The effect of placebo on indirect and direct blood pressure measurements.

We have conducted a series of experiments which clearly show that intra-arterial ambulatory blood pressure is not altered by the administration of placebo. This finding is in contrast to the apparent fall in pressure that appears when the indirect (Korotkoff) method is used. The evidence suggests that the apparent fall in indirect pressure is due to abolition of the orienting reflex with time. Therefore, clinical trials of antihypertensive drugs using the intra-arterial method do not need a placebo control, whereas a placebo control is essential in all trials using the indirect technique for measuring blood pressure.