Effects of active muscle mass size on cardiopulmonary responses to exercise in congestive heart failure

Previous studies from this laboratory demonstrated that in healthy young men, cardiac output is closely coupled to oxygen uptake during dynamic exercise, regardless of its mode or relative intensity, whereas other physiologic responses such as heart rate, blood pressure and ventilation are inversely related to the size of the active muscle mass when expressed as functions of oxygen uptake. The purpose of the current investigation was to determine whether congestive heart failure alters the pattern of physiologic responses to various modes of arm and leg exercise in proportion to the size of the active muscle mass. Cardiopulmonary responses to four modes of dynamic work (one arm curl, one arm cycle ergometry, one leg cycle ergometry and two leg cycle ergometry) were characterized in terms of absolute and relative intensities (oxygen uptake and mode-specific percent of peak oxygen uptake, respectively) in middle-aged men with congestive heart failure and control groups of healthy subjects and patients after myocardial infarction without heart failure. Peak oxygen uptake was reduced to the greatest extent in patients with heart failure for large muscle mass work (-13% for curl, -32% for one arm and one leg cycle ergometry and -37% for two leg cycle ergometry; p < 0.05 versus the normal group for the three modes of ergometry). This finding was paralleled by a markedly blunted slope for the cardiac output-oxygen uptake relation for leg but not arm exercise that was only partially compensated for by a widened arteriovenous oxygen difference. Blood pressure expressed as a function of oxygen uptake remained inversely related to active muscle mass size in all groups of subjects despite attenuation of systolic pressure for heavy large muscle mass effort in the group with heart failure. Pulmonary ventilation at a given metabolic rate was not influenced by active muscle mass size. Thus, saturation of capacity for systemic oxygen transport occurs in conjunction with blunted cardiac output reserve in patients with heart failure during exercise involving a smaller muscle mass than in healthy subjects. The basic inverse relation between size of the active muscle mass and blood pressure at a given metabolic rate is not altered by aging or reduced cardiac reserve. The muscle mass effect on ventilation seen in young healthy subjects disappears with aging.