BACKGROUND Obstructive sleep apnoea (OSA) causes recurrent rises in blood pressure during sleep, and recent community surveys have suggested a link between mild OSA and diurnal hypertension. The fact that OSA and hypertension share some risk factors, as well as problems accurately quantifying OSA severity, have diluted the power of such studies. This study tries to circumvent some of these problems by measuring the overnight change in blood pressure and relating it to relevant measures of the severity of upper airway obstruction on the same night.

RESULTS Data were available from 224 couples (448 subjects). On average, systolic BP fell 8 mm Hg from evening to morning. Only hypoxic dips (>4% Sao ₂ dips/h) and the measure of degree of respiratory effort were significant independent predictors of this overnight change in systolic BP, together accounting for 7–10% of the variation (p<0.0001). Dividing the subjects into quartiles according to the respiratory effort overnight showed a progressive reduction in the fall of systolic BP overnight: 13.6, 10.8, 7.3, and 5.6 mm Hg, lowest to highest quartiles.

CONCLUSIONS This study suggests that increased respiratory effort during sleep (seen in OSA and related syndromes of increased upper airway resistance during sleep) offsets the normal fall in BP that occurs overnight, even within this community population. This may be one of the mechanisms by which hypertension is carried over into the waking hours in patients with OSA.