Exercise intolerance in patients with chronic left heart failure: relation to oxygen transport and ventilatory abnormalities.

Circulatory, metabolic, and ventilatory responses to maximal and submaximal symptom-limited exercise were studied in 13 patients with chronic stable heart failure. Maximal exercise was sustained 6.5 +/- 0.6 minutes (mean +/- standard error of the mean) and increased minute oxygen consumption (VO2) to 940 +/- 65 ml/min, whereas submaximal exercise was sustained for 15.4 +/- 2.3 minutes and increased VO2 to 825 +/- 49 ml/min (both p less than 0.01 compared with maximal exercise). Both exercise protocols were terminated because of fatigue and both were associated with reduced cardiac output relative to VO2, marked systemic oxygen extraction (80 +/- 2% maximal versus 78 +/- 2% submaximal) and similarly elevated blood lactate concentrations (37 +/- 4 mg/dl maximal versus 36 +/- 4 mg/dl submaximal), suggesting inadequate oxygen delivery to working muscle. Minute ventilation during both types of exercise was also more than twice normal relative to carbon dioxide production. However, during submaximal exercise, dyspnea was noted in only 3 patients despite these ventilatory abnormalities. During maximal exercise, dyspnea was noted in 11 patients but did not force termination of exercise or preclude achievement of marked systemic oxygen extraction and lactate production. These data suggest that patients with chronic stable cardiac failure are limited during both maximal and submaximal exercise primarily by inadequate oxygen transport to working muscle.