Autonomic effects of suggestive placebo interventions to increase or decrease blood pressure: a randomized controlled...

Placebo effects on pain and other subjective parameters are well-established, but the evidence for placebo effects on autonomic functions is scarce. Our randomized-controlled trial aimed to investigate autonomic responses after a suggestive placebo intervention intended to increase or decrease blood pressure (BP).

92 healthy subjects inhaled a placebo spray with the prior suggestion that it contained an effective drug to either increase or decrease BP, or the information that a placebo was administered (controls). BP, heart rate, stroke volume, peripheral resistance, heart rate variability and skin conductance level were monitored 30min before and after placebo administration. The expected and the subjectively perceived drug effect were measured by means of visual analog scales.

We found no statistically significant differences between the groups with respect to BP, heart rate, stroke volume, total peripheral resistance and heart rate variability responses to the verbal suggestions. Skin conductance response was more pronounced in the BP decrease group compared with controls (p=0.04), but this finding might be due to chance, given the multiple tests. Within the total study sample, BP, total peripheral resistance, low frequency power of heart rate variability and skin conductance were significantly higher after the placebo spray independent of the associated suggestions. Subjects in the BP increase and BP decrease condition had higher ratings of the expected and the subjectively perceived drug effect compared with controls (all p<0.05).

We found no evidence that specific verbal suggestions during placebo interventions affect BP in healthy subjects.