1. Obstructive sleep apnoea and snoring are associated with daytime hypertension. It is uncertain whether this association is directly due to the disturbed sleeping respiration or the result of confounding variables, particularly obesity, smoking and alcohol intake.

2. Ambulatory blood pressure and echocardiographic left ventricular muscle mass were measured in 19 patients with obstructive sleep apnoea, 19 men who snore without apnoea and 38 control subjects matched for age, sex, body mass index, smoking and alcohol intake. Ambulatory blood pressure was also measured before and after treatment in 11 patients with obstructive sleep apnoea and their matched control subjects.

3. Compared with matched control subjects, untreated obstructive sleep apnoea and snoring were not associated with an increase in daytime blood pressure. A daytime elevation of either systolic or diastolic blood pressure of > 3.8 mmHg due to obstructive sleep apnoea or snoring was excluded with 95% confidence in each of the study groups. Daytime blood pressure was also unchanged when obstructive sleep apnoea was treated with nasal continuous positive airway pressure. Night-time blood pressure was not significantly different in the patients with obstructive sleep apnoea or the snorers when compared with their matched control subjects. However, a fall in night-time systolic blood pressure was seen in the patients with obstructive sleep apnoea after treatment [fall in systolic blood pressure −6.3 (SD 8.2) mmHg, P < 0.02].

4. Left ventricular diameter, wall thickness and calculated mass were similar in each of the study groups and their matched control groups.

5. Compared with well-matched control subjects, daytime ambulatory blood pressure is not increased in patients with obstructive sleep apnoea or snoring and these patients do not show left ventricular hypertrophy. Night-time ambulatory blood pressure may be raised by obstructive sleep apnoea since it falls with treatment.

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