Arrhythmia inducibility and ventricular vulnerability in a chronic feline infarction model

Effects of dofetilide on electrical dispersion and arrhythmias in post-infarcted anesthetized dogs

An increase in dispersion of myocardial refractoriness has been shown to coincide with a greater risk of inducible ventricular arrhythmias. We compared the dispersion of electrophysiologic parameters and antiarrhythmic effects of dofetilide (0.03, 0.1, 0.3 and 1 mg/kg i.v.) in post-infarcted anesthetized dogs. Animals were tested for inducibility of arrhythmias using a programmed electrical stimulation (PES) protocol, and divided into inducible (I) and non-inducible (NI) groups. In addition, myocardial vulnerability was measured using ventricular fibrillation thresholds (VFT), as well as susceptibility to sudden cardiac death (SCD). Dofetilide significantly increased ventricular effective refractory periods (ERP) and monophasic action potential durations (APD) in a dose-dependent manner. The standard deviation of ERP, which was used as an index of dispersion of refractoriness, increased from sham (control value of 5.4 +/- sd 2.5 ms), non-inducible (control value of 11.0 +/- 5.5 and 8.0 +/- 3.7 ms for vehicle and dofetilide groups, respectively) and inducible states (control value of 17.3 +/- 6.2 and 21.6 +/- 7.1 ms for vehicle and dofetilide groups, respectively). However, dofetilide treatment did not alter dispersion of refractoriness over the dose range studied. Dofetilide did not significantly increase inducibility in the NI group (2 out of 8 [25%] compared to 0 out of 9 [0%] in vehicle treated animals). In the I group, dofetilide (0.3 mg/kg) treated animals converted 2 out of 7 (29%) to NI, and 5 out of 7 (71%; significant at p < 0.05) to a NI or non sustained ventricular tachycardia. There were no significant changes in VFT following the last dose of dofetilide given. Dofetilide did not significantly affect SCD survival (88% and 29% in the NI and I group, respectively) relative to vehicle (66% and 50% in the NI and I group, respectively). Although infarct sizes were significantly greater in the I groups, there was no difference between vehicle and dofetilide animals within these groups. In conclusion, dofetilide increased ERP and APD values, but did not affect dispersion of refractoriness. Thus, changes in dispersion of refractoriness may be used as a marker for inducibility in untreated animals, but it did not predict the antiarrhythmic effects observed with dofetilide.

Inducible monomorphic sustained ventricular tachycardia in the conscious pig

Sustained monomorphic ventricular tachycardia (VT) is of clinical importance but has not been readily modeled in conscious animals. Eleven pigs had myocardial infarction induced by pulling snares previously placed around the left anterior descending (LAD) coronary artery. Six days after occlusion, bipolar pacing catheters were inserted in the right ventricular apex for induction of VT. Testing was repeated in conscious pigs on 6 out of 8 to 19 days after infarction. Monomorphic VT was induced in each animal during each session, using three to four extrastimuli. VT was terminated by burst pacing in 74% of trials; average VT rate was 362 +/- 26 beats/min. VT was prevented in four of eight animals by procainamide and in five of eight animals by magnesium, but was not prevented by lidocaine or metoprolol. The model may be useful in the study of potentially malignant ventricular tachyarrhythmias, important prodromes for sudden death.

Day to day reproducibility of electrically inducible ventricular arrhythmias in survivors of acute myocardial infarction

Day to day reproducibility of the response to programmed ventricular stimulation has not been evaluated in survivors of acute myocardial infarction. Programmed ventricular stimulation was performed prospectively on 2 consecutive days in 56 patients on an average of 12 +/- 5 days (range 7 to 29) after an acute myocardial infarction. No patient had a history of documented or suspected sustained ventricular tachycardia or fibrillation occurring greater than 48 h after infarction. During initial programmed ventricular stimulation, 21 patients had induction of sustained ventricular tachycardia or fibrillation (Group I), and 35 patients had induction of either nonsustained ventricular tachycardia or no ventricular tachycardia (Group II). Repeat programmed ventricular stimulation in Group I patients induced sustained ventricular tachycardia or fibrillation in 16 of 21 patients (reproducibility 76%); the maximal induced response in the other 5 patients was nonsustained ventricular tachycardia in 2 patients and fewer than six repetitive ventricular responses in 3 patients. The day to day reproducibility was significantly higher for inducible sustained ventricular tachycardia of cycle length greater than or equal to 240 ms compared with rapid sustained ventricular tachycardia of cycle length less than 240 ms (100% versus 44%, p less than 0.009) or ventricular fibrillation (100% versus 43%, p less than 0.009). Repeat programmed ventricular stimulation in Group II patients did not induce sustained ventricular arrhythmias in 31 of 35 patients (reproducibility 89%). Thus, in survivors of acute myocardial infarction, inducible slow sustained ventricular tachycardia was a highly reproducible finding, whereas inducibility of rapid sustained ventricular tachycardia and ventricular fibrillation showed a significant day to day variability.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

The effect of thromboxane inhibition on vulnerability to ventricular fibrillation in the acute and chronic feline infarction models

Inhibition of the enzyme that synthesizes thromboxanes may protect against the development of ventricular fibrillation (VF) during acute myocardial ischemia. This study was carried out to test this hypothesis with a new thromboxane synthetase inhibitor, and to extend the studies to alternative animal models of myocardial infarction. In a series of acute experiments, 19 cats were pretreated with 10 mg/kg of U-63557A (a dose that produced greater than 75% reduction in thromboxane B2 [TxB2] levels) or saline before abrupt left anterior descending coronary artery occlusion. Seven of the nine control animals suffered spontaneous VF associated with a 77% fall in VF threshold compared with the treated animals, of which 2 of 10 had spontaneous VF and in which VF threshold fell by only 45% (p less than 0.025). Despite a similar extent of TxB2 inhibition in another set of nine animals, U-63557A failed to protect against a fall in VF threshold during coronary reperfusion. Finally, chronic changes in VF threshold and inducibility of sustained ventricular tachycardia by programmed stimulation were assessed in a group of eight animals. The lowering of VF threshold and inducibility of ventricular tachycardia seen in the control state were not influenced by treatment with U-63557A. Thus protection against infarct-related VF by TxB2 inhibition is a property shared by more than one pharmacologic agent. Arrhythmias generated by reperfusion or induced in a more chronic setting may not be thromboxane-dependent. These results have important implications for the planning of studies designed to assess the antiarrhythmic potential of drugs that inhibit thromboxane synthesis.