James W. Covell

Factors Influencing Infarct Size Following Experimental Coronary Artery Occlusions

The purpose of this study was the determination of whether hemodynamic and pharmacologic factors influence the extent and severity of myocardial necrosis produced by coronary occlusion. In 48 dogs, 10 to 14 epicardial leads were recorded on the anterior surface of the left ventricle in the distribution and vicinity of the site of occlusion of a branch of the left anterior descending coronary artery. The average S-T segment elevation for each animal was determined at 5-min intervals after occlusion. This elevation was used as an index of the presence and severity of myocardial ischemic injury. The number of sites showing this elevation provided an additional measure of the size of the injured area. Occlusion alone raised the average S-T segment elevation from 0.22 ± 0.04 to 3.32 ± 0.37 mv (SEM). Isoproterenol, ouabain, glucagon, bretylium, and tachycardia given prior to a repeated occlusion increased the severity and extent of ischemic injury, while propranolol decreased it. Elevation of arterial pressure with methoxamine reduced the occlusion-induced S-T segment elevation, and lowering of the mean arterial pressure by hemorrhage had the opposite effect. In 19 additional experiments, propranolol, isoproterenol, and alterations in arterial pressure produced similar alterations in S-T segment elevation when these interventions were applied as long as 3 hr after ligation. In a third group of dogs, myocardial creatine phosphokinase (CPK) activity was determined 24 hr after occlusion at the same sites at which epicardial electrocardiograms were taken. Depression of myocardial CPK activity in injured portions of the left ventricle 24 hr after coronary artery ligation correlated well with S-T segment elevation in the same sites 15 min after ligation. Moreover, isoproterenol increased and propranolol decreased the area of depression of myocardial CPK activity. We conclude that the hemodynamic status and neurohumoral background at the time of occlusion and for up to 3 hr thereafter can alter the extent and severity of myocardial ischemic injury and myocardial necrosis.

Precordial S-T segment elevation mapping: An atraumatic method for assessing alterations in the extent of myocardial ischemic injury

Abstract A noninvasive technique for evaluating the extent of myocardial ischemic injury after experimental coronary artery occlusion was devised and applied to study alterations in the extent of injury produced by hemodynamic and pharmacologic interventions. The technique was then extended to the assessment of myocardial ischemic injury in patients with acute myocardial infarction. In 7 closed chest dogs, electrocardiograms were recorded from 15 sites on the chest wall before and after intermittent occlusions of the left anterior descending coronary artery. There was no S-T segment elevation before the occlusion; 15 minutes after occlusion the sum of S-T segment elevations (ΣS-T) averaged 15.0 ± 3.0 mm (SEM, 1 mm deflection = 0.1 mv), and an average of 4.2 ± 0.6 sites exhibited elevations exceeding 0.1 mv (NS-T). Occlusions occurring during administration of isoproterenol (0.25 μg/kg per min) increased ΣS-T to 51.0 ± 9.0 mm and NS-T to 10.6 ± 0.9, whereas occlusions occurring after administration of propranolol (1 mg/kg) decreased ΣS-T to 3.0 ± 1.5 mm and NS-T to 0.2 ± 0.2. In 8 dogs the extent of ischemic injury, manifested by S-T segment changes, was decreased by propranolol and norepinephrine and increased by hemorrhagic hypotension and isoproterenol, applied up to 6 hours after occlusion. Reproducible S-T segment maps, using 35 surface electrodes, were obtained in 19 patients with acute myocardial infarction. In 15 patients studied serially, ΣS-T decreased from 54.25 ± 7.00 to 38.50 ± 6.30 mm and NS-T from 18.7 ± 2.5 to 12.3 ± 2.8, respectively, during a 24 hour period. However, in 3 patients in whom ventricular fibrillation, arterial hypotension and further ischemic pain occurred, ΣS-T and NS-T increased whereas in another patient propranolol decreased ΣS-T and NS-T. Thus, precordial mapping, both in dogs and patients, shows changes parallel to those measured by the epicardial technique and should provide a useful clinical tool for determining acute changes in the extent of ischemic injury.

Transverse Shear Along Myocardial Cleavage Planes Provides a Mechanism for Normal Systolic Wall Thickening

Recent studies in humans and other species show that there is substantial transverse shear strain in the left ventricular myocardium, and others have shown transverse myocardial laminae separated by cleavage planes. We proposed that cellular rearrangement based on shearing along myocardial cleavage planes could account for > 50% of normal systolic wall thickening, since < 50% can be explained by increases in myocyte diameter. To test this hypothesis, we measured strains at two sites with different cleavage-plane anatomy in eight open-chest dogs. Columns of radiopaque markers were implanted in the left ventricular anterior free wall and septum. Markers were tracked with biplane cineradiography, and strains were quantified by using finite deformation techniques. Hearts were perfusion-fixed with glutaraldehyde, and cleavage-plane orientations at the bead sites were measured in three orthogonal planes. At subendocardial sites of the anterior left ventricular wall, where the cleavage planes approach the endocardium obliquely from the apical side of the surface normal in the longitudinal-radial plane (-67 +/- 11 degrees), systolic longitudinal-radial transverse shear (E23) was positive (0.14 +/- 0.08). At the septal sites where the subendocardial cleavage planes approach the endocardium obliquely from above the surface normal (44 +/- 12 degrees), E23 was negative (-0.12 +/- 0.08). The differences in cleavage-plane angle and E23 at the two sites were each highly significant (P < .0005). At both sites, the transverse shear strain accompanied substantial systolic wall thickening at the subendocardium (anterior, E33 = 0.44 +/- 0.16; septum, E33 = 0.22 +/- 0.14). These data are not representative of the behavior in midwall and outer wall sites, where cleavage-plane orientation was not consistently different between anterior left ventricle and septum. Our data indicate that rearrangement of myocytes by slippage along myocardial cleavage planes is in the correct direction and of sufficient magnitude in the subendocardium (inner third) to account for a substantial proportion (> 50%) of systolic wall thickening. Furthermore, three-dimensional reconstruction of the myocardial laminae and local comparison with maximum strain vectors indicate that for the inner third of the ventricular wall the maximum shear deformation is a result of relative sliding between myocardial laminae.

Effect of Glucose-Insulin-Potassium Infusion on Myocardial Infarction following Experimental Coronary Artery Occlusion

The effects of glucose-insulin-potassium (GIK) infusion and glucose (G) infusion started 30 min after experimental coronary occlusion and the combination of GIK and propranolol (P) started 3 hours after coronary occlusion on the development of myocardial infarction were studied in 37 dogs. Fifteen minutes after the coronary occlusion, epicardial electrocardiograms were recorded at 10-15 sites; 24 hours later transmural specimens were obtained from the same sites for determination of myocardial creatine phosphokinase (CPK) activity and the evaluation of morphologic changes. In the control group (normal saline infusion) the relationship between S-T-segment elevation (mv) 15 min after occlusion and CPK activity (IU/mg of protein) 24 hours later was: log CPK = −0.064 S-T + 1.24; r = 0.81. In the GIK group, the infusion was begun 15 min following epicardial mapping, and sites with the same S-T-segment elevations showed less CPK depression than did the control group: log CPK = −0.022 S-T + 1.25. The G group also showed less CPK depletion than the control group but to a somewhat lesser extent than the GIK group (log CPK = −0.030 S-T + 1.20). The group receiving GIK and P 3 hours after occlusion also showed less CPK depression than did the control group (log CPK = −0.034 S-T + 1.26). Histologic analysis in 24-hour specimens showed that sites which exhibited S-T-segment elevation 15 min after occlusion showed normal histology in 3% of specimens obtained from control dogs, while the other 97% showed early signs of myocardial infarction. However, in the GIK group, 36% of the specimens with S-T-segment elevation prior to the infusion were histologically normal 24 hours later, while in the G group 30% were normal, and in the GIK and P group 17% were normal. Electron microscopy confirmed the morphologic changes observed by light microscopy. Thus, in the presence of experimental coronary occlusion, GIK exerts a protective effect against myocardial ischemia and reduces the extent of myocardial necrosis. G alone acts similarly but to a lesser degree, while a beneficial effect can also be demonstrated when GIK and P are started 3 hours after the onset of coronary occlusion.

Laminar fiber architecture and three-dimensional systolic mechanics in canine ventricular myocardium

Previous studies suggest that the laminar architecture of left ventricular myocardium may be critical for normal ventricular mechanics. However, systolic three-dimensional deformation of the laminae has never been measured. Therefore, end-systolic finite strains relative to end diastole, from biplane radiography of transmural markers near the apex and base of the anesthetized open-chest canine anterior left ventricular free wall (n = 6), were referred to three-dimensional laminar microstructural axes reconstructed from histology. Whereas fiber shortening was uniform [-0.07 +/- 0.04 (SD)], radial wall thickening increased from base (0. 10 +/- 0.09) to apex (0.14 +/- 0.13). Extension of the laminae transverse to the muscle fibers also increased from base (0.08 +/- 0. 07) to apex (0.11 +/- 0.08), and interlaminar shear changed sign [0. 05 +/- 0.07 (base) and -0.07 +/- 0.09 (apex)], reflecting variations in laminar architecture. Nevertheless, the apex and base were similar in that at each site laminar extension and shear contributed approximately 60 and 40%, respectively, of mean transmural thickening. Kinematic considerations suggest that these dual wall-thickening mechanisms may have distinct ultrastructural origins.Previous studies suggest that the laminar architecture of left ventricular myocardium may be critical for normal ventricular mechanics. However, systolic three-dimensional deformation of the laminae has never been measured. Therefore, end-systolic finite strains relative to end diastole, from biplane radiography of transmural markers near the apex and base of the anesthetized open-chest canine anterior left ventricular free wall ( n = 6), were referred to three-dimensional laminar microstructural axes reconstructed from histology. Whereas fiber shortening was uniform [-0.07 ± 0.04 (SD)], radial wall thickening increased from base (0.10 ± 0.09) to apex (0.14 ± 0.13). Extension of the laminae transverse to the muscle fibers also increased from base (0.08 ± 0.07) to apex (0.11 ± 0.08), and interlaminar shear changed sign [0.05 ± 0.07 (base) and -0.07 ± 0.09 (apex)], reflecting variations in laminar architecture. Nevertheless, the apex and base were similar in that at each site laminar extension and shear contributed ∼60 and 40%, respectively, of mean transmural thickening. Kinematic considerations suggest that these dual wall-thickening mechanisms may have distinct ultrastructural origins.

Control of myocardial oxygen consumption: relative influence of contractile state and tension development

Myocardial oxygen consumption was measured in 11 anesthetized, open-chest dogs in order to compare in the same heart the relative influence on oxygen usage of tension development and the contractile or inotropic state, as reflected in V(max.) the maximum velocity of shortening of the unloaded contractile elements. The isovolumetrically contracting left ventricle was studied with left ventricular volume, heart rate, and systemic perfusion rate controlled. Wall tension, contractile element velocity, and V(max) were calculated. Peak developed tension was increased at a constant V(max) by increasing ventricular volume, and the effect on oxygen consumption was determined. Oxygen utilization was then redetermined at an increased V(max) but at a constant peak developed tension by infusing norepinephrine (0.76 to 7.6 mug/min) and decreasing ventricular volume to match the tension existing before norepinephrine infusion. Oxygen consumption consistently increased with increases in both developed tension and V(max) with the following multiple regression equation relating these variables: myocardial oxygen consumption (mul/beat per 100 g in LV) = K + 0.25 peak developed tension (g/cm(2)) + 1.43 V(max) (cm/sec). These data indicate that the oxygen cost of augmentation of contractility is substantial, can be independent of any change in fiber shortening, and is similar in order of magnitude to the effect of alterations in tension development

Coronary Artery Reperfusion: II. REDUCTION OF MYOCARDIAL INFARCT SIZE AT 1 WEEK AFTER THE CORONARY OCCLUSION

The question of whether or not the size of an area of myocardial infarction, measured at 1 wk after coronary occlusion, can be influenced by coronary artery reperfusion was examined in dogs. In seven control experiments the anterior descending coronary artery was ligated, while in seven other studies the occlusion was released after 3 hr. In all animals calibrated photographs were used to assess the zone of hypoperfusion and the acutely injured area of epicardial ST segment elevation, as well as the extent of damage at postmortem 1 wk later. In control dogs, the gross infarct size at postmortem averaged 63.8+/-7.3% of that predicted from the acutely injured zone. However, in reperfused hearts the average gross infarct size at 1 wk was only 10.2+/-4.4% of that predicted. Transmural specimens were obtained at autopsy for histology and measurement of myocardial creatine phosphokinase (CPK) activity from sites initially used for epicardial electrocardiography. In control animals, there was a direct relationship between the degree of ST segment elevation and the degree of cell necrosis in transmural histologic sections. ST segment elevation also predicted myocardial CPK (international units per milligram protein): log CPK = - 0.0613 ST + 1.17 (r = 0.66, n = 56 sites). In the reperfused animals, log CPK = - 0.166 ST + 1.36 (r = 0.69, n = 46 sites) showing almost complete preservation of CPK activity at 1 wk, sparing being most prominent in the epicardial zone. Similarly, there was a good correlation between myocardial CPK activity and the histological assessment of cell destruction, the degree of cell damage = - 0.152 CPK + 3.86 (r = 0.86; n = 102 sites). Thus, control dogs showed severe myocardial CPK depletion and histologic evidence of extensive cell destruction, whereas animals subjected to coronary artery reperfusion had little CPK depletion and much less evidence of myocardial cell necrosis 1 wk later.

Assessment of Cardiac Contractility The Relation Between the Rate of Pressure Rise and Ventricular Pressure During Isovolumic Systole

It was considered that the relationship between dp/dt and simultaneously developed pressure during the course of isovolumic contraction might afford a more accurate measure of contractility than the maximum rate of intraventricular pressure rise (peak dp/dt). In six cat papillary muscles contracting isometrically from any given preload, the ratio of the rate of tension development (dT/dt) to simultaneously occurring isometric tension always varied directly with the contractile state. This ratio rose slightly as preload was increased, but it was not affected by changes in afterload. In 17 experiments in an intact canine heart preparation in which left ventricular end-diastolic pressure was constant, dp/dt at any given pressure during isovolumetric contraction again was observed to be a function of the contractile state when the latter was enhanced by norepinephrine and acetylstrophanthidin. High-fidelity left ventricular pressures and dp/dt were recorded throughout isovolumic contraction in eight patients. Isoproterenol and exercise raised the level of dp/dt at any given pressure prior to ejection. Interventions known to alter ventricular loading but not to influence the contractile state, such as elevation of ventricular end-diastolic pressure by leg raising and increases in aortic pressure induced by methoxamine, did not influence this relation significantly. In conclusion, the determination of dp/dt and intraventricular pressure throughout isovolumic contraction in the presence of variable arterial pressure and small changes of preload provides a useful, simple, and experimentally verified approach to the assessment of alterations of the contractile state of the heart in intact man.

Diastolic Geometry and Sarcomere Lengths in the Chronically Dilated Canine Left Ventricle

In 10 dogs left ventricular geometry and ultrastructure were examined 3 to 12 weeks after chronic ventricular dilatation was induced by means of a large arteriovenous shunt. Following cardiac catheterization, the hearts were arrested and fixed in diastole at the left ventricular end-diastolic pressures (LVEDP) which existed in the beating heart. Transmural LVEDP were increased in all animals (avg. 27 mm Hg). The diastolic pressure-volume relationship was shifted to the right, the average end-diastolic volume of the chronically dilated ventricles (avg. 103 ml) being larger than that of previously studied acutely dilated ventricles (avg. 72 ml, P<.01), but calculated diastolic wall stress values were not different. Sarcomere lengths in the chronically dilated hearts averaged 2.19±.02 μ (SE) (range 2.11 to 2.27), a value near the apex of the sarcomere length-tension curve, but not significantly different from sarcomere lengths in acutely dilated ventricles. Slippage between myofibrils, reflected by a loss of normal alignment of the Z lines, appeared to be one mechanism underlying this adaptation. The findings indicate that sarcomeres in the canine left ventricular wall are remarkably resistant to chronic as well as to acute overstretch. They further imply that little or no Frank-Starling reserve mechanism was available in these ventricles and raise the possibility that a descending limb of the hearts performance as a pump need not reflect a descending limb at the sarcomere level.

Left Ventricular Performance Following Correction of Free Aortic Regurgitation

Left ventricular (LV) myocardial contractility, or inotropic state, was characterized in terms of the instantaneous relations between velocity of circumferential fiber shortening (VCF), determined cineangiographically, and LV wall tension (hoop stress), calculated from LV dimensions and pressure, in five patients before and 7 to 10 mo after aortic valve replacement for free aortic regurgitation. Preoperatively the cardiac index was reduced or the LV end-diastolic pressure was markedly increased (or both occurred) in four patients, in each of whom depression of inotropic state was documented by a reduced VCF at maximum wall tension, ranging from 0.13 to 1.07 circumferences (circ)/sec (normal, > 1.40 circ/sec) at wall tensions of 318 to 464 g/cm2 (normal, 178 to 417 g/cm2). In one patient in whom LV end-diastolic pressure and cardiac index were normal preoperatively, VCF was 1.66 circ/sec at a maximum tension of 440 g/cm2. Following operation, LV end-diastolic pressure fell in the four patients with depressed inotropic state (average decrease, 23 mm Hg) and cardiac index increased (average increase, 0.93 L/min/m2). However, no change in the tension-velocity relation was observed postoperatively, VCF ranging from 0.27 to 1.14 circ/sec in these patients, indicating that no change had occurred in the inotropic state. In addition, a fixed abnormality in diastolic LV pressure-volume characteristics, determined from preoperative and postoperative measurements of pressure and radius during diastole, had occurred in patients with depressed myocardial function. In contrast, in the patient with normal myocardial function, LV end-diastolic radius was reduced by 33% postoperatively while end-diastolic pressure was unchanged, suggesting reversal of stress relaxation, or creep, following relief of volume overload.