Objective: To examine the submaximal and maximal indices of the exercise response of patients with hypertrophic cardiomyopathy.
Design and setting: Prospective examination of cardiopulmonary responses to ramp exercise test of a consecutive group of patients with hypertrophic cardiomyopathy attending a cardiomyopathy outpatient clinic.
Methods: 50 patients aged 12 to 76 years (mean (SD) 35 (14)) with diagnosis of hypertrophic cardiomyopathy performed incremental cycle ergometry; 22 sedentary volunteers (seven female, 15 male) aged 14 to 58 years (mean (SD) 31 (12)) served as controls. Respiratory gas was continuously sampled from the mouth-piece, and its concentration profile phase aligned to the respired air flow signals. Following analogue to digital conversion, gas exchange variables were computed breath by breath and the data were averaged every 30 seconds for graphic display. A 12 lead ECG was monitored continuously and recorded every three minutes during the exercise.
Results: Both the peak oxygen uptake attained on the test (VO2 peak) and anaerobic threshold were reduced in patients with hypertrophic cardiomyopathy compared with the control group (p < 0.0001). In 29 patients (59%) the VO2 peak was less than 60% and only two patients achieved a peak above 80% of their predicted values. The anaerobic threshold was below 60% of the predicted value in 31 patients and above 80% in only three patients. The slope of oxygen uptake/work rate relation (delta VO2/delta WR) was decreased in 16 patients (32%). The maximum oxygen pulse (VO2/HR) was reduced as a percentage of the predicted value, and became flat at high work rates in 32 patients. There was a significant correlation between anaerobic threshold and VO2 peak (p < 0.0001), work efficiency (p < 0.0001), and maximum oxygen pulse (p < 0.0001). The slope of change in ventilation against change in carbon dioxide output (delta VE/delta VCO2) for the subanaerobic threshold range was increased in 36 patients (72%) and was inversely correlated with anaerobic threshold (p < 0.0002).
Conclusions: There were severe abnormalities in maximal and submaximal indices of pulmonary gas exchange in a cohort of hypertrophic cardiomyopathy patients attending a referral cardiovascular clinic. The pattern of the abnormalities suggests that a reduced stroke volume response, ventilation/perfusion mismatch, and abnormal peripheral oxygen utilisation are the possible mechanisms of exercise intolerance.