Airway blood flow responses to temperature and humidity of inhaled air.

We determined the effect of breathing cold dry air (-39 degrees C, 0.1% relative humidity, RH) and warm humid air (43 degrees C, 100% RH) on airway mucosal blood flow (Qaw) in normal human subjects (n = 8, age 25-53 years) at rest. Qaw was measured with a dimethylether uptake technique which reflects blood flow in the mucosa of large airways corresponding to a 50 ml anatomical dead-space segment extending distally from the trachea. Mean Qaw was 10.1 +/- 1.9 ml min-1 (mean +/- S.D.) during room air breathing (25 degrees C, 70% RH) and decreased to 4.7 +/- 2.1 ml min-1 during cold dry air breathing (p < 0.05). Within 20 min of resuming room air breathing, mean Qaw had returned to baseline. Breathing warm humid air had no significant effect on mean Qaw (8.2 +/- 1.4 ml min-1). These results indicate that quiet breathing of frigid air causes vasoconstriction in central airways.