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 CO₂ tension (P _aCO ) 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 CO₂ output (V'_CO ); and/or 2) the ventilatory equivalent for CO₂ at the lactate threshold (V'_E/V'_CO [Formula: see text]) or its minimum value (V'_E/V'_CO 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 [Formula: see text] and V'_E/V'_CO min provide a rigorous index of an elevated V _D/V _T when P _aCO is known (or can be assumed) to be regulated. In contrast, a high V'_E-V'_CO 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 is actually regulated during rapid-incremental exercise (as is the case for steady-state moderate exercise); and whether V'_E/V'_CO [Formula: see text] or V'_E/V'_CO min provide accurate reflections of the true asymptotic value of V'_E/V'_CO , to which the V'_E-V'_CO slope approximates at very high work rates.