Ward SA;

European respiratory review : an official journal of the European Respiratory Society [Eur Respir Rev] 2021 Apr 13; Vol. 30 (160). Date of Electronic Publication: 2021 Apr 13 (Print Publication: 2021).

“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.