Experimental myocardial infarction. II. Acute depression and subsequent recovery of left ventricular function: serial measurements in intact conscious dogs

Left ventricular function and coronary artery anatomy before and after myocardial infarction; a study of six cases

Six patients underwent cardiac catheterization before and after occurrence of a myocardial infarction. Results from the two procedures allowed the quantitation of changes in coronary artery anatomy and left ventricular performance associated with myocardial infarction.

Left ventricular biplane or single plane angiography and selective coronary angiography were used to evaluate coronary artery anatomy, left ventricular end diastolic pressure (LVEDP), left ventricular end diastolic volume (LVEDV), end systolic volume (LVESV), and systolic ejection fraction (SEF) under resting conditions.

Four patients had developed occlusion of the artery supplying the area of infarction. In five cases new or progressive contraction abnormalities occurred. One patient had no change in contraction pattern or SEF. Systolic ejection fraction fell in three patients, with no change in LVEDV. In two patients LVEDV rose and SEF fell.

These data demonstrate that a wide spectrum of functional abnormalities is associated with myocardial infarction. Infarction was always associated with significant coronary artery stenosis, but not necessarily associated with occlusion. The SEF and contractile pattern were the indicators of left ventricular dysfunction which most frequently deteriorated.

Changes in left ventricular performance from early after acute myocardial infarction to the convalescent phase

Left ventricular performance was assessed in 22 patients early (within the first 2 days) after acute myocardial infarction (AMI) and again during the convalescent phase (3-5 weeks later). Left ventricular end-diastolic pressure (LVEDP) fell from the early to the late study by an average of 10.7 mm Hg in the 12 (55%) group A patients. In these patients, cardiac index (CI) and left ventricular stroke work (LVSW) increased by an average of 0.65 liters/min/m 2 and 63.8 g-m, respectively, and the mean right atrial pressure fell by 2.4 mm Hg. These changes were statistically significant ( P < 0.05). Left ventricular (LV) function was improved in this group of patients.

LVEDP increased in the other 10 patients and they were subdivided into groups B and C depending on whether CI increased or decreased. In six patients (27%), LVEDP increased by 5 mm Hg and CI increased by 0.4 liters/min/m 2 (group B); the P value for these changes was < 0.05. Group B patients also had an increase of mean arterial pressure and left ventricular ejection time, and thus the alterations in LVEDP and CI could have resulted from an increase of arterial pressure. In four patients (18%), LVEDP increased by 11.5 mm Hg, CI decreased by 0.9 liter/min/m 2 , and LVSW was reduced by 34.5 g-m (group C). Although the decreases of CI and LVSW were not statistically significant in group C patients, LV function had deteriorated. In the follow-up period, three of the four patients in group C died and the fourth patient is in functional class IV.

Therefore, in the convalescent period ventricular function had improved in 55%, and had deteriorated in 18% of cases. The changes in LV function could not have been predicted on the basis of the clinical features, systolic time intervals, or the initial hemodynamic findings. Patients whose ventricular function deteriorated during convalescence had a very poor prognosis.

Evolution of abnormalities in left ventricular function after acute myocardial infarction

Twelve patients with acute myocardial infarction (AMI) were studied by left ventricular (LV) catheterization, echocardiography, and volume manipulation (tourniquets, phlebotomy, and dextran infusion) within 24 hours of infarction (study 1) and 3 weeks later (study 2). Three patients also were studied 6-10 months later (study 3). Cardiac index rose (from 2.50 to 3.15 liters/min/m 2 ) and right atrial pressure fell (from 9.4 to 5.3 mm Hg) from study 1 to study 2. LV end-diastolic pressure (LVEDP) remained elevated (21.8 mm Hg at study 1 and 20.9 mm Hg at study 2) and was actually higher at study 2 in six patients who were otherwise improved. At study 1, end-diastolic ventricular diameter was within normal limits in five patients with clinical LV failure, four of whom had grossly elevated LVEDP. At study 2, ventricular diameter remained normal in three patients with elevated LVEDP but no LV failure. At study 3, LVEDP was still elevated in two patients and chamber diameter normal in two and slightly increased in one. Ventricular function curves (LVEDP vs stroke index) constructed from results during volume manipulation showed no consistent improvement during convalescence.

These data indicate that clinical improvement after AMI is not usually associated with a fall in LVEDP or a normalization of the impaired LV function curve. Normal ventricular diameter indicates normal ventricular volume which, in the presence of an elevated LVEDP, suggests a persistent decrease in LV compliance.

Functional development of the coronary collateral circulation during coronary artery occlusion in the conscious dog

We studied changes in the coronary collateral circulation during coronary artery occlusion in 14 conscious dogs by: a) determining simultaneous changes in peripheral coronary pressure (PCP) and retrograde flow (RF) after abrupt coronary artery occlusion; b) correlating these functional indices with quantitative anatomic indices (AI) of coronary collateral development (Menick et al: Am Heart J 82:503-510, 1971); and c) observing changes in these indices after repeated reocclusions of a coronary artery. These dogs were subjected to left circumflex coronary artery (LCCA) occlusions for 2 hours to 8 days; pressure tubes were implanted in the aorta and LCCA, the latter tube placed distal to an occlusive cuff for PCP and RF measurements. Afterwards the animals were sacrificed, their hearts injected with a modified Schlesinger's gelatin mass, and AI determined. During 2 to 24 hour LCCA occlusions (11 dogs) mean PCP rose to levels 50 to 80% of prevailing aortic pressure. During repreated 2- to 24-hour occlusions (2 dogs) in the same dog, the rate at which PCP rose increased. Retrograde flow was unchanged during 2- to 24-hour occlusions. Anatomic indices of these dogs were in the same range as those observed in unoccluded controls. When LCCA occlusion was maintained for more than 4 days (3 dogs), mean PCP rose during the first 24 hours and then remained stable; RF did not change until 4 days into occlusion and then increased. Anatomic indices of dogs occluded for more than 4 days were significantly greater (P < 0.001) than those of the 2- to 24-hour occlusion groups. Our study shows that: a) the early PCP rise after occlusion is not associated with an increase in RF, b) RF is a better index of collateral function and c) RF correlated well with the anatomic development of the collateral bed.

Determination of the site, extent, and significance of regional ventricular dysfunction during acute myocardial infarction

Infarct site, extent, and the degree of associated asynergy are major determinants of the hemodynamic consequences of myocardial infarction. Although conventional electrocardiography and vectorcardiography are routinely employed in assessing the location and size of infarction, they are relatively nonspecific. The newer techniques of high-frequency electrocardiography and isopotential mapping offer promise but have yet to undergo systematic evaluation. A rough measure of the extent of infarction is obtained from serum enzyme measurements. However, they furnish no information with regard to localization.

The region of infarction may be detected by precordial scanning following the intravenous or intracoronary injection of a radioisotope. The infarct may be revealed as an area of decreased perfusion (cold spot) or as an area to which a specific radioactive label is bound (hot spot). With the availability of newer radionuclides such as 43 potassium and the use of computer techniques, a more precise means of localizing and quantifying myocardial infarction may become available.

Optimal definition of asynergy is obtained with contrast angiography. However, the risk of this procedure has limited its use, to date, in acute myocardial infarction. Apex- and kinetocardiography, chest X-ray, and fluoroscopy often suggest regional ventricular dysfunction, but these techniques are not sufficiently specific. Newer noninvasive methods for objectively evaluating regional ventricular dysfunction are ECG-gated cardiac scintiphotography and radarkymography. With ECG-gated scintiphotography, end-diastolic and end-systolic cardiac isotope images are obtained following intravenous injection of 99m technetium-albumin. From these images, assessment of asynergy and extent and location of infarct can be made. With radarkymography, heart-wall motion is assessed and quantitated by tracking segments of the cardiac silhouette visualized on a cinefluorogram. These techniques are ideally suited to the acutely ill patient. Echocardiography is another noninvasive technique with potential application to the study of asynergy. However, at present, only posterior-wall motion can be measured.

At the time of surgery regions of infarction may be localized by means of chemical indicators (fluoroscein), isotope techniques, or epicardial electrocardiographic mapping. Recently much has been learned about the hemodynamics of myocardial infarction. Through the use of the techniques described, further insight into regional ventricular abnormality and extent and localization of myocardial infarction could be obtained. With this information better approaches to therapy and prognosis could be developed.

Many students of the coronary circulation must have noted that the ventricular zone affected by ligating a large coronary branch not only appears cyanotic and dilated, but that it seems to alter in its mode of contraction. The detailed and sequential changes in contraction are not easily followed by the unaided eye and so far have not been recorded myographically. The reasons for this were the lack of an adequate and suitable myograph and a technique for the application of one to a limited ventricular surface so that records obtained represent, at least reasonably well, changes in muscle length and not predominantly artifacts due to position changes, thrusts and vibrations of the vigorously beating ventricle. 1

Experimental myocardial infarction. X. Efficacy of glucagon in acute and healing phase in intact conscious dogs: effects on hemodynamics and myocardial oxygen consumption

This study was designed to test the efficacy of glucagon in the treatment of hemodynamic abnormalities of acute and healing experimental canine myocardial infarction. Myocardial infarction was produced in intact, conscious dogs by gradual inflation of a balloon cuff device implanted around the left anterior descending coronary artery 1 to 2 weeks prior to the study. Hemodynamic and metabolic effects of 50 µg/kg of glucagon were assessed serially in the control state, 1 hour after myocardial infarction and again 1 week later. In the control state glucagon improved cardiac performance and increased myocardial oxygen consumption. One hour after acute myocardial infarction glucagon improved cardiac performance and reduced the degree of left ventricular failure, without any increase in myocardial oxygen consumption. Similar effects of glucagon were noted in the healing phase of myocardial infarction. It is postulated that in this animal model in the presence of heart failure due to myocardial infarction there are reciprocal changes in the factors that increase myocardial oxygen consumption (glucagon-induced inotropy) and decrease oxygen consumption (fall in ventricular end-diastolic volume and wall stress), resulting in no net change in oxygen requirement.

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.

Experimental myocardial infarction. 8. Chronotropic augmentation of cardiac function in left ventricular failure of acute and healing stages in intact conscious dogs

The hemodynamic effects of tachycardia induced by atrial pacing were investigated in left ventricular failure of acute and healing experimental myocardial infarction in 20 intact, conscious dogs. Myocardial infarction was produced by gradual inflation of a balloon cuff device implanted around the left anterior descending coronary artery 10-15 days prior to the study. 1 hr after acute myocardial infarction, atrial pacing at a rate of 180 beats/min decreased left ventricular end-diastolic pressure from 19 to 8 mm Hg and left atrial pressure from 17 to 12 mm Hg, without change in cardiac output. In the healing phase of myocardial infarction 1 wk later, atrial pacing decreased left ventricular end-diastolic pressure from 17 to 9 mm Hg and increased the cardiac output by 37%. This was accompanied by evidence of peripheral vasodilation. In two dogs with healing anterior wall myocardial infarction, left ventricular failure was enhanced by partial occlusion of the circumflex coronary artery. Both the dogs developed pulmonary edema. Pacing improved left ventricular performance and relieved pulmonary edema in both animals. In six animals propranolol was given after acute infarction, and left ventricular function deteriorated further. However the pacing-induced augmentation of cardiac function was unaltered and, hence, is not mediated by sympathetics.The results show that the spontaneous heart rate in left ventricular failure of experimental canine myocardial infarction may be less than optimal and that maximal cardiac function may be achieved at higher heart rates.

Experimental myocardial infarction. IV. Reduction of left ventricular compliance in the healing phase

Compliance of the infarcted left ventricle was studied in dogs 3-5 days after occlusion of the left anterior descending coronary artery. Compliance was assessed from postmortem pressure-volume curves and from pressure-length measurements (mercury-in-silastic segment length gauges) made both in vivo and postmortem. Postmortem pressure-volume curves showed reduced compliance compared to sham-operated animals. Postmortem pressure-length curves of infarcted and adjacent normal myocardium indicated that the diminished total compliance could be attributed to an increase in stiffness of the infarcted area. This was confirmed by in vivo end-diastolic pressure-length changes produced by transient aortic occlusion. The infarcted area was akinetic, showing neither contraction nor aneurysmal bulging. In addition, anesthetized dogs with infarcts, when compared with sham-operated animals, had similar left ventricular end-diastolic volumes (indicator dilution method), but higher left ventricular end-diastolic pressures. Taken with previous observations, which show that systolic aneurysmal bulging is uniformly present at the onset of ischemia, these results indicate that stiffening of the ischemic myocardium occurs during the first 5 days after infarction, and show that elevation of left ventricular filling pressure does not necessarily signify ventricular dilatation. The results also suggest a mechanism whereby ventricular performance may improve during recovery from acute myocardial infarction.

Experimental myocardial infarction. VI. Efficacy and toxicity of digitalis in acute and healing phase in intact conscious dogs

Use of digitalis in myocardial infarction is controversial. To determine the efficacy and toxic threshold, serial infusions of 3 mug/kg per min of acetyl-strophanthidin were given to six intact conscious dogs 24 hr before and 1 hr, 2 days, and 7 days after myocardial infarction induced by inflation of a balloon cuff implanted on the left anterior descending coronary artery. Within 1 hr after myocardial infarction, heart rate increased by 28%. Left ventricular end-diastolic pressure increased from 7 to 20 mm Hg, and stroke volume decreased by 25%. At this time acetylstrophanthidin caused no beneficial hemodynamic change, 1 wk later, the heart rate and left ventricular end-diastolic pressure had declined toward normal but remained elevated. At this time, acetylstrophanthidin lowered left ventricular end-diastolic pressure by 25%, and increased the stroke volume and cardiac output by 25% and 21% respectively, without any change in heart rate or aortic pressure. Tolerance to acetylstrophanthidin, defined as appearance of ventricular tachycardia, declined the 1st hr after myocardial infarction by 24% (P<0.05) from the control level of 43 +/-4 mug/kg (SEM), but subsequently returned to control.Thus, immediately after myocardial infarction, tolerance to acetylstrophanthidin was reduced, and left ventricular failure was not ameliorated. 1 wk later in the healing phase of myocardial infarction, tolerance to acetylstrophanthidin returned to normal and left ventricular performance was improved by this drug. The study suggests a limited therapeutic role for digitalis in the treatment of left ventricular failure in the acute phase immediately after myocardial infarction, but beneficial effects may occur in the healing phase 1 wk later.

Left ventricular performance at high end-diastolic pressures in isolated, perfused dog hearts

In the isolated, blood-perfused heart of the dog, left ventricular developed pressure and developed mean wall stress were observed while the ventricle contracted at a constant, nearly isovolumic afterload and while end-diastolic pressure was raised to levels exceeding 100 mm Hg. Coronary perfusion pressure was maintained at the level of the peak systolic pressure. Dilatation of the mitral ring and consequent mitral regurgitation were avoided by left atrial plication. Normalized graphs of percent of peak developed pressure against end-diastolic pressure showed that developed pressure rose abruptly with diastolic pressure, peaked at a diastolic pressure of approximately 30 mm Hg, and declined 14.7% (±0.9 SE) at an end-diastolic pressure of 100 mm Hg. Likewise, developed mean wall stress rose abruptly with diastolic pressure, peaked at a higher diastolic pressure of approximately 50 mm Hg, and declined only 7.5% (±0.8 SE) from this peak at an end-diastolic pressure of 100 mm Hg. Similar findings were observed in hearts acutely depressed with propranolol. Electron micrographs showed sarcomere length to average 2.275µ and 2.300µ in ventricles fixed in diastole while subjected to pressures of 61 and 100 mm Hg, respectively, after potassium arrest, confirming the findings illustrated by the normalized graphs. These observations imply that in the isolated heart of the dog there is no loss of ventricular performance attributable to a descending limb of the Frank-Starling mechanism until the end-diastolic pressure exceeds 60 mm Hg and that this loss is minimal at diastolic pressures as high as 100 mm Hg.