure

The effects of enalaprilat (MK-422), an angiotensin converting enzyme inhibitor, were compared to those of SCRIP, a renin inhibitor, in experimentally induced left ventricular failure. In anesthetized dogs, acute left ventricular failure was induced by repeated embolization, via the left main coronary artery, with 50 microns plastic microspheres. Embolization significantly increased left ventricular enddiastolic pressure from 6 +/- 1 to 14 +/- 1 (p less than 0.05) mm Hg and decreased both left ventricular maximal dP/dt (3,135 +/- 338 to 1,636 +/- 126 mm Hg/second, p less than 0.05) and cardiac output (3.0 +/- 0.3 to 1.6 +/- 0.1 liters per minute, p less than 0.05). Embolization also significantly reduced heart rate and mean arterial pressure. These parameters remained stable after induction of heart failure. Forty-five minutes after embolization, 16 dogs received enalaprilat (100 microns/kg intravenously) and six dogs received SCRIP (100 microns/kg intravenously followed by 10 microns/kg per minute). Both agents caused similar reductions in left ventricular end-diastolic pressure (21 percent versus 26 percent) and total peripheral resistance (25 percent versus 32 percent) and rise in peak positive cardiac contractility, as measured by (dP/dt)/P, (12 percent versus 11 percent). The data suggest that inhibition of angiotensin II formation by two agents, each or which acts at a different point in the cascade, results in similar beneficial hemodynamic effects in dogs with acute left ventricular failure. In addition, angiotensin converting enzyme inhibition failed to further increase sodium excretion and glomerular filtration rate caused by embolization. In summary, inhibition of angiotensin II production by two different inhibitors of the renin system causes an improvement in left ventricular performance in a model of acute experimental left ventricular failure.

Beneficial effect of amrinone on myocardial oxygen consumption during acute left ventricular failure in dogs

In 11 dogs ischemic left ventricular failure characterized by a 30 percent reduction in cardiac output and a left ventricular end-diastolic pressure of 18 mm Hg or more was produced by proximal occlusion of the left anterior descending coronary artery followed by serial occlusions of the distal left circumflex coronary artery. Administration of amrinone in an intravenous bolus injection followed by a constant infusion produced improvements in cardiac output (from 1.62 +/- 0.50 to 2.19 +/- 0.52 liters/min [mean +/- standard deviation], p less than 0.05), left ventricular end-diastolic pressure (from 21.6 +/- 3.5 to 11.0 +/- 5.4 mm Hg, p less than 0.05) and peak positive rate of rise of left ventricular pressure [dP/dt] (from 1,264 +/- 241 to 1,800 +/- 458 mm Hg.s-1, p less than 0.05). These improvements were maintained throughout the 20 minute period of therapy. No significant alteration in heart rate or arterial pressure was noted. In parallel with the hemodynamic improvement myocardial oxygen consumption improved to 0.094 +/- 0.05 and 0.092 +/- 0.04 vol.min-1.g-1 after 2 and 20 minutes, respectively, of amrinone compared with 0.124 +/- 0.05 during left ventricular failure (both p less than 0.05). The effects of amrinone on left ventricular failure are due to augmented contractility and mild systemic vasodilatation. The reduction in myocardial oxygen consumption during amrinone-treated left ventricular failure presumably results from a reduction in ventricular wall tension that more than offsets the effect of an increase in contractility.

Increased exercise tolerance after oral diltiazem, a calcium antagonist, in angina pectoris

Effects of diltiazem, a recently introduced calcium antagonist, on exercise performance were studied in nine coronary disease patients with effort angina. The duration of exercise before the onset of angina and the time to the onset of ischemic ST depression 2 hours after 90 mg of oral diltiazem were compared with those 2 hours after oral placebo and a few minutes after 0.3 mg of sublingual nitroglycerin. Diltiazem prolonged the duration of exercise in all nine patients (average 2.5 minutes, p less than 0.001) and delayed the onset of ischemic ST depression (average 2.4 minutes, p less than 0.001). The increment of the duration of exercise and the time to the onset of ischemic ST depression following 90 mg of oral diltiazem were almost equivalent to that following sublingual nitroglycerin. These results in fixed coronary atherosclerosis indicate the clinical antianginal efficacy of diltiazem which persists for at least 2 hours after oral administration.

Comparative influence of ouabain, norepinephrine and heart rate on myocardial oxygen consumption and inotropic state in dogs

The purpose of this study was to compare the myocardial oxygen cost of augmented inotropic state produced by ouabain, norepinephrine, or increased heart rate. This problem was examined in dogs using an isovolumically contracting left ventricular preparation. Inotropic state was measured as the maximum observed contractile element velocity at the lowest common level of wall stress (MAX V). Peak left ventricular wall stress was maintained constant in each dog so that it would not influence changes in myocardial oxygen consumption (MVO2). Ouabain (4 X 10(-2) mumoles/Kg.) and norepinephrine (2 X 10(-3) mumoles/Kg./minute) always augmented inotropic state (MAX V) and increased MVO2. The positive slopes of the regression of MVO2 on MAX V for ouabain (45.4 +/- 12.5 microliter/beat/100 Gm./muscle length/sec; mean +/- SEM) and norepinephrine (34.5 +/- 5.6 microliter/beat/100 Gm./muscle length/sec; mean +/- SEM) were not significantly different, indicating that for an equal augmentation of inotropic state, ouabain increases myocardial oxygen demands to the same extent as does norepinephrine. When the results with ouabain or norepinephrine were compared to results obtained by altering heart rate, it was found that increasing inotropic state by these pharmacologic agents is more costly in terms of myocardial energy demands than when inotropic state is enhanced by increasing heart rate.

Spatial and nonspatial influences on the TG-ST segment deflection of ischemia. Theoretical and experimental analysis in the pig

Spatial and nonspatial aspects of TQ-ST segment mapping were studied with the solid angle theorem and randomly coded data from 15,000 electrograms of 160 anterior descending artery occlusions each of 100-s duration performed in 18 pigs. Factors analyzed included electrode location, ischemic area and shape, wall thickness, and increases in plasma potassium (K(+)). Change from control in the TQ-ST recorded at 60 s (DeltaTQ-ST) was measured at 22 ischemic (IS) and nonischemic (NIS) epicardial sites overlying right (RV) and left (LV) ventricles. In IS regions, DeltaTQ-ST decreased according to LV > septum > RV and LV base > LV apex. In NIS regions, LV sites had negative (Neg) DeltaTQ-ST which increased as LV IS border was approached. However, RV NIS had positive (Pos) DeltaTQ-ST which again increased as RV IS border was approached. With large artery occlusion IS area increased 123+/-18%, DeltaTQ-ST at IS sites decreased (-38.1+/-3.6%), and sum of DeltaTQ-ST at IS sites increased by only 67.3+/-10.3%. In RV NIS Pos DeltaTQ-ST became Neg. With increased K(+), DeltaTQ-ST decreased proportionately to log K(+) (r = 0.97+/-0.01) at IS and NIS sites on the epicardium and precordium. TQ-ST at 60 s was obliterated when K(+) = 8.7+/-0.2 mM. All findings were significant (P < 0.005) and agreed with the solid angle theorem. Thus, a transmembrane potential difference and current flow at the IS boundary alone are responsible for the TQ-ST. Nonspatial factors affect the magnitude of transmembrane potential difference, while spatial factors alter the position of the boundary to the electrode site.

Effects of allopurinol, propranolol and methylprednisolone on infarct size in experimental myocardial infarction

With use of a canine model of occlusion of the left anterior descending coronary artery and an intracellular lactic dehydrogenase stain to measure infarct size directly, the effects of allopurinol, methylprednisolone sodium succinate and propranolol were studied. Allopurinol did not influence the extent of myocardial necrosis, whereas both methylprednisolone and propranolol significantly reduced myocardial infarct size. Possible mechanisms of action and clinical applicability of these agents are discussed.

The relationship between regional myocardial perfusion at rest and arteriographic lesions in patients with coronary atherosclerosis

Measurements of mean left ventricular (LV) and regional myocardial blood flow rates were made at rest in 161 patients with 133Xe and a multiplecrystal scintillation camera. Myocardial perfusion rates were correlated with assessments of the degree of coronary artery disease made from the arteriograms obtained during the same studies. In patients with normal coronary arteries without heart failure, the presence of hypertension, aortic stenosis, or aortic insufficiency was not associated with changes in mean LV perfusion from the control value of 61+/-7 ml/100 g-min. However, mean LV perfusion was significantly reduced in patients with normal coronary arteries who had cariomyopathy and impaired ventricular performance. Mean LV perfusion was not significantly different from control values in patients with "mild" coronary artery disease (less than 50% obstruction) or in patients with significant isolated disease (greater than 50% obstruction) of the left anterior descending (lad) artery. Significant reductions in mean LV perfusion were found in patients with greater than 50% obstruction of two coronary arteries (LAD + right or LAD + circumflex) and in patients with triple-vessel disease. The average perfusion rate for regions distal to LAD obstructions in patients with isolated LAD disease was not lower than the LAD perfusion in control patients, but was significantly reduced in patients with LAD + right coronary artery disease (43+/-14 ml/100 g-min). In the latter group average perfusion distal to the LAD lesion was significantly lower than the average regional perfusion rate for the remainder of the LV. However, the mean blood flow rate for the remainder of the LV was also significantly lower than control values despite the lack of significant circumflex disease. The data demonstrate that the presence of radiographically "mild" or significant isolated LAD coronary disease is not associated with reductions in mean LV perfusion at rest, but that mean LV perfusion is reduced in the presence of significant disease of two or three coronary artieries. None of the patients experienced angina during the resting studies and most had clinical evidence of ventricular failure. The observation of depressed LV perfusion in this group, as in the patients with cardiomyopathy, raises the possibility that a lowered resting blood supply may be adequate for a reduced level of performance of a diseased ventricle. The lack of selective reductions of regional perfusion at rest in the majority of the patients with LAD lesions suggests that regional myocardial blood flow must be measured during an intervention which increases myocardial oxygen consumption in order to assess the physiological significance of lesions which are observed at coronary arteriography.

Cardiac responses to snout immersion in trained dogs

1. Four dogs were trained to immerse their snout voluntarily for durations up to 30 sec. Indwelling instrumentation was implanted to measure blood flow velocity in the circumflex branch of the left coronary artery, to sample blood from the left atrium and coronary sinus for the determination of P(O2), P(CO2), pH, oxygen saturation and haemoglobin concentration, and to pace the heart. An index of myocardial oxygen consumption was calculated by multiplying the mean flow velocity by the arteriovenous difference in oxygen content.2. Mean coronary flow velocity decreased significantly during simulated diving by 26 +/- 27% (+/- S.D.). The range of decrease in seventeen out of twenty experiments was from -5 to -81%. Heart rate decreased by 48 +/- 7% and this bradycardia was abolished by I.V. atropine.3. Coronary sinus oxygen saturation increased significantly with snout immersion (three dogs) and arteriovenous difference decreased from 67 +/- 10 to 47 +/- 5%. The index of myocardial oxygen consumption decreased by 42 +/- 19%. This decrease was attenuated slightly by beta-blockade but was abolished by cardiac pacing in three out of four experiments.4. The present study indicates that the heart consumes oxygen at a considerably reduced rate during simulated diving and therefore plays a direct role in the overall conservation of oxygen. This response appears to result primarily from a negative chronotropic effect induced by increased vagal tone.

Effect of inhibition of lipolysis on infarct size after experimental coronary artery occlusion

Recent studies have demonstrated a depressant effect of increased delivery of FFA to the hypoxic heart. Because catecholamines are released in acute myocardial infarction, it is likely that lipolytic activity is increased. The purpose of this study was to determine whether inhibition of hormone-sensitive lipases influence the extent and severity of myocardial ischemic injury produced by coronary occlusion. Myocardial infarction was produced by occlusion of the left anterior descending coronary artery in open-chest dogs. 15 min later a surface map of S-T segments was obtained with the use of 10-14 epicardial leads in the distribution area of the occluded artery. Average S-T segment elevation of all sites was used as an index of myocardial ischemic injury. Before coronary occlusion, the average S-T segment elevation was 0.3+/-0.2, which increased to 4.1+/-0.7 mV (SEM, 12 dogs) after occlusion. Inhibition of lipolytic activity by beta-pyridyl-carbinol before repeated coronary occlusion reduced the occlusion-induced S-T segment elevation to 2.1+/-0.6 mV (P < 0.001). When arterial concentrations of FFA were raised by i.v. infusion of a triglyceride emulsion and heparin, average S-T segment elevation after coronary occlusion increased from 1.2+/-0.7 to 2.2+/-0.8 mV (P < 0.05) in animals treated with beta-pyridyl-carbinol, which suggests an unfavorable effect of circulating FFA in this setting. Isoproterenol given before a repeated occlusion increased the severity and extent of the ischemic injury. The effect of isoproterenol on the occlusion-induced S-T segment elevation was reduced, however, when the lipolytic effect of the drug was inhibited by beta-pyridyl-carbinol. Our study suggests that beta-pyridyl-carbinol during acute coronary artery occlusion may be of importance in reducing the extent and severity of myocardial ischemic injury.

Extent of regulation of the heart's contractile state in the conscious dog by alteration in the frequency of contraction

The effects of alterations in the frequency of contraction over the range from 94 to 220/min on left ventricular pressure, diameter, and dP/dt were studied in 10 dogs instrumented with ultrasonic diameter ganges and miniature pressure gauges. The same dogs were studied on separate days in the conscious state, after general anesthesia with pentobarbital Na, 30 mg/kg, and in the conscious state after pretreatment with propranolol, 3 mg/kg. End diastolic diameter was maintained constant during alterations in frequency by infusing saline intravenously. The maximum increases in peak dP/dt and dP/dt/P in the conscious state were 14 and 10%, respectively. After anesthesia, raising the frequency of contraction from 122 to 220/min caused maximum increases in peak dP/dt and dP/dt/P of 36 and 30%, respectively. In the conscious state after cardiac depression by propranolol, the maximum increases in peak dP/dt and dP/dt/P were 23 and 23%, respectively. Thus, increasing the frequency of contraction of the normal heart of the conscious dog causes only a slight inotropic effect, but this effect is significantly greater in the presence of myocardial depression produced by anesthesia with pentobarbital Na or in the conscious animal after a myocardial-depressing dose of propranolol.

Effects of cardiac depression and of anesthesia on the myocardial action of a cardiac glycoside

The effects of ouabain (G-strophanthin) 20 mug/kg, on left ventricular (LV) pressure (P), diameter (D), velocity of contraction (dD/dt), and dP/dt were studied in conscious dogs instrumented with ultrasonic diameter gauges and miniature pressure gauges. The effects of ouabain were compared on separate occasions in the same dogs after cardiac depression with propranolol, 3.0 mg/kg, and also after general anesthesia with Na pentobarbital, 30 mg/kg. Maximal pressor effects were observed in the first 10 min, but maximal effects on the contractile state occurred at 30 min after ouabain. At this time, in conscious dogs, ouabain had increased LV isolength systolic pressure by 5%, LV isolength velocity by only 9%, and LV (dP/dt)/P by 21%, while end systolic diameter (ESD) decreased slightly and end diastolic diameter (EDD) and heart rate (HR) were unchanged. After anesthesia, ouabain increased LV systolic pressure by 8%, velocity 32%, (dP/dt)/P by 47%, and ESD decreased by 1.2 mm while EDD rose slightly and HR fell by 26 beats/min. Returning HR to control with atrial pacing decreased EDD 0.9 mm below control. After cardiac depression with propranolol, ouabain caused responses similar to those observed in the anesthetized dogs. Thus, the cardiac glycoside was found to exert only minor inotropic effects on the nonfailing heart of conscious dogs but far more striking inotropic responses in the anesthetized state.

Left ventricular performance after acute myocardial infarction

Current knowledge concerning the major hemodynamic features of acute myocardial infarction has been reviewed and discussed in relation to present concepts of cardiac pathophysiology. The physical examination provides a great deal of information and new, noninvasive methods promise to supplement the bedside appraisal of left ventricular function. Direct hemodynamic methods of serially monitoring patients with acute myocardial infarction are finding increasing application and recently have added considerably to our understanding of this condition. Certain limitations in the use of the central venous pressure, pulmonary arterial pressure, and cardiac output in appraising left ventricular function have become apparent, but together with direct catheterization of the left ventricle such hemodynamic studies have now provided limited correlations between the clinical picture and various hemodynamic patterns. It is becoming increasingly clear that most of these features, including cardiogenic shock, probably reflect varying degrees of left ventricular failure. These initial findings and interpretations will require confirmation, however, and so far insufficient objective data are available concerning the natural history of acute myocardial infarction and its responses to various forms of therapy. The aims of investigations now being carried in specialized Myocardial Infarction Research Units and other cardiovascular research centers, are to gain such further understanding of the pathophysiology of this disease and to aid in its clinical management by developing accurate indirect monitoring techniques as well as new forms of therapy.