Infarct size reduction by propranolol before and after coronary ligation in dogs

Reduction of creatine kinase and creatine kinase-MB indexes of infarct size by intravenous verapamil

In a prospective, controlled study, 29 patients were randomly allocated to receive intravenous verapamil, 5 to 10 mg/hour, for 2 days starting at a mean of 8 hours after the onset of myocardial infarction. Twenty-five patients received no specific treatment and served as control subjects. Left ventricular (LV) filling pressure in all patients was initially less than 15 mm Hg. Age, infarct localization and hemodynamic values on admission (Swan-Ganz catheter) were comparable in both groups. Maximal creatine kinase (CK) and creatine kinase-MB (CK-MB) values were markedly lower in the verapamil group than in the control group (CK 547 vs 703 U/liter, p less than 0.05; CK-MB 51 vs 68 U/liter, p less than 0.025), as was infarct weight (48 vs 65 g-Eq, p less than 0.03; CK-MB 31 vs 49 g-Eq, p less than 0.005). Arterial blood pressure was 10% lower in the verapamil group than in the control group. Systemic vascular resistance and LV filling pressure remained unchanged. Verapamil reduced myocardial infarction size by about 30% in patients without LV failure and the arterial pressure was reduced.

Comparative effects of propranolol, timolol and metoprolol on myocardial infarct size after experimental coronary artery occlusion

The effects of equiblocking doses of three beta-adrenergic blocking agents, propranolol, timolol and metoprolol, on myocardial infarct size were evaluated in 28 dogs after acute experimental coronary artery occlusion. Heart rate, arterial pressure and arterial free fatty acid concentration were measured in an attempt to evaluate their effects on the extent of myocardial injury. The zone at risk of infarction in each dog 1 minute after left anterior coronary artery occlusion was assessed by injecting highly radioactive albumin microspheres into the left atrium, and the hypoperfused zone was determined by autoradiography. After 15 minutes, the dogs were randomized into four groups: control dogs (n = 7), propranolol-treated dogs (1.2 mg/kg intravenously, n = 7), timolol-treated dogs (0.2 mg/kg intravenously, n = 7) and metoprolol-treated dogs (1.2 mg/kg intravenously, n = 7). After 6 hours, the dogs were killed. The left ventricle was sliced and stained with triphenyl-tetrazolium chloride for measurement on infarct size. The same slices were then autoradiographed for measurement of the hypoperfused zone. The percent of hypoperfused zone that evolved to infarction (the ratio of infarct size to hypoperfused zone) was 90.4 +/- 1.9% in the control group, 72.4 +/- 2.4% in the propranolol-treated dogs (p less than 0.05 versus control group); 57.9 +/- 4.4% in the timolol-treated dogs (p less than 0.01 versus control group; p less than 0.05 versus propranolol) and 54.4 +/- 3.7% in the metoprolol-treated dogs (p less than 0.01 versus control group; p less than 0.05 versus propranolol). Thus, propranolol, timolol and metoprolol reduced myocardial infarct size in dogs by 20, 36 and 40%, respectively, after experimental coronary artery occlusion. Metoprolol and timolol protected the ischemic myocardium more effectively than did propranolol.

Usefulness of intravenous propranolol in predicting left anterior descending blood flow during anterior myocardial infarction

The effect of propranolol on precordial ST-segment elevation was studied in 24 patients with acute anterior myocardial infarction. The electrocardiographic response to the drug was correlated with the early angiographic appearance of the left anterior descending coronary artery (LAD). After a 30-minute observation period, intravenous propranolol (average dose 3.5 +/- 2.2 mg) was given a mean of 2.8 +/- 1.9 hours after the onset of persistent chest pain. Coronary angiography was performed 3.6 +/- 2.0 hours after the onset of symptoms. Patients were classified into 2 groups according to the angiographic findings. Group A consisted of 7 patients with a stenotic but patent LAD and 1 patient with excellent collateral blood flow to that area. Group B consisted of 16 patients with a completely occluded LAD and poor or absent collateral blood flow. Patients in group A showed a mean reduction in precordial ST-segment elevation of 77 +/- 18% and patients in group B showed a mean reduction of 13 +/- 14% (p less than 0.005). Left ventricular ejection fraction at discharge was 0.6 +/- 0.07 in group A and 0.37 +/- 0.08 in group B (p less than 0.001). Thus, the electrocardiographic response to intravenous propranolol given early in the course of acute anterior myocardial infarction predicts the presence of blood flow to the infarcting zone. The combination of residual blood flow and reduction of ST-segment elevation secondary to propranolol is associated with preservation of ventricular function.

Effects of beta-adrenergic inhibition on scar formation after myocardial infarction

In view of clinical interest in the efficacy of beta-adrenergic blockade during acute myocardial infarction (AMI), we have determined the long-term effect of therapy on scar formation after experimental myocardial ischemia. Intact anesthetized dogs underwent acute occlusion of the left anterior descending coronary artery, by means of a balloon catheter, which permitted monitoring of the aortic-peripheral coronary artery pressure gradient during the 4-hour period of balloon inflation. Practolol administration was begun 15 minutes after the onset of ischemia in group A. Control animals (group B) received procainamide to approximate the antiarrhythmic action of beta blockade. Only group A exhibited significant reduction in the ST segments during acute ischemia. Chronic therapy was maintained for 1 month and the mature scar formed in the myocardium was assessed after 4 months. The extent of subendocardial scar was similar in both groups but subepicardial scar formation was significantly less in group A. There was also a significant decrease in the percentage of total myocardium involved with scar in this treatment group. Although thinning of the left ventricular wall was similar for both groups in the central scar region, this process was significantly reduced at the lateral margin in group A. Thus, specific beta-receptor blockade during acute myocardial ischemia and sustained during the repair process can result in a reduced quantity and altered distribution of mature scar.

Enhancement of salvage of reperfused myocardium by early beta-adrenergic blockade (timolol)

Although reperfusion of severely ischemic myocardium with thrombolytic agents or surgery has shown reduction in infarct size, the time after coronary occlusion during which reperfusion can salvage ischemic myocardium is limited. To determine whether beta-adrenergic blockade could enhance the salvage of ischemic myocardium by reperfusion, the left anterior descending coronary artery was occluded in 18 anesthetized dogs. An in vivo area at risk was determined by injecting technetium-99m-labeled albumin microspheres into the left atrium 5 minutes after occlusion and carrying out radioautography to define the poorly perfused tissue. Fifteen minutes after coronary occlusion, the dogs were randomized either to a control (saline-treated) group (n = 8) or to a timolol-treated group (n = 10). Timolol was administered until a decrease of 20% in heart rate or blood pressure occurred (mean total dose = 0.85 +/- 0.22 mg/kg +/- standard error of the mean). Coronary occlusion was maintained for 3 hours and was followed by 3 hours of reperfusion in both groups. At the end of 6 hours, infarct size was defined by triphenyltetrazolium chloride staining and masses of infarct and risk were calculated. Percent left ventricular mass at risk was similar for both groups (control = 20.9 +/- 2.4%, timolol-treated = 23.7 +/- 2.1%, p = not significant). Mass of necrosis/mass at risk was significantly smaller in the timolol-treated reperfusion group (27.3 +/- 2.7%) versus saline reperfusion alone (46.5 +/- 5.6%) (p less than 0.005). Thus, beta-adrenergic blockade administered early after coronary occlusion results in substantial enhancement of the salvage achieved by reperfusion alone.

Effects of ortho-iodo sodium benzoate on acute myocardial ischemia, hemodynamic function, and infarct size after coronary artery occlusion in dogs

Ortho-iodo sodium benzoate (OISB) decreases the affinity of blood for oxygen, thus enhancing potential tissue oxygen delivery. To test the hypothesis that a change in oxygen affinity would ameliorate regional myocardial ischemic injury resulting from occlusion of the left anterior descending (LAD) coronary artery, experiments were carried out in 55 anesthetized dogs which received an intravenous infusion of OISB. In Protocol I studies (n = 9), preocclusion intravenous infusion of OISB (500 mg/kg) reduced epicardial S-T segment elevation 15 minutes after coronary occlusion, while a similar volume of normal saline solution did not affect this index of ischemic damage. In Protocol II experiments, 34 dogs were randomized to either an OISB or saline group, after which the LAD was ligated, the chest closed, and the animal allowed to recover from anesthesia. Myocardial infarction (MI) size was assessed after the animal died or was killed 8 to 24 hours later, and was found to be 29% smaller in dogs receiving OISB. In 6 dogs, blood P50 (the partial oxygen pressure at which hemoglobin is 50% saturated with oxygen) was increased by OISB infusion, confirming that its administration effected a rightward shift in the oxyhemoglobin dissociation curve. Protocol III studies assessed the effects of OISB on cardiac hemodynamic function and acute myocardial ischemic damage when infusion was begun 15 minutes after LAD occlusion: average epicardial S-T segment elevation was not altered by saline solution, but decreased when OISB was infused during the last 15 minutes of myocardial ischemia. Reductions in heart rate, left ventricular dP/dt, and cardiac output were observed in 7 dogs during OISB infusion, but there were no changes in these measurements during coronary occlusion in 5 dogs receiving a constant infusion of saline solution. There were no changes in regional myocardial blood flow (microsphere technique) to either ischemic or nonischemic zones in either the saline control or OISB treatment groups. Thus, both acute myocardial ischemic injury (assessed by epicardial electrocardiographic mapping) and ultimate MI size are reduced when OISB is infused before experimental coronary artery occlusion. OISB also reduces myocardial ischemic injury when its administration is begun 15 minutes after coronary occlusion, while effecting decreases in heart rate, left ventricular contractility, and cardiac output.

Role of beta-adrenergic blockade in the therapy of patients with myocardial infarction

Beta-adrenergic blockade appears to have beneficial effects on ischemic myocardium, both in experimental animals and in patients. There is substantial evidence that beta blockade, when induced promptly after coronary occlusion, is capable of limiting the size of experimentally produced infarcts. Although many mechanisms of action have been proposed, the bulk of available evidence suggests that beta blockade reduces the severity of myocardial ischemia by reducing myocardial oxygen demands. Intravenous and oral therapy with beta-blockers is safe when patients are carefully selected and observed. Invasive hemodynamic monitoring is not usually necessary for safe use of these drugs. Preliminary reports based on studies with several beta-blockers indicate that early intervention might limit infarct size or even prevent its occurrence, particularly in patients with evidence of increased sympathetic activity. However, before early beta blockade can be recommended as standard therapy for uncomplicated myocardial infarction, these findings should be confirmed in rigorously controlled clinical trials utilizing several techniques for assessment of infarct size. The most significant finding with beta-blockers in patients with myocardial infarction has been that these drugs can improve survival when administered to patients on a long-term basis after infarction. Although the mechanism of this protective effect is not yet understood, the results are clear enough to warrant the routine administration of a beta-blocker to patients who have had infarction and who have no contraindications to such therapy.

Infarct size restriction in cats by the beta-adrenergic blocker timolol

The beta-adrenergic antagonist, timolol, was previously shown to exert a protective effect in cats subjected to 5 h of myocardial ischemia. The present study was designed to determine the effect of timolol on infarct size in cats 24 h after coronary occlusion. Timolol (25 microgram/kg) or vehicle (0.9% NaCl) was administered 0.5, 5, 10, 15 and 20 h after acute ligation of the left anterior descending coronary artery. There was no significant difference in arterial blood pressure or heart rate in MI cats receiving timolol or vehicle. Timolol markedly decreased S-T segment elevation at 2-12 h (P less than 0.05). Left ventricular weights in MI + vehicle cats (8.5 +/- 0.7 g, n = 6) were similar to timolol-treated MI cats (8.9 +/- 0.4 g, n = 7). However, the percent of the left ventricular myocardium infarcted, determined by nitroblue tetrazolium staining, was significantly less (P less than 0.001) in timolol MI cats compared to saline-treated cats, 9.8 +/- 1.2% (n = 7) vs. 18.9 +/- 1.8% (n = 6), respectively. Hemodynamic or cytoprotective actions of timolol do not appear to explain these results. Rather, the mechanism of infarct size reduction by timolol is probably explained by antagonism of beta-receptor-mediated metabolic effects.

Comparative investigation of the effects of metoprolol, propranolol, practolol, and verapamil in the acute phase of experimental myocardial infarction

Myocardial infarction in rats was produced by ligation of the left coronary artery. To ensure exact comparison of drug effect, the extent of the myocardial zone excluded from the coronary circulation was determined in each animal, and the experimental data were related to it. For this purpose, the hearts were perfused with Evans blue, and after the photometric determination of the dye content of the hearts the percentage of ischemic myocardium was calculated. With metoprolol, propranolol, and verapamil a significant increase of the survival times was obtained (min/% of non-ischemic myocardium). Metoprolol and propranolol also significantly increased the survival rates. None of the beta-blockers exerted an antiarrhythmic effect. The arrhythmias were prevented by higher doses of the calcium antagonist verapamil which, however, decreased the survival times. All beta-blocking agents delayed the typical elevation of the ST-segment in the electrocardiogram, and reduced the increase of the activity of the serum creatine kinase. Propranolol and metoprolol antagonized the blood pH decrease obtained after coronary occlusion. Results concerning heart rate, and arterial and central venous pressures are also reported. - The findings with metoprolol, especially, indicate that the essential mechanism in the therapeutic action of beta-blockers is their ability to block the cardiac beta 1-receptors.

Cardiac effects of prostaglandins during global ischemia in isolated perfused cat hearts

Prostaglandin E1,PGD2, and 16,16 dimethyl prostaglandin E2 were studied in an isolated perfused cat heart preparation during normal conditions and in myocardial ischemia. Under ischemic perfusion, prostaglandin E1 showed some protective action on the release of creatine kinase into perfusate during ischemia, but none of the prostaglandins studied prevented increases in perfusate creatine kinase after reperfusion. Prostaglandin E1 also showed significant membrane stabilizing activity reducing the rate of lysosomal hydrolase release from cat liver lysosomes. Prostaglandin E1 may be beneficial in myocardial ischemia due to its membrane stabilizing action and perhaps to other effects but it did not exert significant protective effects on reperfusion injury of the ischemic myocardium under conditions of these experiments.

Coronary artery reperfusion in acute myocardial infarction: beneficial effects of intracoronary streptokinase on left ventricular salvage and performance

Coronary angiography was performed on hospital admission in 37 patients with acute myocardial infarction (AMI). Thirty patients had total occlusion of the infarct-related coronary artery and seven patients had severe proximal stenoses with poor distal flow. In 20 of 30 patients with total occlusion, intracoronary (IC) infusion of streptokinase (SK) resulted in reperfusion of the distal coronary artery. Left ventricular (LV) performance was assessed before coronary angiography and at discharge from the hospital by use of gated cardiac blood pool imaging techniques. In patients evidencing reperfusion of the infarct-related coronary artery, mean (+/- SD) left ventricular ejection fraction (LVEF) increased from admission through discharge (46% +/- 15% to 55% +/- 10%, p = 0.002). In contrast, LVEF did not change from admission through discharge in patients with severe proximal stenoses alone or in patients with total occlusion who did not demonstrate reperfusion following SK administration (47% +/- 17% vs 49% +/- 18%, p = ns). In an additional 14 control patients with AMI who were not evaluated with coronary angiography, LVEF did not change from admission through discharge (46% +/- 12% vs 48% +/- 14%, p = ns). Quantitative thallium-201 perfusion imaging demonstrated an increase (p less than 0.05) in thallium uptake in the infarct segment following coronary artery reperfusion. In contrast, thallium uptake did not change (p = ns) in the infarct segment in patients not evidencing angiographic coronary artery reperfusion. These data support the following : (1) Coronary artery thrombus occurs frequently in AMI and can be lysed by IC SK, and (2) reperfusion with IC SK in patients with evolving myocardial infarction results in myocardial salvage and improved LV performance through hospital discharge.

Combined administration of nitroglycerin and propranolol to patients with acute myocardial infarction

Hemodynamic effects of combined nitroglycerin and propranolol administration were investigated in patients with acute myocardial infarction. After nitroglycerin infusion decreased the mean arterial pressure by 20 mm Hg for one hour, nitroglycerin was continued, and patients were given 0.033 mg/kg of propranolol every five minutes for a total dose of 0.1 mg/kg, or until there was a decrease in heart rate to less than 60 beats/min, an increase in left ventricular filling pressure (LVFP) to greater than 15 mm Hg, or a decrease in systolic arterial pressure to less than 85 mm Hg. Seven of eight patients with initial LVFP less than or equal to 15 mm Hg and three of seven with initial LVFP greater than 15 mm Hg received 0.1 mg/kg of propranolol. Propranolol significantly decreased heart rate. Although pressure time/minute decreased significantly, the magnitude of its decrease was small, suggesting only a minimal effect on myocardial oxygen demands. The LVFP increased after giving propranolol but remained less than the control value. Simultaneous administration of nitroglycerin likely prevented further increases, since LVFP increased after cessation of nitroglycerin infusion, and three patients subsequently had pulmonary edema. Propranolol administration resulted in a significant increase in peripheral vascular resistance and a decrease in cardiac output.

Intracoronary infusion of streptokinase in patients with acute myocardial infarction: effects of reperfusion on left ventricular performance

Cardiac catheterization and coronary angiography were performed on hospital admission in 32 consecutive patients with acute myocardial infarction. Twenty-six patients had total occlusion of an infarct-related coronary artery and six had severe proximal stenosis with poor distal flow. In 18 of the 26 patients with total occlusion, intracoronary infusion of streptokinase resulted in reperfusion of the distal coronary artery. Seventeen of these 18 patients had severe coronary arterial stenosis at the site of the previous total occlusion. Hemodynamic indexes of left ventricular performance and ejection fraction determined by gated cardiac blood pool imaging did not change immediately after reperfusion (p [probability] = not significant [NS]). The mean (+/- standard deviation) left ventricular ejection fraction increased significantly (p = 0.007) from admission (44 +/- 15 percent) to hospital discharge (55 +/- 7 percent) in patients evidencing reperfusion of the occluded coronary artery. It did not change (p = NS) in this time span in the patients with severe stenosis alone, in those with total occlusion not demonstrating reperfusion after administration of streptokinase or in an additional 10 control patients with acute myocardial infarction not evaluated with coronary angiography. These data suggest that (1) coronary arterial thrombus is frequent in acute myocardial infarction and can be lysed by intracoronary streptokinase; (2) reperfusion with intracoronary streptokinase in acute myocardial infarction results in improved left ventricular performance between admission and hospital discharge.

Importance of the source of hyaluronidase preparations in determining protective effect on ischemic heart muscle in acute myocardial infarction

Using a very sensitive new technique, the effectiveness of hyaluronidase (HYL) in reducing infarct size (IS) was compared employing HYL obtained from bovine testicles and from fungi. One minute after coronary artery occlusion in dogs, highly radioactive microspheres were injected for autoradiographic assessment. The animals were then randomized into a control group and several HYL-treated groups. Six hours later all hearts were divided into 20 to 25, 3 mm-thick slices which were incubated in TTC to measure IS and thereafter autoradiographed to delineate the hypoperfused zone (HZ). The percent of HZ that evolved to necrosis (IS/HZ) was determined in each animal. In the control group, IS/HZ was 89.9 +/- 4.2% and was reduced by 17% (p less than 0.05) in the group treated with 500 units/kg of bovine HYL. With 500 units/kg of fungal HYL, IS/HZ was not reduced, but the higher dose of 5000 units/kg reduced IS/HZ by 26% (p less than 0.01). Thus dependence of HYL source is documented relative to the effectiveness of this salutary intervention for protecting ischemic heart muscle and limiting necrosis in acute myocardial infarction.

Randomized controlled trial of vasodilator therapy after myocardial infarction

Hemodynamic changes and mortality and morbidity were compared in a randomized controlled trial of sodium nitroprusside after acute myocardial infarction. Fifty patients with a mean pulmonary capillary wedge pressure of more than 20 mm Hg within 24 hours of acute infarction were randomly assigned to one of two groups: 25 patients treated with nitroprusside and 25 treated with furosemide. Nitroprusside rapidly produced a sustained decrease in systemic vascular resistance and increase in cardiac index (thermodilution). After 1 hour the cardiac index had increased 16 +/- 3 (mean +/- standard error of the mean) percent (p less than 0.001) compared with a decrease of 7 +/- 3 percent with administration of furosemide (p less than 0.01). Differences in systemic vascular resistance and cardiac index in the two groups persisted throughout the 48 hour treatment period (p less than 0.001). Pulmonary capillary wedge pressure decreased rapidly with nitroprusside and slowly with furosemide so that, although it was significantly lower in the former group overall (p less than 0.001), by 48 hours the values were not different. Although beneficial acute hemodynamic effects of nitroprusside were demonstrated, there was no difference in mortality or in morbidity assessed clinically, by chest X-ray film, echocardiogram or graded treadmill stress testing after 6 months or 1 year.

Deleterious influence of hypothyroidism on evolving myocardial infarction in conscious dogs

To study the influence of hypometabolism on evolving myocardial infarction in a model with intact autoregulation, we investigated 53 awake dogs after coronary artery occlusion. Severe hypothyroidism was induced by the intravenous administration of 131I. Animals were instrumented to obtain hemodynamic measurements, and regional myocardial blood flow was measured with radioactive microspheres. Infarct size was determined by the creatine kinase depletion method, and dysrhythmia analysis was performed from 24-h Holter monitor tapes in animals matched for infarct size. The microarchitecture of hypothyroid myocardium was determined by the electron microscope. Before coronary occlusion, mean systemic pressure in hypothyroid dogs was reduced by 14% and cardiac output reduced by 32%, with no change in left ventricular end-diastolic pressure, first derivative of left ventricular pressure rise, (dP/dt), or heart rate. After coronary occlusion, there was deterioration in hemodynamic measurements in both groups, with lower absolute levels of mean systemic blood pressure and cardiac output obtained in hypothyroid dogs. Hypothyroidism was detrimental to evolving infarction with a 36% increase in infarct size present in hypothyroid dogs (30 +/- 2%) compared to euthyroid controls (22 +/- 3%), P less than 0.05. Dysrhythmias were more severe in hypothyroid dogs. There were no changes in the relationship between regional myocardial blood flow and the extent of infarction after coronary occlusion. Abnormalities in microarchitecture were present in hypothyroid dog myocardium. Severe hypometabolism in this model was associated with alterations in hemodynamics, more severe dysrhythmias and changes in microarchitecture. The combined effect of these alterations resulted in an overall detrimental influence of hypothyroidism on evolving myocardial necrosis in this model.

Effect of oral propranolol on rest, exercise and postexercise left ventricular performance in normal subjects and patients with coronary artery disease

The effect of beta-adrenergic blockade with oral propranolol on resting, exercise and postexercise ventricular performance was evaluated using multiple-gated equilibrium cardiac blood and pool images in normal volunteers and patients with coronary artery disease. Propranolol produced no detectable effect on basal left ventricular function in normal subjects at doses producing intermediate (160 mg propranolol/day) and maximal (434 +/- 99 mg propranolol/day) beta blockade and in patients with coronary artery disease at clinically effective antianginal doses (162 +/- 47 mg propranolol/day). During exercise, a dose-related, negative inotropic effect was observed in normal subjects: 160 mg propranolol/day produced a small but statistically insignificant decline in exercise left ventricular performance, whereas maximal beta blockade significantly depressed the left ventricular response to exercise. In patients with coronary artery disease, propranolol's effect on exercise ventricular performance depended on the presence or absence of ischemic dysfunction during exercise. In patients with an ischemic functional response to exercise, propranolol significantly improved regional and global performance during and after exercise; in coronary artery disease patients with a normal response to exercise, propranolol had no significant effect on exercise and postexercise ventricular function. These results imply increased sensitivity to the effects of beta blockade in ischemic myocardium. In coronary artery disease patients with an abnormal response to exercise and in normal volunteers during beta blockade, propranolol's effect on exercise left ventricular performance was independent of changes in ventricular preload and after load related to heart rate and blood pressure.

Reduction of CK and CK-MB indexes of infarct size by intravenous nitroglycerin

The effect of i.v. nitroglycerin administration on indexes of infarct size was examined in 31 patients with acute myocardial infarction. Serial creatine kinase (CK) and CK-MB isoenzyme determinations were used to calculate infarct size. Twenty-nine patients served as controls. Two subgroups of the study group were formed to evaluate differences between early and late intervention. In the first group (n = 22), continuous infusion of nitroglycerin over 48 hours was initiated within 8 hours (mean 4.5 hours) after the onset of symptoms. Peak CK activity for the nitroglycerin-treated patients (n = 9) in this subgroup was 544 U/1 vs 871 U/1 for the controls (n = 13) (p less than 0.05). The rate of CK release was reduced from 79 to 33 U/1.hr (58%), as was total CK and CK-MB release (p less than 0.02). Calculated infarct size was 69 gEq in the controls and 48 gEq in patients receiving nitroglycerin (CK-MB: 69 vs 43 gEq, p less than 0.05). In the late intervention subgroup, nitroglycerin therapy was begun more than 8 hours (mean 12.8 hours) after the onset of symptoms. Here, too, use of the agent was associated with lower peak CK and CK-MB levels as well as a reduction in calculated infarct size (p less than 0.05). Hemodynamic measurements, recorded every 4 hours, showed that nitroglycerin also reduced left ventricular filling pressure significantly and cardiac output increased. Blood pressure fell slightly, and systemic vascular resistance declined. The results indicate that i.v. nitroglycerin reduces CK and CK-MB release and thus calculated infarct size in both early and late intervention.

The effects of intravenous nitroglycerin on hemodynamics, coronary blood flow and morphologically and enzymatically estimated infarct size in conscious dogs

Nitroglycerin (TNG) decreases ST-segment elevation accompanying myocardial ischemia, but its effect on morphometrically and enzymatically estimated infarct size (IS) has not been defined. Accordingly, coronary occlusion was produced in 92 conscious dogs; 65 survived for 24 hours. Thirty-three received TNG (200-300 microgram/min i.v. for 8 hours) and the results were compared with those in 32 untreated dogs. Coronary blood flow (CBF) was measured with tracer microspheres (141Ce, 85Sr and 95Nb) 5 minutes after occlusion before TNG, 20 minutes after TNG and again at 8 hours. Mean blood pressure decreased from 103 to 84 mm Hg with TNG, vs 99 to 94 mm Hg in controls (p > 0.02). Nitroglycerin increased CBF in the subendocardium of ischemic areas by 45% (0.09 to 0.13 ml/min/g). The dogs were sacrificed after 24 hours and IS was estimated morphometrically (25 +/- 1% vs 27 +/- 1% of left ventricular weight) and from myocardial CK depletion (23 +/- 1% vs 24 +/- 1%) were similar for the two groups. Thus, despite increased subendocardial CBF, prolonged i.v. TNG did not decrease infarct size, although a 15% difference would have been detected with this sample size. TNG may relieve coronary spasm but does not appear to be beneficial with sustained coronary occlusion.

Coronary steal-induced increase in myocardial infarct size after pharmacologic coronary vasodilation

This study was performed to determine if maximal coronary arterial vasodilation of nonischemic areas would produce an increase in myocardial infarct size through a "steal" of collateral flow from an ischemic region. Myocardial infarction was produced by a 2 hour occlusion and reperfusion of the distal left anterior descending coronary artery in anesthetized dogs. Five minutes after occlusion, 7 dogs were given saline solution, and in 12 dogs the coronary vasodilator chromonar (8 mg/kg, intravenously) was administered. Chromonar produced a significant increase (p less than 0.05) in blood flow to nonischemic regions and a concomitant decrease in flow to ischemic areas. Associated with these changes in flow was an elevation in total release and peak plasma creatine kinase compared with values in saline-treated control dogs. Myocardial infarct size determined with nitroblue tetrazolium stanining was significantly increased (p less than 0.05). These demonstarte that maximal coronary vasodilation of nonischemic areas can result in an extension of myocardial infarction by a steal of collateral flow away from the ischemic region.

Changes in diastolic time with various pharmacologic agents: implication for myocardial perfusion

Diastolic time (DT) is calculated as the cycle length (RR) minus electromechanical systole (QS2). The ratio of DT (RR-QS2) to RR interval times 100, or the percent diastole (%D), varies nonlinearly with heart rate (HR), increasing rapidly with decreasing HR. The effect of commonly used cardioactive agents on %D was studied in five groups of normal subjects. In group 1 (n = 12), propranolol (160 mg daily) increased %D from 55.9 +/- 1.7 to 64.7 +/- 1.3 (p less than 0.001) by slowing HR. In group 2 (n = 12), dobutamine (2.5 micrograms/kg/min) increased %D from 56.4 +/- 1.4 to 61.8 +/- 1.3 (p less than 0.005) by shortening the QS2. In group 3 (n = 10), Cedilanid-D (1.6 mg i.v.) increased %D from 55.5 +/- 1 to 63.2 +/- 0.7 (p less than 0.001), both by slowing the HR and shortening the QS2. In group 4 (n = 12), isoproterenol (2 micrograms/min) increased HR and shortened the QS2 significantly. The net result was a significant reduction of %D from 56.1 +/- 1.4 to 53.5 +/- 1.1, (p less than 0.05). In group 5 (n = 15), a 100-mg bolus of i.v. lidocaine did not have a significant effect on %D. This study indicates that cardiovascular drugs may have significant effects on the relative duration of diastole either by affecting HR or the duration of systole. This may have clinical implications for patients with coronary artery disease and patients with left ventricular hypertrophy, since in both cases coronary flow in mostly diastolic.

[IX. Randomized study of the effect of nitroglycerin on CK and CK-MB infarct size. Preliminary report (author's transl)]

In 38 patients with acute myocardial infarction the effect of nitroglycerin on infarct size was studied. Patients were randomized into two groups. 16 patients received continuous nitroglycerin infusions of 0.6 to 6.0 mg/h (mean 2.3 mg/h) over a 48 h period, 22 patients received no specific therapy and served as control. Nitroglycerin was given in the mean 12 +/- 5 (+/- 1 SD) hours following onset of chest pain and 8 +/- 5 after the increase of CK values. Infarct size was determined according to the time activity curve of creatine kinase (CK) and of its myocardial isoenzyme (CK-MB). In all but one patient hemodynamic parameters (left ventricular filling pressure, blood pressure, cardiac index) were measured. The mean infarct size was 51 +/- 30 CK-g-equiv. in control patients, and 48 +/- 33 g in nitroglycerin treated patients. Infarct size as calculated from CK-MB values was 60 +/- 36 g (n=16) in control, and 52 +/- 41 g (n=11) in treated patients. At left ventricular filling pressure values (LVFP) below 20mm Hg infarct size amounted to 43 +/- 30 g (n=12) in control, and to 41 +/- 32 g (n=11) in the nitroglycerin group. At LVFP values above 20 mmHg infarct size was 61 +/- 29 g (n=10) in control as opposed to 64 +/- 32 g (n=5) in treated patients. There was no difference between infarct size as predicted during the first 7 h and the observed infarct size. - Despite the known beneficial effect of nitroglycerin on hemodynamics and on myocardial ischemia, infarct size seems not to be greatly reduced, however, intervention occurred fairly late (12 h). In early intervention beneficfial effects seem likely.

Limitation of myocardial infarct size: review of experimental basis and clinical trials

Limitation of myocardial injury after coronary occlusion is an exciting, recently recognised, but as yet unconfirmed, possibility in the treatment of myocardial infarction. Review of the experimental basis for the concept indicates that although manipulation of the ST segment of the electrocardiogram is not sufficient proof that myocardial necrosis can be limited, more robust experimental methods have confirmed the efficacy of a variety of interventions in experimental coronary occlusion. The effect of myocardial infarct size in determining acute prognosis and complications has been confirmed in clinical studies though the importance of infarct size lessens after recovery. Reported attempts at limiting myocardial necrosis in patients with myocardial infarction have so far not been sufficiently convincing to merit universal clinical application. Results of large scale randomised trials currently underway are awaited with interest.

Effect of propranolol and nitroglycerin plus methoxamine on transmural creatine kinase activity after acute coronary occlusion

Transmural creatine kinase activity was determined 5 hours after acute occlusion of the left anterior descending coronary artery in 27 open chest anesthetized dogs. In seven dogs, propranolol, 2 mg/kg, was given intravenously over a 10 minute period 10 minutes after occlusion. In 10 dogs, nitroglycerin, 300 microgram/min, was infused intravenously for 1 hour 10 minutes after occlusion. Methoxamine, 300 to 500 microgram, was administered to return blood pressure and heart rate to prenitroglycerin levels. In untreated dogs, there was a distinct transmural gradient of creatine kinase activity in the ischemic region from subepicardium to subendocardium: nonischemic subepicardium 1,187 +/- 50 international units (IU)/g versus ischemic subepicardium 1,054 +/- 46 IU/g and nonischemic subendocardium 1,170 +/- 53 IU/g versus ischemic subendocard;um 766 +/- 42 IU/g, respectively. Administration of propranolol did not affect the transmural creatine kinase gradient after 5 hours of occlusion. In contrast, nitroglycerin plus methoxamine significantly (P less than 0.05) decreased subendocardial creatine kinase depletion after 5 hours of occlusion (776 +/- 42 versus 978 +/- 47 IU/g). These findings demonstrate the unique capability of nitroglycerin plus methoxamine to protect the subendocardium during ischemic insult.

Effect of propranolol on enzymatic and histochemical estimates of infarct size in experimental myocardial infarction

Acute myocardial infarction was produced in anesthetized dogs by ligation of the left anterior descendens coronary artery. Propranolol (2 mg/kg i.v.) administered 4 hours post ligation was examined for its ability to reduce infarct size estimated by histochemical and enzymatic methods. There was a signficant correlation between these two methods in their estimation of infarct size. Treatment with propranolol significantly decreased infarct size estimated with both methods. It is concluded that some portion of the myocardium can be protected against infarction by pharmacologic intervention as late as 4 hours after the onset of coronary artery occlusion.

Protective effect of propranolol in threatened myocardial infarction

Propranolol 0.1 mg/kg intravenously followed by 320 mg orally over 27 h was given to 20 randomly selected patients within 4 h of the onset of suspected myocardial infarction unaccompanied by diagnostic electrocardiographic changes. Patients given propranolol had fewer completed infarcts as assessed by serial electrocardiograms, a lower frequency of serum-creatine-kinase levels above the normal range, and lower peak serum-creatine-kinase levels than 23 control subjects. This evidence suggests that threatened myocardial infarction can in some cases be prevented by early beta-adrenoceptor blockade.

Long-term preservation of ischemic myocardium in the dog by hyaluronidase

The administration of hyaluronidase is a promising intervention to protect the ischemic myocardium in man, but evidence of the extent to which it may reduce the ultimate size of an infarct is not well-defined. Hence, open chest, anesthetized dogs were randomized into 10 control dogs which received saline and eight treated dogs which received three doses of hyaluronidase (500 NF units/kg I.V.) at 15 minutes, 2 hours and 24 hours after occlusion of the left anterior descending coronary artery (CAO). Regional myocardial blood flow (RMBF) assessed by the microsphere technique was measured 12 minutes after CAO. The chest was then closed and the dogs were allowed to recover. Twenty-one days after CAO, the hearts were excised, divided into 1 cm thick slices and incubated in triphenyl tetrazolium chloride. Infarct size was then determined by planimetry. The left ventricular myocardium was divided into multiple samples for RMBF analysis. In control dogs 23.2 +/- 2% of the left ventricle was infarcted, compared to only 9 +/- 2.8% (P less than 0.001) in hyaluronidase-treated dogs. RMBF in noninfarcted myocardium directly adjacent to the infarct was similar to that in the normal zone remote from the infarct in the control dogs; however, in the hyaluronidase-treated dogs, blood flow in the myocardium adjacent to the infarct was significantly reduced to 68% of normal (P less than 0.01) in the outer myocardial wall and to 86% of normal (P less than 0.02) in the inner myocardial wall, which indicates that this tissue, at least in some part, was in jeopardy, but was salvaged by hyaluronidase. Epicardial electrocardiographic data showed that three weeks after CAO, Q waves were less frequent and smaller in hyaluronidase compared to untreated dogs. Preservation of the frequency and magnitude of R waves was greater in the hyaluronidase-treated group at three weeks. We conclude that hyaluronidase resulted in long-term preservation of the ischemic myocardium.

The wavefront phenomenon of ischemic cell death. 1. Myocardial infarct size vs duration of coronary occlusion in dogs

Irreversible ischemic myocardial cell injury developes in an increasing number of cells as the duration of coronary occlusion is prolonged. The present study quantitates myocardial necrosis produced by 40 minutes, 3 hours, or 6 hours of temporary circumflex coronary occlusion (CO) followed by 2 to 4 days of reperfusion, or by 24 or 96 hours of permanent circumflex ligation in pentobarbital anesthetized open chest dogs. After 40 minutes of ischemia, myocyte necrosis was subendocardial but with increasing duration of coronary occlusion, irreversible injury progressed as a wavefront toward the subepicardium. Transmural necrosis was 38 +/- 4% after 40 min, 57 +/- 7% after 3 hours, 71 +/- 7% after 6 hours and 85 +/- 2% after 24 hours of ischemic injury. These results document the presence of a subepicardial zone of ischemic but viable myocardium which is available for pharmacologic or surgical salvage for at least three and perhaps six hours following circumflex occlusion in the dog.