General principles of antiarrhythmic therapy for ventricular tachyarrhythmias

Transient ventricular fibrillation and myosin heavy chain isoform profile

The present paper deals with spontaneous ventricular defibrillation in mammals and the possibility to facilitate its occurrence. Clinical and experimental evidence suggest that in the majority of cases, ventricular fibrillation (VF) is permanent, requiring defibrillation by electric shock. However, a growing number of reports show that VF can terminate spontaneously in various mammals, including human beings.The mechanisms involved in spontaneous ventricular defibrillation are controversial. Available reports imply that intracellular Ca²⁺ overload is the key event triggering VF and preventing its reversal. Since the sarcoplasmatic reticulum is the main intracellular Ca²⁺ regulating organelle and the activity of the cardiac SR Ca²⁺ ATPase (SERCA 2a) is its prime element of Ca²⁺ sequestration, spontaneous ventricular defibrillation likely requires high level of SERCA 2a activity. We suggest that mammalian hearts with high SERCA 2a activity defibrillate spontaneously and those with low activity only after its enhancement. Since high SERCA 2a activity is co-expressed with the myosin heavy chain (MHC) isoform V1, we assumed that those hearts preferentially expressing V1 MHC are able to defibrillate spontaneously. Hearts with small amounts of V1 MHC and correspondingly lower level of SERCA 2a activity can only defibrillate following administration of compounds that augment SERCA 2a activity and prevent intracellular Ca²⁺ overload.

Synthesis and evaluation of in vivo activity of diphenylhydantoin basic derivatives

During the search for antiarrhythmic agents among amide derivatives of phenytoin, compound 7 {3-ethyl-1-[2-hydroxy-3-(4-phenyl-piperazin-1-yl)-propyl]-2,4-dioxo-5,5-diphenyl-imidazolidine} was selected as it showed antiarrhythmic as well as antihypertensive activity. Treating this compound as a lead, new derivatives 8-19 were synthesised, differing in piperazine phenyl ring substitution (2-, 3-, 4-Cl, 2-CH3O) as well as in hydantoin N3 alkyl chain (ethyl, ethyl acetate or ethyl 2-propionate). The obtained compounds in form of hydrochlorides 7a-19a were examined for prophylactic antiarrhythmic and antihypertensive properties. Compounds containing ethyl 2-propionate moiety (17a, 18a) exhibited the highest antihypertensive properties. Water-soluble compounds, containing 2-methoxyphenylpiperazine group (11a, 19a), showed strong antiarrhythmic properties in adrenaline-induced arrhythmia; compound 9a {1-[3-(4-(3-chloro-phenyl)-piperazin-1-yl)- 2-hydroxy-propyl]- 3-ethyl-2,4-dioxo-5,5-diphenyl-imidazolidine hydrochloride} exhibited the highest antiarrhythmic activity in barium chloride arrhythmia model.

Modulation of intracellular Ca(2+) concentration by tedisamil, a class III antiarrhythmic agent, in isolated heart preparation

New class III antiarrhythmic/defibrillating compound tedisamil was shown to facilitate termination of atrial and ventricular fibrillation in experimental as well as clinical conditions. However, class III-related inhibition of K(+) current associated with prolongation of repolarization can not solely explain its defibrillating ability. Following recent findings it was hypothesized that defibrillating effect of tedisamil is likely due to its sympathomimetic feature linked with modulation of intracellular calcium. Results of this study obtained in isolated heart preparation showed that elevated intracellular Ca(2+) free concentration was decreased by administration of tedisamil in concentration that did not induce Q-T interval prolongation. Due to species differences the effective concentration was in rat 10(-7) M, while in guinea pig 10(-5) M. On the contrary, further dramatic increase of elevated Ca(2+) was detected upon administration of tedisamil in concentration that markedly prolonged Q-T interval (10(-5) M in rat). It is concluded that defibrillating ability of tedisamil is most likely associated with attenuation of abnormal and harmful intracellular Ca(2+) elevation (that is highly arrhythmogenic) than with prolongation of APD or Q-T interval.

Are the antiarrhythmic-defibrillating effects of D-sotalol due to or despite the prolongation of the action potential duration?

These results support our hypothesis that class III compounds, with a positive inotropic effect, increase intercellular coupling and synchronization, mainly by preventing intracellular Ca overload. They act as defibrillating compound, similar to cAMP and adrenaline, most probably due to their so called sympathomimetic effect. In our opinion, their cardioprotective effects, resembling cardioversion, are not related to their ability to prolong APD and ERP. Moreover, we suggest that any compound that possesses these sympathomimetic effects, but without inducing the arrhythmogenic prolongation of APD, may exhibit a potent, safety and more efficient antiarrhythmic - defibrillating ability.

Advances in antiarrhythmic therapy

The therapy of cardiac arrhythmias in small animals can be confusing and challenging. This article reviews the current concepts of cardiac rhythm, including impulse generation, automaticity, and conduction in normal and diseased cardiac tissues. The mechanisms of arrhythmogenesis (abnormal automaticity and triggered events) and automatic modulation of cardiac arrhythmias are also discussed. Finally, a review of the clinical management of specific cardiac arrhythmias provides the practicing veterinarian with the current concepts of cardiac rhythm control.

Importance of beta blockade in the therapy of serious ventricular arrhythmias

Beta blockers have traditionally been considered relatively poor antiarrhythmic agents for patients with ventricular arrhythmias. This view is based on the observations that beta blockers are less effective in suppressing spontaneous ventricular ectopy or inducible ventricular arrhythmias than are the class I and class III agents. However, there are convincing data that beta blockers can have a clinically important antiarrhythmic effect and prevent arrhythmic and sudden death. Beta blockers have multiple potential effects that can contribute to a therapeutic antiarrhythmic action, including an antiadrenergic/vagomimetic effect, a decrease in ventricular fibrillation threshold, and prevention of a catecholamine reversal of concomitant class I/III antiarrhythmic drug effects. Postinfarction trials, recent congestive heart failure studies, and observations in patients who are at risk for sustained ventricular arrhythmias all suggest a potent antiarrhythmic effect of beta blockade.