Supplemental oxygen curbs morning BP rise in OSA
Overnight supplemental oxygen attenuated the rise in morning blood pressure (BP) experienced by patients with moderate to severe obstructive sleep apnoea (OSA) following withdrawal of continuous positive airway pressure (CPAP), according to the SOX* trial.
“This is important because many patients, especially those with few symptoms, are unable to tolerate using CPAP treatment and other treatments may be needed for these individuals,” said lead author Dr Chris Turnbull of University of Oxford in Oxford, UK. In particular, the findings hold implications for OSA patients with resistant hypertension, according to Turnbull, as elevated BP place them at a higher risk for stroke and heart attack.
The double-blind, cross-over trial randomized 25 patients (mean age 62.7 years, 84 percent male) with moderate to severe OSA to receive overnight supplemental oxygen (flow rate 5 L/minute) or air (sham exposure) for 2 weeks after CPAP was withdrawn, before crossing over the other treatment group. [Am J Respir Crit Care Med 2018;doi:10.1164/rccm.201802-0240OC]
Compared with regular air exposure, supplemental oxygen attenuated the morning BP rise with significant reductions in mean systolic BP by 6.6 mm Hg (p=0.008) and diastolic BP by 4.6 mm Hg (p=0.006).
Intermittent hypoxia was also significantly less frequent during supplemental oxygen treatment than regular air exposure (median oxygen desaturation index, -23.8/hour; p<0.001).
However, supplemental oxygen had no significant effects on apnoea-hypopnoea index (AHI, a surrogate marker of arousal) and subjective or objective measures of daytime sleepiness.
“Therefore, intermittent hypoxia, and not recurrent arousals, appears to be the dominant cause of daytime increases in BP in OSA,” said Turnbull and co-authors.
While exploratory outcome of the inflammatory biomarker hsCRP** was not significantly different between groups, mean venous base levels (a marker of hypercapnia) were significantly increased from baseline by 3.1 mmol/L (p<0.001) with supplemental oxygen vs regular air.
“Although hypercapnia was not particularly marked in this study … the longer-term safety of oxygen therapy in OSA therefore needs careful consideration,” the researchers cautioned, noting that supplemental oxygen could worsen hypercapnia in a previous study, especially in OSA patients with obesity hypoventilation overlap. [Thorax 2014;69:346-353]
Two other randomized trials, they noted, showed no significant effects of supplemental oxygen on BP rise in OSA, but these studies excluded OSA patients with the most severe hypoxia, which according to the authors, “is probably the group most likely to benefit from oxygen treatment.”
“CPAP has a greater effect on BP in patients with resistant hypertension,” said Turnbull and co-authors. “Thus, the effect of supplemental oxygen on BP should be assessed in patients with OSA and resistant hypertension, with significant nocturnal intermittent hypoxia, where CPAP is not indicated or tolerated, but with careful monitoring of carbon dioxide levels.”