Ambulatory monitoring uncorrected for placebo overestimates long-term antihypertensive action. Systolic Hypertension in Europe (SYST-EUR) Trial Investigators.

This study compares blood pressure (BP) changes during active antihypertensive treatment and placebo as assessed by conventional and ambulatory BP measurement. Older patients (> or = 60 years, n=337) with isolated systolic hypertension by conventional sphygmomanometry at the clinic were randomized to placebo or active treatment consisting of nitrendipine (10 to 40 mg/d), with the possible addition of enalapril (5 to 20 mg/d) and/or hydrochlorothiazide (12.5 to 25 mg/d). At baseline, clinic systolic/diastolic BP averaged 175/86 mm Hg and 24-hour and daytime ambulatory BPs averaged 148/80 and 154/85 mm Hg, respectively. After 13 months (median) of active treatment, clinic BP had dropped by 22.7/7.0 mm Hg and 24-hour and daytime BPs by 10.5/4.5 and 9.7/4.3 mm Hg, respectively (P<.001 for all). However, clinic (9.8/1.6 mm Hg), 24-hour (2.1/1.1 mm Hg), and daytime (2.9/1.0 mm Hg) BPs decreased also during placebo (P<.05, except for daytime diastolic BP); these decreases represented 43%/23%, 20%/24%, and 30%/23% of the corresponding BP fall during active treatment. After subtraction of placebo effects, the net BP reductions during active treatment averaged only 12.9/5.4, 8.3/3.4, and 6.8/3.2 mm Hg for clinic, 24-hour, and daytime BPs, respectively. The effect of active treatment was also subject to diurnal variation (P<.05). Changes during placebo in hourly systolic and diastolic BP means amounted to (median) 21% (range, -1% to 42%) and 25% (-3% to 72%), respectively, of the corresponding changes during active treatment. In conclusion, expressed in millimeters of mercury, the effect of antihypertensive treatment on BP is larger with conventional than with ambulatory measurement. Regardless of whether BP is measured by conventional sphygmomanometry or ambulatory monitoring, a substantial proportion of the long-term BP changes observed during active treatment may be attributed to placebo effects. Thus, ambulatory monitoring uncorrected for placebo or control observations, like conventional sphygmomanometry, overestimates BP responses in clinical trials of long duration.