Which aspects of breathing during sleep influence the overnight fall of blood pressure in a community population?

Background-Obstructive sleep apnoea (OSA) causes recurrent rises in blood p ressure during sleep, and recent community surveys have suggested a link be tween mild OSA and diurnal hypertension. The fact that OSA and hypertension share some risk factors, as well as problems accurately quantifying OSA se verity, have diluted the power of such studies. This study tries to circumv ent some of these problems by measuring the overnight change in blood press ure and relating it to relevant measures of the severity of upper airway ob struction on the same night. Methods-Men born between 1930 and 1960 and their wives living in a market t own north of Oxford were identified from a GP practice register. Enough cou ples were recruited to provide approximately 10 (20 individuals) per year o f birth. Subjects were visited at home where a questionnaire was administer ed, anthropometric measurements made, blood pressures taken (including by t he subject), and sensors applied for a subsequent overnight sleep study. Th e sleep study measured indices of hypoxia, snoring, autonomic arousal, degr ee of respiratory effort; the last two of these derived from measurements o f pulse transit time (indirect beat to beat blood pressure). After waking t he following morning, the subjects took their own blood pressures again. Results-Data were available from 224 couples (448 subjects). On average, sy stolic BP fell 8 mmHg 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 ac counting for 7-10% of the variation (p<0.0001). Dividing the subjects into quartiles according to the respiratory effort overnight showed a progressiv e 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 sl eep (seen in OSA and related syndromes of increased upper airway resistance during sleep) offsets the normal fall in BP that occurs overnight, even wi thin this community population. This may be one of the mechanisms by which hypertension is carried over into the waking hours in patients with OSA.