Abstract
"Ventilatory efficiency" is widely used in cardiopulmonary exercise testing to make inferences regarding the normality (or otherwise) of the arterial CO(2) tension (P (aCO(2)) ) and physiological dead-space fraction of the breath (V (D)/V (T)) responses to rapid-incremental (or ramp) exercise. It is quantified as: 1) the slope of the linear region of the relationship between ventilation (V'(E)) and pulmonary CO(2) output (V'(CO(2)) ); and/or 2) the ventilatory equivalent for CO(2) at the lactate threshold (V'(E)/V'(CO(2)) [Formula: see text]) or its minimum value (V'(E)/V'(CO(2)) min), which occurs soon after [Formula: see text] but before respiratory compensation. Although these indices are normally numerically similar, they are not equally robust. That is, high values for V'(E)/V'(CO(2)) [Formula: see text] and V'(E)/V'(CO(2)) min provide a rigorous index of an elevated V (D)/V (T) when P (aCO(2)) is known (or can be assumed) to be regulated. In contrast, a high V'(E)-V'(CO(2)) slope on its own does not, as account has also to be taken of the associated normally positive and small V'(E) intercept. Interpretation is complicated by factors such as: the extent to which P (aCO(2)) is actually regulated during rapid-incremental exercise (as is the case for steady-state moderate exercise); and whether V'(E)/V'(CO(2)) [Formula: see text] or V'(E)/V'(CO(2)) min provide accurate reflections of the true asymptotic value of V'(E)/V'(CO(2)) , to which the V'(E)-V'(CO(2)) slope approximates at very high work rates.