Case control study of 24-hour ambulatory blood pressure in patients with obstructive sleep apnoea and normal matched control subjects

There is considerable debate regarding the relationship between obstructive sleep apnoea (OSA) and hypertension. Many studies attempting to assess the association, have produced conflicting results, which may be due to variable allowance for confounding variables such as upper body obesity, alcohol and smoking. We performed a case control study of 24-hour ambulatory blood pressure (ABP) monitoring in patients with OSA, compared to closely matched controls from the general population, to assess whether OSA is an independent contributor to diurnal hypertension. 45 sleep clinic patients with moderate to severe OSA and excessive daytime sleepiness were matched to 45 controls without evidence of OSA on a sleep study. Matched variables included age, body mass index (BMI), alcohol and cigarette consumption, treated hypertension and history of ischaemic heart disease. Upper body obesity was measured using waist:hip and waist:height ratios. 24-hour ABP recordings were performed in all subjects, and before treatment in the patients with OSA. Compared to controls, OSA patients had significantly increased mean (SD) diastolic blood pressure (DBP) during both the daytime (87.4 (10.2) versus 82.8 (9.1) mmHg, p<0.05) and night-time (78.6 (9.3) versus 71.4 (8.0), p<0.001), and higher systolic blood pressure (SBP) at night (119.4 (20.7) versus 110.2 (13.9), p<0.05). There was less nocturnal reduction in SBP ("dipping") in patients with OSA (day to night fall SBP, 13.5 v 21.0mmHg, p<0.005). The relatively increased nocturnal SBP in patients with OSA persisted briefly into the morning. Compared to closely matched control subjects, patients with OSA have increased DBP during both day and night, and increased SBP at night. The magnitude, of these differences is such that by extrapolation it should carry increased relative risk of cardiovascular morbidity. Further studies are required to establish whether cardiovascular morbidity is in fact raised and whether treatment of OSA reduces blood pressure and its complications.