Different relations between infarct size and occluded bed size in barbiturate-anesthetized versus conscious dogs

Limitation of infarct size and ventricular remodeling in patients with completely reperfused anterior acute myocardial infarction--the potential role of ischemia time

BACKGROUND

Experimental studies suggest that coronary reperfusion does not result in appreciable myocardial salvage beyond 3 to 4 h.

HYPOTHESIS

The present study was undertaken to examine the potential role of ischemia time as a determinant of infarct size and cardiac function in humans.

METHODS

Ninety patients (69 men, 21 women, aged 61 +/- 1 years) presented within 24 h of onset of a first anterior infarct had ST-segment elevation on electrocardiogram. All patients underwent coronary intervention within 24 h of onset of symptoms and obtained complete reperfusion of the infarct-related artery.

RESULTS

Infarct size expressed as a percentage of the area at risk (IS/RA) and left ventricular end-diastolic volume (LVEDV) were significantly (p < 0.017) smaller and left ventricula rejection fraction (LVEF) assessed by left ventriculography (35 +/- 4 days) was significantly higher in patients treated within 4 h after onset (IS/RA:55 +/- 4%, LVEDV: 127 +/- 7 ml, LVEF: 62 +/- 2%) than in those treated 4 to 12 h (97 +/- 2%, 140 +/- 13 ml, 52 +/- 3%) and 12 to 24 h (93 +/- 2%,163 +/- 14 ml, 49 +/- 5%) after symptom onset. Left ventricular end-diastolic volume was significantly smaller in patients treated 4 to 12 h after onset than in those treated 12 to 24 h after onset.

CONCLUSIONS

Patients with < 4 h of myocardial ischemia exhibited significant myocardial salvage and better left ventricular function and patients with 4 to 12 h of myocardial ischemia exhibited significantly smaller LVEDV than those with more prolonged ischemia, although there was no difference in final infarct size.

Cyclooxygenase-2 does not mediate late preconditioning induced by activation of adenosine A1 or A3 receptors

Recent studies have demonstrated that the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and the adenosine A3 receptor agonist N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA) produce a delayed phase of protection against infarction similar to the late phase of ischemic preconditioning (PC). However, the mechanism for adenosine A1 or A3 receptor-induced late PC remains unknown. The goal of this study was to determine whether the delayed cardioprotective effects of adenosine A1 or A3 receptors are mediated by cyclooxygenase-2 (COX-2), which is an obligatory mediator of ischemic PC. We found that COX-2 protein expression (Western blotting) did not increase 24 h after the administration of either CCPA (100 microg/kg iv) or IB-MECA (300 microg/kg iv) compared with controls. To probe the role of constitutive COX-2 expression, conscious rabbits were subjected to 30-min coronary occlusion followed by 72-h reperfusion. Twenty-four hours before the occlusion, the rabbits were pretreated with CCPA (100 microg/kg iv) or IB-MECA (300 microg/kg iv). Both CCPA and IB-MECA resulted in a marked (approximately 47%) reduction in infarct size vs. controls [36.2 +/- 4.0% of the risk region (n = 9), 31.2 +/- 4.7% (n = 9), and 59.5 +/- 3.8% (n = 9), respectively; P < 0.05], similar to that induced by the late phase of ischemic PC [31.8 +/- 3.2% (n = 9)]. The selective COX-2 inhibitor N-(2-[cyclohexyloxy]4-nitrophenyl)methanesulfonamide (NS-398, 5 mg/kg), which abolished the protective effect of ischemic late PC, failed to block the protection of either CCPA or IB-MECA, indicating that COX-2 does not mediate the delayed protection of either CCPA or IB-MECA [CCPA + NS-398, 29.1 +/- 3.4% (n = 7); IB-MECA + NS-398, 34.9 +/- 2.9% (n = 8)]. NS-398 in itself did not affect infarct size [54.9 +/- 3.7% (n = 9)]. Taken together, these results demonstrate that, in contrast to ischemia-induced late PC, the mechanisms of adenosine A1 or A3 receptor-induced late PC is independent of COX-2.

Nuclear factor-kappaB plays an essential role in the late phase of ischemic preconditioning in conscious rabbits

Although it is recognized that late preconditioning (PC) results from upregulation of cardioprotective genes, the specific transcription factor(s) that govern this genetic adaptation remains unknown. The aim of this study was to test the hypothesis that the development of late PC is mediated by nuclear factor-kappaB (NF-kappaB) and to elucidate the mechanisms that control the activation of NF-kappaB after an ischemic stimulus in vivo. A total of 152 chronically instrumented, conscious rabbits were used. A sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles, which elicits late PC, induced rapid activation of NF-kappaB, as evidenced by a marked increase in p65 content (+164%; Western immunoblotting) and NF-kappaB DNA binding activity (+306%; electrophoretic mobility shift assay) in nuclear extracts isolated 30 minutes after the last reperfusion. These changes were attenuated 2 hours after ischemic PC and resolved by 4 hours. Competition and supershift assays confirmed the specificity of the NF-kappaB DNA complex signals. The mobility of the NF-kappaB DNA complex was shifted by anti-p65 and anti-p50 antibodies but not by anti-c-Rel antibodies, indicating that the subunits of NF-kappaB involved in gene activation after ischemic PC consist of p65-p50 heterodimers. Pretreatment with the NF-kappaB inhibitor diethyldithiocarbamate (DDTC; 150 mg/kg IP 15 minutes before ischemic PC) completely blocked the nuclear translocation and increased DNA binding activity of NF-kappaB. The same dose of DDTC completely blocked the cardioprotective effects of late PC against both myocardial stunning and myocardial infarction, indicating that NF-kappaB activation is essential for the development of this phenomenon in vivo. The ischemic PC-induced activation of NF-kappaB was also blocked by pretreatment with Nomega-nitro-L-arginine (L-NA), a nitric oxide synthase (NOS) inhibitor, N-2-mercaptopropionyl glycine (MPG), a reactive oxygen species (ROS) scavenger, chelerythrine, a protein kinase C (PKC) inhibitor, and lavendustin A, a tyrosine kinase inhibitor (all given at doses previously shown to block late PC), indicating that ischemic PC activates NF-kappaB via formation of NO and ROS and activation of PKC- and tyrosine kinase-dependent signaling pathways. A subcellular redistribution and increased DNA binding activity of NF-kappaB quantitatively similar to those induced by ischemic PC could be reproduced pharmacologically by giving the NO donor diethylenetriamine/NO (DETA/NO) (at a dose previously shown to elicit late PC), demonstrating that NO in itself can activate NF-kappaB in the heart. Taken together, these results provide direct evidence that activation of NF-kappaB is a critical step in the signal transduction pathway that underlies the development of the late phase of ischemic PC in conscious rabbits. The finding that four different pharmacological manipulations (L-NA, MPG, chelerythrine, and lavendustin A) produced similar inhibition of NF-kappaB suggests that this transcription factor is a common downstream pathway through which multiple signals elicited by ischemic stress (NO, ROS, PKC, tyrosine kinases) act to induce gene expression. To our knowledge, this is the first demonstration that NO can promote NF-kappaB activation in the heart, a finding that identifies a new biological function of NO and may have important implications for various pathophysiological conditions in which NO is involved and for nitrate therapy.

The early and late phases of ischemic preconditioning: a comparative analysis of their effects on infarct size, myocardial stunning, and arrhythmias in conscious pigs undergoing a 40-minute coronary occlusion

The effectiveness of the late phase of ischemic preconditioning (PC) in protecting against myocardial infarction and the concomitant contractile dysfunction after sustained ischemia remains unclear. The early and late phases of PC have not been compared using the same protocol in the same experimental model; furthermore, the late phase of PC has not been assessed in the conscious state in a large animal preparation. The goal of this study was to directly compare the effects of early and late PC on myocardial infarct size and postischemic dysfunction in chronically instrumented, conscious pigs. Four groups of pigs were subjected to a 40-minute coronary occlusion followed by 3 days of reperfusion. Group 1 (n=7) served as control. Group 2 (n=6) was subjected to ten 2-minute occlusion/2-minute reperfusion cycles 25 minutes before the 40-minute occlusion (early PC). Groups 3 (n=7) and 4 (n=4) were subjected to 10 and 25 cycles, respectively, of 2-minute occlusion/2-minute reperfusion 24 hours before the 40-minute occlusion (late PC). Infarct size averaged 45.1+/-5.9% of the region at risk in control pigs, was reduced by 79% (to 9.4+/-3.2%) in group 2, but did not differ in groups 3 (33.3+/-4.8%) and 4 (38.8+/-8.2%) versus group 1. Power analysis demonstrated that there was an 80% probability of detecting a 40% decrease in infarct size in groups 3 and 4 versus group 1. The recovery of systolic wall thickening (measured with ultrasonic crystals) after the 40-minute occlusion was poor in groups 1, 3, and 4 but markedly enhanced in group 2 throughout the 3 days of reperfusion; this beneficial effect could have been due to limitation of infarct size, alleviation of stunning, or both. Thus, a series of ten 2-minute coronary occlusions had a profound (approximately 80%) early infarct-limiting effect, which was associated with a marked functional benefit. This protection, however, disappeared 24 hours later and could not be reinstituted by increasing the number of PC coronary occlusions to 25. The incidence and duration of ventricular tachycardia after reperfusion was not changed by either early or late PC; no conclusions could be drawn regarding ventricular fibrillation or ischemia-induced ventricular tachycardia, since these arrhythmias did not occur in control animals. Taken together, the present results demonstrate striking differences between the early and late effects of PC: In conscious swine subjected to a sustained coronary occlusion, a PC protocol that induces powerful protection during the early phase of PC fails to induce any protection during the late phase, indicating either that a late protective effect of PC does not exist or that, if it exists, it must be weaker than the early protective effect.

Direct evidence that the hydroxyl radical plays a pathogenetic role in myocardial "stunning" in the conscious dog and demonstration that stunning can be markedly attenuated without subsequent adverse effects

Recent studies suggest that the hydroxyl radical (.OH) plays a pathogenetic role in postischemic ventricular dysfunction (myocardial "stunning"). This concept, however, is predicated exclusively on results obtained in anesthetized open-chest preparations, which are subject to the confounding influence of many unphysiological conditions and in which both myocardial stunning and free radical generation are greatly exaggerated. The lack of supporting evidence in more physiological animal models represents a major limitation of the .OH hypothesis of stunning. Furthermore, concern has been raised that myocardial stunning may be a period of "rest" necessary for full recovery, so that attenuation of the early phase of stunning by antioxidant therapy may have subsequent detrimental effects on the resting function and/or on the return of myocardial contractile reserve. To address these issues, in phase 1 of this study conscious unsedated dogs undergoing a 15-minute coronary artery occlusion received an intravenous infusion of normal saline (n = 22), of the .OH scavenger N-2-mercaptopropionyl glycine (MPG, n = 17), or of the iron chelator desferrioxamine (DF, n = 14). Compared with control dogs, the dogs treated with MPG or DF exhibited significantly greater postischemic wall thickening throughout the first 6 hours of reperfusion; the total deficit of wall thickening during this time interval was reduced 50% by MPG and 50% by DF. The magnitude of this beneficial effect was a function of the severity of ischemia, so that the dogs with the lowest collateral flows had the greatest improvement of wall thickening. The accelerated recovery produced by MPG and DF in the first 6 hours was not followed by any deterioration of resting wall thickening at 24 or 48 hours. Furthermore, in dogs treated with MPG or DF, the increase in wall thickening elicited by maximal inotropic stimulation (isoproterenol or dopamine) was similar before stunning and shortly after resting wall thickening had normalized (24 or 48 hours after reflow); thus, despite the fact that most of the early postischemic dysfunction had been eliminated by antioxidant therapy, there was no subsequent impairment of either resting function or contractile reserve. In phase 2, production of free radicals (measured with the spin trap alpha-phenyl N-tert-butyl nitrone) was markedly (> 80%) inhibited by the same doses of MPG and DF that attenuated stunning in phase 1.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Postischemic myocardial "stunning". Identification of major differences between the open-chest and the conscious dog and evaluation of the oxygen radical hypothesis in the conscious dog

Recent studies suggest that oxygen-derived free radicals contribute to the pathogenesis of postischemic myocardial dysfunction (myocardial "stunning"). This concept, however, is predicated exclusively on results obtained in open-chest preparations, which are subject to the confounding influence of many unphysiological conditions. The lack of supporting evidence in more physiological animal models represents a major persisting limitation of the oxy-radical hypothesis of myocardial stunning. The goal of this study was to address two fundamental (and related) questions: 1) Does the open-chest animal model alter the phenomenon of myocardial stunning? 2) If so, how valid are the concepts, derived from such a model, regarding the pathogenetic role of oxy-radicals? In part 1 of the study, myocardial stunning after a 15-minute coronary occlusion was compared in 30 pentobarbital-anesthetized open-chest dogs and in 19 conscious dogs. For any given level of collateral flow during occlusion, the recovery of systolic wall thickening after reperfusion was markedly less in open-chest animals. In an additional group of five open-chest dogs, a close inverse relation was noted between body temperature and postischemic wall thickening, indicating that the recovery of the stunned myocardium in acute experiments may vary markedly depending on how temperature is controlled. Because of these major differences between open-chest and conscious dogs, the oxy-radical hypothesis needs to be tested in the latter model. Thus, in part 2 of the study, conscious unsedated dogs undergoing a 15-minute coronary occlusion were randomized to an intravenous infusion of either saline (19 coronary occlusions) or superoxide dismutase (SOD) plus catalase (CAT) (21 coronary occlusions). Despite the fact that the plasma levels of SOD and CAT declined rapidly after reperfusion, postischemic wall thickening was significantly greater in treated compared with control dogs throughout the first 6 hours of reflow. Thus, a brief (60-minute) infusion of SOD and CAT produced a sustained improvement of recovery of contractility. The magnitude of this beneficial effect was a function of the severity of ischemia: the lower the collateral perfusion, the greater the improvement effected by the enzymes. The accelerated recovery produced by SOD and CAT was not followed by any deterioration of contractility, suggesting that postischemic dysfunction is not a teleologically "protective" phenomenon. In conclusion, the severity of myocardial stunning is greatly exaggerated by the unphysiological conditions present in the barbiturate-anesthetized open-chest dog.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin converting enzyme inhibitors improve contractile function of stunned myocardium by different mechanisms of action

Angiotensin converting enzyme (ACE) inhibitors enhance contractile function of myocardium "stunned" by a brief episode of coronary artery occlusion, yet their mechanism(s) of action remain unresolved. In addition to possible hemodynamic effects, ACE inhibitors may stimulate the synthesis of cardioprotective prostaglandins. Furthermore, the beneficial effects of ACE inhibitors that contain a sulfhydryl group may be due in part to the ability of thiol compounds to act as nonspecific antioxidants or direct scavengers of cytotoxic oxygen-derived free radicals. To investigate this question we compared the effects of (1) the sulfhydryl-containing ACE inhibitor zofenopril, (2) the sulfhydryl-containing stereoisomer of captopril (SQ 14,534) with essentially no ACE inhibitor properties, (3) the nonsulfhydryl-containing ACE inhibitor enalaprilat, and (4) solvent alone, given at the time of reperfusion, on recovery of contractile function after 15 minutes of coronary occlusion in the anesthetized open-chest dog. Segment shortening in control animals remained depressed or "stunned" after reperfusion, recovering to only -5 +/- 12% of baseline preocclusion values at 3 hours after reperfusion. In contrast, all three treatment agents attenuated postischemic dysfunction: segment shortening was restored to 33 +/- 12%, 54 +/- 6%, and 83 +/- 5% of baseline values at 3 hours after reflow in dogs treated with SQ 14,534 (p less than 0.05), zofenopril (p less than 0.01), and enalaprilat (p less than 0.01), respectively (all vs control value). These improvements in segment shortening did not appear to be the result of altered oxygen supply or demand after reperfusion, inasmuch as no significant differences in systemic hemodynamic parameters or myocardial blood flow were observed among the groups. In the second phase of the study, we found that the improved contractile function associated with enalaprilat treatment could largely be reversed by infusion of the potent cyclooxygenase inhibitor indomethacin: segment shortening was reduced from 69 +/- 12% at 2 hours after treatment/reperfusion to 38 +/- 12% at 2 hours after indomethacin infusion (p less than 0.01 vs 2 hours after reperfusion). Infusion of indomethacin had no effect, however, on the improved contractile function associated with zofenopril treatment. We therefore conclude that sulfhydryl- versus nonsulfhydryl-containing agents enhance contractile function of stunned myocardium by different mechanisms of action: enalaprilat attenuates postischemic dysfunction at least in part by a prostaglandin-mediated mechanism, whereas the salutary effects of zofenopril and SQ 14,534 may be due in part to the antioxidant properties of the sulfhydryl moiety.

Chronic hypertension and left ventricular hypertrophy facilitate induction of sustained ventricular tachycardia in dogs 3 hours after left circumflex coronary artery occlusion

The purpose of this study was to investigate the electrophysiology of acute ischemia in hypertrophic as compared with nonhypertrophic myocardium. Left circumflex coronary artery occlusion was produced in anesthetized open chest dogs. Of 40 dogs studied, 22 were normotensive and 18 had chronic hypertension produced by a single kidney renal clamp procedure. Recordings of electrograms and extrastimulus testing were performed in endocardial and epicardial sites in both normal and ischemically damaged zones documented by triphenyltetrazolium chloride. In the hypertrophy group, there was greater endocardial to epicardial conduction delay in ischemic zones, mean +/- SEM 57 +/- 4 ms versus 31 +/- 2 ms in the normotensive group (p less than 0.05). Also, sustained monomorphic ventricular tachycardia was inducible in seven of eight dogs with hypertrophy and in none of eight normotensive dogs surviving to 3 h. Entrainment and several observations during induction were consistent with reentrant ventricular tachycardia. To exclude hypertension alone as an etiology of tachycardia, five normotensive dogs without inducible monomorphic tachycardia remained unchanged during hypertension produced with low doses of phenylephrine or descending aortic occlusion. Thus, the electrophysiologic response to ischemia is altered in hypertrophied myocardium, which predisposes to rapid sustained monomorphic ventricular tachycardia.

Spatial and temporal characteristics of circumferential flow-function relations during acute myocardial ischemia in the conscious dog

In the anesthetized open-chest dog the ischemic area produced by coronary occlusion is surrounded by an area of nonischemic contractile dysfunction, identified as the functional border zone. To establish whether a similar functional border zone exists in the conscious animal during acute regional ischemia and to determine its spatial dimensions and temporal changes, we performed simultaneous two-dimensional echocardiography and radioactive microsphere studies in nine chronically instrumented dogs. We produced circumferential flow-function maps at 22.5-degree intervals over the full circumference of the left ventricle at the midpapillary muscle level during control conditions, 5 minutes after left circumflex occlusion, and 2.5 hours after left circumflex occlusion. After occlusion there was no change in left ventricular end-diastolic area, an increase in left ventricular end-systolic area (p less than 0.01), and a decrease in left ventricular area ejection fraction (p less than 0.01). The circumferential extent of left ventricular dysfunction was 197 +/- 11 degrees (mean +/- SEM) at 5 minutes of left circumflex occlusion, whereas the extent of subendocardial hypoperfusion was 144 +/- 6 degrees (p less than 0.0005). This produced a functional border zone measuring 54 +/- 8 degrees, or 25% of the nonischemic myocardium, which did not change over the 2.5-hour occlusion period. Despite a modest but significant decrease in wall thickening (70 +/- 6% to 43 +/- 6%; p less than 0.01) in the functional border zone, there was no difference in subendocardial blood flow between the functional border zone and the control nonischemic area. We conclude that a discrete functional border zone exists in the conscious dog during acute regional ischemia produced by circumflex coronary occlusion, which does not change during the early evolution of myocardial infarction. The functional border zone likely contributes to minor overestimation of infarct size in the early hours after myocardial infarction if extent of left ventricular dysfunction is used as an index of infarction in humans.

Effects of residual stenosis on infarct size and regional transmural myocardial blood flow after reperfusion

This study in dogs was designed to determine the effects of residual stenosis on infarct size and on the transmural distribution of coronary flow in the central and peripheral ischemic perfusion bed. A plastic shunt containing a Doppler flow probe was inserted between the left anterior descending coronary artery and the subclavian artery. The dogs were divided into two groups. Group 1 (N = 7) underwent total shunt occlusion for 2 hours followed by reperfusion at 50% of control flow for 2 hours. Group 2 (N = 8) underwent 2 hours of total occlusion followed by 2 hours of total reperfusion. Regional blood flow was measured by radiolabeled microspheres, and infarct areas were quantitated with triphenyl tetrazolium chloride staining. Infarct sizes expressed as a percentage of the left ventricle or as a percentage of perfusion territory were significantly (p less than 0.05) smaller in animals with total reperfusion (group 2) than in dogs with partial reperfusion (group 1). Endocardial flows in the central infarct zone were significantly higher in dogs with total reperfusion than was observed with partial reperfusion; epicardial flows were not significantly different. In the peripheral region both endocardial flows and epicardial flows with total reperfusion were significantly higher than with partial reperfusion. These studies suggest that residual stenosis after thrombolysis may increase infarct size and reduce endocardial flow in the central infarct zone and transmural flow in the peripheral zone.

Determinants of infarct size during permanent occlusion of a coronary artery in the closed chest dog

One hundred nine dogs subjected to experienced experimental coronary occlusion were retrospectively examined in an attempt to identify factors influencing infarct size. A coronary artery was occluded by an embolus. The field of the occluded artery (zone at risk) was determined by subsequent autoradiography of 141-cerium-labeled microspheres that were injected into the left ventricle 2 minutes after embolization. Fifty-eight dogs were analyzed after 24 hours of embolization and 51 after 48 hours. Infarct size (assessed by tetrazolium staining) was directly proportional to the size of the zone at risk in both groups. The percent of the risk zone that developed infarction was independent of risk zone size in both the 24 and 48 hour groups. No differences were seen between male and female animals or between the dogs with left circumflex or left anterior descending artery occlusion. Infarct size was also independent of the heart rate-systolic pressure product at the time of coronary occlusion, and of the time of year. Eighty percent of the variability in the portion of the risk zone that infarcted in this population could be explained by the level of collateral flow in the risk zone 2 minutes after embolization. The linear regression between the percent of the risk zone that developed infarction and collateral flow was the same in slope and intercept between the 24 and 48 hour groups. The correlation between infarct size and collateral flow was slightly better when collateral flow was expressed as a percent of flow in the nonischemic zone than when it was expressed in absolute terms.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction in experimental infarct size by recombinant human superoxide dismutase: insights into the pathophysiology of reperfusion injury

To determine the importance of reperfusion injury and the ability of the free-radical scavenger recombinant human superoxide dismutase (h-SOD) to prevent it, open-chest dogs underwent 90 min of proximal circumflex coronary artery occlusion, and only at the moment of reperfusion received either h-SOD (400,000 IU bolus into the left atrium followed by a 300,000 IU iv infusion over 1 hr) or saline. After 48 hr the surviving animals were killed and measurements were made of the risk region (by postmortem angiography) and infarct size (by gross pathology). All measurements were made by investigators blinded to treatment given, and the code was broken only at the end of the study. Hemodynamic variables and collateral flow during ischemia were similar in the two groups. Infarct size in control animals (n = 8) averaged 22.4 +/- 3.1% of the left ventricle and 52.2 +/- 7.1% of the risk region, compared with 13.3 +/- 0.8% of the left ventricle and 33.6 +/- 2.1% of the risk region in h-SOD-treated dogs (n = 8) (p less than .05). Infarcts in treated animals were not only smaller, but also exhibited a distinctive "patchiness," suggesting protection along vascular distributions. Furthermore, analysis of the relationship between infarct size and collateral flow measured during ischemia in the two groups indicated that protection by h-SOD was greatest in animals with the lowest collateral flows. This study supports the concept that reperfusion of ischemic myocardium results in a separate component of cell damage, presumably linked to the generation of oxygen free radicals on reflow. Since the h-SOD preventable reperfusion component of injury was most pronounced in hearts with the most severe ischemia, scavenging of oxygen radicals at the time of reflow may offer a novel and particularly promising therapeutic approach for the protection of ischemic myocardium.

Increased myocardial infarct size by thiopental after coronary occlusion in the dog

The effect of a single dose (10 mg/kg) of intravenous thiopental (TP), during acute myocardial infarction, on infarct size was studied in conscious dogs randomized 10 minutes after left circumflex coronary artery occlusion to either the TP group (n = 10) or a control group given 0.9% saline solution (n = 10). During the first hour following therapy, myocardial blood flow (microspheres), arterial pressure, left atrial pressure, and arterial blood gases were similar in the two groups, but the heart rate (140 +/- 3 vs 110 +/- 3 bpm; p less than 0.001) and rate-pressure product (15,090 vs 12,210 bpm X mm Hg; p less than 0.025) were greater in the TP group. Infarct size (planimetry) and occluded bed size (postmortem coronary arteriography) measured 2 days later revealed that: the slope of the relation between infarct and occluded bed mass, as a percentage of the left ventricle (% LV) was greater with TP than with saline solution (1.10 vs 0.61; p less than 0.001); excluding hearts (four TP and three saline solution) with small occluded beds (less than 22% LV), infarcts were also larger with TP (n = 6) than with saline solution (n = 7), both as a percentage of the left ventricle (26.4 vs 12.2%; p less than 0.02) or occluded bed (61.5 vs 28.9%; p less than 0.005); and transmural and endocardial extents of the infarcts on topographic maps were greater with TP than with saline solution. In 12 other conscious dogs, increasing the heart rate between 10 and 70 minutes after left circumflex coronary artery occlusion to the average rate of the TP group (140 bpm) by atrial pacing resulted in infarcts larger than those in control dogs but similar to those in the TP group. Thus, TP therapy after left circumflex occlusion increased infarct size in dogs. This effect appeared to be due mainly to the increased heart rate, probably via increased myocardial oxygen demands.

Disparity of reperfusion arrhythmias after reversible myocardial ischemia in open chest and conscious dogs

Myocardial reperfusion after brief, reversible ischemia is frequently associated with malignant arrhythmias in experimental animals. These observations have been extrapolated to humans despite being restricted to anesthetized, open chest preparations. No data are available regarding the incidence of reperfusion arrhythmias after reversible (less than 20 minutes) ischemia in the conscious state. Thus, reperfusion arrhythmias after a 15 minute occlusion of the left anterior descending coronary artery were compared in 24 open chest dogs (17 anesthetized with pentobarbital and 7 with chloralose plus urethane) and 25 conscious, unsedated, trained dogs. The incidence of all rhythm disorders (single premature ventricular complexes, pairs, ventricular tachycardia and fibrillation) was markedly and significantly lower in conscious than in either pentobarbital- or chloralose-anesthetized dogs. The disparity was not accounted for by differences in coronary collateral flow, coronary reactive hyperemia or occluded bed size. The conscious animals, however, exhibited lower heart rates and arterial pressures during reperfusion than did the open chest dogs, suggesting a lower level of adrenergic stimulation, which might have contributed to the reduced incidence of reperfusion arrhythmias. Coronary reperfusion after 15 minutes of occlusion is unlikely to precipitate ventricular tachyarrhythmias in the conscious, trained dog, even after severe ischemia. The occurrence of these rhythm disorders in anesthetized models may reflect the influence of surgical trauma or excessive adrenergic activity, or both. Reperfusion arrhythmias after reversible ischemia may be considerably less common in the clinical setting than previously postulated on the basis of open chest animal experiments.