Introduction Athletes who train regularly can develop left ventricular (LV) hypertrophy, which can be difficult to differentiate from hypertrophic cardiomyopathy. We aimed to use cardiovascular magnetic resonance (CMR) tissue characterisation to assess the cardiac effects of complete detraining.

Methods Athletes (n=26) with a limb fracture that prohibited training for >1 month had identical CMR protocol including volumetric analysis and T1 mapping on Philips Acheiva 3.0 T<1 week after their injury and again after one and three months.

Results Athletes exercised for 7.7±2.3 hours per week and were predominantly male (21/26). LV mass at baseline was 131±27 g, decreasing to 120±23 g after one month detraining (p=0.003). It did not decrease further by three months 122±21 g (p=NS). Reduction in LV mass after one month was mediated by regression of the cardiac myocyte compartment (100±22 g to 90±18 g, p<0.001) with no change in the cardiac extracellular compartment (32±5 g to 31±6 g, p=0.25). There were corresponding increases in native T1 (1226±29 ms to 1237±29 ms, p=0.048) and (ECV 24.4±2.3% to 25.8±2.4%, p=0.003). Baseline cardiac myocyte mass indexed to body surface area was predictive of which athletes would have >10% decrease in LV mass (n=9) after one month detraining (AUC 0.90, 95% CI: 0.71 to 0.98, p<0.0001) with optimal cut-off baseline myocyte mass index >54 g/m² (67% sensitivity, 94% specificity).

Conclusion Regression of LV hypertrophy occurs within one month of complete detraining and is mediated by regression of cardiac myocyte mass rather than extracellular compartment. In the athlete with unexplained LV hypertrophy increased myocyte mass has the potential to diagnose physiological remodelling without the need for detraining.