Effect of eucapnic hypoxia on bronchomotor tone and on the bronchomotor response to dry air in asthmatic subjects.

Because hypoxia has been shown to cause bronchoconstriction and to potentiate bronchomotor responsiveness in animals, we investigated whether hypoxia has similar effects in subjects with asthma. We measured specific airway resistance (SRaw; the mean of 5 sequential readings taken 30 s apart) before and immediately after each of 15 asthmatic subjects breathed a mixture of 8% O2 in N2 until hemoglobin saturation (SaO2; by ear oximetry) fell to 80% or less for at least 2 min. We maintained end-tidal CO2 at resting levels, the temperature of the inspirate at 22.0 +/- 1.1 degrees C, and the dew point at 18.5 +/- 1.6 degrees C (mean +/- SD). The SaO2 fell to 70 +/- 8%; minute ventilation rose to 28.4 +/- 8.5 L/min, and heart rate rose by 27 +/- 6 beats/min. The SRaw did not increase significantly in the group (baseline SRaw, 6.61 +/- 2.36; posthypoxia SRaw, 6.69 +/- 2.21 L X cm H2O/L/s) or in any subject. To determine if hypoxia increases bronchomotor responsiveness, we also compared the responses to eucapnic hyperpnea with dry air and with dry gas mixtures of 7 to 10% O2 in N2 in a randomized, double-blind sequence in 9 of the subjects. We measured SRaw in each subject before and after stepwise increases in minute ventilation, for 3 min at each level, until SRaw doubled or until the subject's maximal voluntary ventilation was achieved. The SaO2 fell to 82% or less at each level of ventilation with the hypoxic gas mixture. The 2 stimulus-response curves thus obtained did not differ in any subject.(ABSTRACT TRUNCATED AT 250 WORDS)