Surgical stress index in response to pacemaker stimulation or atropine.

BACKGROUND The surgical stress index (SSI) is a new monitoring tool for the assessment of nociception during general anaesthesia. It is calculated based on the heart beat interval and the pulse wave amplitude. Correlation of SSI with nociceptive stimuli and opioid effect-site concentrations has been demonstrated, but the influence of isolated modulation of heart rate (HR) on SSI is still unclear. The aim of this study was to evaluate the effect on SSI of atropine administration and cardiac pacing. METHODS In 18 anaesthetized ASA III ICU patients, either repetitive cardiac pacemaker stimulation or administration of atropine (10 microg kg(-1)) was performed, and the effect on SSI, arterial pressure, spectral entropy, and bispectral index was analysed. RESULTS Cardiac pacing at 100 beats min(-1) was followed by an increase in SSI from 26 [17-35 (10-41)] to 59 [53-72 (48-78)] {median [inter-quartile range (range)]} (P=0.0006), whereas other variables remained unaffected. Also, atropine administration increased SSI from 27 [20-34 (16-39)] to 58 [48-70 (41-81)] (P=0.007) without significant effect on other variables except HR. A recalibration of SSI during cardiac pacing leads to a significant decrease in SSI to 49 [40-52 (36-57)] (P=0.03), whereas recalibration after atropine administration had no effect. CONCLUSIONS SSI values measured in patients receiving atropine or in patients with pacemakers should be interpreted cautiously.