DAYTIME AND NIGHTTIME AMBULATORY BLOOD PRESSURES SHOULD BE CALCULATEDOVER THE TRUE SLEEP-WAKING CYCLE AND NOT OVER ARBITRARY PERIODS

The present study was done to compare the values of mean daytime and n ighttime blood pressure (BP) calculated over arbitrary periods to thos e calculated over the true retiring and rising times of the individual patients. A total of 88 individuals, including 55 untreated hypertens ives (office BP > 140/90 mm Hg) and 33 normotensives, were recruited. Ambulatory BP was monitored over 24 h during the normal routine of the patient. The patient was requested to trigger a recording on going to bed and on rising in the morning to clearly identify these periods. T he mean daytime and nighttime values were calculated over arbitrarily defined periods (6 AM to 10 PM daytime and 10 PM to 6 AM nighttime) an d as a function of the true retiring and rising times of the individua l patients. The true daytime BP was significantly higher than the valu e calculated over the arbitrary period and the true nighttime BP was s ignificantly lower than the value calculated between 10 PM and 6 AM (p aired t test, P <.05). Employing a cutoff value of 135/85 mm Hg for da ytime BP, a significant proportion of patients classified as normotens ive when daytime BP was calculated over arbitrary periods were in fact classified as hypertensive from the values calculated over true retir ing and rising times.