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The difference between clinic and ambulatory average daytime blood pressures is frequently taken as a surrogate measure of the ‘white-coat effect’ (i.e. the pressor reaction triggered in the patient by the physician's visit).To assess the reproducibility of this difference and its relationship with clinic and average ambulatory daytime blood pressure levels.These issues were addressed with two large groups of subjects in whom both clinic and ambulatory blood pressures were measured, namely 783 outpatients with systolic and diastolic essential hypertension [Group 1, aged 50.8 ± 9.4 years (mean ± SD)], participating in standardized Italian trials of antihypertensive drugs, and 506 elderly patients (group 2, age 71 ± 7 years) with isolated systolic hypertension, participating in the European Syst-Eur trial.The clinic-daytime blood pressure difference for the essential systolic and diastolic hypertensive patients (group 1) was 13.6 ± 14.3 mmHg for systolic and 9.1 ± 8.6 mmHg for diastolic blood pressure (P always < 0.01). This difference for the elderly patients with isolated systolic hypertension (group 2) was 21.2 ± 16.0 mmHg for systolic and only 1.3 ± 10.2 mmHg for diastolic blood pressure (P < 0.01 and P < 0.05, respectively). In both studies little or no systematic clinic-daytime difference could be observed for heart rate. The reproducibility of the clinic-daytime blood pressure difference, tested for 108 essential systolic and diastolic hypertensive patients from group 1 and 128 isolated systolic hypertensives from group 2, was invariably lower than that both of daytime and of clinic blood pressure values. Finally, the clinic-daytime blood pressure difference was progressively higher for increasing levels of clinic blood pressure and progressively lower for higher levels of ambulatory daytime blood pressure.Thus, the clinic-daytime blood pressure difference has a limited reproducibility; depends not only on clinic but also on daytime average blood pressure, which means that its size is a function of the blood pressure criteria employed for selection of the patients in a trial; and is never associated with a systematic clinic-daytime difference in heart rate, which further questions its use as a reliable surrogate measure of the true pressor response induced in the patient by the doctor's visit.