The present experiment investigated whether administration of placebo affected heart rate variability during heat pain. It was hypothesized that the ratio of low frequent to high frequent (LF/HF) heart rate variability would decrease after administration of an inert substance together with information that it was a powerful painkiller. In a within subjects design, 20 participants (10 females) were tested on two separate days, one day for the placebo condition and one day for control. In the placebo condition, the participants received two capsules containing 75mg lactose each during the second of five pain tests, with information that the pills were a high dose of a standard overthe-counter analgesic with high pain analgesic effect on heat pain. In the
control condition, the same subjects underwent the same five pain tests, but without placebo administration. Pain tests consisted of heat pain (+46C, duration 240 seconds) to the forearm. ECG was recorded continuously for spectral analysis of heart rate variability. Subjective pain intensity, pain unpleasantness, stress and arousal were measured on VAS scales during each pain test. In addition, mood was measured by the SAM. Results revealed that the LF/HF ratio during painful stimulation decreased significantly in the
placebo condition compared to the control condition after placebo administration (F(1,19)=8.76, p<.01), but this effect was only significant for the first two pain tests after placebo administration. Moreover, a main effect of placebo on heart rate variability was found (F(1,19)=8.43, p<.01), with lower ratio of LF/HF in the placebo condition. There was lower pain intensity (F(1,19=14.99, p<.01) and pain unpleasantness (F(1,19)=4.74, p=.04) in the placebo condition compared to the control condition. Subjective stress during pain was decreased after placebo administration, and there was lower subjective stress in the placebo condition compared to the control condition (F(1,19)=13.06, p<.01). There were no significant effects on arousal or mood. The results from the present experiment suggest that placebo analgesia is accompanied by a reduction in cardiac autonomic activation and a reduction of subjective negative emotions.