Study design

This was a prospective observational case‐control study conducted in the Cardiovascular Performance Program Laboratory at Massachusetts General Hospital, Boston, MA, USA. We recruited male participants with established HCM and healthy age‐ and sex‐matched controls without left ventricular hypertrophy. All participants underwent two study visits. During the first study visit, participants completed a questionnaire that included demographics, personal and family medical history, current medication use, exercise exposure including current duration and type of exercise per week, and dietary and lifestyle habits. This was followed by measurement of anthropometric data and performance of both a comprehensive resting transthoracic ECG and a maximal effort cardiopulmonary exercise test (MECPET). During the second study visit, participants completed an incremental submaximal cycle ergometer exercise test with blood sampling designed to assess concentrations of plasma catecholamines as described below.