Effects of sedation produced by thiopentone on responses to nasal occlusion in female adults.

To test the hypothesis that full wakefulness is an important factor in the control of the route of breathing in adult humans, we have studied the responses to nasal occlusion before and during sedation with thiopentone in 14 female subjects. A tightly fitting partitioned face mask separated the nasal and oral breathing routes. Nasal and oral breathing were identified from changes in carbon dioxide concentration and airway pressure in the different compartments of the face mask. Arterial oxygen saturation (SpO2) was monitored simultaneously. Eleven of 14 subjects breathed only through the nose (nasal breathers) both before and during sedation. In these subjects, the time required to initiate oral breathing in response to nasal occlusion during sedation was significantly longer than that before sedation (mean 37.7 (SD 15.5) s vs 3.2 (1.3) s (P < 0.01)). Also, there was a significant difference (P < 0.01) in the smallest values of SpO2 attained during nasal occlusion before (98.0 (0.8)%) and after (89.3 (4.3)%) sedation. In adult humans the ability to maintain adequate ventilation by switching from the nasal to the oral route in response to nasal occlusion is greatly impaired during sedation, probably because of the impairment of conscious influence on the control of the palatal muscles.