HERING-BREUER REFLEXES IN HIGH-ALTITUDE INFANTS

1. Pulmonary ventilation was found to be similar in high-altitude and low-altitude newborn infants, but the breathing pattern was deeper and slower at high altitude (Mortola, J.P., Frappell, P.B., Frappell, D.E ., Villena-Cabrera, N., Villena-Cabrera, M., Pena, F., Am Rev Respir D is 1992, 46: 1206-9). We questioned the contribution of vagal reflexes to these differences in breathing pattern. 2. Measurements were perfo rmed on high-altitude (La Paz, Bolivia, 3600-4050 m, inspired O-2 pres sure similar to 92 mmHg, n = 34) and low-altitude infants (Santa Cruz, Bolivia, 400 m, PIO2 similar to 141 mmHg, n = 26). The strength of th e Hering-Breuer inspiratory inhibitory reflex was estimated from the i nspiratory time during a respiratory effort against airways closed at end-expiration (T-loccl). The strength of the Hering-Breuer expiratory facilitatory reflex was estimated from the expiratory duration when a irways were occluded during expiration (T-Eoccl). 3. T-loccl was signi ficantly longer than the open-airways T-l at both low and high altitud e, but significantly more so (similar to 14%) at high altitude. T-Eocc l was longer than open-airways T-E in both groups of infants, but sign ificantly less so at high altitude, whether T-Eoccl was compared betwe en occlusions of similar tidal volume (on average, T-Eoccl at high alt itude was 79% of that at low altitude) or similar airway pressure (87% ). 4. The results suggest that at high altitude the contribution of th e phasic volume-dependent vagal input to the inspiratory off-switch th reshold is higher, and that the tonic vagal expiratory facilitation is lower, than at low altitude, presumably because of hypoxia. The large r V-T and longer T-E of the HA infant cannot be explained by these dif ferences in vagal pulmonary reflexes.