Awakenings change results of nighttime ambulatory blood pressure monitoring

OBJECTIVE: Investigate the effect of distinguishing nighttime and sleep on nocturnal blood pressure results in ambulatory blood pressure monitoring. METHODS: We recruited 36 patients, 29 men, with suspected obstructive sleep apnea/hypopnea syndrome attending a sleep clinic for diagnostic polysomnography and who agreed to wear a Spacelabs 90207 ambulatory blood pressure monitor during polysomnography. Their mean age was 45+/-11 years; body mass index (BMI), 30.8+/-5.4 kg/m; apnea-hypopnea index, 35+/-29 AH/h; 13 had a history of hypertension. A microphone attached to the ambulatory blood pressure monitor recorded its sounds in the polygraph and allowed us to classify each ambulatory blood pressure monitoring measurement as being made in electrographically-determined wake (e-wake) or sleep state (e-sleep). RESULTS: Patients were asleep during (mean+/-SD) 61+/-24% (range 0-100%) of the 14+/-1 nighttime blood pressure measurements. Systolic and diastolic ambulatory blood pressure monitoring readings were significantly higher during e-wake (121+/-12/73+/-9 mmHg) than during total nighttime (119+/-11/70+/-8 mmHg) and e-sleep (116+/-13/68+/-9 mmHg). On the basis of nighttime measurements, 22 patients (61%) had nocturnal hypertension. On the basis of measurements made during e-sleep, nocturnal hypertension was diagnosed in 12 patients (33%; chi2=5.54; P=0.018). A multiple linear regression model showed that the percentage of measurements made in e-sleep was the only variable that significantly explained the difference between nighttime and e-sleep blood pressure figures, when controlling for sex, age, BMI, apnea-hypopnea index, and lowest SaO2. CONCLUSION: During ambulatory blood pressure monitoring, nighttime blood pressure readings are higher than during e-sleep and this changes dipping and nocturnal hypertension classification.