The placebo effect in ambulatory blood pressure monitoring

The reduction of clinic blood pressure by placebo tablets in clinical trials of antihypertensive drugs is well established and health authorities require a placebo arm in these trials in order to know how much of the total reduction of blood pressure is attributable to the drug. Most authors now favor the hypothesis that, in contrast to clinic blood pressure measurements, ambulatory blood pressure monitoring (ABPM) is not subject to a placebo effect. Scientific evidence so far shows that ABPM recordings are affected by a negligible and clinically irrelevant placebo effect in short-term trials. Nevertheless, the results of the SYST-EUR study suggest that placebo treatment induces a reduction of blood pressure in old patients with isolated systolic hypertension even when blood pressure is measured by 24 h ABPM, and raise the question of whether a placebo arm is required in clinical trials when ABPM is used to measure changes in blood pressure. In this article, the evidence supporting and contradicting the hypothesis that placebo exerts an effect on ABPM and the possible explanations for the discrepancy are reviewed. My position is that clinical trials on antihypertensive drugs using ABPM should be designed with the same standards as those using clinic blood pressure measurements; that is, randomized, double-blind, placebo-controlled studies. This should be an absolute requirement in long-term trials. Short-term trials lasting no more than 12 weeks, particularly those focused on examining average changes in 24 h diastolic blood pressure, may be designed without a placebo arm.