Performance horses as a model for exercise-associated cardiac arrhythmias and sudden cardiac death

Figures

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  Fig. 1. Comparison of the causes of sudden death of human athletes (top row) and equine athletes (bottom row) identified at post-mortem examination. In humans, there is a larger percentage of athletes who have underlying cardiac abnormalities (all shades of red combined), compared with horses where a large percentage are necropsy-negative (grey). The “Cardiac – Miscellaneous” category in Dennis et al. 2018 is notably larger than other studies due to inclusion of adult (>35 years of age) athletes, and therefore a greater percentage of coronary artery disease which is less common in younger populations. “Cardiac – Miscellaneous” in equine studies include mild, focal areas of fibrosis, of unknown clinical significance [1,8,10,11,59,60]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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  Fig. 2. Purkinje cells stained with hematoxylin and eosin within the myocardium of a 7-month-old foal (A, B) and adult cat (C) for comparison. (A) Full thickness slice (4× magnification) of the left ventricular free wall. Arrowheads show the sub-endocardial location of Purkinje cells while black arrows denote the location of an intramural Purkinje fiber. (B) Enlargement of A showing the Purkinje cells located between the black arrows in the sub-endocardium (20× magnification). (C) Purkinje cells located in the sub-endocardium of an adult cat (black arrow, 40× magnification). Key: End – endocardium; Epi – epicardium.

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  Fig. 3. Diagrammatic representation of the orientation of the heart and genesis of a base-apex ECG in horses and humans. Approximate location of the heart (red outline) in the chest and mean electrical axis (arrow) in humans (A) and horses (B). Additionally, the placement of positive and negative electrodes to capture the main electrical vector of each species is shown with the corresponding ECG for humans (C) and horses (D). Note that despite placement of the electrodes being in the same positions relative to the orientation of the heart, the ECG signals are inverted due to differences in the pattern of ventricular activation. An approximate scale bar shows the relative duration of the average QT interval between horses and horses at approximately resting heart rate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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  Fig. 4. Diagrammatic representation of the right ventricular monophasic action potential in humans (A), and horses (B). Ion channels known (humans) or thought to be (horses) responsible for the primary ion currents in each phase of the action potential are listed. Action potential of the horse is based on work by De Clercq et al. [50]. Ion channels in blue font are hypothesized based on mRNA studies and have not been confirmed in horses. Additionally, there are potentially other unidentified channels contributing to the action potential in horses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)