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 ) or its minimum value ( V ′ E / V ′ CO 2 min), which occurs soon after 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 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 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.