Effect of common dead space on VA/Q distribution in the dog

Abstract

Several previous studies have shown worsening ventilation-perfusion (VA/Q) relationships in humans during heavy exercise at sea level. However, the mechanism of this deterioration remains unclear because of the correlation with ventilatory and circulatory variables. Our hypothesis was that the decrease in the series dead space-to-tidal volume ratio during exercise might be partly responsible because mixing in the common dead space can reduce apparent inequality. We tested this notion in 10 resting anesthetized normocapnic dogs passively hyperventilated by increase tidal volume and a) inspired CO2 or b) external dead space. We predicted less apparent VA/Q inequality in condition b because of mixing in the added dead space. After base-line measurements, conditions a and b were randomly assigned, and after a second set of base-line measurements they were repeated in the reverse order in each dog. VA/Q inequality was measured by the multiple inert gas elimination technique. Comparison of conditions a and b demonstrated that additional external dead space improved (P less than 0.001) the blood flow distributions as hypothesized [log standard deviation of perfusion = 0.49 +/- 0.02 (SE) in condition b and 0.61 +/- 0.03 in condition a with respect to 0.52 +/- 0.03 at base line]. This study suggests that the increased tidal volume during exercise could uncover VA/Q inequality not evident at rest because of the higher ratio of common dead space to tidal volume at rest.

Impact of Ventilation Frequency and Parenchymal Stiffness on Flow and Pressure Distribution in a Canine Lung Model

“…Dead space demonstrated a near-linear dependence on P̄ tp , with values consistent with previous estimates for a dog of this size 38 . Total gas volume in the model demonstrated a curvilinear relationship with P̄ tp as shown in Figure 3-B, consistent with nonlinear parenchymal stiffening of the acini (Figure 2).…”

Heterogeneity and matching of ventilation and perfusion within anatomical lung units in rats

“…Another mechanism that could be used to homogenize V̇ A /Q̇ inequalities across the lung is to mix expired and inspired gases through a larger common deadspace (Ross and Farhi, 1960; Tsukimoto et al, 1990). The average anatomical deadspace as a fraction of tidal volume in our animals can be calculated if a resting tidal volume is estimated from the literature.…”

Ventilation-perfusion inequality during normoxic and hypoxic exercise in the emu

“…Various mechanisms of the increase in V /Q inequality have been proposed, including 1) a reduced common dead space-to-tidal volume ratio and therefore reduced admixture of the expired air, unmasking existing V /Q heterogeneity present at rest (27); 2) nonuniform pulmonary vasoconstriction and increased pulmonary arterial pressure (4); 3) interstitial edema (22,23); or 4) ventilatory time-constant inequality (27). None of these mechanisms have been investigated in exercising birds; however, the rather nonelastic structure of the avian lung may reduce the importance of those mechanisms relying on pressure changes in the pulmonary tissue and/or microenvironment.…”