Relationship between the extent of the hypoperfused zone of the myocardium and the occurrence of ventricular fibrillation

Rat Models of Ventricular Fibrillation Following Acute Myocardial Infarction

A number of animal models have been designed in order to unravel the underlying mechanisms of acute ischemia-induced arrhythmias and to test compounds and interventions for antiarrhythmic therapy. This is important as acute myocardial infarction (AMI) continues to be the major cause of sudden cardiac death, and we are yet to discover safe and effective treatments of the lethal arrhythmias occurring in the acute setting. Animal models therefore continue to be relevant for our understanding and treatment of acute ischemic arrhythmias. This review discusses the applicability of the rat as a model for ventricular arrhythmias occurring during the acute phase of AMI. It provides a description of models developed, advantages and disadvantages of rats, as well as an overview of the most important interventions investigated and the relevance for human pathophysiology.

Sudden Cardiac Death in a Case of Non-Dominant Coronary Artery Obstruction Without Depressed Left Ventricular Function

Acute myocardial infarction complicated with lethal cardiac arrhythmia remains the major cause of sudden death. The possible clinical presentation leading to lethal ventricular arrhythmia has been demonstrated but the data are limited. The previous study revealed no significant correlation between sudden cardiac death and the location of coronary obstruction site. And the possible mechanism of sudden cardiac death in non-dominant coronary artery obstruction is unclear. We presented a case of acute myocardial infarction with mid left circumflex artery occlusion complicated with new onset atrial fibrillation initially. The rhythm degenerated into ventricular fibrillation immediately and sudden cardiac death occurred. After resuscitation, he received coronary angioplasty, and the rhythm recovered to sinus after the occluded coronary artery reopened. We thick new onset atrial fibrillation could be a potential risk factor leading to sudden death in acute myocardial infarction with obstruction of non-dominant coronary artery. Control of ventricular rate and early restoration of sinus rhythm may be potential benefit.

Cardioprotective effects of acute isovolemic hemodilution in a rat model of transient coronary occlusion*

OBJECTIVES

Following isovolemic hemodilution (AIH), lowering blood viscosity induces acceleration of erythrocyte velocity resulting in improved tissue oxygen delivery. Using a rat model of myocardial infarct, we tested the hypothesis that AIH would attenuate myocardial damage due to transient coronary occlusion.

DESIGN

Prospective, randomized, and controlled animal study.

SETTING

Animal research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

All rats were subjected to 30 mins of left coronary artery occlusion followed by 48 hrs of reperfusion. Before the ischemic period, the anesthetized rats were randomly allocated to undergo either 15 mins of waiting (controls) or AIH to achieve a hematocrit of 30% (AIH-CO) by stepwise blood withdrawal and isovolemic compensation with 6% hydroxyethylstarch 200-0.5.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic variables were comparable in the two groups, except for higher indexes of stroke volume in the AIH-CO group. During coronary occlusion and the reperfusion period, AIH resulted in a lower incidence of fatal ventricular tachyarrhythmia (17% vs. 50% in control group, p < .05) and higher survival at 48 hrs of postreperfusion (83% vs. 42%, p < .05).Preischemic hemodilution significantly attenuated myocardial damage as shown by lower release of cardiac troponin I and reduction in myocardial infarct size as measured by tetrazolin staining. Histologic examination revealed no difference regarding peri-ischemic infiltration with neutrophil granulocytes.

CONCLUSIONS

Our data provide the first experimental demonstration that preischemic moderate AIH confers cardioprotection and improves survival in a rat model of myocardial infarct.

Incidence and prognosis of early primary ventricular fibrillation in acute myocardial infarction--results of the Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto Miocardico (GISSI-2) database

Primary ventricular fibrillation (VF) complicating acute myocardial infarction (MI) predicts short-term mortality. The broad category of patients with primary VF might include subgroups with different outcomes. It is still not certain whether early-onset (< or =4 hours) primary VF is a risk predictor, and information on correlates of these early fibrillations is scarce. This study sought to prospectively analyze the incidence and prognosis of early, as opposed to late (time window >4 to 48 hours) primary VF and retrospectively identify predisposing factors for early-onset primary VF. We analyzed the incidence and recurrence rate of early and late primary VF in 9,720 patients with a first acute MI, treated with thrombolytics, enrolled in the Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto Miocardico (GISSI)-2 trial. The independent prognostic significance of early and late primary VF was assessed by logistic regression analysis. The incidence rates of early and late primary VF were 3.1% and 0.6%, respectively; recurrence rates were 11% and 15%, respectively. The 2 variables most closely related to early primary VF were hypokalemia and systolic blood pressure < 120 mm Hg on admission. Patients with early primary VF had a more complicated in-hospital course than matched controls. Both early (odds ratio [OR] 2.47, 95% confidence interval [CI] 1.48 to 4.13) and late primary VF (OR 3.97, 95% CI 1.51 to 10.48) were independent predictors of in-hospital mortality. Postdischarge to 6-month death rates were similar for both primary VF subgroups and controls. Primary VF, irrespective of its timing, was an independent predictor of in-hospital mortality. Postdischarge to 6-month prognosis was unaffected by the occurrence of either early or late primary VF.

Arrhythmias and death after coronary artery occlusion in the rat. Continuous telemetric ECG monitoring in conscious, untethered rats

BACKGROUND

The onset of acute myocardial infarction (MI) is accompanied by a rapid increase in electrical instability and often fatal ventricular arrhythmias. The aim of this study was to assess the continuous arrhythmia profile during the initial 48 hours after coronary artery ligation in the rat in relation to time course, mortality, and infarct size.

METHODS AND RESULTS

Continuous ECG recording were obtained in 26 conscious, untethered rats for 24 hours before and 48 hours after coronary ligation by use of an implantable telemetry system. All episodes of ventricular tachycardia and fibrillation were counted and their durations summed. Infarct size was measured at 48 hours after MI or after spontaneous death. After ligation, two distinctly active arrhythmogenic periods developed (A1, 0 to 0.5 hours; A2, 1.5 to 9 hours), each followed by a quiescent phase of low ectopy (Q1, 0.5 to 1.5 hours; Q2, 10 to 48 hours). The total mortality rate of 65% was found within the two active periods, with 13 of 15 deaths occurring in A2. Rats with larger infarcts (> or = 50%) and nonsurvivors tended to have increased arrhythmia frequency and duration compared with both animals with smaller MIs (< 50%) and survivors.

CONCLUSIONS

Two distinct arrhythmogenic periods occur in rats with acute MI that may be caused by different mechanisms and correspond to the bimodal arrhythmia time course seen in dogs and humans after acute MI. Telemetric monitoring of the ECG in the conscious rat after infarction will be useful in assessment of the differential effects of therapeutic interventions on these two arrhythmogenic periods and in the study of potential mechanisms for the spontaneous resolution of ventricular ectopy and risk of sudden death.

Sustained beneficial effects of gallopamil (D600) on size of myocardial infarction 24 hours after coronary artery occlusion in dogs

To examine whether gallopamil (D600), a methoxy derivative of verapamil, has sustained beneficial effects on the ischemic myocardium, its effects on the size of myocardial infarction determined 6 hours (protocol 1) and 24 hours (protocol 2) after left anterior descending coronary artery occlusion were compared in anesthetized, open-chest dogs. To quantify the extent of the hypoperfused zone, Tc-99m- or In-111-albumin microspheres were injected into the left atrium 1 minute after occlusion. Fifteen minutes after occlusion, dogs were randomly assigned to a control group or a gallopamil-treated group that received immediately after assignment 0.08 mg/kg of gallopamil followed by a continuous infusion of 0.2 mg/kg/hr for 6 hours. Six or 24 hours after occlusion, the left ventricle was cut into 3 mm thick slices for triphenyltetrazolium chloride staining and autoradiography. There were no differences in the extent of the hypoperfused zone among the four groups. In both protocols 1 and 2 the ratio of the extent of myocardial necrosis to the extent of the hypoperfused zone was significantly smaller in the treated groups (56.7 +/- 6.7% [n = 8], p less than 0.01 and 72.3 +/- 5.3% [n = 6], p less than 0.05 for protocols 1 and 2, respectively) than in the control groups (100.7 +/- 6.0% [n = 7] and 95.2 +/- 4.3% [n = 5] for protocols I and II, respectively). Thus gallopamil administered early after coronary artery occlusion had beneficial effects on the ischemic myocardium, which were sustained for at least 24 hours after the onset of infarction.

Factors that determine the occurrence of reperfusion arrhythmias

The determinants of reperfusion arrhythmias were investigated in 63 open-chest dogs undergoing a 25-minute coronary artery occlusion followed by reperfusion. Heart rate correlated positively with the occurrence of reperfusion ventricular tachycardia (VT) and ventricular fibrillation (VF). Collateral flow during ischemia (radioactive microspheres) exhibited a strong negative correlation with the incidence of both VT and VF upon reperfusion. Importantly, a sensitive coupling was present, whereby small differences in flow were associated with large differences in rhythm disorders. The rise in intramyocardial CO2 tension (another index of severity of ischemia) was greater in dogs exhibiting reperfusion VT (p less than 0.001) and VF (p less than 0.08); however, this variable was significantly correlated with collateral flow (r = -0.57, p less than 0.01). The size of the occluded coronary bed, determined by postmortem perfusion, was not consistently related to VT; within a given range of occluded bed sizes, the incidence of VT was inversely related to collateral flow. Thus, reperfusion-induced VT is relatively independent of the size of the occluded bed, and is determined primarily by the degree of myocardial hypoperfusion. In contrast, VF did not develop with occluded beds less than 25% of the left ventricular mass; above this critical occluded bed size, its incidence was inversely related to collateral perfusion. Reperfusion VF is therefore determined by the association of a large occluded bed with a poor collateral flow. This study identifies and systematically analyzes three major determinants of reperfusion arrhythmias: (1) the severity of antecedent ischemia, estimated either from the degree of flow reduction or the rise in intramural CO2 tension; (2) the amount of ischemic/reperfused myocardium; and (3) the heart rate. In addition to conceptual interest and clinical implications, the findings have important implications for the design of future studies aimed at evaluating antiarrhythmic interventions in experimental models.

Comparative effects of nicardipine, a new calcium antagonist, on size of myocardial infarction after coronary artery occlusion in dogs

To examine whether nicardipine, a dihydropyridine derivative, limits size of myocardial infarction, and to compare the protective effects of nicardipine administered before and early and late after coronary artery occlusion, 99mTc-labeled albumin microspheres were injected into the left atrium during 5 min temporary coronary artery occlusion to determine the extent of the hypoperfused zone (the area at risk). The coronary arteries were then reperfused for 45 min before 6 hr permanent coronary artery occlusion. Fifteen minutes before permanent occlusion, dogs were randomly assigned to a control group (n = 11), a pretreatment group (n = 9), which received at this point 10 micrograms/kg of nicardipine as a loading dose followed by a continuous infusion of 8 micrograms/kg/hr for 6 hr, an early treatment group (n = 9), in which nicardipine treatment was initiated 15 min after occlusion, or a late treatment group (n = 8), in which nicardipine administration was delayed for 3 hr. Six hours after coronary artery occlusion, the hearts were excised and the left ventricle of each was cut into 3 mm thick slices and stained with triphenyltetrazolium chloride. The extent of myocardial necrosis was measured by planimetry of the unstained areas. Thereafter, the same slices were autoradiographed and the extent of the hypoperfused zone was measured by planimetry of the "cold spot." The extent of the hypoperfused zone was identical among the four groups. In the control group, the ratio of the extent of myocardial necrosis to the extent of the hypoperfused zone was 95.8 +/- 3.8% (mean +/- SEM). However, it was significantly smaller in the pretreatment group (59.9 +/- 13.3%, p less than .05) and the early treatment group (49.0 +/- 10.6%, p less than .01) than in the control group. In the late treatment group, this value was not different from that in the control group (86.5 +/- 7.1%). There was a close inverse correlation between reduction of infarct size and the extent of the hypoperfused zone in the pretreatment and early treatment groups. Thus, nicardipine administered before or early after coronary artery occlusion limited infarct size by 37% to 49%, whereas when administration was delayed for 3 hr infarct size was not reduced. Furthermore, nicardipine had more striking effects on the ischemic myocardium of dogs with small hypoperfused zones than on that of dogs with large hypoperfused zones.

Protective activity of defibrotide against lethal acute myocardial ischemia in the cat

Defibrotide (D) is a natural polydeoxyribonucleotide from mammalian lungs with profibrinolytic and antithrombotic activities. D also has PGI2-stimulating and tissue plasminogen activator (TPA)-releasing activities, but has no anticoagulant properties. The protective effects of D were demonstrated very recently in a model for non-lethal ischemia in the cat. In the experiments reported here Defibrotide was tested in a model for acute myocardial ischemia leading to ventricular fibrillation (VF) and death of the cat. Occlusion of the coronary artery (LAD) at its origin induced VF and death in 17 of 20 control cats. When cats were treated with D (32 mg Kg-1, bolus i.v., + 32 mg Kg-1 h-1, i.v., after LAD occlusion) 19 of 20 animals survived until the end of experiments. D also prevented changes in plasma and myocardial CPK, hemodynamics and ECG. D was compared with a variety of pharmacological agents which are used clinically for specific cardiovascular diseases. The ability of D to promote considerable generation of PGI2 from vascular walls plus its ability to prevent the decreases in CPK-activity and ATP in the myocardial tissue may have roles in its beneficial effects against ischemic heart in the cat. However, the mechanism/s of the substantial protective effect of D against cardiac death has still to be clarified.

Does infarct size influence loss of embolised 15-micrometer microspheres from ischaemic myocardium?

The relationship between myocardial infarct size and loss of 15-micron microspheres from ischaemic tissue was investigated in anaesthetized cats. Radioactive microspheres were injected in the left atrium before and 5 h after left anterior descending coronary artery occlusion. Left ventricular hypoperfused zone (HZ) averaged 36.6% and infarct size (IS) 31.6%. Thus, 86% of HZ evolved into necrosis. Preocclusion blood flow was lower in ischaemic (1.62 ml/min per g) compared with non-ischaemic myocardium (2.09, p = 0.002), indicating 22% microsphere loss. In ischaemic subendocardium, oedema (3.7%) could account for the apparent loss. In ischaemic subepicardium, oedema was less pronounced and 18% physical sphere loss occurred. Subepicardial loss increased in proportion to IS and IS/HZ ratio (r2 = 0.71; p less than 0.005). Non-entrapment of 15-micron spheres in coronary circulation averaged 0.6%, and preocclusion spheres appeared in coronary sinus blood throughout the ischaemic period. In systemic circulation, non-entrapment during injection of preocclusion spheres was 7.8%, but only 1.8% 5 h later. Release of postocclusion spheres took place during KCl injection. Thus, myocardial ischaemia is associated with alterations in microvascular function allowing release of entrapped 15-micron spheres. Also, the magnitude of microsphere loss per gram tissue is related to infarct size.

The effect of tricyclic antidepressants on ventricular fibrillation and collateral blood supply following acute coronary occlusion

In previous studies, we showed that dibenzepin HCl (D) and other tricyclic antidepressants (TCAD), given either before or during occlusion of the left anterior descending artery (LAD), decreased the incidence of ventricular fibrillation (VF) following occlusion and reperfusion. Moreover, once VF develops in treated animals, it changes into a transient type, reverting spontaneously to a sinus rhythm. In the treated cats, retrograde perfusion of the occluded coronary artery was observed, most likely as a result of increased collateral blood flow. This latter effect is the subject of the present study. The LAD was occluded at its origin in 43 cats, 28 of which were treated either with D or with 5-iminodibenzyl HCl; the remaining 15 were untreated controls. Two hours after the occlusion, methylene blue was injected into the left atrium to determine color demarcation between the perfused and unperfused myocardium, and the cat was then killed. After fixing for 2 or 3 days in 4% formaldehyde, the hearts were sectioned transversely. The results showed that in the 15 control cats, the blood-supplied (blue) area ranged between 16% and 56% of the left ventricular muscle (mean 39%), while in the 28 treated cats the blue area was between 44% and 83% (mean 66%). These results clearly indicate the beneficial effect of TCAD on the blood supply of the occluded area and can explain, in part, the ability of these drugs to prevent VF even if infused after the coronary occlusion, and their protective effect against VF following reperfusion. No other antiarrhythmic drugs have been shown to possess this latter action.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the new calcium antagonist nisoldipine (BAY k-5552) on myocardial infarct size limitation in conscious dogs

The effect of the new calcium antagonist nisoldipine (BAY k-5552) on myocardial infarct size was studied in four groups of conscious dogs undergoing acute left anterior descending coronary artery occlusion. Group I received placebo for 48 hours before and for 24 hours after occlusion; group II received placebo before and nisoldipine (0.3 mg/kg orally every 6 hours) after occlusion; group III received nisoldipine before and placebo after occlusion; and group IV received nisoldipine before and after occlusion. Infarct size was quantified with the tetrazolium red staining technique. Infarcted ventricular mass was 24.5 +/- 6.6% (mean +/- SD) for group I (control), 21.4 +/- 4.4% for group II (p = NS against control), 13.9 +/- 4.5% for group III (p less than 0.05), and 14.1 +/- 4.0% for group IV (p less than 0.05). Post occlusion sudden death was 30% in non-pretreated dogs and 0% in pretreated dogs (p less than 0.001). We conclude that prophylactic oral treatment with nisoldipine decreases infarct size and lowers the incidence of sudden death in conscious dogs undergoing acute coronary occlusion.

Ventricular fibrillation

Ventricular fibrillation is the most common mechanism of sudden unexpected cardiac death in persons with asymptomatic or symptomatic coronary artery disease. The electrophysiologic mechanisms reviewed in this article include: automaticity of pacemaker fibers, transformation of nonpacemaker into pacemaker fibers, "injury" currents and reentry. Some of the conditions facilitating ventricular fibrillation include bradycardia, long QT syndrome, electrocution, electrolyte imbalance, drugs, sympathetic stimulation and myocardial ischemia. Electrophysiologic studies during acute myocardial ischemia suggest that the earliest activity at the onset of arrhythmia may originate at the sites of the surviving Purkinje fibers or at the epicardial rim. Reentrant arrhythmias arising in ischemic myocardium are attributed to nonhomogeneous distribution of local hyperkalemia and acidosis.

Lack of correlation between occlusion and reperfusion arrhythmias in the cat

Recently it has been stated that in dogs absence of arrhythmias during coronary artery occlusion identifies a subgroup at almost no risk for developing ventricular fibrillation (VF) during reperfusion. A potential implication would be that prevention of ischemic arrhythmias may also prevent the most severe reperfusion arrhythmias. This concept is at variance with available clinical evidence. In order to reexamine this problem the left anterior descending coronary artery was occluded for 20 minutes in 41 anesthetized cats; the incidence and type of arrhythmias during occlusion and during the first minute of reperfusion were analyzed. Five animals had VF during occlusion and were not resuscitated. Twenty animals had ischemic arrhythmias, and among them the incidence of reperfusion VF was 50%. Sixteen animals did not have ischemic arrhythmias, but they did have a very similar incidence of reperfusion VF (44%). Thus, in this feline preparation, absence of ischemic arrhythmias did not help to predict a favorable outcome at the moment of reperfusion. This disparity with previous studies does not depend on the protocol used, but it may be partly due to species difference. These results demonstrate a lack of correlation between reperfusion VF and ischemic arrhythmias and provide further support for the concept of different mechanisms involved in occlusion and reperfusion arrhythmias.

Postinfarction sudden death: significance of inducible ventricular tachycardia and infarct size in a conscious canine model

The relationship between inducible ventricular tachycardia in the convalescent phase of myocardial infarction and subsequent spontaneous ventricular fibrillation is uncertain. Thirty conscious instrumented dogs underwent programmed ventricular stimulation 5 days after anterior infarction; 15 had inducible ventricular tachycardia and 15 were noninducible. Following programmed ventricular stimulation, the application of a 150 uA current to the intima of the proximal circumflex artery initiated intimal damage, thrombosis, and acute ischemia of the posterolateral wall. After 20 minutes of ischemia, 73% inducible and 15% noninducible animals developed ventricular fibrillation (p less than 0.005) without previous hypotension. At 24 hours, 7% inducible and 85% noninducible animals survived (p less than 0.001). Anterior infarct size (percentage of left ventricular mass) was much larger in inducible (24.7 +/- 1.7%) than in noninducible (5.3 +/- 1.1%) (p less than 0.001) animals. Inducible ventricular tachycardia following infarction was highly predictive of spontaneous ventricular fibrillation during a later ischemic episode in this model. The mass of previously injured myocardium was a critical determinant of both.

Correlates of reperfusion ventricular fibrillation in dogs

To elucidate determinants of reperfusion ventricular fibrillation (VF), regional myocardial blood flow, ATP, creatine phosphate (CP), heart rate and blood pressure were compared in 2 groups of anesthetized dogs: those that fibrillated spontaneously upon release of a 15-minute coronary artery occlusion (VF group, n = 8) and those that did not fibrillate when reperfused (No VF group, n = 27). Arterial pressure and heart rate before and during coronary artery occlusion were similar in both groups. Ischemic endo- and epicardial ATP values, measured at the end of the occlusion period, were reduced approximately 20% of nonischemic values in both groups. In contrast, CP (nmol . mg protein-1) within the ischemic zone was significantly lower in the VF group in both the epicardium (14.3 +/- 1.6 in the VF group vs 22.8 +/- 2.5 in the No VF group, p less than 0.01) and the endocardium (9.0 +/- 2.0 in the VF group vs 18.7 +/- 1.8 in the No VF group, p less than 0.01). Furthermore, epi- and endocardial regional myocardial blood flow in the center of the ischemic zone during occlusion was significantly lower in VF dogs than in No VF dogs. Epicardial flow was 0.06 +/- 0.03 ml X min-1 X g-1 in VF dogs vs 0.44 +/- 0.06 in No VF dogs (p less than 0.001) and endocardial flow was 0.03 +/- 0.02 ml X min-1 X g-1 in VF dogs vs 0.23 +/- 0.04 ml X min-1 X g-1 in No VF dogs (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)