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 tension ( ) and physiological dead-space fraction of the breath ( / ) responses to rapid-incremental (or ramp) exercise. It is quantified as: 1) the slope of the linear region of the relationship between ventilation (') and pulmonary CO output (' ); and/or 2) the ventilatory equivalent for CO at the lactate threshold ('/' [Formula: see text]) or its minimum value ('/' 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 '/' [Formula: see text] and '/' min provide a rigorous index of an elevated / when is known (or can be assumed) to be regulated. In contrast, a high '-' slope on its own does not, as account has also to be taken of the associated normally positive and small ' intercept. Interpretation is complicated by factors such as: the extent to which is actually regulated during rapid-incremental exercise (as is the case for steady-state moderate exercise); and whether '/' [Formula: see text] or '/' min provide accurate reflections of the true asymptotic value of '/' , to which the '-' slope approximates at very high work rates.