Background: Premature depolarisations are commonly recorded during and immediately after strenuous exercise in racing horses. However, the clinical significance of these findings remains uncertain. Previous studies, during treadmill exercise, have identified an association between exercise-associated cardiac arrhythmia and hypercapnia and hyperlactatemia. However, associations between arrhythmia and lower and upper airway disease are unclear, with conflicting results reported in the few studies performed to date.
The aim of this study was to investigate potential associations between exercise-associated arrhythmia and the presence of tracheal mucus and / or blood in horses during routine race training.
Study design: Retrospective observational study.
Methods: Thoroughbred horses in race training had exercising electrocardiograms (ECGs) recorded during fast work. Post-exercise endoscopy was performed within 2h after exercise.
Results: A total of 78 examinations were performed in 43 horses. Of these, 49 ECGs were considered diagnostic and were included in the analysis. Exercise-associated arrhythmias were classified as none (n=10), isolated premature depolarisations (n=20) or complex arrhythmias. i.e. with couplets, triplets or salvos (n=19). No horses experienced atrial fibrillation. On endoscopy 41% of cases had excess tracheal mucus (grade 2 or more); median (IQR) grade = 1(1,2) and 41% had evidence of exercise-induced pulmonary haemorrhage (EIPH, grade 1 or more); median grade = 0 (0,1). Horses with excess tracheal mucus had 3.2 times increased odds of having complex arrhythmia (p=0.053). No significant associations were identified between cardiac arrhythmia and EIPH.
Clinical significance: Horses with excess tracheal mucus appear to be at increased risk of having complex arrhythmia during exercise, supporting the need for early identification and treatment of horses with increased tracheal mucus.