Ventilation/carbon dioxide output relationships during exercise in health

“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))[Image: see text]) or its minimum value (V′(E)/V′(CO(2))min), which occurs soon after [Image: 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))[Image: 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))[Image: 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.