[Mechanism of hypoxemia after methacholine challenge]

Hypoxemia after the treatment of asthmatic attack continues for a while, and can even become worse after the treatment. This hypoxemia has been attributed to inhomogeneity of ventilation-to-perfusion ratios in the lungs of asthmatic patients. We studied post-hyperventilation hypoxemia in subjects with hypoxemia after a methacholine challenge test. Nineteen subjects who were hyperresponsive and hypoxemic after a methacholine challenge were studied. The methacholine challenge with incremental doses was stopped when the respiratory resistance doubled. Thereafter, oxygen and carbon dioxide concentrations in the expired air, mouth flow (VE), and oxygen saturation (using a pulse oximeter, SpO2) were monitored continuously. End-tidal PCO2 (PETCO2) as well as respiratory exchange ratio (R) were thus obtained. In 14 subjects, VE significantly increased from 8.6 +/- 1.2 (mean +/- SD) to 13.4 +/- 2.6 l/min and then decreased gradually, while PETCO2 significantly decreased from 38.5 +/- 2.3 to 29.1 +/- 4.4 Torr and then increased gradually, SpO2 decreased from 97 +/- 1.3 to 89 +/- 2.4% concomitant with a significant decrease in R from 0.86 +/- 0.04 to 0.67 +/- 0.05. In contrast, in 5 subjects VE significantly increased from 7.5 +/- 2.8 to 12.4 +/- 1.6 l/min and then remained unchanged, while PETCO2 significantly decreased from 37.9 +/- 1.3 to 31.3 +/- 3.9 Torr and then remained unchanged, SpO2 decreased from 98 +/- 1.1 to 92 +/- 1.9% with no change in R (from 0.84 +/- 0.06 to 0.87 +/- 0.06).(ABSTRACT TRUNCATED AT 250 WORDS)