Ventilation/carbon dioxide output relationships during exercise in health

Abstract

"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 [Formula: see text]) or its minimum value (V'_E/V'_CO2 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'_CO2 [Formula: see text] 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 [Formula: see text] 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.

FIGURE 1

Schematic representation of a) ventilation (V′_E), b) ventilatory equivalent for CO₂ (V′_E/V′_CO2) and c) physiological dead space fraction of the breath (V_D/V_T) as a function of CO₂ output (V′_CO2) during rapid-incremental (or ramp) exercise for work rates where arterial CO₂ tension (P_aCO2) is assumed to be stable. m: V′_E–V′_CO2 slope; c: V′_E intercept. Modified with permission from a, b) [15] and c) [16].

FIGURE 2

Alveolar end-tidal P_CO2 and P_O2 (P_ACO2, P_AO2), ventilation (V′_E), CO₂ output (V′_CO2), O₂ uptake (V′_O2), arterial [HCO₃⁻] and arterial pH as a function of work rate for a rapid-incremental cycle-ergometer exercise test. The lactate threshold occurs when arterial [HCO₃⁻] starts to buffer the metabolic acidosis, evidenced in the accelerating V′_CO2 response (left vertical dashed line). As V′_E continues to respond in proportion to V′_CO2, it therefore increases out of proportion to V′_O2, and P_AO2 therefore begins to rise. P_ACO2 remains stable until respiratory compensation for the metabolic acidosis develops (right vertical dashed line; isocapnic buffering). Beyond this point, V′_E increases at a greater rate than V′_CO2, and P_ACO2 therefore starts to decreases (respiratory compensation). Reproduced with permission from [3].

FIGURE 3

Schematic representation of a) the effects of a hypothetical intervention that increased the V′_E–V′_CO2 slope (m) with no change in the V′_E intercept (c) on ventilation (V′_E) and b) ventilatory equivalent for CO₂ (V′_E/V′_CO2) as a function of CO₂ output (V′_CO2) during rapid-incremental (or ramp) exercise for work rates where arterial CO₂ tension (P_aCO2) is assumed to be stable. Solid profiles: pre-intervention; dashed profiles, post-intervention.

FIGURE 4

Schematic representation of a) the effects of a hypothetical intervention that increased the V′E-intercept (c) with no change in the V′_E–V′_CO2 slope (m) on ventilation (V′_E) and b) ventilatory equivalent for CO₂ (V′_E/V′_CO2) as a function of CO₂ output (V′_CO2) during rapid-incremental or ramp exercise for work rates where arterial CO2 tension (P_aCO2) is assumed to be stable. Solid profiles: pre-intervention; dashed profiles, post-ntervention.

FIGURE 5

The V′_E–V′_CO2 relationship for ramp exercise in which arterial CO₂ tension (P_aCO2) has been demonstrated to increase progressively up to the lactate threshold [25, 56, 57] (dashed line), compared with that obtaining with P_aCO2 constrained not to increase (solid line). Note the decreased slope in the former condition. Data taken from [56].

FIGURE 6

The corresponding V′_E/V′_CO2 relationships related to figure 5. Note the depressed V′_E/V′_CO2 profile consequent to the progressive P_aCO2 increase (circles, dashed curve), compared with that obtaining with arterial CO₂ tension (P_aCO2) constrained not to increase (squares, solid curve). As a result, V′_E/V′_CO2min was decreased in the former condition.