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-aCO2) 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'(CO2)); and/or 2) the ventilatory equivalent for CO2 at the lactate threshold (V'(E)/V'(CO2)(LT) over cap) or its minimum value (V'(E)/V'(CO2)min), which occurs soon after (LT) over cap but before respiratory compensation. Although these indices are normally numerically similar, they are not equally robust. That is, high values for V'(E)/V'(CO2)(LT) over cap and V'(E)/V'(CO2) min provide a rigorous index of an elevated V-D/V-T when P-aCO2 is known (or can be assumed) to be regulated. In contrast, a high V'(E) - V'(CO2) 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-aCO2 is actually regulated during rapid-incremental exercise (as is the case for steady-state moderate exercise); and whether V'(E)/V'(CO2)(LT) over cap or V'(E)/V'(CO2) min provide accurate reflections of the true asymptotic value of V'(E)/V'(CO2), to which the V'(E)/V'(CO2) slope approximates at very high work rates.