From this discussion, it can be hypothesized that the deterioration in functional status from normal subjects to class D patients may be associated with a gradual reduction in the level of daily activity and consequently in peripheral skeletal muscle oxidative capacity . This, in turn, could account for the increase in La/VO2 steepness that occurred as functional status decreased. Our study, however, did not address whether abnormal lactate kinetics contribute to a reduction in exercise tolerance or whether they are secondary to declining functional status. Although abnormal lactate kinetics during exercise may be only a marker of poor peripheral muscle function associated with chronic inactivity it could possibly be involved in early exercise termination by increasing the burden on the respiratory system. The clinical importance of this mechanism remains uncertain because no significant relationship was found between peak plasma lactate concentrations and VE within each functional class of patients. However, it is conceivable that the ventilatory response to a given degree of lactic acidosis may vary markedly among individuals .