Vulnerability to ventricular fibrillation in patients with clinically manifest ventricular tachycardia

Risk stratification after acute myocardial infarction in the reperfusion era

Historically, risk stratification for survivors of acute myocardial infarction (AMI) has centered on 3 principles: assessment of left ventricular function, detection of residual myocardial ischemia, and estimation of the risk for sudden cardiac death. Although these factors still have important prognostic implications for these patients, our ability to predict adverse cardiac events has significantly improved over the last several years. Recent studies have identified powerful predictors of adverse cardiac events available from the patient history, physical examination, initial electrocardiogram, and blood testing early in the evaluation of patients with AMI. Numerous studies performed in patients receiving early reperfusion therapy with either thrombolysis or primary angioplasty have emphasized the importance of a patent infarct related artery for long-term survival. The predictive value of a variety of noninvasive and invasive tests to predict myocardial electrical instability have been under active investigation in patients receiving early reperfusion therapy. The current understanding of the clinically important predictors of clinical outcomes in survivors of AMI is reviewed in this article.

Programmed electrical stimulation of the ventricle: an efficient, sensitive, and specific protocol

A relatively simple and efficient ventricular programmed electrical stimulation (PES) protocol was developed, capable of achieving high degrees of sensitivity and specificity. In a series of 481 subjects, 1, 2, and 3 extrastimuli (ES) were used successively during sinus rhythm and ventricular pacing at two drive cycle lengths, at one or more ventricular sites, together with rapid ventricular pacing, and other maneuvers such as isoproterenol infusion. Three ES were used immediately after two ES at each drive rate, rather than returning after completion of the protocol with two ES. Using the protocol, appropriate arrhythmias could be induced in 88% of all patients with ventricular fibrillation, 84% of all patients with sustained ventricular tachycardia (91% with underlying coronary disease), and 58% of patients with severe nonsustained ventricular tachycardia. There were significant differences in inducibility between patients whose ventricular arrhythmias were due to coronary artery disease and other causes. In contrast, sustained ventricular arrhythmias (all ventricular fibrillation) could be induced in only 5% of a control group of control patients, for a specificity of 95%. The protocol described is simpler and more efficient than those that use exhaustive testing of two ES before going to three ES. Three ES during sinus rhythm proved to be the most productive step, with a higher yield ratio (true: false-positives) than two ES or three ES during pacing, especially at faster rates. Greater efficiency is also achieved by leaving the timing of an extrastimulus just beyond its effective refractory period when an additional extrastimulus is to be added, compared to protocols in which the extrastimulus is moved later in the cycle and then decremented in tandem with the additional extrastimulus. Coupling intervals less than 200 msec produced some false-positives, but fewer overall than intervals greater than or equal to 200 msec, and with yield ratios comparable to other protocol steps. The protocol described meets NASPE standards for ventricular programmed stimulation protocols, and with its demonstrated specificity and relative simplicity and efficiency may be useful as a model for groups not yet committed to an alternative protocol.

The effect of beta-adrenergic blockade on vulnerability to ventricular fibrillation and inducibility of ventricular arrhythmia in short- and long-term feline infarction models

Previous investigation, predominantly in the short-term canine model, has documented a potent antifibrillatory effect of beta-adrenergic blockade. To determine whether the protection afforded by beta blockade is species- and model-specific, we studied 23 chloralose-anesthetized cats. Eight animals were studied over a short term and underwent serial determinations of the ventricular fibrillation (VF) threshold prior to and 1 minute after occlusion of the left anterior descending coronary artery (LAD) and immediately following reperfusion of a 10-minute occlusion. Beta-blocking doses of intravenous propranolol (P) (0.5 mg/kg) attenuated the fall in VF threshold during acute ischemia. Increasing the dose of P to 1 mg/kg did not provide further protection, nor did P protect against reperfusion VF. The other 15 animals underwent a preliminary surgical procedure during which the LAD was completely and irreversibly occluded (nine animals) or in which a sham procedure was performed (six animals). Two weeks later, we measured ventricular refractoriness at several left ventricular sites, ventricular inducibility using programmed electrical stimulation, and VF thresholds both before and after administration of intravenous P (1 mg/kg). Ventricular refractory periods in the infarcted zones were significantly increased compared with normal sites and with values obtained in sham-operated animals. In addition, VF thresholds in the infarcted animals were lower than those obtained in the sham-operated group. Before treatment, a reproducible sustained ventricular tachyarrhythmia was induced by means of programmed stimulation in seven of the nine chronically infarcted animals but in none of the sham-operated animals (p less the 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus-induced critical point. Mechanism for electrical initiation of reentry in normal canine myocardium

The hypothesis was tested that the field of a premature (S2) stimulus, interacting with relatively refractory tissue, can create unidirectional block and reentry in the absence of nonuniform dispersion of recovery. Simultaneous recordings from a small region of normal right ventricular (RV) myocardium were made from 117 to 120 transmural or epicardial electrodes in 14 dogs. S1 pacing from a row of electrodes on one side of the mapped area generated parallel activation isochrones followed by uniform parallel isorecovery lines. Cathodal S2 shocks of 25 to 250 V lasting 3 ms were delivered from a mesh electrode along one side of the mapped area to scan the recovery period, creating isogradient electric field lines perpendicular to the isorecovery lines. Circus reentry was created following S2 stimulation; initial conduction was distant from the S2 site and spread towards more refractory tissue. Reentry was clockwise for right S1 (near the septum) with top S2 (near the pulmonary valve) and for left S1 with bottom S2; and counterclockwise for right S1 with bottom S2 and left S1 with top S2. The center of the reentrant circuit for all S2 voltages and coupling intervals occurred at potential gradients of 5.1 +/- 0.6 V/cm (mean +/- standard deviation) and at preshock intervals 1 +/- 3 ms longer than refractory periods determined locally for a 2 mA stimulus. Thus, when S2 field strengths and tissue refractoriness are uniformally dispersed at an angle to each other, circus reentry occurs around a "critical point" where an S2 field of approximately 5 V/cm intersects tissue approximately at the end of its refractory period.

The role of lipid peroxidation in pathogenesis of arrhythmias and prevention of cardiac fibrillation with antioxidants

The present paper shows the arrhythmogenic effect of a direct induction of lipid peroxidation (LP) on isolated auricles; it is demonstrated that preendured stress potentiates this effect, while antioxidants prevent it. Subsequently, in studying the mechanism of the LP arrhythmogenic effect it was established that stress, like the LP induction, disorders the activity of Na, K-ATPase and accelerates thermodenaturation of this enzyme which plays a key role in maintaining the transmembrane potential and the electrical stability of the heart. Antioxidants prevent the enumerated shifts. Based on these data, the antioxidant BHT was successfully applied for prevention of the fall in cardiac fibrillation threshold in stress and experimental myocardial infarction, and also for prevention of cardiac fibrillation itself under acute ischemia and reoxygenation of the heart.

Arrhythmia inducibility and ventricular vulnerability in a chronic feline infarction model

Ventricular tachyarrhythmias are the cause of sudden cardiac death in ischemic heart disease. Reliable animal models are necessary to study techniques for identifying individuals at risk and to develop effective modes of therapy. The purpose of the present study was to evaluate the inducibility of ventricular tachyarrhythmias and vulnerability to ventricular fibrillation and to correlate these findings with changes in ventricular refractoriness in a chronic feline model. Twelve conditioned cats were randomly divided into two groups: group A, sham-operated controls (n = 5); or group B, permanent occlusion of the left anterior descending coronary artery (n = 7). Two weeks later, the following measurements were made: (1) assessment of refractory periods at several ventricular sites; (2) inducibility to ventricular tachyarrhythmias; and (3) determination of ventricular fibrillation threshold. After electrophysiologic testing, the animals were killed and the hearts were studied histologically. Ventricular fibrillation thresholds were significantly lower in group B compared with group A (13 +/- 3 vs 46 +/- 9 mA; p less than 0.01). One of the sham-operated controls had induction of nonsustained ventricular tachycardia, while six of the group B animals had reproducible, inducible ventricular tachyarrhythmias (p less than 0.01). There was a significant dispersion in effective refractory periods between normal and infarcted sites in group B (46 +/- 6 msec) not seen in group A (12 +/- 2 msec, p less than 0.01). The group A cats demonstrated minimal damage to the myocardium or cardiac architecture. Group B cats demonstrated extensive, transmural, homogeneous infarcts of approximately 30% of the anterior wall of the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)