Subject specific effects of hyperpnea but not hypocapnia on airway conductance.

We investigated the effects of hypocapnia in normal subjects on airway tone while controlling airway cooling and drying. We hypothesized that airway tone is positively related to the degree of hypocapnia. Participants (8; 2 women) underwent 3 protocols consisting of 20 min of hyperpnea (breathing frequency = 20 breaths min-1; tidal volume = 2.5 L) and 10 min recovery. End-tidal PCO2 was maintained at +1 Torr above rest (ISO; 37.9 ± 1.2 Torr), 8 Torr below resting values (H-8; 29.2 ± 1.7 Torr) or 15 Torr below resting values (H-15; 23.2 ± 2.9 Torr). Breath-by-breath lung conductance (GL) was calculated from flow, volume, and esophageal pressure. GL responses to hyperpnea varied widely across subjects. However, individual responses during ISO correlated highly with responses during H-8 (r = 0.976, p < 0.001) and H-15 (r = 0.952, p < 0.001), with the magnitude of change inversely related to basal GL (r = -0.555, p = 0.006). Thus, inter-subject variation in GL was due to hyperpnea, with no detectable effect of hypocapnia.