Inducible monomorphic sustained ventricular tachycardia in the conscious pig

Hibernating Myocardium

The inability to reproduce spontaneous ventricular fibrillation in an animal model of chronic coronary artery disease has limited advances in understanding mechanisms of sudden cardiac death (SCD). Swine with hibernating myocardium arising from a chronic left anterior descending coronary artery (LAD) occlusion have a high rate of SCD that parallels the poor clinical survival of medically treated patients with hibernating myocardium. Kaplan-Meier analysis (n=426) demonstrated a cumulative mortality of 49% after 5 months that was almost entirely attributable to spontaneous SCD. Using implantable loop recorders, ventricular fibrillation was documented as the arrhythmic mechanism of death in all animals (n=10) and was usually preceded by ventricular tachycardia (n=8). Physiological studies before SCD (n=7) demonstrated total LAD occlusion and collateral-dependent myocardium (n=5), excluding acute occlusion as a major trigger of arrhythmia. The physiological substrate of hibernating myocardium was present before SCD, with reductions in LAD perfusion (SCD 0.79+/-0.13 versus 0.80+/-0.08 mL/min per g) and wall thickening (SCD 28+/-3% versus 22+/-3%) that were similar to survivors (n=14). Triphenyltetrazolium chloride infarcts among animals with SCD were infrequent (4 of 32) and small, averaging 4.6% of LV mass. Histology (n=4) showed postmortem changes but no acute inflammation nor contraction band necrosis. These data support the notion that hibernating myocardium is a pathophysiological substrate at high risk of SCD. This is independent of changes in functional stenosis severity, acute myocardial necrosis, or fibrotic scar. Thus, regional adaptations that promote myocyte survival in the setting of chronic repetitive ischemia result in a substrate with enhanced vulnerability to lethal arrhythmias and SCD.

Significance of magnesium in congestive heart failure

Electrolyte balance has been regarded as a factor important to cardiovascular stability, particularly in congestive heart failure. Among the common electrolytes, the significance of magnesium has been debated because of difficulty in accurate measurement and other associated factors, including other electrolyte abnormalities. The serum magnesium level represents < 1% of total body stores and does not reflect total-body magnesium concentration, a clinical situation very similar to that of serum potassium. Magnesium is important as a cofactor in several enzymatic reactions contributing to stable cardiovascular hemodynamics and electrophysiologic functioning. Its deficiency is common and can be associated with risk factors and complications of heart failure. Typical therapy for heart failure (digoxin, diuretic agents, and ACE inhibitors) are influenced by or associated with significant alteration in magnesium balance. Magnesium therapy, both for deficiency replacement and in higher pharmacologic doses, has been beneficial in improving hemodynamics and in treating arrhythmias. Magnesium toxicity rarely occurs except in patients with renal dysfunction. In conclusion, the intricate role of magnesium on a biochemical and cellular level in cardiac cells is crucial in maintaining stable cardiovascular hemodynamics and electrophysiologic function. In patients with congestive heart failure, the presence of adequate total-body magnesium stores serve as an important prognostic indicator because of an amelioration of arrhythmias, digitalis toxicity, and hemodynamic abnormalities.

A closed-chest pig model of sustained ventricular tachycardia

The goal of this study was to develop and explore a closed-chest animal model of sustained VT. Seven of 11 domestic pigs had successful induction of myocardial infarction by injection of agarose gel microbeads into the left anterior descending coronary artery through an inflated balloon angioplasty catheter. Four of the first five pigs died and seem to represent a "learning experience." During a 3- to 50-day follow-up period, each pig underwent 1-3 electrophysiological studies. Sustained, monomorphic VT was induced 1-4 times in 5 of the 7 pigs (a total of 19 episodes), was reproducible during the same study in all pigs, and could be repetitively induced during successive studies in some. Ventricular fibrillation was induced less frequently (nine episodes) and was successfully terminated by DC shock in eight episodes. We conclude that a closed-chest pig model of VT is feasible and is associated with a relatively high induction rate of sustained, monomorphic, and reproducible VT and a relatively low mortality rate.

A review of magnesium, acute myocardial infarction and arrhythmia

Many years ago, experimental medicine accumulated substantial evidence that magnesium (Mg) balance was important for a stable cardiovascular system. Recent clinical interest was aroused by evidence of decreased mortality in patients with acute myocardial infarction (AMI), treated with Mg infusions. Pharmacologic actions of Mg include its antiarrhythmic, antivasospastic and other important cardiovascular effects, substantiating the rationale for its use in AMI. Direct pharmacologic effect of this ion, rather than compensation of hypomagnesemia frequently encountered during acute ischemic injury, has been suggested to account for the above benefits. Several trials studied the efficacy of early Mg therapy in decreasing mortality from AMI while most of the data point to improved survival, a few trials could not demonstrate any benefit of Mg. The reported rate of complications with this therapy is low though the potential for serious side effects exists. Larger studies of Mg in AMI are expected to resolve the existing controversy.

Decreases in repetitive extrasystole threshold in the conscious pig with myocardial infarct were reversed by tyrosine

Reports indicate that the administration of tyrosine, the precursor amino acid for catecholaminergic neurotransmitters, may be beneficial under conditions of physiologic stress. We studied the effects of tyrosine on vulnerability to ventricular arrhythmia in conscious pigs with healing myocardial infarcts, and sham operated (intact) pigs. Mean arterial pressure and heart rate were measured via chronically implanted aortic catheters. The repetitive extrasystole threshold (defined as the energy in milliamperes (ma) needed to cause a spontaneous ventricular beat following a premature beat induced by an electrical impulse), was measured via a bipolar pacing catheter placed during instrumentation surgery in the apex of the right ventricle. One week after infarct, the myocardial infarct group was studied before and ninety minutes after the administration of tyrosine (8 mg/kg iv). Before tyrosine, the myocardial infarct group had a significantly lower repetitive extrasystole threshold (12 +/- 1 ma) compared to the intact group (19 +/- 2 ma). Ninety minutes after tyrosine, the repetitive extrasystole threshold in the myocardial infarct group was 17 +/- 1 ma. The availability of tyrosine did not alter the repetitive extrasystole threshold in the intact group. Thus, vulnerability to ventricular arrhythmia was enhanced in pigs with recent myocardial infarction. Tyrosine, which can be nutritionally manipulated, may reduce myocardial vulnerability to arrhythmia after infarct.