Hypoxic pulmonary vasoconstriction does not explain all regional perfusion redistribution in asthma

Abstract

Rationale: Regional hypoventilation in bronchoconstricted patients with asthma is spatially associated with reduced perfusion, which is proposed to result from hypoxic pulmonary vasoconstriction (HPV). Objectives: To determine the role of HPV in the regional perfusion redistribution in bronchoconstricted patients with asthma. Methods: Eight patients with asthma completed positron emission tomographic/computed tomographic lung imaging at baseline and after bronchoconstriction, breathing either room air or 80% oxygen (80% O2) on separate days. Relative perfusion, specific ventilation (sV), and gas fraction (Fgas) in the 25% of the lung with the lowest specific ventilation (sVlow) and the remaining lung (sVhigh) were quantified and compared. Measurements and Main Results: In the sVlow region, bronchoconstriction caused a significant decrease in sV under both room air and 80% O2 conditions (baseline vs. bronchoconstriction, mean 6 SD, 1.02 6 0.20 vs. 0.35 6 0.19 and 1.03 6 0.20 vs. 0.32 6 0.16, respectively; P, 0.05). In the sVlow region, relative perfusion decreased after bronchoconstriction under room air conditions and also, to a lesser degree, under 80% O2 conditions (1.02 6 0.19 vs. 0.72 6 0.08 [P, 0.001] and 1.08 6 0.19 vs. 0.91 6 0.12 [P, 0.05], respectively). The Fgas increased after bronchoconstriction under room air conditions only (0.99 6 0.04 vs. 1.00 6 0.02; P, 0.05). The sVlow subregion analysis indicated that some of the reduction in relative perfusion after bronchoconstriction under 80% O2 conditions occurred as a result of the presence of regional hypoxia. However, relative perfusion was also significantly reduced in sVlow subregions that were hyperoxic under 80% O2 conditions. Conclusions: HPV is not the only mechanism that contributes to perfusion redistribution in bronchoconstricted patients with asthma, suggesting that another nonhypoxia mechanism also contributes. We propose that this nonhypoxia mechanism may be either direct mechanical interactions and/or unidentified intercellular signaling between constricted airways, the parenchyma, and the surrounding vasculature.