Effects of myocardial revascularization following acute coronary occlusion in pigs

Reappearance of myocytes in ovine infarcts produced by six hours of complete ischemia followed by reperfusion

BACKGROUND

In this study we tested the hypothesis that delayed reperfusion of ischemic myocardium-too late to save myocytes-attenuates infarct expansion and improves collagen synthesis.

METHODS

The hypothesis was tested in a sheep model of anteroapical infarction that has no collateral blood flow to the area at risk. After coronary ligation or arterial occlusion for 1 or 6 hours, sheep had serial hemodynamic and quantitative echocardiographic studies before and after infarction and 2, 5, 8, and 12 weeks later. Hearts were examined by light and electron microscopy at 2 and 12 weeks; hydroxyproline and ratios of type I/III collagen were measured at 12 weeks.

RESULTS

After coronary occlusion, left ventricular (LV) function progressively decreased and size increased to form an anteroapical aneurysm. After 1 hour of ischemia, neither resting LV size nor function changed; the infarct contained a midmyocardial scar between epicardial and endocardial muscle. After 6 hours of ischemia, LV function was significantly better than that in nonperfused sheep. Two weeks after 6 hours of ischemia, no viable myocytes were visible by light microscopy, but electron micrographs showed rare intact nucleated myocytes with scarce cytoplasmic myofibrils. At the 12th week epicardial and endocardial myocytes reappeared in the infarct. Infarct collagen type I/III ratios were 1.2 in reperfused groups and 0.7 in nonperfused sheep.

CONCLUSIONS

Delayed reperfusion causes loss and subsequent reappearance of ovine epicardial myocytes, improves collagen type I/III ratios, and attenuates LV dilatation and loss of function. One hypothesis to explain the reappearance of myocytes is that reperfusion partially reverses an incomplete apoptotic process.

Benefits of late coronary artery reperfusion on infarct expansion progressively diminish over time: relation to viable islets of myocytes within the scar

To define the time limit and mechanism of the effects of late coronary artery reperfusion on infarct expansion, rats were randomized into one of four groups: permanent left coronary artery occlusion; and 2, 8, and 16 hours of left coronary artery occlusion followed by reperfusion. Two weeks after coronary occlusion, morphometric and histologic analyses were performed. Benefits of late reperfusion on infarct expansion progressively diminished after increasingly long periods of coronary occlusion and were minimal but present after 16 hours of coronary occlusion. The extent of the benefits of late reperfusion on infarct expansion were related to preservation and hypertrophy of small islets of still viable myocytes located mainly in the subepicardium of the scar.

Aspirin enhances the benefits of late reperfusion on infarct shape. A possible mechanism of the beneficial effects of aspirin on survival after acute myocardial infarction

BACKGROUND

The time window of the benefits of late reperfusion on infarct shape is limited. In rats, these benefits diminish in a wave front over time, with minimal benefits when reperfusion follows 16 hours of coronary occlusion. The mechanism of the benefits of aspirin on survival after acute myocardial infarction is unknown. The purpose of this study was to test the ability of aspirin to enhance the benefits of late coronary artery reperfusion on infarct shape and to examine the mechanism of the benefits of aspirin on infarct shape.

METHODS AND RESULTS

Rats were entered into two different protocols, the morphometric and the histological protocols. In the morphometric protocol, rats were randomized into two groups: the aspirin group, in which rats underwent left coronary artery occlusion followed by treatment with aspirin (12 mg/kg i.v.), and the control group, in which rats underwent left coronary artery occlusion followed by treatment with placebo. Rats in both groups were reperfused 8 hours after coronary occlusion. Rats in the aspirin group received aspirin in the drinking water (12 +/- 2 mg/kg daily). Morphometric analysis was performed 2 weeks after acute myocardial infarction. In the histological protocol, rats underwent the same randomization, coronary occlusion, and reperfusion protocols. Hearts were removed 24 hours after coronary occlusion, and microvessels were assessed for patency. Infarct size expressed as a percent of circumference was similar in the aspirin and placebo treatment groups (28 +/- 2% versus 33 +/- 3%, P = NS). Septal thickness was also similar in both groups (1.8 +/- 0.1 versus 2.1 +/- 0.1 mm, P = NS for aspirin versus placebo). The aspirin-treated group had thicker infarcts compared with the placebo-treated group (0.8 +/- 0.1 versus 0.5 +/- 0.1 mm, P < .05) and less expanded infarcts (expansion index, 1.2 +/- 0.1 versus 2.0 +/- 0.2, P < .05). Aspirin was associated with increased patency of the microvessels in the infarcted area compared with the placebo group (96% versus 64% of microvessels patent, P < .001).

CONCLUSIONS

Aspirin enhances the benefit of late coronary artery reperfusion on infarct shape after 8 hours of coronary occlusion. The benefits of aspirin on infarct shape after late reperfusion are related to increased patency of the microvessels in the infarcted area.

Ventricular remodeling and infarct expansion

Infarct expansion, defined as an alteration in the ventricular topography due to thinning and lengthening of the infarcted segment, develops within the first few hours of the acute symptoms, mostly in patients with a large, transmural, anterior myocardial infarction. Shape changes, peculiar to risk region location and due to disparity in regional ventricular architecture, could be posited as the first step in the process of infarct expansion, with various cellular mechanisms contributing to subsequent continued early and late ventricular dilation. Because the increase in left ventricular volume is expected to be linearly dependent on the extent of the infarction, limiting infarct size, by thrombolysis, would proportionally reduce enlargement of the cavity. The effect of thrombolysis on left ventricular volume, however, seems not to be completely accounted for by the lessening effect of reperfusion on infarct size, because data suggest a restraining effect of reperfusion on the process of ventricular dilation in addition to the lessening effect on infarct size. If this turns out to be true, then the achievement of a patent vessel even beyond the time period when that patency may be expected to salvage myocardium would be further justified. Theoretical predictions substantiate the potential effectiveness in restraining ventricular dilation of stiffening of the necrotic region alone, independently of myocardial salvage in infarcted patients. The process of progressive ventricular dilation involves not only a primary alteration in function of the infarcted region, but also a time-dependent secondary change in the noninfarcted tissue itself, finalized to restore stroke volume despite a persistently depressed ejection fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Late reperfusion for acute myocardial infarction limits the dilatation of left ventricle without the reduction of infarct size

BACKGROUND

While previous clinical studies have shown a possible beneficial effect of the reperfusion performed at a relatively late phase of acute myocardial infarction ("late reperfusion") in preventing left ventricular enlargement, the mechanism has not been clarified.

METHODS AND RESULTS

Of 89 patients with an initial anterior myocardial infarction, reperfusion was successful in 69. These 69 were divided into three groups according to the time required to achieve reperfusion after the onset of symptoms: early-reperfused (< 3 hours from the onset to reperfusion; n = 22), intermediate-reperfused (3 to 6 hours from the onset to reperfusion; n = 28), and late-reperfused (> 6 hours from the onset to reperfusion; n = 19). The 20 patients whose infarct-related artery were occluded in the acute phase as well as 1 month later was classified as nonreperfused. Infarct size, evaluated as defect volume by 201Tl single-photon emission computed tomography 1 month after the onset, was 1593 +/- 652 units (mean +/- SD) in the late-reperfused group, significantly larger (P < .05) than that of the intermediate-reperfused (1066 +/- 546 U) or the early-reperfused groups (372 +/- 453 U) but not different from that of the nonreperfused group (1736 +/- 562 U). Wall motion abnormality index as well as global ejection fraction evaluated by left ventriculography 1 month after the onset showed that late reperfusion did not preserve the left ventricular wall motion and function. These results indicate that the earlier reperfusion decreased the size of the infarction and preserved left ventricular function, whereas late reperfusion (> 6 hours after onset) did not limit infarct size or preserve left ventricular function. In contrast, the end-diastolic volume index did not differ significantly among the early-reperfused (50 +/- 15 mL/m2), intermediate-reperfused (54 +/- 14 mL/m2), and late-reperfused (53 +/- 19 mL/m2) groups; those were significantly smaller than that of the nonreperfused group (68 +/- 12 mL/m2; P < .05). Left ventriculographic data obtained in both the acute and chronic phase in 39 patients showed that left ventricular volumes increased significantly during the course of myocardial infarction only in the nonreperfused group.

CONCLUSIONS

Late reperfusion appeared to prevent ventricular dilatation acute myocardial infarction independent of the limitation of infarct size.

Conversion of induced ventricular tachycardia by single and serial chest thumps: a study in domestic pigs 1 week after experimental myocardial infarction

A single chest thump (CT) is widely accepted in the emergency treatment of ventricular asystole, whereas there exists controversy about this method for the interruption of ventricular tachycardia (VT). Hitherto, delivering serial chest thumps (SCTs) has been described only once for the treatment of VT. A systematic analysis for interruption of VT by CT or SCTs (or both) is lacking. We have therefore investigated this subject in five domestic pigs after experimental myocardial infarction. Manual conversion was attempted in 20 induced VTs. Six VTs were converted by CT, seven VTs were converted by the first SCTs, and six VTs were converted by the last of multiple (two to seven) SCTs. The overall success was 95%. There were no serious complications. The rate of successful SCTs exceeded the rate of VT by 10% to 126%. The technique of SCTs, the mechanisms of manual conversion, and the controversial opinions regarding the value of precordial thumping in asystole, VT, and ventricular fibrillation are discussed. SCTs should be practiced only very cautiously in patients until further experience is available.

Limitation of myocardial infarct expansion by reperfusion independent of myocardial salvage

Very early reperfusion after coronary occlusion can reduce infarct size and preserve left ventricular function. Whether later reperfusion is of benefit is unclear. We studied the effect of very early reperfusion with myocardial salvage and "late" reperfusion without myocardial salvage on infarct expansion and aneurysm formation. Sixty-eight rats underwent left coronary artery ligation and were randomized to 30 min reperfusion, 2 hr reperfusion, or permanent coronary artery ligation. The animals were killed and the hearts examined at 2 weeks. Thirty minute reperfusion reduced infarct size, extent of transmurality, and infarct expansion. Two hour reperfusion reduced neither infarct size nor transmurality but inhibited infarct expansion. The results of this study in the rat preparation suggest a beneficial effect of "late" reperfusion on infarct expansion independent of myocardial salvage. If these results are confirmed in human beings, the period after myocardial infarction during which a patient can be considered for thrombolytic therapy or reperfusion by percutaneous transluminal coronary angioplasty could be greatly prolonged.

Effects of pindolol therapy on the size of experimentally induced myocardial infarction in the pig

The effect of pindolol on experimental myocardial infarction was studied in a pig model. Intravenous application of 0.05 mg pindolol per kg body weight was initiated one hour after coronary ligation and repeated at 12-hour intervals for five days. No significant difference in infarct size could be found between pindolol-treated animals (20.4 +/- 0.6% SEM of whole ventricular mass, n = 6) and untreated controls (20.5 +/- 1.2% SEM, n = 9). Hemodynamic data did not change significantly throughout the experiment. These results differ in part from those reported by other investigators: The disagreement may be due to the specific pharmacological properties of the applied drug, to variations in the dosages of beta blockers, as well as to differences in the study design.

Coronary thrombolysis and infarct size reduction after intravenous infusion of recombinant tissue-type plasminogen activator in nonhuman primates

Occlusive thrombus was produced by thrombin-induced coagulation in the left anterior descending coronary artery (LAD) of 16 open-chest baboons. In six control animals, occlusive thrombosis persisting over a period of 4 h as evidenced by coronary arteriography resulted in large transmural infarction (63.1 +/- 3.5% of the perfusion area). In 10 animals, tissue-type plasminogen activator obtained by recombinant DNA technology (rt-PA) was infused systemically at a rate of 1,000 IU (10 micrograms)/kg per min for 30 min after 30-80 min of coronary thrombosis. Reperfusion occurred within 30 min in nine animals. In one animal, intravenous infusion was followed by an intracoronary infusion at the same rate, which resulted in thrombolysis within 8 min. In the rt-PA group, mean duration of occlusion before reperfusion was 77 +/- 24 min. Reocclusion occurred in one animal. Recanalization resulted in an overall reduction of infarct size (37.8 +/- 5.9%, P less than 0.05 versus controls). Residual infarction was related to the duration of occlusion (r = 0.80, P less than 0.01). Reperfusion was associated with reduced reflow. Myocardial blood flow in the perfusion area of the LAD was only 70% of normal after 4 h despite perfect angiographic refilling. The infusion of rt-PA was not associated with systemic activation of the fibrinolytic system, fibrinogen breakdown, or clinically evident bleeding. It is concluded that intravenous infusion of rt-PA may recanalize thrombosed coronary vessels without inducing systemic lysis. The extent of residual infarction is closely related to the duration of coronary artery occlusion before thrombolysis.

Consequences of myocardial reperfusion following temporary coronary occlusion in pigs; effects on morphologic, biochemical and haemodynamic findings

The effects of myocardial reperfusion have been examined following a 1 h coronary occlusion and compared to a permanent coronary ligation in pigs. Haemodynamic investigations were carried out throughout the surgical intervention and repeated after 7 days. Cellular injury was estimated by serial serum enzyme determinations (creatin phosphokinase, alpha-hydroxybutyric dehydrogenase, aspartate aminotransferase, lactic dehydrogenase) during the first 5 postoperative days; infarct size was assessed morphometrically by a histochemical staining procedure 1 week after the temporary or permanent coronary occlusion. A linear correlation was found between the logarithmically plotted peak serum activity of AST, HBDH, CPK and the morphometrically determined infarct size. Based upon enzyme and morphometrical studies no significant difference could be detected between the two experimental groups. In the animals subjected to transient coronary occlusion, however, the development of a ventricular aneurysm had been prevented to early and sustained reperfusion. Early re-establishment of coronary circulation appears to accelerate the proliferation of a more resistant granulation tissue into the infarcted area. Cardiac performance was not improved by myocardial reperfusion.