Effect of Supplemental Oxygen on Blood Pressure in Obstructive Sleep Apnea (SOX): A Randomised, CPAP Withdrawal Trial

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Obstructive sleep apnoea (OSA) is associated with systemic hypertension. Either overnight intermittent hypoxia, or the recurrent arousals that occur in OSA, could cause the daytime increases in blood pressure (BP).


To establish the role of intermittent hypoxia in the increased morning BP in patients with OSA.


Randomised, double-blinded, cross-over trial assessing the effects of overnight supplemental oxygen versus air (sham) on morning BP, following continuous positive airway pressure (CPAP) withdrawal in patients with moderate to severe OSA. The primary outcome was the change in home morning BP following CPAP withdrawal for 14 nights, oxygen versus air. Secondary outcomes included oxygen desaturation index (ODI), apnoea hypopnoea index (AHI), subjective (Epworth sleepiness score) and objective (Oxford sleep resistance test) sleepiness.

Measurements and main results:

Supplemental oxygen virtually abolished the BP rise following CPAP withdrawal and, compared to air, significantly reduced the rise in mean systolic BP (-6.6mmHg; 95% confidence interval or CI -11.3 to -1.9; p=0.008), mean diastolic BP (-4.6mmHg; 95% CI -7.8 to -1.5; p=0.006), and median ODI (-23.8/h; interquartile range -31.0, -16.3; p<0.001), following CPAP withdrawal. There was no significant difference, oxygen versus air, in AHI, subjective or objective sleepiness.


Supplemental oxygen virtually abolished the rise in morning BP during CPAP withdrawal. Supplemental oxygen substantially reduced intermittent hypoxia, but had a minimal effect on markers of arousal (including AHI), subjective or objective sleepiness. Therefore intermittent hypoxia, and not recurrent arousals, appears to be the dominant cause of daytime increases in BP in OSA.


Clinical trial registration available at www.isrctn.com, ID ISRCTN 17987510.

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