Upper airway collapsibility during general anaesthesia predicts severity of sleep apnoea

Upper airway (UAW) collapsibility during wakefblness is poorly related to collapsibility during sleep, presumably because of differences in respiratory drive, muscle tone, reflex gain and cortical influences. We hypothesise that the differences would be less between general anaesthesia and sleep and that UAW behaviour during anaesthesia may be predictive of its behaviour during sleep. Methods: During anaesthesia we studied the pressure-flow relationships of the UAW in 22 healthy subjects immediately after minor surgery. Each subject was maintained at a stable anaesthetic depth (end-tidal isofluorane = 0.80i0.1%, SD} while supine and spontaneously breathing on nasal CPAP (mouth occluded) with mask pressure (PN) set to 11.6±2.4 cmH:0. Periodically PN was variably reduced and the relationship between PN and maximal inspirator}flow (Vi) determined. The critical closing pressure (Peril) was defined as the PN at which flowwas abolished. Two to 5 weeks following surgery each subject underwent overnight polysomnography to determine apnoea-hypopnea index (AMI). Results: The airway remained hypotonie (absent intramuscular genioglossus EMC activity) throughout each study in all subjects. Progressive reductions in PN resulted in the appearance of flow limitation. Vi during flow-limited breaths decreased linearly with decreasing PN (r = 0.98±0.02), consistent with starling resistor behaviour. Peril ranged from -4.6 to 9.8 cmH20 (mean 1.2±2.9 cmHiO). AMI ranged from 0.5 lo 31.6 /hr (mean 7.2±8.8 /hi). Logistic regression analyses demonstrated a high positive predictive power (82%) of Peril for the presence of sleep apnoea (AHI>10). Odds ratios w-ere larger for REM (1.75) than NREM sleep (1.52) indicating a stronger association with Peril. Conclusion: The degree of UAW collapsibility during anaesthesia appears to be related to the propensity for sleep-disordered breathing. The predictive power is greatest for REM sleep where, like anaesthesia, the UAW is hypotonie.