Pulmonary Gas Exchange in Severe Chronic Asthma: Response to 100% Oxygen and Salbutamol

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Abstract
Ventilation-perfusion ( A/ ) inequality has been evaluated using the multiple inert gas technique in nine nonsmoking patients (mean ± SD, age 56 ± 10 yr) with stable, severe, chronic asthma (partially reversible airway obstruction; baseline FEV1, 39 ± 10% predicted) before and during 100% O2 breathing and then 15 min after three puffs (300 µg) of inhaled salbutamol. The aim of this study was to investigate whether this type of asthma was associated with a different pattern of a/ inequality from that observed in acute episodes and in particular to determine whether the a/ pattern was fixed or could be altered by bronchodilator agents or O2 breathing. The predominant pattern of a/ distribution was broad and unimodal but without shunt ( a/ = 0) or low a/ areas ( a/ < 0.1 to > 0.005). The amount of a/ inequality as assessed by the dispersion of the distribution of pulmonary bloodflow (log SDQ) was not great (log SDQ, 0.77 ± 0.09), and no correlation was found with the degree of airway obstruction, PaO2 or AaPO2. During 100% O2 breathing, a/ inequality worsened (from log SDQ of 0.77 ± 0.09 to 1.11 ± 0.21, p = 0.01) with an increase in the perfusion of low a/ units (from 0.43 ± 0.66% to 6.3 ± 6.5%, p = 0.02) but still no development of shunt. This suggests the presence of hypoxic pulmonary vasoconstriction breathing air, possibly contributing to the preservation of a/ relationships. Inhaled salbutamol improved FEV1 by 35%, but had no effect on a/ relationships nor on PaO2. These findings may reflect preferential distribution of the aerosol to those pulmonary areas already well ventilated or, alternatively, no effect of salbutamol on those abnormalities responsible for the gas exchange defects.