Myocardial Fibrosis is Present in Thoroughbred Racehorses with Sudden Cardiac Death

Background: Sudden cardiac death (SCD) due to ventricular arrhythmia occurs in intensively trained human athletes. Horses undergo cardiac adaptations to training in a similar manner to human athletes and are a naturally occurring model for the Athlete's Heart. In rodent studies, athletic training induces myocardial fibrosis and this is a substrate for arrhythmia. The extent of myocardial fibrosis is yet to be quantified in other animal models such as the horse. Methods: Cardiac tissues were harvested at post mortem from horses without structural heart disease that died from sudden unexplained cardiac death (SCD, n = 8 TBH), non-cardiac death (NC, n = 8 TBH), or healthy untrained horses (UT, n = 10 brumbies). Global and interstitial fibrosis was assessed in the right ventricular free wall (RVFW), left ventricular free wall (LVFW) and left ventricular papillary muscle (LVPM). Groups were compared by ANOVA and students t-test. Results: At each site examined, mean global and interstitial fibrosis was highest in the SCD group. In LVPM, mean interstitial fibrosis was higher in SCD (4.05 ± 1.9%) than NC (1.87 ± 1.0%), p = 0.002, and UT (1.73 ± 0.4%), p = 0.001. In LVFW, mean interstitial fibrosis was higher in SCD (5.00 ± 3.5%) than UT (2.41 ± 0.6%), p = 0.038. Conclusion: Myocardial fibrosis was greater in horses with SCD compared to similarly trained horses with death from a non-cardiac cause, or untrained horses. Myocardial fibrosis could be a substrate for arrhythmia and SCD in this species. These results may have application to intensively trained human athletes.