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

Biomechanics of infarcted left Ventricle-A review of experiments

Myocardial infarction (MI) is one of leading diseases to contribute to annual death rate of 5% in the world. In the past decades, significant work has been devoted to this subject. Biomechanics of infarcted left ventricle (LV) is associated with MI diagnosis, understanding of remodelling, MI micro-structure and biomechanical property characterizations as well as MI therapy design and optimization, but the subject has not been reviewed presently. In the article, biomechanics of infarcted LV was reviewed in terms of experiments achieved in the subject so far. The concerned content includes experimental remodelling, kinematics and kinetics of infarcted LVs. A few important issues were discussed and several essential topics that need to be investigated further were summarized. Microstructure of MI tissue should be observed even carefully and compared between different methods for producing MI scar in the same animal model, and eventually correlated to passive biomechanical property by establishing innovative constitutive laws. More uniaxial or biaxial tensile tests are desirable on MI, border and remote tissues, and viscoelastic property identification should be performed in various time scales. Active contraction experiments on LV wall with MI should be conducted to clarify impaired LV pumping function and supply necessary data to the function modelling. Pressure-volume curves of LV with MI during diastole and systole for the human are also desirable to propose and validate constitutive laws for LV walls with MI.

Preservation of cardiac function in left ventricle cardiac hypertrophy using an AAV vector which provides VEGF-A expression in response to p53

Here we present the application of our adeno-associated virus (AAV2) vector where transgene expression is driven by a synthetic, p53-responsive promoter, termed PG, used to supply human vascular endothelial growth factor-A165 (VEGF-A). Thus, p53 is harnessed to promote the beneficial expression of VEGF-A encoded by the AAVPG vector, bypassing the negative effect of p53 on HIF-1α which occurs during cardiac hypertrophy. Wistar rats were submitted to pressure overload induced by thoracic aorta coarctation (TAC) with or without concomitant gene therapy (intramuscular delivery in the left ventricle). After 12 weeks, rats receiving AAVPG-VEGF gene therapy were compared to those that did not, revealing significantly improved cardiac function under hemodynamic stress, lack of fibrosis and reversal of capillary rarefaction. With these functional assays, we have demonstrated that application of the AAVPG-VEGF vector under physiologic conditions known to stimulate p53 resulted in the preservation of cardiac performance.

Use of afterload hemodynamic stress as a practical method for assessing cardiac performance in rats with heart failure

After myocardial infarction, the hemodynamics under basal conditions might appear to be unaltered, which makes it difficult to identify cardiac dysfunction by the usual approaches. Thus, we tested the response to sudden afterload stress in infarcted rats with apparently normal ejection function. Control (CT) and infarcted (MI) Wistar rats with various MI sizes were submitted to echocardiography 30 days after coronary occlusion, followed by assessment of hemodynamics under basal conditions and during a pharmacologically induced sudden pressure overload (phenylephrine 15-25 microg/kg, i.v.). Coronary occlusion resulted in cardiac remodeling proportional to MI size, although several functional parameters such as systolic pressure (SP), stroke volume (SV), and stroke work (SW) of all MI rats were similar to those of CT rats. However, the afterload stress that was produced led to a relative preservation of SV and an increase of SW in CT rats; MI rats exhibited a significant reduction in SV and SW generation, although global cardiac function was normal under basal conditions, as indicated by regular echocardiography and hemodynamics assessment. Thus, we propose the use of sudden pharmacologically induced afterload stress as a practical and efficient procedure for identifying impaired performance of the heart in anesthetized rats, providing an additional physiological variable to be evaluated in experimental therapeutic studies.

Structure and Mechanics of Healing Myocardial Infarcts

Therapies for myocardial infarction have historically been developed by trial and error, rather than from an understanding of the structure and function of the healing infarct. With exciting new bioengineering therapies for myocardial infarction on the horizon, we have reviewed the time course of structural and mechanical changes in the healing infarct in an attempt to identify key structural determinants of mechanics at several stages of healing. Based on temporal correlation, we hypothesize that normal passive material properties dominate the mechanics during acute ischemia, edema during the subsequent necrotic phase, large collagen fiber structure during the fibrotic phase, and cross-linking of collagen during the long-term remodeling phase. We hope these hypotheses will stimulate further research on infarct mechanics, particularly studies that integrate material testing, in vivo mechanics, and quantitative structural analysis.

Noninvasive evaluation of pulmonary capillary wedge pressure in patients with acute myocardial infarction by deceleration time of pulmonary venous flow velocity in diastole

OBJECTIVES

This study investigates the correlation between deceleration time of diastolic pulmonary venous flow (PV-DT) and of early filling mitral flow (LV-DT), and pulmonary capillary wedge pressure (PCWP) in patients with acute myocardial infarction (AMI).

BACKGROUND

An earlier study suggests that Doppler-derived LV-DT provides an accurate means of estimating PCWP in postinfarction patients with left ventricular systolic dysfunction. Furthermore, recent studies have suggested that PCWP correlates better with PV-DT than with LV-DT. However, the value of PV-DT and LV-DT for assessment of PCWP in patients with AMI has not been evaluated.

METHODS

In 141 consecutive patients with AMI, we measured PV-DT and LV-DT by Doppler echocardiography, and compared these variables with PCWP measured using a Swan-Ganz catheter.

RESULTS

There was a weak negative correlation between the LV-DT and PCWP (r = -0.54). Although the sensitivity of < or =130 ms in LV-DT in predicting > or =18 mm Hg in PCWP was high (86%), its specificity was low (59%). On the other hand, a very close negative correlation was found between PV-DT and PCWP (r = -0.89). The sensitivity and specificity of < or =160 ms in PV-DT in predicting > or =18 mm Hg in PCWP were 97% and 96%, respectively.

CONCLUSIONS

In patients with AMI, Doppler-derived PV-DT showed a stronger correlation with PCWP than LV-DT.

Changes of the left ventricle after myocardial infarction--estimation with cine magnetic resonance imaging during the first six months

BACKGROUND

In recent years, the interest of cardiologists has focused increasingly on the morphologic and functional changes of the left ventricle after myocardial infarction (MI), due to their great prognostic significance for the patient.

HYPOTHESIS

The aim of this study was to evaluate changes in left ventricular morphology and function during the first 6 months following MI.

METHODS

In all, 61 patients (17 women, 44 men, age 36-83 years) were examined with cine magnetic resonance imaging (CMRI) 1, 4, and 26 weeks after myocardial infarction. Thirty-two patients had anterior MI and 29 patients had posterior MI. According to enzyme-derived infarct weight, 15 patients had small infarcts (< 20 g), 19 had intermediate-sized infarcts (20-40 g), and 27 patients had large infarcts (> 40 g). CMRI was performed in the true short axis of the left ventricle. In each examination, left ventricular end-diastolic and end-systolic volume indices (LVEDVI, LVESVI), stroke volume index (LVSVI), ejection fraction (LVEF), and regional thickness, mass, and motility of the myocardial wall-diastolic thickness (IDdia), infarct mass (IM) and motility (IMOT) of the infarct area and diastolic and systolic thickness (VDdia, VDsys), muscular mass (VM), and motility (VMOT)-were determined. In addition, patients were divided into subgroups according to New York Heart Association (NYHA) functional status at baseline.

RESULTS

In the total group, LVEDVI increased from 73.9 +/- 23.5 ml/m2 to 85.4 +/- 28.1 ml/m2 (p < 0.001) and LVESVI from 40.5 +/- 19.4 ml/m2 to 51.2 +/- 29.0 ml/m2 (p < 0.001). In the subgroups the development depended on infarct size and location. LVSVI and LVEF remained more or less constant except for large anterior infarctions. All changes of the myocardial wall depended on infarct size and location: In all patients IDdia decreased from 10.4 +/- 1.6 mm to 8.9 +/- 1.7 mm (p < 0.001), IMOT from 2.0 +/- 1.6 mm to 0.5 +/- 2.9 mm (p < 0.001). IM increased from 41 +/- 21 g to 45 +/- 25 g (p < 0.001). In the total group, VDdia increased from 11.9 +/- 1.6 mm to 12.4 +/- 1.8 mm (p < 0.05), VDsys from 16.6 +/- 2.5 mm to 17.2 +/- 3.1 mm (p < 0.05). In the subgroups changes varied: VDdia and VDsys decreased markedly in large anterior wall infarctions. VM increased in the total cohort from a mean of 246 +/- 66 g to 276 +/- 80 g (p < 0.001). VMOT decreased from 7.1 +/- 2.4 mm to 6.3 +/- 2.7 mm (p < 0.05). Loss of motility was most pronounced in anterior infarctions. The volume-mass ratio, a measure of the success of compensation of volume increase by myocardial hypertrophy, decreased in small infarcts, remained unchanged in intermediate infarcts, and increased in large infarcts. There was a trend toward improvement of the NYHA functional status during the observation period.

CONCLUSIONS

Changes of the left ventricular chamber during the first 6 months following MI are dependent on its size and location, with large anterior infarctions having the worst course. Myocardial wall remodeling is also dependent on infarct size and location, and the volume-mass ratio increases in the presence of large areas of necrosis, indicating the non-compensatory effect of myocardial hypertrophy. However, these changes have no clinical effect during the first half year after MI.

The effect of nifedipine on ventriculoarterial coupling in old myocardial infarction

The effect of nifedipine on ventriculoarterial coupling was examined in 8 patients with old myocardial infarction who showed a depressed ejection fraction (37 +/- 7%). Left ventricular (LV) pressure and LV volume were determined simultaneously by micromanometer and conductance catheter, respectively. We measured the slope (Ees) of the end-systolic pressure-volume relation during transient inferior vena caval occlusion, the slope (Ea) of the end-systolic pressure-stroke volume relation, the ratio of Ea to Ees (Ea/Ees), and the work efficiency (the ratio of external work to the systolic pressure-volume area) at baseline and after the sublingual administration of nifedipine (10 mg). Nifedipine slightly increased the heart rate from 71 +/- 14 to 78 +/- 17 beats/min. Although nifedipine had little effect on Ees (2.54 +/- 0.68 vs 2.47 +/- 0.62 mmHg/ml/m2, ns), it significantly decreased Ea from 3.47 +/- 1.16 to 2.37 +/- 0.54 mmHg/ml/m2. Consequently, Ea/Ees decreased from 1.42 +/- 0.47 to 0.97 +/- 0.31 and work efficiency increased from 48 +/- 12 to 59 +/- 13% after nifedipine administration. These data suggest that nifedipine reduces afterload (Ea) and improves left ventriculoarterial coupling without depressing left ventricular contractility in patients with failing hearts due to old myocardial infarction.

Reproducibility of symptom-limited exercise-stress testing performed 2 and 6 weeks after myocardial infarction

To assess the reproducibility of variables with prognostic value from exercise testing, two symptom-limited treadmill exercise tests were performed in 76 consecutive patients at 2 weeks (predischarge) and 6 weeks after myocardial infarction. In addition, cardiac catheterization was performed at 6 weeks. Exercise duration showed a moderate increase from 7.9 +/- 4.4 min to 8.8 +/- 3.0 min (NS). The rate-pressure product increased from 22,377 +/- 5491 to 24,832 +/- 7261 (P less than 0.001). Reproducibility of ST-segment depression was dependent on the initial response: among the group of 25 patients with ST-segment depression at 2 weeks, only 13 (52%) patients had a reproducible result, whereas among the group of 51 patients without initial ST-segment depression, 40 (78%) patients showed reproducibility. There was no difference in coronary anatomy or ejection fraction between the groups with and without reproducibility results. Among the 30 patients with initial ST-segment elevation, 15 (50%) patients showed reproducibility, while among the 46 patients without initial ST-segment elevation, 42 (91%) patients showed reproducibility: the ejection fraction was significantly higher in the latter group than in the group of patients with lower reproducibility. Thus predischarge exercise testing in postinfarction patients identifies a different group of patients at risk compared to exercise testing after 6 weeks, due to considerable variation between the two tests.

Effect of cilazapril on regional left ventricular wall thickness and chamber dimension following acute myocardial infarction: in vivo assessment using MRI

The primary goal of the current study was to assess in situ, using magnetic resonance imaging, the effect of a new angiotensin-converting enzyme inhibitor, cilazapril, in reducing left ventricular remodeling after acute myocardial infarction. Three groups of animals were investigated: (1) sham-operated rats (n = 19); (2) infarcted rats receiving no treatment (n = 23); and (3) infarcted rats receiving cilazapril (100 mg/L drinking water, n = 20). Treatment with cilazapril began on the third day postocclusion and continued for 3 to 4 months. Myocardial infarction was produced by ligation of the left coronary artery, and electrocardiographic (ECG)-gated short-axis images were acquired 3 to 4 months later. Sham-operated animals were subjected to the same procedure but the left coronary artery was not ligated. From the image acquired in the middle of the left ventricle (equatorial slice), left ventricular wall thicknesses, chamber diameters, and surface area measurements of the cavities were determined. At autopsy examination, infarct size and tissue water content were determined. The results demonstrate that magnetic resonance imaging has the potential to assess in situ the alterations of left ventricular dimensions and mass after acute myocardial infarction and can be used to document the influence of therapeutic interventions. Cilazapril provided protection against the deleterious remodeling changes such as ventricular dilation and wall thinning consequent to acute myocardial infarction.

Functional significance of hypertrophy of the noninfarcted myocardium after myocardial infarction in humans

Hypertrophy of the noninfarcted left ventricle as a chronic response to myocardial infarction has been demonstrated in animals and at autopsy in humans. However, the functional significance of postmyocardial infarction hypertrophy is a subject of dispute. The purpose of this study was to determine the time course of development of postmyocardial infarction hypertrophy of the noninfarcted myocardium in humans and to assess its functional significance. Subcostal view, two-dimensional echocardiograms were recorded at rest and during peak exercise, 6 and 40 weeks postmyocardial infarction in 45 patients (16 anterior, 20 inferior, nine non-Q wave infarcts), for measurement of left ventricular mass and ejection fraction. The left ventricular mass index increased from 94 +/- 30 to 118 +/- 27 g/m2 (p less than 0.01) during the time of the two studies. There was a significant correlation between the change in left ventricular mass index and improved resting ejection fraction (r = 0.48, p less than 0.001) and exercise ejection fraction (r = 0.48, p less than 0.001) at the follow-up study. Of the 32 patients who increased their left ventricular mass index greater than 7%, 18 improved their rest ejection fraction greater than 0.05 units and 17 improved their exercise ejection fraction greater than 0.05 units. Conversely, of the 13 patients who failed to increase their left ventricular mass index, only three improved their rest ejection fraction and one improved the exercise ejection fraction (Fisher's exact test, p less than 0.05). We reached three conclusions. First, in humans, significant hypertrophy of the noninfarcted myocardium can be detected by two-dimensional echocardiography, 9 months postmyocardial infarction.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of functional recovery to scar contraction after myocardial infarction in the canine left ventricle

We have previously reported that regional wall motion abnormalities in a canine model of acute myocardial infarction may show substantial improvement in the first 6 weeks after infarction. To determine whether the mechanism of this improvement in function is the result of scar contraction within the infarct, we studied the relationship between changes in regional wall motion defined by cross-sectional echocardiography and the regional concentration of radioactive microspheres injected immediately before coronary occlusion and sampled 6 weeks after occlusion. Eight dogs underwent serial echocardiographic and microsphere blood flow measurements immediately before and 30 minutes, 48 hours, 1 week, 3 weeks, and 6 weeks after ligation of the left anterior descending or the left circumflex coronary artery. Wall motion and blood flow were measured in the short-axis section of the left ventricle at the level of the midpapillary muscle in each 10-degree radial segment around the circumference of the ventricle. Infarct histology was assessed at 6 weeks by means of the same radial coordinate system. Control data were collected in a similar manner from four dogs that underwent sham operations and had no histologic evidence of infarction. In all of the animals with infarcts, but not in the sham animals, the calculated preocclusion endocardial and epicardial blood flow values in the histologic infarct zone (252 +/- 44 and 168 +/- 17 ml/min/100 gm, respectively, mean +/- SEM) were significantly higher than those in the normal opposite wall (endocardial: 106 +/- 3 ml/min/100 gm, p less than 0.01); epicardial: 108 +/- 3 ml/min/100 gm, p less than 0.01. The location and circumferential extent of myocardium showing this elevation of preocclusion blood flow correlated well (r = 0.93, p less than 0.001) with the location and circumferential extent of the histologic infarct. The amount of wall motion abnormality, measured from the "correlation plot area," decreased significantly from its maximum value of 39 +/- 3 degrees at 48 hours after coronary occlusion to 3 +/- 1 degrees (p less than 0.001) at 6 weeks after occlusion. The ratio of the preocclusion transmural blood flow in the infarct zone to that in the noninfarct zone, a measure of the condensation of the microspheres injected before coronary occlusion, and therefore of the degree of scar contraction at 6 weeks, correlated well (r = 0.83, p less than 0.01) with the recovery of wall motion 6 weeks after infarction.(ABSTRACT TRUNCATED AT 400 WORDS)

Evolution of the temporal contraction sequence after acute experimental myocardial infarction

The effect of infarct maturation on the temporal sequence of contraction within infarct zones has not previously been described. Accordingly, the time-varying pattern of contraction within ischemic/infarct zones was studied with use of cross-sectional echocardiography in 17 dogs at 10 min to 6 weeks after acute experimental myocardial infarction. Left ventricular short-axis images were digitized from end-diastole to end-systole and endocardial fractional radial change along 36 evenly spaced rays was calculated. The circumferential extent of dyskinesia and the number of rays that exhibited maximal dyskinesia were determined for each decile of the normalized contraction sequence. Between 10 min and 1 week after infarction, the greatest circumferential extent of dyskinesia occurred between the 3rd and 4th deciles of the normalized contraction sequence. However, as the infarct matured, the greatest spatial expanse of dyskinesia was noted to occur progressively earlier in the contraction sequence (second decile at 6 weeks), and the extent of mid- to late-systolic dyskinesia decreased markedly. Whereas end-systolic dyskinesia was present in 30% to 50% of ischemic/infarct zone rays from 10 min to 48 h, end-systolic dyskinesia was no longer observed at 6 weeks. Similarly, the maximal amplitude of dyskinesia was most commonly observed during midsystole from 10 min to 48 h, but occurred progressively earlier as the infarct matured, falling during the first decile at 6 weeks after infarction. These data suggest that maximal circumferential extent and amplitude of dyskinesia occur progressively earlier in the systolic contraction sequence as the infarct matures.(ABSTRACT TRUNCATED AT 250 WORDS)

Can new inodilators displace digitalis in the therapy of congestive heart failure?

New inodilators that possess both positive inotropic and vasodilator actions have many favorable effects in patients with congestive heart failure, even in those with refractory heart failure. These effects are expected to prevent myocardial injury, improve peripheral circulation, depress the excessive endogenous neurohumoral activation, and, finally, improve the quality of life, and increase lifespan. However, experience with new inodilators has only begun. Several questions remain to be answered before these drugs can be widely used with safety, including whether life-threatening adverse effects appear, mortality rate is lessened, and drug tolerance occurs. The therapeutic level of the dose and the relation between the effectiveness of the drug and the degree of the severity of heart failure should also be established. Therefore, long-term, randomized, double-blind, placebo-controlled clinical trials will be necessary before the new inodilators can take the place of digitalis and thus become the mainstay of the therapy of congestive heart failure.

Rate and extent of recovery of left ventricular function in patients following acute myocardial infarction

To determine the rate and extent of recovery of left ventricular (LV) performance following acute myocardial infarction (MI), peak aortic blood acceleration was measured serially in 26 patients and in 11 normal volunteers with a continuous wave Doppler placed suprasternally. In patients, Doppler measurements were made 20 +/- 2 hours after the acute onset of chest discomfort and were repeated daily for 6 consecutive days. Infarction patients were divided into two groups. Group I consisted of 15 patients who did not have a previous MI and whose present course was not complicated by congestive heart failure (CHF). Group II consisted of 11 patients who had either a previous MI or developed CHF during the present admission. Peak acceleration in the normal volunteers showed minimal daily variations over a period of 6 days. Peak acceleration in the entire group of 26 MI patients increased from 13 +/- 3 m/sec/sec on the day of admission (day 1) to 18 +/- 6 m/sec/sec on day 6 (p less than 0.001). In group I, peak acceleration increased from 13 +/- 4 to 20 +/- 6 m/sec/sec between day 1 and day 6 (p less than 0.001). In group II, however, peak acceleration was 12 +/- 2 m/sec/sec on day 1 and increased to only 15 +/- 4 m/sec/sec on day 6 (NS). These results indicate that LV performance, based upon peak acceleration of blood in the ascending aorta, improves markedly within 6 days in patients suffering their first MI uncomplicated by CHF.(ABSTRACT TRUNCATED AT 250 WORDS)

Early and late global and regional left ventricular function after experimental transmural myocardial infarction: relationships of regional wall motion, wall thickening, and global performance

Experimental myocardial infarction of the posterolateral wall of the left ventricle was produced in dogs by injecting 80 micron microspheres into the left circumflex coronary artery to determine changes in regional myocardial function after infarction, to examine how the changes in regional myocardial function relate to the changes in global left ventricular function, and to examine the relationship between regional wall motion and wall thickening after myocardial infarction. Serial measurements of global ventricular and regional myocardial function were made in six dogs before and during 20 days after infarction, with the use of M-mode echocardiography and chronic Swan-Ganz catheter implantation. One hour after infarction, stroke volume index had decreased 49% from baseline, percent fractional shortening had decreased 52%, lateral wall motion had decreased 80%, and lateral wall thickening had decreased 100%. By 6 days after infarction, stroke volume index had increased 41% from its low point, percent fractional shortening had increased 34%, and lateral wall motion had increased 100% toward but not to baseline. Lateral wall thickening did not return following infarction. Peak and end-systolic circumferential wall stresses and systemic arterial blood pressure remained stable. End-systolic diameter increased acutely (36%) after infarction and did not change during the 20-day time period, while end-diastolic diameter gradually increased, resulting in the increase in percent fractional shortening. In conclusion, after posterolateral wall infarction, wall motion can return without an improvement in regional myocardial function, presumably because the infarcted region stiffens, allowing it to be pulled inward.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional vascular adjustments during recovery from myocardial infarction in rats

Left ventricular function and systemic regional blood flow (radioactive microspheres, 15 +/- 5 mu) were studied 1, 3, 10 or 42 days after left coronary occlusion in conscious rats. One day after coronary occlusion, vascular resistance in the skeletal muscle and cutaneous beds increased while stroke work and left ventricular systolic pressure were depressed. Regional blood flow and hemodynamic data were similar for sham and infarction groups at 3 and 10 days after surgery, except for left ventricular end-diastolic pressure, which was significantly increased in rats with infarction (sham versus infarct: 11.5 +/- 1.0 versus 18.4 +/- 3.2 at day 3 and 12.2 +/- 1.4 versus 19.9 +/- 3.2 at day 10) (p less than 0.05). At 42 days after myocardial infarction, manifest heart failure occurred as documented by decreased cardiac output and left ventricular systolic pressure and elevated left ventricular end-diastolic pressure and vascular resistance in the cutaneous, skeletal muscle and renal beds. In a separate group of animals with moderate (33.2 +/- 2% of left ventricle) and large infarctions (45 +/- 1.3% of left ventricle), regional blood flow was compared with the sham group. Rats with a large infarct demonstrated significant (p less than 0.05) reduction in flow to kidney, gut and liver. In rats with a medium sized infarct, only renal blood flow was significantly reduced. It is concluded that in this model of myocardial infarction, early cardiocirculatory depression is followed by a partially compensated state with increased left ventricular end-diastolic pressure and subsequent systemic and regional vasoconstriction which, in turn, may contribute to late deterioration of heart failure.

Left ventricular dysfunction and altered autonomic activity: a possible link to sudden cardiac death

There is now a growing body of clinical evidence that suggests a strong association between left ventricular dysfunction and sudden cardiac death in patients recovering from myocardial infarction. The mechanisms underlying this association remain to be determined. Alterations within the autonomic nervous system may represent one factor that links an impairment in cardiac function to an increased mortality. Since ventricular dysfunction would tend to reduce stroke volume, an increased sympathetic and/or decreased parasympathetic efferent activity may compensate for this fall in stroke volume by increasing heart rate and/or the force of contraction (inotropic state) in an attempt to maintain a more normal cardiac output. Similar changes in autonomic activity are, in fact, known to increase the vulnerability to ventricular fibrillation. Therefore, I propose that myocardial infarction induces changes in cardiac function which in turn elicits autonomic efferent changes. As a consequence of these compensatory reflex changes the heart becomes less electrically stable and thereby more prone to lethal arrhythmias.

Reversible and irreversible changes in the dog heart during acute left ventricular failure due to experimental multifocal ischaemia

Acute left ventricular (LV) failure was induced in closed-chest pentobarbital anaesthetized dogs (n = 15), by injection of 50 micron plastic microspheres into the main left coronary artery. There were marked reductions of cardiac output and peak LV dp/dt after the embolization, while LV end-diastolic pressure (LVEDP) increased markedly. Biopsies were taken 1, 2, 3, 4, 6, 8, 10, 36, 48 hours, 7 and 14 days after injection of microspheres. Histological examination of the left ventricle revealed multiple ischaemic lesions distributed throughout the are supplied by the left main coronary artery. Oilred O staining revealed deposition of fine lipid droplets in the ischaemic cardiac muscle cells bordering on the necrotic areas. Ultrastructurally the lipid-containing cells showed numerous vacuoles localized in association with the mitochondria. The vacuoles could be observed already 1 hour after embolization and increased in number up to 48 hours. At the end of the observation period at two weeks, the number of lipid droplets was markedly reduced and the heart regained its functional activity. It is concluded that the myocardial lesions induced by coronary embolization of plastic microspheres were associated with acute left ventricular pump failure and consist of multiple foci of damaged myocardium with a central core of necrotic tissue. In the periphery of these lesions there were myocytes with lipids and other signs of light and moderate ischaemic injury. Our suggestion is that these cells represent a region of intermediate injury of "border zone" cells that are a potentially salvageable myocardium.

Radionuclide assessment of regional left ventricular function in acute myocardial infarction

To determine changes in global and regional left ventricular function following acute myocardial function, 17 patients underwent radionuclide angiography at 3 and 10 days post infarction. Five patients had nontransmural myocardial infarction and 12 had transmural infarction (six anterior and six inferior). There were no previous infarctions in 16 (94%) patients. Regional ejection fractions were calculated by dividing the left ventricle into four quadrants using the geometric center of the left ventricle on the end-diastolic frame as a reference point. At 3 days post infarction, 8 of 17 (47%) patients had an abnormality of global left ventricular ejection fraction (LVEF), whereas 16 of 17 (94%) patients had abnormalities of one or more regional ejection fractions (p less than 0.01). Between 3 and 10 days, global LVEF did not change (51% to 49%, p = NS). However, there were significant changes in 23 of 68 (34%) regional LVEFs. These changes did not relate to type, ECG location, creatine kinase (CK) size of infarction, or initial global LVEF. These data suggest that regional LVEF is a sensitive technique for identifying segmental dysfunction associated with myocardial infarction. In addition, significant changes occur in regional LV function during acute myocardial infarction despite stable serial global LV performance.

The natural history of regional dysfunction in a canine preparation of chronic infarction

We used a canine preparation of experimental infarction to study the natural course of echocardiographically defined regional wall motion abnormalities in the 6 weeks after acute coronary ligation. Eight dogs underwent serial short-axis echocardiographic evaluation and microsphere blood flow determinations at control, and 30 min, 48 hr, 1 week, 3 weeks, and 6 weeks after acute coronary artery ligation. Wall motion analysis and blood flow calculations were applied to 10 degree radial segments of the left ventricle (short axis) and correlated to the size and extent of infarction as defined histologically at 6 weeks. All animals had at least 50% transmural histologic infarction. The ratio of flow in infarcted tissue vs noninfarcted myocardium fell to 0.40 +/- 0.13 for endocardium and 0.56 +/- 0.13 for epicardium at 30 min after ligation, but recovered to 0.83 +/- 0.15 for endocardium and 1.12 +/- 0.11 for epicardium by 6 weeks. The maximum circumferential extent of abnormal regional wall motion was observed at 48 hr after infarction (mean circumferential extent = 51%), but was resolved to a significant extent by 6 weeks (circumferential extent = 21%, p less than .001). Four animals had virtually normal wall motion by 6 weeks after infarction. Segment-by-segment regional dysfunction correlated highly with the regional transmural reduction in blood flow of 20% )r = .89, p = .0001) for the experiment as a whole, but the echocardiogram tended to underestimate the size of the histologic infarct at 6 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular performance in rats with myocardial infarction and failure

Ventricular performance was assessed in rats three weeks following coronary artery ligation and the subsequent production of a wide range of infarct sizes. The entire spectrum of ventricular dysfunction was observed, from minimal impairment to overt congestive heart failure. Rats with small infarcts ejected normal baseline and volume-stressed forward outputs from a modestly dilated ventricular chamber. Rats with moderate infarcts exhibited normal baseline hemodynamics but had a reduced reserve flow capacity when challenged with a volume load despite considerable ventricular dilatation. Rats with large infarcts demonstrated frank congestive heart failure with elevations in both left and right ventricular filling pressures and consequent right ventricular hypertrophy; marked reductions in both baseline and volume-stressed forward outputs; and ventricular volumes that were twice those of rats without infarcts. Thus, a progressive impairment in ventricular performance and an increase in chamber volume occurred in relation to infarct size in rats with healed myocardial infarction.

The functional implications of scintigraphic measures of myocardial ischemia and infarction

To compare serial functional and perfusion scintigraphic changes after myocardial infarction, we performed left ventricular (LV) cineangiograms and thallium (TI)-201 myocardial perfusion scintigrams before and 1 hour, 2 days, 9 days, and 1 month after closed chest coronary occlusion in 14 dogs as survival permitted. Survivors were studied with technetium-99m (stannous) pyrophosphate (TcPYP) scintigrams at 48 hours, and at postmortem examination infarction was documented and measured after nitroblue tetrazolium (NBT) staining. The TcPYP image was abnormal in 10 dogs, each of which had infarcts on NBT staining measuring 3 to 23 gm. In all 14 dogs, perfusion scintigrams became abnormal and LV ejection fraction (EF) fell when measured within 48 hours of occlusion. In the nine late survivors studied over 1 week after the event, perfusion scintigrams and EF improved in those which developed infarcts and normalized in those without infarction. The decrement in LVEF after coronary occlusion generally showed serial improvement and correlated with the size of the defect in the accompanying TI-201 scintigram (r = 0.74). TI-201 defect size seen in late studies correlated well with NBT infarct size (r = 0.89) and TcPYP image infarct size (r = 0.82), as it did with the decrement in LVEF noted in late studies (r = 0.86). The results suggest that early perfusion scintigrams together with TcPYP images may be useful for estimating the amount of reversible dysfunction after coronary occlusion.

Myocardial healing and repair after experimental infarction in the rabbit

Adequacy of healing after acute myocardial infarction may determine the incidence of postmyocardial infarction rupture and ventricular aneurysm. Accordingly, in 36 rabbits, from 1 to 8 days after coronary ligation, and in 18 shams, we measured collagen formation and mechanical resistance of the infarcted left ventricle to stretch and rupture. Prolyl hydroxylase, an intracellular enzyme of collagen synthesis, increased from control activity of 3970 +/- 431 to 9224 +/- 643 counts/min per mg (cpm/mg) extractable protein (P less than 0.01) at 48 hours and was nearly maximal at 3 days postmyocardial infarction (14,518 +/- 2,030 cpm/mg, P less than 0.01). Lysyl oxidase, an extracellular collagen cross-linkage enzyme, increased from control activity of 29.6 +/- 4.8 to 74.7 +/- 18.8 cpm/mg extractable protein (P less than 0.01) at 72 hours and peaked at 121.5 +/- 7.3 (P less than 0.01) 4-6 days postmyocardial infarction. Hydroxyproline, a measure of collagen content, increased from control of 2.8 +/- 0.2 to 5.3 +/- 0.6 mg/g dry weight (P less than 0.05) at 72 hours and continued to increase at 8 days postmyocardial infarction (14.5 +/- 1.7 mg/g dry weight; P less than 0.01). When enzyme activities and hydroxyproline content were expressed relative to other reference bases, including DNA, tissue protein, dry weight, and total left ventricle, similar results were obtained. The mechanical properties of the infarcted left ventricle were determined by filling a balloon in the excised left ventricle until rupture. The rupture threshold in the normal left ventricle, [664 +/- 43 mm Hg (n = 16)], was not significantly different from that of the infarcted left ventricle on days 1-8 postmyocardial infarction. However, left ventricular rupture occurred more often through the myocardial infarction on days 1-4 postmyocardial infarction (59%) than on days 6 and 8 (18%; P = 0.03) when collagen content had significantly increased. Wall stress at the point of rupture in left ventricles from shams and normals was 30 +/- 2 g/mm2; tensile strength in isolated left ventricle muscle strips was 25 +/- 4 g/mm2 and in isolated scar strips at 7 days postmyocardial infarction was 59 +/- 7 g/mm2. The passive stiffness of the infarcted left ventricle increased from control of 61 +/- 5 to 94 +/- 6 mm Hg/100 microliters (P less than 0.05) at 4 days and 100 +/- 7 mm Hg/100 microliters (P less than 0.01) at 6 days postmyocardial infarction. Stiffness correlated with hydroxyproline content over the 8 days postmyocardial infarction (r = 0.599; P less than 0.001). Thus, the acutely infarcted ventricle was highly resistant to rupture during the initial 48 hours postmyocardial infarction, before any increase in collagen occurred. This result suggests that the preinfarction collagen content has an important role in preventing rupture. After 72 hours postmyocardial infarction, collagen synthesis appeared to be a determinant of infarct stiffness and resistance of the infarcted ventricle to rupture.

Anterior transmural myocardial infarction: effects of surgical coronary reperfusion on global and regional left ventricular function

Global and regional left ventricular function were assessed before and after surgical coronary reperfusion in 54 patients surviving anterior transmural myocardial infarction. Two groups were identified. Group I (n = 34) was treated within 4.8 +/- 0.7 (mean +/- standard deviation) hours of onset of symptoms of anterior transmural myocardial infarction, and Group II (n = 20) was treated 9.2 +/- 4.8 hours from the onset of symptoms (p less than 0.01). On study entry, the two groups were similar in all characteristics except global left ventricular ejection fraction (48 +/- 9 versus 42 +/- 13%, p less than 0.05). Regional ejection fraction was obtained by computer-assisted planimetry from ventriculographic tracings at end-systole and end-diastole. The anterior wall was divided into four equal segments from the apex (area 1) to base (area 4). Areas 2 and 3 defined the midportion of the anterior wall of the left ventricle. This yielded four fractional changes expressed as ejection fraction in percent. Global and regional ejection fractions (from apex to base) of the anterior wall significantly improved in Group I (from 48 +/- 9 to 55 +/- 11%; 7 +/- 17 to 18 +/- 20%; 12 +/- 14 to 25 +/- 18%; 25 +/- 15 to 38 +/- 17%; and 39 +/- 13 to 41 +/- 12%) (p less than 0.05, except for the basal area), but only to a minor degree in Group II (from 42 +/- 13 to 45 +/- 16%; 9 +/- 10 to 13 +/- 15%; 10 +/- 10 to 17 +/- 10%; 27 +/- 16 to 32 +/- 14%; and 37 +/- 10 to 36 +/- 13%) (all p values were not significant [NS] except for region 2). These data suggest significant enhancement of global function and regional wall motion in selected patients if surgical reperfusion is performed within 6 hours from the onset of symptoms of anterior infarction. Little improvement can be expected when the procedure is instituted later than 6 hours from peak symptoms, although improvement in some patients occurs if adequate collateral perfusion or nontotal left anterior descending coronary occlusion is present. In spite of functional improvements, some contractile deficit persisted throughout the period studied even when successful reperfusion was achieved early during evolving anterior transmural myocardial infarction.

Effect of regional ischemia on the left ventricular end-systolic pressure-volume relationship of isolated canine hearts

We studied the effects of regional ischemia on the left ventricular isovolumic end-systolic pressure-volume relationship (ESPVR) in six excised, blood-perfused canine ventricles. We created different extents of regional ischemia by ligating various branches of the coronary arteries while keeping the coronary arterial pressure constant (80 mm Hg). The extent of regional ischemia (Rm) relative to the total mass of the left ventricular myocardium was determined by regional myocardial blood flow measured by the radioactive microsphere technique. With regional ischemia, the ESPVR shifted rightward without significant change in slope in the physiologic end-systolic pressure range. In the subphysiological end-systolic pressure range, however, its slope became lower than control. In order to quantify the degree of the rightward shift, we measured the extrapolated volume axis intercept (Vo) by fitting a straight line to the ESPVR in the physiological range under control and ischemic conditions. The shift in Vo (delta Vo) associated with ischemia was linearly correlated with Rm (delta Vo = 50.7Rm-0.6, n = 28, r = 0.944, P less than 0.001). We conclude that the major effect of acute regional ischemia on the ESPVR in the physiological pressure range is a parallel rightward shift. This forms a striking contrast to the effect of global ischemia (under which only the slope is affected without a substantial change in Vo).

Spontaneous changes in thallium-201 myocardial perfusion imaging after myocardial infarction

To examine regional myocardial perfusion after myocardial infarction, 26 patients underwent exercise electrocardiographic testing with thallium-201 myocardial perfusion imaging 3 weeks and 3 months after infarction. At 3 weeks, 9 of 26 patients (35%) had myocardial ischemia by exercise electrocardiographic testing, whereas 18 of 26 (69%) had ischemia by thallium-201 imaging. The thallium-201 scintigrams were scored by dividing each image, in 3 views, into 5 segments, using a 5-point scoring scheme. The exercise thallium-201 score was 44.3 +/- 1.2 and increased to 47.3 +/- 1.2 in the redistribution study (p less than 0.001). Three months after infarction, although there was a significantly greater rate-pressure product which would predict a larger ischemic defect and a decrease in the stress thallium-201 score, the stress score was improved (48.3 +/- 1.1, p less than 0.001). The redistribution score was similar, that is, 48.9 +/- 1.0. The improvement in thallium-201 myocardial perfusion was associated with a loss of stress-induced ischemia in 8 patients (30%). These results indicate that spontaneous improvements in thallium-201 myocardial perfusion imaging may occur after myocardial infarction.

Contractile reserve and left ventricular function in regional myocardial ischemia in the dog

Contractile activity remaining in a region made ischemic by acute occlusion of the left anterior descending coronary artery (LAD) was assessed in dogs relative to its role in maintaining left ventricular (LV) function. Compensatory increases in contractility of normal myocardium were eliminated by treating all dogs with reserpine (3 mg/kg) to deplete their catecholamine stores. LV function was determined by measuring stroke volume while increasing the LV filling pressure with a shunt from the aorta to left atrium. Heart rate and mean aortic pressure were kept constant. LV function was studied after occlusion of the LAD alone and after the selective infusion of potassium chloride (1 mEq/ml) into the LAD to raise the regional extracellular potassium concentration to 30 mEq/ml. The reduction in LV function induced by LAD ligation was less than the reduction caused by abolishing contraction in the entire zone supplied by the LAD with infusion of potassium. The totally cardioplegic zone induced by potassium amounted to 20.3-39.8% of the LV mass. At an LV end-diastolic pressure of 12 mm Hg, stroke volume (SV) was reduced in proportion to the size of the cardioplegic zone: -SV (% volume) = -1.55% (% of LV mass) + 120.1 (r = -0.69, p less than 0.005). Thus, a dyskinetic zone of 35% of the left ventricle reduced stroke volume by 34% when adrenergic compensation was blocked. We conclude that residual transmural contractility exists in the ischemic region of myocardium subserved by an obstructed LAD and contributes significantly to LV function.

Left ventricular diastolic pressure-volume relations in rats with healed myocardial infarction. Effects on systolic function

To determine the effects of healed myocardial infarction on the diastolic compliance of the left ventricle, we studied 36 rats 26 days after left coronary artery ligation. Peak cardiac output and stroke volume were measured under ether anesthesia during volume loading, and peak left ventricular developed pressure was determined during occlusion of the ascending aorta. During a slow infusion of saline into the potassium-arrested left ventricle, diastolic pressure and volume were measured continuously over the pressure range -5 to 30 mm Hg. Infarct size was determined by planimetry of serial sections taken from each heart at 1-mm intervals from apex to base. In rats with healed infarcts, left ventricular volume was increased in proportion to infarct size and the diastolic pressure-volume relationship was shifted so that at pressures below 2.5 mm Hg volume was increased, resulting in an increased ventricular compliance in this low pressure range. Above this pressure, the slopes of the pressure-volume curves were similar in rats with and without infarctions. Peak cardiac output and pressure-generating capacity were impaired in proportion to infarct size. This impairment of cardiac performance correlated with the infarct size-related increase in diastolic volume, which served to offset the reduction in flow generating capacity caused by systolic dysfunction, while contributing directly to the impairment of pressure generating capacity.

Myocardial infarct size and ventricular function in rats

To define the relationship between infarct size and ventricular performance, we performed hemodynamic studies in rats 21 days after left coronary artery occlusion. Ventricular performance was assessed under ether anesthesia by measurements of baseline hemodynamics and stressed performance as determined by the peak cardiac output and stroke volume obtained during intravenous volume loading and by the peak left ventricular developed pressure obtained during occlusion of the ascending aorta. Infarct size was determined by planimetry of the endocardial circumference of each of four histological slices of the left ventricle. Rats with small (4-30%) myocardial infarctions had no discernible impairment in either baseline hemodynamics or peak indices of pumping and pressure-generating ability when compared to the sham-operated, noninfarcted rats. Rats with moderate (31-46%) infarctions had normal baseline hemodynamics but reduced peak flow indices and developed pressure. Rats with infarctions greater than 46% had congestive heart failure, with elevated filling pressures, reduced cardiac output, and a minimal capacity to respond to pre- and after load stresses. The entire spectrum of postinfarction ventricular function was observed, from no detectable impairment to congestive failure. In this model of histologically healed myocardial infarction, the impairment of left ventricular function was directly related to the loss of myocardium.

Experimental myocardial infarction. 14. Accelerated myocardial stiffening related to coronary reperfusion following ischemia

In six dogs with surgically opened chests, segmental mechanical function was determined by measuring segment length using mercury-in-Silastic gauges attached to the epicardial surface of the left ventricular wall. Following coronary arterial occlusion the amplitude of the resulting paradoxical systolic bulge was quantitated in terms of "muscle lengths", defined as the ratio of the amplitude of the segment length over the end-diastolic segment length (EDSL). From an excursion of 0.176 +/- 0.029 muscle lengths at six hours of ischemia, the amplitude of the bulge decreased abruptly to 0.125 +/- 0.024 muscle lengths after 15 minutes of coronary reperfusion (P less than 0.05) but maintained paradoxical expansion in systole. Segmental "effective stiffness", calculated at the same periods of time from end-diastolic pressure-length relationships during transient pressure loading of the left ventricle, showed a reciprocal change, increasing from 1.416 +/- 0.161 to 2.051 +/- 0.238 mm Hg/% deltaEDSL (P less than 0.05). These data indicate that the degree of paradoxical bulging of an ischemic segment is affected by its pressure-length characteristics (distensibility) and that a rapid decrease both in the amplitude of the bulge and in distensibility occurs during reperfusion. The mechanism is uncertain but may relate to either myocardial edema or myofibrillar contracture.

Left ventricular hemodynamics and function in acute myocardial infarction: studies during the acute phase, convalescence and late recovery

The left ventricular hemodynamics of 70 patients with acute myocardial infarction were determined from measurements of pulmonary arterial end-diastolic pressure, cardiac index, mean arterial pressure and heart rate during the acute phase(first study, 5 hours after admission), 4 to 6 weeks later (second study, during convalescence) and in 35 percent of all subjects 6 to 12 months after the acute infarction (third study). Serial analysis of serum creatine kinase was carried out during the acute phase. The peak CK value normalized for body surface area was used as a rough index of the extent of the acute myocardial necrosis. The condition of all survivors of the acute stage improved. Patients with only slightly reduced left ventricular performance during the acute stage recovered to nearly normal during convalescence. The condition of patients with greatly reduced left ventricular function also improved but remained impaired during convalescence. In all patients the main changes in left ventricular hemodynamics occurred within the first 4 to 6 weeks; there was almost no further alteration during the following 9 months.

[The Contractile Function of the Surviving Heart Muscle Following Coronary Occlusion (author's transl)]

Following acute myocardial infarction the functional load of the surviving heart muscle does increase considerably, leading to an increased release of adrenergic neurotransmitters with a consequent decrease in endogenous catecholamine stores. Within the first 24 h following infarction, a temporary decline in the high-energy phosphate content is observed in the surviving heart muscle; futhermore, a reduction in lactate extraction is noted. In the intact organism an increased shortening of the surviving heart muscle is noted as a consequence of the altered ventricular geometry and the increased release of catecholamines. If these effects are excluded by means of isolation and analysis in vitro, a decrease in contractile function could be demonstrated in the surviving heart muscle in the early phase following infarction; the response to positive inotropic interventions was depressed as well. These changes are reversible; six weeks following infarction a normal contractile behaviour is observed.

Regional myocardial function and dimensions early and late after myocardial infarction in the unanesthetized dog

Pairs of ultrasonic dimension gauges and a micromanometer implanted in the subendocardium of the left ventricles of unanesthetized dogs were used to analyze serial changes in hemodynamic status and segmental function for up to 4 weeks after permanent circumflex coronary artery occlusion. Regional function was studied in control segments and in segments identified as marginal (hypokinetic) and ischemic. In three dogs, after transient regional dysfunction, no myocardial infarction developed, whereas in five dogs regional dysfunction at 3 hours after occlusion was followed by the development of persistent dysfunction and infarction. Left ventricular end-diastolic segment length (EDL) changes over time; EDL of the control segments increased progressively, but in marginal segments EDL was 12% below control and in the ischemic segments 30% below control by 4 weeks. Progressive increases in percent active shortening occurred in control segments; but holosystolic bulging was replaced by akinesia in ischemic segments, and persistent reduction in shortening was present in marginal segments at 4 weeks. Correlations were found between percent scar and reductions in percent shortening, EDL, and the ratio of change in diastolic length to change in diastolic pressure. These methods have detected hyperfunction in normal regions and variable segmental loss of contractile function, together with reduction of subendocardial dimensions and changes that may reflect decreased diastolic compliance in ischemic regions. We conclude that this model for the conscious animals may be useful for studying the influence of therapy on the extent of myocardial damage after experimental coronary occlusion.

Vascular and cardiac contractile reserve in the dog heart with chronic multiple coronary occlusions

Nineteen mongrel dogs survived chronic occlusion of the left circumflex and of the right coronary artery without infarction due to the timely development of a collateral circulation. Only 38 per cent of the conductance of the arteries before occlusion was restored by collateral vessels. In these animals and in 15 control dogs with normal coronary arteries myocardial contractility, contractility reserve, and myocardial blood flow were studied. The same was done in dogs with chronic coronary artery occlusion after aortocoronary bypass. Myocardial blood flow was determined woth the tracer microsphere technique. Contractility reserve was tested and defined as isovolumetric left ventricular pressure and dp/dt max with norepinephrine infusion and cross-clamping of the aorta. Contractile reserve was not significantly different between normal dogs and dogs with chronic coronary artery occlusion before and after aortocoronary bypass. Myocardial blood flow during control conditions was homogenously distributed in all three groups studied. The ratio of blood flow to the endocardium and the epicardium was not significantly different from inity. Coronary reserve was determined at peak reactive hyperemia following a 20 second period of coronary artery occlusion, with ongoing norepinephrine infusion. Under these conditions subendocardial fow in normal dogs rose by a factor of 7.9 while subepicardial flow increased 7.4 times. In dogs with chronic occlusion of two coronary arteries the increase of myocardial flow was nonnomogenous; subendocardial flow to areas supplied by a normal coronary artery rose by a factor of 7.0 while subepicardial flow increased 5.7 times control. Subendocardial collateral flow rose by a factor of 2.4 and subepicardial collateral flow increased 3.5 times control. In normal dogs norepinephrine alone did not result in maximal coronary flow but only 57 per cent thereof. Dogs with chronic coronary occlusion, however, required the entire coronary reserve in areas that were supplied by a normal coronary artery, whereas areas supplied by collaterals became ischemic. Opening of an aortocoronary bypass restored normal flow to previously ischemic areas, and reduced the flow to areas supplied by a normal artery. With the bypass open no differences existed between normal dogs and those with two occluded coronary arteries. We conclude that the norepinephrine-stimulated contractile reserve of hearts with chronic coronary occlusion was comparable to that of normal hearts; however, norepinephrine forced these hearts to spend the entire flow reserve of the remaining normal artery while producing ischemia in collateral-dependent areas. The same dose of norepinephrine did not require the entire flow reserve of normal dogs.

Serial measurements of left ventricular ejection fraction by radionuclide angiography early and late after myocardial infarction

The left ventricular ejection fraction was determined serially with radioisotope angiography in 63 patients with acute myocardial infarction. After the peripheral injection of a bolus of technetium-99m, precordial radioactivity was recorded with a gamma scintillation camera and the ejection fraction calculated from the high frequency left ventricular time-activity curve. Since this technique requires no assumptions with respect to left ventricular geometry, it is particularly useful in patients with segmental left ventricular dysfunction. Serial measurements during the first 5 days after hospital admission were made in 50 patients, 30 of whom were studied during the subsequent 2 to 39 months (mean 19.9 months). Late follow-up serial studies were also performed in an additional 13 patients who had only one measurement of the left ventricular ejection fraction during the early postinfarction period. Early after infarction, the left ventricular ejection fraction was normal (more than 0.52) in only 15 of the 63 patients, and averaged 0.52 +/- 0.05 (standard deviation) in the 27 patients with an uncomplicated infarct. The ejection fraction was reduced in 24 patients with mild to moderate left ventricular failure (0.40 +/- 0.05, P less than 0.0001) and in the 12 patients with overt pulmonary edema (0.33 +/- 0.07, P less than 0.0001). In 35 patients the ejection fraction correlated with the mean pulmonary arterial wedge pressure (r = 0.72). In 15 patients with normal left ventricular wall motion by heart motion videotracking, the ejection fraction was significantly higher (0.53 +/- 0.08) than in the 26 patients with regional left ventricular dysfunction (0.41 +/- 0.10, P less than 0.0001). During the early postinfarction period, the left ventricular ejection fraction improved in 55 percent of patients and remained unchanged or decreased in 45 percent. A further increase in the ejection fraction was noted in 61 percent of patients during the late follow-up period. Patients with an initially low or decreasing ejection fraction had a significantly greater incidence of early mortality and left ventricular dysfunction (P less than 0.02) than those whose ejection fraction was normal or improved to normal early after infarction. These data indicate that the ejection fraction is a sensitive indicator of left ventricular function after acute myocardial infarction and that serial measurements are helpful in predicting early mortality and morbidity.

Failure of hyaluronidase to alter the early course of acute myocardial infarction in pigs

The effect of hyaluronidase on the early course of acute myocardial infarction was evaluated in closed chest anesthetized pigs. One hour after balloon catheter occlusion of the left anterior descending coronary artery, hyaluronidase (500 units/kg body weight) was rapidly infused in 10 animals while 9 received no treatment. The animals were than observed over the next 4 hours. Cardiac output, heart rate, mean arterial pressure and left atrial pressure were not significantly affected by treatment. Heart rate increased and arterial pressure decreased in each group to a comparable degree of 5 hours, but left atrial pressure and cardiac output were unaffected. Precordial S-T segment mapping revealed no significant difference between the two groups. S-T segments rose to a comparable degree in each group and peaked before 1 hour. Hyaluronidase had no acute effects on the S-T segment map in the first 30 minutes after infusion or during the subsequent return of the map toward control level. Slightly lower S-T segments in the hyaluronidase-treated group at 5 hours was of borderline significance but was attributed to factors other than the drug intervention. Changes in ventricular wall motion were assessed angiographically, and all animals manifested akinetic or dyskinetic segments. A significant reduction in shortening fraction of involved segments was seen after occlusion, but no difference was observed between the two groups at 5 hours. Shortening fraction of the combined anterior and anteropical segments decreased from 66 +/- 10 to 20 +/- 6 percent at 5 hours in the hyaluronidase group (no. = 7) whereas in the control group (no. = 6) it decreased from 68 +/- 6 to 28 +/- 9 percent. Comparable increases in end-diastolic volume were also present at 5 hours in each group. Volumes increased from 80.6 +/- 5.1 to 97.5 +/- 6.4 ml3 at 5 hours (P less than 0.05) in the hyaluronidase-treated group (no. = 10) compared with 86.9 +/- 8.9 to 104.8 +/- 11.0 ml3 (P less than 0.05) in the control group (no. = 8). Hyaluronidase did not alter the early course of acute myocardial infarction in pigs. Species differences may contribute to different results reported to date.

Experimental myocardial infarction in the cat. I. Reversible decline in contractility of noninfarcted muscle

The contractile state of the noninfarcted myocardium was examined in adult cats after myocardial infarction produced by ligation of several branches of the left coronary artery. At 2 days, 7 days, and 6 weeks after infarction, and after determination of intracardiac pressures, papillary muscles were exicised from the noninfarcted segment of the right ventricle and attached to a myograph for analysis of contractile function. One week after infarction there was a decline in actively developed force at Lmax, caused by a decrease in the rate of force development. In addition, the response to procedures that augment myocardial contractility, such as paired stimulation and increasing the frequency of electrical stimulation, was significantly depressed. Two days after infarction, changes were less significant, although similar in direction. Six weeks after infarction, developed force at Lmax had returned to normal values. The response to procedures augmenting contractility also had returned to normal. There appears to be a distinct, reversible loss of contractility in the remaining viable myocardium in the early phase after experimental infarction.

Experimental myocardial infarction. XIII. Sequential changes in left ventricular pressure-length relationships in the acute phase

Diastolic pressure-length relationships of an ischemic region of the canine left ventricle were measured over a six-hour period following left anterior descending coronary artery ligation, and their evolution was compared with the extent of systolic aneurysmal bulging. Normalized ischemic segment length excursion, which after coronary artery ligation may be taken as a measure of systolic aneurysmal bulging, increased during the first hour after ligation but thereafter declined toward control values. Concurrently, reciprocal changes were demonstrated in the slope of the end-diastolic pressure-length curves obtained during transient pressure loading of the left ventricle. These data show that the magnitude of acute systolic aneurysmal bulging followed experimental coronary artery ligation is determined not only by loss of contractile function, but also by changes in passive pressure-length relationships of the myocardium. Moreover, the results indicate that development of akinesis in experimental ischemia, heretofore demonstrated only in the chronic phase of infarction, may begin within hours of the onset of myocardial ischemia.

Experimental myocardial infarction: XII. Dynamic changes in segmental mechanical behavior of infarcted and non-infarcted myocardium

The mechanical behavior of ischemic myocardium was studied in anesthetized open chest dogs. In each animal, a small well localized myocardial infarction was produced by ligation of a single ventricular branch of the left circumflex coronary artery. Serial in situ measurements of segment length were made by mercury-in-Silastic gauges sutured directly to the left ventricular surface. After coronary ligation, systolic aneurysmal bulging of the ischemic segment was uniformly noted. This was quantified as follows: normalized segment length change in this region, expressed in muscle lengths (where muscle lengths = phasic segment length amplitude/end-diastolic segment length), immediately increased from 0.06 +/- 0.01 (standard error of the mean) to 0.10 +/- 0.02 muscle lengths (+67 percent, P less than 0.02). Over a 6 hour period, muscle lengths progressively declined to near control values, but retained an aneurysmal contour. End-diastolic segment length increased 5 percent above control values after coronary occlusion and remained fixed at this level for 6 hours. In contrast, noninfarcted myocardium exhibited no significant changes in muscle length or end-diastolic segment length. These studies demonstrate that the degree of systolic aneurysmal bulging in infarcted myocardium, although initially great, resolves within 6 hours but retains an aneurysmal contour. These findings are consistent with either partial return of contractility or diminished local compliance, but persistence of an aneurysmal shape favors the latter mechanism. The fixed increase in end-diastolic segment length suggests that "stress-relaxation" takes place in the infarcted region. It is possible that diminished compliance in zones of infarction, previously noted after several days, begins within a few hours after the onset of ischemia.

Influence of dobutamine on regional myocardial blood flow and ventricular performance during acute and chronic myocardial ischemia in dogs

The data from this study document that dobutamine is a powerful inotropic agent in anesthetized dogs with acute myocardial ischemia and in awake, unsedated ones with chronic myocardial infarction. Dobutamine significantly increases heart rate at relatively small doses in anesthetized dogs with acute myocardial ischemia but considerably larger amounts of dobutamine are required to significantly increase heart rate in awake, unsedated dogs with myocardial infarction. Dobutamine also significantly increases regional myocardial blood flow to all areas of the heart at 20mug/kg/min in both anesthetized dogs with acute myocardial ischemia and awake, unsedated ones with myocardial infarction. However, in anesthetized dogs 20mug/kg/min of dobutamine significantly increases epicardial ST-segment elevation during acute myocardial ischemia. Propranolol prevents the inotropic and chronotropic effects of dobutamine in both anesthetized and awake, unsedated dogs. This study suggests that during experimental acute myocardial ischemia dobutamine given at doses that significantly increase heart rate and contractility may increase the extent of myocardial damage. The data also suggest that this agent should be of value in the setting of severe myocardial depression without associated severe coronary artery disease to increase cardiac contractility at doses that do not markedly alter heart rate. The hemodynamic and coronary blood flow effects of dobutamine in patients with and without severe coronary artery disease should be evaluated.

Biventricular dynamics during quantitated anteroseptal infarction in the porcine heart

The porcine heart has been shown to have close anatomic similarity to the human heart and was used as the experimental model in this study to gain further understanding of the early responses of both ventricles during acute anteroseptal myocardial infarction. High fidelity pressure and flow data were measured and multiple preejection and ejection variables were calculated for both ventricles. Infarct weight and distribution in both ventricles were quantitated. The standard infarction resulted from single stage ligation of the left anterior descending coronary artery just beyond its midpoint and second left ventricular branch. It comprised an average of 15.8 percent of total ventricular myocardium with an infarct/perfused ratio of 0.62 and a periinfarction transition zone of 7.5 mm, and involved significant portions of both ventricles and the interventricular septum. Performance characteristics of both ventricles were altered significantly by anteroseptal infarction and involved all phases of contraction--end-diastole, isovolumic systole and ventricular ejection. Although contractile alterations in the right ventricle were significant, they were somewhat delayed, yielding relatively low correlation coefficients with analogous left ventricular contractile indexes. These correlations became quite distinct during specific ventricular stresses. Comparison of anterolateral and anteroseptal infarction, matched in terms of infarct size, indicated that the right ventricular changes in the latter were related to direct involvement of the right ventricular free wall and septum rather than secondary to left ventricular alterations.

Left atrial transport function in myocardial infarction. Importance of its booster pump function

After myocardial infarction (MI), left ventricular (LV) end-diastolic pressure (EDP) is higher than mean pulmonary artery wedge pressure because of powerful atrial contraction. To evaluate the significane of atrial contraction to left ventricular function we studied 10 control (C) patients without cardiac disease and 17 patients from three to six weeks after acute myocardial infarction. Cardiac catheterization with simultaneous left ventricular diastolic pressure (DP) and left ventricular cineangiograms were obtained. Left ventricular volumes and pressure were (mean +/- SD): (SEE ARTICLE). Although left ventricular stroke volume was lower in the patients with myocardial infarction than in the control subjects (46 versus 56 ml/m2), atrial contraction contributed more to left ventricular filling during diastole (which is the same as left ventricular stroke volume) in the patients with myocardial infarction than in the controls (16 versus 10 ml/m2). The average atrial contribution to left ventricular end-diastolic volume was 11.9 per cent (C), 15.4 per cent (MI); to left ventricular end-diastolic pressure 20 per cent (C), 38.7 per cent (MI); and to left ventricular stroke volume 21.7 per cent (C), 35.1 per cent (MI). Atrial contribution to left ventricular stroke volume was 56 per cent in patients with a cardiac index less than or equal to 2.0 liters/min/m2 and 31 per cent in those with a cardiac index greater than 2 liters/min/m2 (p less than 0.01). Atrial contraction contributed 35 per cent to left ventricular stroke volume in patients with normal end-diastolic volume and in those with increased end-diastolic volume and 10 per cent to end-diastolic volume in patients with increased end-diastolic volume (p less than 0.001). In patients with myocardial infarction, atrial contraction made a large contribution to left ventricular filling and stroke volume irrespective of the type of left ventricular functional derangement that was present. The "booster pump" function of the atrium cannot be ignored in assessing left ventricular performance.

The influence of hypertonic mannitol on regional myocardial blood flow during acute and chronic myocardial ischemia in anesthetized and awake intact dogs

The influence of hypertonic mannitol on regional myocardial blood flow and ventricular performance was studied during acute myocardial ischemia in awake, unsedated and in anesthesized dogs and after myocardial infarction in awake unsedated dogs. Regional myocardial blood flow was measured with radioactive microspheres. Generalized increases in regional myocardial blood flow occurred after mannitol in all of the different animal models studied. The increases in coronary blood flow after mannitol were just as impressive in the nonischemic regions as in the ischemic portion of the left ventricle in all of the different models that were examined in this study. Improvement in regional myocardial blood flow to the ischemic area of the left ventricle after mannitol was associated with a reduction in ST segment elevation during acute myocardial ischemia in anesthetized dogs. The increases in regional myocardial flow after mannitol were also associated with increases in contractility, but the increases in flow appeared to be more impressive than the changes in contractility. The data obtained demonstrate that mannitol increases regional coronary blood flow to both ischemic and nonischemic myocardium in both anesthetized and awake, unsedated, intact dogs with acute and chronic myocardial ischemia and that mannitol reduces ST segment elevation during acute myocardial ischemia in anesthetized dogs. Thus the results suggest that under these circumstances the increases in regional myocardial blood flow after mannitol are of physiological importance in reducing the extent of myocardial injury. Since coronary blood flow increased to nonischemic regions the increases in regional myocardial flow demonstrated in this study after mannitol cannot be entirely explained by the mechanism of reduction in ischemic cell swelling.