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., Peña, F., Am Rev Respir Dis 1992, 46: 1206–9). We questioned the contribution of vagal reflexes to these differences in breathing pattern.

2. Measurements were performed on high-altitude (La Paz, Bolivia, 3600–4050 m, inspired O2 pressure ∼92 mmHg, n = 34) and low-altitude infants (Santa Cruz, Bolivia, 400 m, Pio2 ∼141 mmHg, n = 26). The strength of the Hering—Breüer inspiratory inhibitory reflex was estimated from the inspiratory time during a respiratory effort against airways closed at end-expiration (TIoccl). The strength of the Hering—Breüer expiratory facilitatory reflex was estimated from the expiratory duration when airways were occluded during expiration (TEoccl).

3. TIoccl was significantly longer than the open-airways TI at both low and high altitude, but significantly more so (∼14%) at high altitude. TEoccl was longer than open-airways TE in both groups of infants, but significantly less so at high altitude, whether TEoccl was compared between occlusions of similar tidal volume (on average, TEoccl at high altitude was 79% of that at low altitude) or similar airway pressure (87%).

4. The results suggest that at high altitude the contribution of the phasic volume-dependent vagal input to the inspiratory off-switch threshold is higher, and that the tonic vagal expiratory facilitation is lower, than at low altitude, presumably because of hypoxia. The larger VT and longer TE of the HA infant cannot be explained by these differences in vagal pulmonary reflexes.

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