Breathing pattern affects airway wall temperature during cold air hyperpnea in humans.

We studied the influence of flow rate on respiratory heat exchange in 9 healthy adult subjects using a new noninvasive technique, the single-breath temperature washout (SBTW) curve. The SBTW curve is a plot of exhaled gas temperature versus exhaled volume during a standard exhalation and consists of an initial rise (within the first 200 ml) to a plateau temperature that persists through the remainder of exhalation. We found that exhaled gas temperatures within the initial expirate were colder at every airway locus than corresponding intra-airway gas temperatures at end-inspiration, suggesting that heat exchange occurs between lumenal gas and the relatively cooler airway walls during exhalation. The SBTW plateau temperatures were: (1) lower after preconditioning the airways with rapid (80 L/min) isocapnic hyperpnea of frigid air than after less rapid (40 L/min) cold-air hyperpnea or after quiet breathing; (2) lower when, after identical airway preconditioning regimens, the SBTW exhalation was performed with a slower (0.5 versus 2.5 L/s) expiratory flow; and (3) lower when SBTW curves were obtained after airway preconditioning using respiratory patterns with larger inspiration-expiration duration (I:E) ratios (5:1 versus 1:5) at fixed minute ventilation and respiratory rate. Our results indicate that the global respiratory gas-wall heat transfer coefficient increases with velocity to the 0.9 power, a finding similar to that in previous studies of turbulent flow in rigid pipes.