Dysrhythmias

Learning objectives

To discuss aetiopathogenesis, clinical findings, diagnosis, management and prognosis for dysrhythmias found in performance horses
Dysrhythmias are an uncommon, but important cause of poor performance and sudden death. Martin et al. (2000) proposed that lameness was the most common cause, with lower airway disease, upper airway disease and then cardiac disease, respectively, in that order, as causes of poor performance in a group of racehorses. Muscle disease was not discussed in this study, but probably is a more common cause than cardiac disease. Twenty-five per cent of the cases in the previous study (Martin et al. 2000) had more than 2 problems: concurrent upper airway disease and dysrhythmias were common, but this finding has not been published elsewhere.
It is worth appreciating that normal horses do have conduction abnormalities at all phases of exercise, but are particularly prevalent in the warm-up and warm-down periods. Ryan et al. (2005) found a significant number of horses in racetraining to have dysrhythmias. Out of 98 horses, 23 (23.5%) had ventricular premature depolarisations (VPD) in one phase, 4 (4%) in 2 phases and one horse (1%) had VPD in 4 of the 5 exercise phases. Three horses had VPD during peak exercise. Seventeen horses (17.3%) had supraventricular premature depolarisations (SVPD) in one exercise phase, 4 (4%) in 2 phases and one horse (1%) in 3 of the 5 exercise phases. Overall, 42 horses (40%) had at least one premature depolarisation, with 8 horses (8.2%, CI 2.7–13.6%) having both SVPD and VPD.
The next challenge then faced by the clinician is deciding when dysrhythmias are likely to be significant. Martin et al. (2000) proposed that they are classified clinically important if >2 isolated premature depolarisations are detected during peak exercise, or if multiple (>5) pairs or paroxysms of premature depolarisations are detected immediately after exercise, but the evidence-based medicine to support these recommendations beyond expert opinion is lacking. […]