Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

Gene expression patterns of the four cardiac chambers in the Thoroughbred horse

Athlete horses' contraction and conduction of the healthy heart influences racing performance. Gene expression patterns in the horse heart are not yet fully investigated. We aim to evaluate the gene expression of the four chambers of the heart overall and with focus on genes involved in the electrophysiology of the heart in Thoroughbred racehorses with no clinical cardiac abnormalities. Tissue was collected from the left atrium (LA), right atrium (RA), left ventricle (LV), and right ventricle (RV). Total RNA was analysed by microarray technique. We compared gene expression in the heart chambers by contrasting atrial against ventricular chambers (chamber related differences), and by contrasting left side to right side (left-to-right related differences). The pathway analyses revealed that RA was characterised by significantly lower expression of genes related to energy derivation and metabolism in comparison to both ventricles and left atria. LA, on the other hand, was characterised by higher expression of genes related to cardiac conduction, and less expression of cardiac morphogenesis, compared to ventricles. The left-to-right related comparisons indicated wider differences between the atria than between the ventricles. Mapping of the genes specifically involved in cardiac conduction and contraction indicated clear chamber related differences. Some potassium channels, KCNE1 and KCNJ2,3 and 4, showed distinct atrial-ventricular specificity and genes involved in calcium regulation were, as a group, more abundant in both atria compared to ventricles. Our results provide a general overview of the gene expression pattern in the healthy racehorse, with particular focus on the cardiac ion channels.

Rational quinidine dosage regimen for atrial fibrillation in Thoroughbred racehorses based on population pharmacokinetics

Introduction

Quinidine (QND) sulfate is an effective treatment for atrial fibrillation (AF) in horses, and several dosage regimens have been proposed to address its wide variability in response and potential adverse effects. The purpose of this study was to analyze the variability in plasma quinidine concentrations using population pharmacokinetics to determine an effective and safe dosage regimen for Thoroughbred horses.

Methods

Six healthy Thoroughbred horses were treated with 20 mg/kg quinidine sulfate dihydrate (16.58 mg/kg QND base) administered PO or 5 mg/kg quinidine hydrochloride monohydrate (4.28 mg/kg QND base) administered IV (single administration), and blood samples were taken regularly. Four healthy horses were treated with 20 mg/kg quinidine sulfate dihydrate administered twice (every 6 h) via PO route. For the other 19 Thoroughbred racehorses that developed AF, blood samples were taken during quinidine therapy. Quinidine concentrations were measured in all plasma samples using liquid chromatography with tandem mass spectrometry, and the data from 29 horses were modeled using a nonlinear mixed-effects model, followed by Monte Carlo simulations (MCS).

Results

The median quinidine concentration for successful sinus rhythm conversion was 2.0 μg/mL (range: 0.5-2.7 μg/mL) in AF horses, while a median concentration of 3.8 μg/mL (range: 1.6-5.1 μg/mL) showed adverse effects. MCS predicted that plasma quinidine concentrations for quinidine sulfate dihydrate PO administration (loading dose: 30 mg/kg, maintenance dose: 6.5 mg/kg q 2 h) reached 1.4, 2.0 and 2.7 μg/mL in 90, 50 and 10% of the horse populations, respectively. Increasing the loading dose to 45 mg/kg and the maintenance dose to 9 mg/kg q 2 h, the plasma concentrations achieved were 1.9, 2.8, and 3.8 μg/mL in 90, 50, and 10% of horse populations, respectively.

Discussion

Using simulations, different empirical dosing regimens were proposed to achieve plasma quinidine concentrations immediately or progressively, representing a tradeoff between optimizing therapeutic effects and minimizing adverse effects. A combination of these dosing regimens is recommended to gradually increase the therapeutic concentration levels of quinidine for safe and effective treatment of AF in racehorses.

Detection of paroxysmal atrial fibrillation preceding persistent atrial fibrillation in a horse using an implantable loop recorder with remote monitoring

Implantable loop recorders (ILRs) are increasingly used in equine cardiology to detect arrhythmias in the context of collapse, poor performance or monitoring for recurrence of atrial fibrillation (AF). However to date, the ILR has never been reported to be used with a remote monitoring functionality in horses, therefore the arrhythmia is only discovered when a clinician interrogates the ILR using dedicated equipment, which might delay diagnosis and intervention. This case report describes the use of an ILR with remote monitoring functionality in a horse with recurrent AF. The remote monitoring consisted of a transmission device located in the stable allowing daily transmission of arrhythmia recordings and functioning messages to an online server, available for the clinician to evaluate without specialised equipment. The ILR detected an episode of paroxysmal AF approximately 3 months after implantation. Seven months after implantation, initiation of persistent AF was seen on an episode misclassified by the ILR as bradycardia, and the horse was retired. This report shows the feasibility and benefits of remote monitoring for ILRs in horses, but also the shortcomings of current algorithms to interpret the equine electrocardiogram.