Airway blood flow responses to temperature and humidity of inhaled air

We determined the effect of breathing cold dry air (−39°C, 0.1% relative humidity, RH) and warm humid air (43°C, 100% RH) on airway mucosal blood flow (Q̇aw) in normal human subjects (n = 8, age 25–53 years) at rest. Q̇aw 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 Q̇aw was 10.1 ± 1.9 ml min−1 (mean ± S.D.) during room air breathing (25°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 Q̇aw had returned to baseline. Breathing warm humid air had no significant effect on mean Q̇aw (8.2 ± 1.4 ml min−1). These results indicate that quiet breathing of frigid air causes vasoconstriction in central airways.