Atrial, SA nodal, and AV nodal electrophysiology in standing horses: normal findings and electrophysiologic effects of quinidine and diltiazem.

This research investigated the effects of quinidine and diltiazem on the electrical activity of the heart in horses. It found that quinidine prolonged the atrial effective refractory period, shortened the AV node’s functional refractory period, and increased the ventricular response rate, while diltiazem suppressed the sinoatrial node, controlled ventricular rate in a rate-dependent manner, and decreased blood pressure in a well-tolerated way.

Research Objective and Methodology

• The primary objective of this study was to better understand the impacts of quinidine and diltiazem on the electrophysiology of the heart in horses. Particularly, the researchers scrutinized the impact of these drugs on atrioventricular (AV) nodal conduction.
• Fourteen healthy horses were involved in the research. The scientists recorded the horses’ arterial blood pressure, surface electrocardiogram, and right atrial electrogram during sinus rhythm and programmed electrical stimulation.
• The experiments were conducted in three stages. First, baseline measurements were taken. Then, quinidine gluconate was administrated at two different dosages. Finally, the horses were coadministered diltiazem, at repeated doses ranging from 0.125 to 1.125 mg/kg until the desired effect was achieved.

Findings and Conclusions

• Quinidine gluconate significantly increased the atrial effective refractory period, decreased the functional refractory period of the AV node, and increased the ventricular response rate during atrial pacing.
• Diltiazem, on the other hand, increased the functional refractory period, suppressed activity of the sinoatrial node, controlled ventricular rate in a rate-dependent fashion, and induced a significant decrease in blood pressure. This decrease, however, was well-tolerated.
• The researchers concluded that standard electrophysiologic techniques are suitable in characterizing drug impacts on horses while standing. Diltiazem proved effective in controlling the ventricular rate in supraventricular tachycardia induced by the pacing model. The potential of diltiazem for heart rate control in horses with atrial fibrillation warrants further investigation.