Ventilation-perfusion distribution in normal subjects

Abstract

Functional values of LogSD of the ventilation distribution (σ(V)) have been reported previously, but functional values of LogSD of the perfusion distribution (σ(q)) and the coefficient of correlation between ventilation and perfusion (ρ) have not been measured in humans. Here, we report values for σ(V), σ(q), and ρ obtained from wash-in data for three gases, helium and two soluble gases, acetylene and dimethyl ether. Normal subjects inspired gas containing the test gases, and the concentrations of the gases at end-expiration during the first 10 breaths were measured with the subjects at rest and at increasing levels of exercise. The regional distribution of ventilation and perfusion was described by a bivariate log-normal distribution with parameters σ(V), σ(q), and ρ, and these parameters were evaluated by matching the values of expired gas concentrations calculated for this distribution to the measured values. Values of cardiac output and LogSD ventilation/perfusion (Va/Q) were obtained. At rest, σ(q) is high (1.08 ± 0.12). With the onset of ventilation, σ(q) decreases to 0.85 ± 0.09 but remains higher than σ(V) (0.43 ± 0.09) at all exercise levels. Rho increases to 0.87 ± 0.07, and the value of LogSD Va/Q for light and moderate exercise is primarily the result of the difference between the magnitudes of σ(q) and σ(V). With known values for the parameters, the bivariate distribution describes the comprehensive distribution of ventilation and perfusion that underlies the distribution of the Va/Q ratio.

Fig. 1.

Examples of end-expiratory concentrations of helium (He; ■), acetylene (Ac; ◆), and dimethyl ether (DME; ▲) normalized by inspired gas concentrations, and model values (open symbols connected by lines) vs. breath number at rest (A) and at moderate exercise (B). The vertical broken line in B marks the estimated breath number at which recirculation of blood occurs, and, in this example, the fit was made to data for Ac and DME for breaths 1–7.

Fig. 2.

Means ± SE values of cardiac output (Q̇) vs. minute ventilation (V̇) for rest and three exercise levels.

Fig. 3.

Means ± SE values of the width of the ventilation (σ_v̇; triangles), perfusion (σ_q̇; squares), and ventilation-perfusion ratio (σ_v̇/q̇; diamonds) for rest (R) and light (L), moderate (M), and heavy (H) exercise levels. The decreases in σ_q̇ and σ_v̇/q̇ at the onset of exercise are statistically significant.

Fig. 4.

Comprehensive distribution of ventilation (F) as a function of ln v̇ and ln q̇ for the parameter values for moderate exercise.

Fig. 5.

MIGET distributions of ventilation [V̇(v̇_A/q̇)] and perfusion [Q̇(v̇_A/q̇)] derived from the comprehensive distribution for moderate exercise. The scale of the ordinate in this figure is different from the conventional scale of the graphs generated by MIGET. Here, the areas under the curves equal total alveolar ventilation (V̇_A) and total perfusion (Q̇).