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Received March 9, 2004
Revised April 13, 2004
Accepted after revision June 14, 2004
1 Brigham and Women's Hospital/Harvard Medical School
2 Adelaide Institute for Sleep Health
* To whom correspondence should be addressed. E-mail: ajordan{at}rics.bwh.harvard.edu.
The termination of obstructive respiratory events is typically associated with arousal from sleep. The ventilatory response to arousal may be an important determinant of subsequent respiratory stability/instability and therefore be involved in perpetuating obstructive respiratory events. In healthy subjects arousal is associated with brief hyperventilation followed by more prolonged hypoventilation on return to sleep. This study was designed to assess whether elevated sleeping upper airway resistance (RUA) alters the ventilatory response to arousal and subsequent breathing on return to sleep in patients with obstructive sleep apnoea (OSA). Inspired minute ventilation (VI), RUA and end tidal CO2 (PETCO2) were measured in 22 patients (11 men, 11 women) with OSA (Mean ± SEM, AHI 48.9 ± 5.9 events·hr-1) during NREM sleep with low RUA (2.8 ± 0.3 cmH2O·l-1·s; Optimal CPAP [continuous positive airway pressure] = 11.3 ± 0.7 cmH2O) and with elevated RUA (17.6 ± 2.8cmH2O·l-1·s; Sub-Optimal CPAP = 8.4 ± 0.8 cmH2O). A single observer, blinded to respiratory data, identified spontaneous and tone-induced arousals of 3-15s duration arising from and returning to stable NREM sleep. VI was compared between CPAP levels before and after arousal in 16 subjects with spontaneous arousals in both conditions. During stable NREM sleep on Sub-Optimal CPAP, PETCO2 was mildly elevated (43.5 ± 0.8 versus 42.5 ± 0.8 Torr). However, baseline VI (7.8 ± 0.3 versus 8.0 ± 0.3 l·min-1) was unchanged between CPAP conditions. For the first three breaths following arousal, VI was higher on Sub-Optimal than Optimal CPAP (First breath: 11.2 ± 0.9 versus 9.3 ± 0.6 l·min-1). The magnitude of hypoventilation on return to sleep was not affected by the level of CPAP and both obstructive and central respiratory events were rare following arousal. Similar results occurred after tone-induced arousals which led to larger responses than spontaneous arousals. VI on the first breath following arousal on Optimal CPAP was greater in men than women (11.0 ± 0.4 versus 7.6 ± 0.6 l·min-1). These results demonstrate that the ventilatory response to arousal is influenced by pre-arousal airway resistance and gender. Whether this contributes to the perpetuation of respiratory events and the pathogenesis of OSA is unclear.
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