Atrial, SA nodal, and AV nodal electrophysiology in standing horses: normal findings and electrophysiologic effects of quinidine and diltiazem.
Authors: Schwarzwald Colin C, Hamlin Robert L, Bonagura John D, Nishijima Yoshinori, Meadows Cheyney, Carnes Cynthia A
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary Atrial arrhythmias represent a significant clinical challenge in equine practice, yet the underlying electrophysiology remains poorly characterised. Schwarzwald and colleagues used invasive electrophysiologic testing in 14 healthy standing horses to establish baseline atrial and atrioventricular nodal properties, then systematically evaluated how quinidine and diltiazem altered electrical conduction using programmed electrical stimulation and continuous monitoring of blood pressure, surface electrocardiography, and right atrial electrograms. Quinidine prolonged atrial refractoriness but paradoxically shortened atrioventricular nodal refractoriness, increasing ventricular response rates during rapid atrial pacing—a finding with important implications for arrhythmia management. Diltiazem demonstrated rate-dependent control of atrioventricular nodal conduction (effective doses ranging from 0.125 to 1.125 mg/kg), suppressed sinoatrial activity in a dose-dependent manner, and successfully controlled ventricular rates without excessive blood pressure compromise. These electrophysiologic findings suggest diltiazem warrants investigation as a rate-control agent for equine atrial fibrillation, particularly in cases where quinidine monotherapy may inadequately limit ventricular response, though clinicians should appreciate the drug's vasodilatory effects and titrate carefully in haemodynamically compromised patients.
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Practical Takeaways
- •Diltiazem can be used effectively to control ventricular response rate during rapid atrial pacing in horses receiving quinidine, offering a practical approach to managing atrial arrhythmias
- •Effective diltiazem dosing in horses requires titration (0.125-1.125 mg/kg IV) and monitoring for blood pressure effects, but side effects were well-tolerated in this study
- •Electrophysiologic testing is feasible in standing horses and can guide selection and optimization of antiarrhythmic therapy for atrial fibrillation
Key Findings
- •Quinidine significantly prolonged atrial effective refractory period and increased ventricular response rate during atrial pacing in horses
- •Diltiazem increased AV nodal functional refractory period and controlled ventricular rate in a rate-dependent manner with effective doses ranging from 0.125 to 1.125 mg/kg
- •Diltiazem caused dose-dependent suppression of sinoatrial node function and significant but well-tolerated decreases in blood pressure
- •Standard electrophysiologic techniques successfully characterize drug effects in standing horses without general anesthesia