Atrial, SA Nodal, and AV Nodal Electrophysiology in Standing Horses: Normal Findings and Electrophysiologic Effects of Quinidine and Diltiazem

Abstract

Background: Although atrial arrhythmias are clinically important in horses, atrial electrophysiology has been incompletely studied.Hypotheses: Standard electrophysiologic methods can be used to study drug effects in horses. Specifically, the effects of diltiazem on atrioventricular (AV) nodal conduction are rate-dependent and allow control of ventricular response rate during rapid atrial pacing in horses undergoing quinidine treatment.Animals: Fourteen healthy horses. Methods: Arterial blood pressure, surface electrocardiogram, and right atrial electrogram were recorded during sinus rhythm and during programmed electrical stimulation at baseline, after administration of quinidine gluconate (10 mg/kg IV over 30 minutes, n 5 7; and 12 mg/kg IV over 5 minutes followed by 5 mg/kg/h constant rate infusion for the remaining duration of the study, n 5 7), and after coadministration of diltiazem (0.125 mg/kg IV over 2 minutes repeated every 12 minutes to effect).Results: Quinidine significantly prolonged the atrial effective refractory period, shortened the functional refractory period (FRP) of the AV node, and increased the ventricular response rate during atrial pacing. Diltiazem increased the FRP, controlled ventricular rate in a rate-dependent manner, caused dose-dependent suppression of the sinoatrial node and produced a significant, but well tolerated decrease in blood pressure. Effective doses of diltiazem ranged from 0.125 to 1.125 mg/kg.Conclusions and Clinical Importance: Standard electrophysiologic techniques allow characterization of drug effects in standing horses. Diltiazem is effective for ventricular rate control in this pacing model of supraventricular tachycardia. The use of diltiazem for rate control in horses with atrial fibrillation merits further investigation.

Electrocardiographic characteristics of trained and untrained standardbred racehorses

“…[24][25][26] In horses, SNRT and His bundle recordings are described infrequently. [27][28][29] However, with the emerging possibility of invasive electrophysiological studies and mapping procedures, these types of recordings have great potential to identify the mechanisms of SAN and AV conduction and possibly the effect of training in horses.…”

“…In humans, recordings of the specialized conduction system of the heart including SNRT, AV, and His bundle recordings commonly are used to assess conduction abnormalities 24‐26 . In horses, SNRT and His bundle recordings are described infrequently 27‐29 . However, with the emerging possibility of invasive electrophysiological studies and mapping procedures, these types of recordings have great potential to identify the mechanisms of SAN and AV conduction and possibly the effect of training in horses.…”

First catheter‐based high‐density endocardial 3D electroanatomical mapping of the right atrium in standing horses

“…Simple catheter‐based atrial electrophysiological procedures, as well as catheter‐based transvenous electrical cardioversion of AF, are well described in horses 12‐16 . In contrast to humans and small animals, where fluoroscopy is often used to navigate catheters, catheter placement is generally guided by electrical signals and echocardiography in horses, as the width of the thorax makes chest fluoroscopy challenging 8 .…”

Advances of Echocardiography in Equines Practice - A Review

“…M-mode and 2D equine echocardiography allow users to see the heart structure and function [6]. The structures nearest to the transducer are displayed at the top of the screen, the dorsal (in the long axis views) and cranial (in the short-axis views) structures of the heart, respectively, are displayed to the right side of the screen Doppler ultrasonography works similarly to Doppler weather radar by showing direction, volume, and speed of fluid movement [7].…”