Cardiac electrophysiological adaptations in the equine athlete—Restitution analysis of electrocardiographic features

Abstract

Exercising horses uniquely accommodate 7–8-fold increases in heart rate (HR). The present experiments for the first time analysed the related adaptations in action potential (AP) restitution properties recorded by in vivo telemetric electrocardiography from Thoroughbred horses. The horses were subjected to a period of acceleration from walk to canter. The QRS durations, and QT and TQ intervals yielded AP conduction velocities, AP durations (APDs) and diastolic intervals respectively. From these, indices of active, λ = QT/(QRS duration), and resting, λ0 = TQ/(QRS duration), AP wavelengths were calculated. Critical values of QT and TQ intervals, and of λ and λ0 at which plots of these respective pairs of functions showed unity slope, were obtained. These were reduced by 38.9±2.7% and 86.2±1.8%, and 34.1±3.3% and 85.9±1.2%, relative to their resting values respectively. The changes in λ were attributable to falls in QT interval rather than QRS duration. These findings both suggested large differences between the corresponding critical (129.1±10.8 or 117.4±5.6 bpm respectively) and baseline HRs (32.9±2.1 (n = 7) bpm). These restitution analyses thus separately identified concordant parameters whose adaptations ensure the wide range of HRs over which electrophysiological activation takes place in an absence of heart block or arrhythmias in equine hearts. Since the horse is amenable to this in vivo electrophysiological analysis and displays a unique wide range of heart rates, it could be a novel cardiac electrophysiology animal model for the study of sudden cardiac death in human athletes.

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

“…In ventricular myocytes, Phase 4 constitutes the “resting phase,” during which the negative resting membrane potential of approximately −90 mV is generated by an outward K + ‐current (I K1 ) through constitutively opened K + ‐channels K IR 2.1 and 2.2 in the myocyte membrane 20,21 . Depolarising current coming from a neighbouring cell can trigger the opening of fast voltage‐gated Na +‐ channels Nav1.5 leading to a rapid cell membrane depolarisation driven by an inward Na + ‐current (I Na ) 22 .…”

“…Ventricular, atrial and nodal cells often react very differently to the pro‐arrhythmic factors implicated in AF and to anti‐arrhythmic drugs. Targeted pharmacological therapies focused on atrial‐specific modulations aim to counteract atrial electric instability without disturbing the normal ventricular activity 20,28 . In horses, less data are available, only monophasic action potentials of the right atrium and ventricle have been recorded in the standing horse 33 …”

“…The amount of data available on AF pathophysiology in the horse is unfortunately limited and it is not clear whether equine AF involves hierarchical or anarchical mechanisms or whether both mechanisms are likely. Techniques such as intracardiac mapping 61–63 and computer‐based ECG analysis 20,64–66 are in their infancy for horses and hopefully should improve our understanding of this complex rhythm disturbance. Imaging techniques 67–69 and molecular biological data 15–17 are still limited, but a lot can be learned from studies in animal models and human patients 21,28 …”

“…The effect of quinidine may also vary in different cardiac regions depending on its affinity for the channels expressed in a particular area. For example, the inhibition of I Kur by quinidine in human is specific to the atria 20 and its effect on action potential duration is therefore higher in this region. Unfortunately, the differential expression of ion channels in the equine heart has been poorly described 15–17 …”

The application of Lempel-Ziv and Titchener complexity analysis for equine telemetric electrocardiographic recordings

“…To eliminate this subjectivity various objective measures have been proposed. In our previous paper we explored use of ECG restitution analysis to evaluate the function of equine cardiovascular system 9 . The restitution analysis has been shown to indicate the integrated mechanoelectrical impacts on the heart and of arrhythmia liability 10 .…”