Mohamed O. Jeroudi

Quantification of myocardial infarction during coronary occlusion and myocardial salvage after reperfusion using cardiac imaging with technetium-99m hexakis 2-methoxyisobutyl isonitrile

Myocardial imaging with technetium-99m hexakis 2-methoxyisobutyl isonitrile was investigated as a means to assess myocardial infarct size during coronary occlusion and to quantify the extent of salvaged myocardium after coronary occlusion followed by reperfusion. Open chest dogs underwent either a permanent coronary artery occlusion (Group 1, n = 16) or a 2 h occlusion followed by reperfusion (Group 2, n = 15). Animals in both groups were killed 48 h after occlusion. During coronary occlusion, 23 of the 25 dogs that survived the coronary occlusions had abnormal myocardial scintigrams. The scintigraphic perfusion defect size correlated well with the pathologic infarct size (r = 0.85 and 0.95 by planar and tomographic imaging, respectively). The planar scintigraphic defect size, but not the tomographic defect size, overestimated the pathologic size. The planar scintigraphic defect size observed during coronary occlusion was markedly reduced 48 h after reperfusion (24.8 +/- 12.8% to 10.6 +/- 9.7% of the left ventricle, p less than 0.003). The uptake of technetium-99m hexakis 2-methoxyisobutyl isonitrile in the ischemic myocardium increased significantly 48 h after reperfusion (p less than 0.003) and correlated with the increase in regional myocardial blood flow, as assessed by radioactive microspheres (r = 0.83, p less than 0.01). Thus, myocardial imaging with technetium-99m hexakis 2-methoxyisobutyl isonitrile allows reliable demonstration of the presence of acute infarction, estimation of infarct size and quantification of the extent of salvaged myocardium after coronary reperfusion.

Demonstration of free radical generation in the "stunned" myocardium in the conscious dog and identification of major differences between conscious and open-chest dogs.

Conscious dogs undergoing a 15-min coronary occlusion were given alpha-phenyl N-tert-butyl nitrone (PBN) and the local coronary venous plasma was analyzed by electron paramagnetic resonance spectroscopy. A prolonged myocardial release of PBN radical adducts was observed, which exhibited a burst in the initial minutes of reflow (peaking at 3 min) and then abated but continued for 1-3 h after reperfusion. Computer simulation revealed the presence of at least two PBN adducts (aN = 15.2 G and a beta H = 6.0 G; aN = 14.6 G and a beta H = 3.0 G), both consistent with the trapping of secondary carbon-centered radicals. No appreciable PBN adduct production was observed when collateral flow exceeded 30-40% of nonischemic flow, indicating that a flow reduction of at least 60% is necessary to trigger free radical reactions. There was a direct relationship between the magnitude of PBN adduct production and the severity of contractile dysfunction (r = 0.77), suggesting that the radicals generated upon reperfusion play a causal role in the subsequent stunning. The total release of PBN adducts after 3 h of reperfusion following a 15-min coronary occlusion was found to be approximately five times greater in open-chest compared with conscious dogs; at the same time, the recovery of wall thickening was markedly less in open-chest dogs. This study represents the first application of spin trapping to a conscious animal model of myocardial ischemia. The results demonstrate (a) that free radicals are generated in the stunned myocardium in the absence of the artificial or abnormal conditions associated with previously used models (isolated hearts, open-chest preparations), and (b) that both the severity of postischemic dysfunction and the magnitude of the attendant free radical production are greatly exaggerated in the open-chest dog, implying that previous conclusions derived from this model may not be applicable to conscious animals or to humans. This investigation also provides a method to measure free radicals in awake animals.

Augmentation of endogenous adenosine attenuates myocardial 'stunning' independently of coronary flow or hemodynamic effects.

BackgroundMounting evidence suggests a protective effect of exogenous adenosine in myocardial ischemia and reperfusion. We tested the hypothesis that augmentation of endogenous adenosine levels, achieved by inhibiting adenosine catabolism and washout, is beneficial in postischemic myocardial dysfunction (“stunning”). Methods and ResultsIn phase I of the study, open-chest dogs undergoing a 15-minute coronary artery occlusion and 4 hours of reperfusion received an intracoronary infusion of either saline (controls, n=23) or 6-(4-nitrobenzyl)-mercapto: purine ribonucleoside (NBMPR, a selective nucleoside transport inhibitor) combined with erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, a potent adenosine deaminase inhibitor) (EHNA+NBMPR, n=15) starting 15 minutes before coronary occlusion and ending 15 minutes after the initiation of reflow. Regional myocardial function (assessed as systolic wall thickening) was similar in control and treated groups at baseline and during ischemia. After reperfusion, however, the dogs treated with EHNA+NBMPR exhibited a significant improvement in the recovery of function, which was evident as early as 30 minutes after restoration of flow and was sustained throughout the rest of the reperfusion phase. The enhanced recovery effected by EHNA+NBMPR could not be attributed to nonspecific factors such as differences in collateral flow during occlusion, coronary flow after reperfusion, arterial pressure, heart rate, or other hemodynamic variables. In phase II of the study, the myocardial content of adenine nucleotides and nucleosides was measured by high performance liquid chromatography in myocardial biopsies obtained serially from open-chest dogs undergoing the same protocol used in phase I. There were no significant differences between control (n=8) and treated (n=9) dogs with respect to myocardial levels of adenosine triphosphate (ATP) at 30 and 60 minutes after reperfusion, indicating that the beneficial effects of EHNA+NBMPR cannot be ascribed to repletion of ATP stores. Compared with controls, dogs treated with EHNA+NBMPR exhibited a much larger increase in myocardial adenosine (6.07±1.47 vs 1.03±0.16 nmol/mg protein, P<.05) and a much smaller increase in inosine (0.52±0.27 vs 3.04±0.54 nmol/mg protein, P<.05) at the end of ischemia, such that the inosine-to-adenosine ratio noted in controls was completely reversed (=6:1 vs =1:6, respectively). In the treated group, adenosine levels remained markedly increased compared with controls up to 1 hour after reperfusion. ConclusionsThis study demonstrates that (1) administration of an adenosine deaminase inhibitor plus a nucleoside transport blocker is remarkably effective in augmenting myocardial adenosine levels during regional ischemia and subsequent reperfusion in vivo, (2) this augmentation of adenosine results in a significant and sustained attenuation of myocardial stunning, and (3) the attenuation of stunning is not due to ATP repletion or to nonspecific actions on hemodynamic variables or coronary flow. These findings suggest that endogenous adenosine production during ischemia serves as an important pathophysiological mechanism that protects against myocardial stunning. The results also suggest that augmentation of endogenous adenosine (without exogenous adenosine administration) represents an effective therapeutic approach to the alleviation of reversible postischemic dysfunction.

Effect of Adenosine on Myocardial ‘Stunning’ in the Dog

Recent evidence suggests a cardioprotective effect of adenosine in myocardial ischemia and reperfusion. The present study was undertaken to determine (1) whether adenosine attenuates myocardial stunning, (2) if so, whether the beneficial effect of adenosine takes place during ischemia or after reperfusion, and (3) whether adenosine preconditions against myocardial stunning. A total of 93 dogs were used. In phase A of the study, open-chest dogs undergoing a 15-minute occlusion of the left anterior descending coronary artery followed by 4 hours of reperfusion received an intracoronary infusion of either saline (group I [control], n = 14), 2 mg/min adenosine from 30 minutes before occlusion until 1 hour after reperfusion (group II, n = 10), or 2 mg/min adenosine from 2 minutes before reperfusion until 1 hour after reperfusion (group III, n = 11). Regional myocardial function (assessed as systolic wall thickening) was similar in the three groups at baseline and during ischemia. After reperfusion, dogs treated with adenosine before, during, and after ischemia (group II) demonstrated a significant improvement in the recovery of function that persisted throughout the 4 hours of reperfusion. In contrast, in dogs treated only during the reperfusion period (group III), the recovery of function was not statistically different from that in control dogs. The enhanced recovery effected by adenosine in group II could not be ascribed to differences in ischemic zone size, collateral flow during occlusion, coronary flow after reperfusion, arterial pressure, heart rate, or other hemodynamic variables. In phase B of the study, dogs received an intracoronary infusion of either saline (group IV [control], n = 6) or adenosine (4 mg/min from 40 to 10 minutes before occlusion [group V, n = 6]). Despite pretreatment with adenosine, the recovery of function in group V was indistinguishable from that in the control group. This study demonstrates that (1) continuous administration of adenosine before, during, and after ischemia results in a significant and sustained attenuation of myocardial stunning; (2) this improved recovery of function cannot be attributed to nonspecific variables, such as collateral flow during coronary occlusion, coronary flow after reperfusion, or other hemodynamic factors, and therefore reflects a direct cardioprotective action of adenosine; (3) the protection against stunning is lost or markedly diminished if adenosine is given only at reperfusion; and (4) administration of adenosine before ischemia does not precondition the myocardium against the stunning induced by a 15-minute occlusion.(ABSTRACT TRUNCATED AT 400 WORDS)

An accurate, nontraumatic ultrasonic method to monitor myocardial wall thickening in patients undergoing cardiac surgery☆

Measurement of systolic wall thickening by sonomicrometry provides an accurate index of regional left ventricular function, but the trauma of crystal insertion limits its widespread clinical use. The first clinical application of a 10 MHz ultrasonic Doppler probe that can be either sutured or applied by suction to the epicardium and can measure wall thickening at any depth of the left ventricular wall is described. In 18 dogs, measurements obtained with the suction probe correlated well (r = 0.97) with those of a previously validated sutured probe. To assess clinical feasibility, the probe was applied to the epicardium of patients undergoing coronary bypass surgery. Good quality wall thickening signals were obtained with no complications. Transmural left ventricular thickening fraction before bypass surgery was 34 +/- 3% (mean value +/- SE) at the mid-ventricular lateral wall, 33 +/- 4% at the anterior basal wall and 26 +/- 4% at the mid-ventricular posterior wall. Right ventricular thickening fraction averaged 25 +/- 3%. Endocardial thickening fraction tended to exceed epicardial thickening fraction, although the difference attained statistical significance (p less than 0.05) only at the anterior basal wall. On average, thickening fraction during the immediate postoperative period remained unchanged compared with the preoperative values, but a marked individual variability was observed, with 7 of 15 patients exhibiting a decrease and 8 an increase. Exteriorization of the wires attached to the sutured probe allowed continuous in situ monitoring of wall thickening in the postoperative period and subsequent removal of the probe. In six patients the crystal was left in place for 48 to 72 h after surgery and then removed without complications; good wall thickening signals were obtained for the entire period during which the probe was implanted. Thus, the Doppler probe is an accurate, atraumatic method for measuring right and left ventricular regional function. Transmural, endocardial and epicardial function can be mapped at various sites during surgery, and post-operatively one can monitor serial changes of regional function and assess the effects of cardioplegia and other therapeutic interventions. This technique should be useful for both investigative and clinical purposes.

Effect of Adenosine on Myocardial Stunning

Myocardial stunning (1), or postischemic myocardial contractile dysfunction, is a mechanical dysfunction that persists after reperfusion despite the absence of irreversible damage (2). The essential point of this definition is that stunning is a fully reversible abnormality. Consequently, concepts derived from animal models of irreversible injury may not be applicable to stunned myocardium (3). In view of the fact that myocardial stunning has been shown to occur in numerous clinical settings (4), such as rest and exercise angina, myocardial infarction with early reperfusion, open-heart surgery, and cardiac transplantation, and in view of its potential to limit the salutary effect of reperfusion therapy, intense research has focused on the pathogenesis of this phenomenon and on the development of effective therapeutic strategies.

Moderate ischemic injury and myocardial stunning

Reperfusion of acutely ischemic myocardium is associated with a constellation of characteristic structural and functional derangements, which range widely in severity [1]. At one end of the spectrum are transient, completely reversible abnormalities such as reperfusion arrhythmias and postischemic myocardial dysfunction or “myocardial stunning” [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. At the other end are severe, irreversible abnormalities such as cell death (infarction). Thus, myocardial stunning should be considered as a mild, sublethal sequela of ischemia/reperfusion injury.

Oxidative Stress in the Pathogenesis of Postischemic Ventricular Dysfunction (Myocardial “Stunning”)

Since its first description in 1982 by Braunwald and Kloner (1982), extensive research has focused on the phenomenon of myocardial “stunning” or postischemic dysfunction, and on the elucidation of its mechanisms. Only a handful of other areas of cardiology in the 1980s have generated so much interest and experimental work in such a short time span. As is often the case with newly described pathophysiological entities, several pathogenetic hypotheses were initially advanced (for review, see Bolli, 1990). At the time of this writing, the “oxyradical” and the “calcium” hypotheses appear to be the most plausible explanations for stunning (for review, see Bolli, 1990).

Postischemic Myocardial Dysfunction (Myocardial “Stunning”): Role of Oxidative Stress

It is now recognized that spontaneous reperfusion after coronary spasm or thrombosis is a common occurrence in patients with coronary artery disease. In addition, coronary reperfusion by means of thrombolytic therapy, percutaneous transluminal angioplasty or bypass surgery is rapidly emerging as the fundamental strategy in the management of acute ischemic syndromes. The recent appreciation of the role of reperfusion in the natural history of coronary artery disease, coupled with the explosive growth of interventional recanalization, has provided the impetus to investigate the consequences of restoring blood flow to the ischemic myocardium. Experimental studies have demonstrated that, although early reperfusion limits or even prevents necrosis, this beneficial effect does not lead to immediate functional improvement; rather, the return of contractility in tissue salvaged by reperfusion is delayed for hours, days or even weeks [1–9], a phenomenon that has been termed “stunned myocardium” [10].

Evidence for Free Radical Generation in Vivo during Cardiac Ischemia and Reperfusion

Myocardial stunning results from reperfusion of a zone of heart muscle that has been subjected to ischemia for a period of less than 20 min (10). Although stunning usually results in temporary disruption of heart function, it does not appear to result in death of myocytes which is characteristic of longer periods of ischemia followed by reperfusion (21). Recovery of viable tissue in the reperfused zone after more extensive periods of ischemia is a function of the time of coronary occlusion. Based on the observation of a number of investigators, it became apparent that reoxygenation of anoxic tissues was detrimental to those tissues, but not as detrimental as would be the case if there were no reoxygenation at all. The oxygen paradox concept was set forth by Hearse and colleagues (8) who determined that there were ultra-structural changes in myocytes which occurred as a result of reoxygenation of ischemic heart tissue (9). Other investigations suggested that oxygen free radicals might be involved in reperfusion injury to tissues in general due to the conversion of xanthine dehydrogenase to xanthine oxidase during ischemia (7). In addition, other possible sources of radical production have been implicated, including activated neutrophils (22,23) and cellular redox functions such as the mitochondrial transport system (17) and eicosanoid metabolism (5,24). Factors which inhibit or modulate the in vitro activities of these various sources of radical production and which have been reported to provide some protection against reperfusion tissue injury when administered in vivo constitute the basis for the implications (prostaglandin synthesis inhibitors, superoxide dismutase (15), oxypurinol (19), free radical scavengers (16), iron chelators (3), etc.).