The effect of posture and anaesthetic technique on the surgical pleth index.

The surgical pleth index has been shown to correlate with surrogate variables of nociception during general anaesthesia, and it has been suggested to be of use as a depth of anaesthesia monitor. However, little is known about confounding factors. As the main determining variables are based on both central and peripheral autonomic regulatory mechanisms, we hypothesised that changing a patient`s posture may produce a marked effect. We studied the effects of posture change in 45 patients who were randomly assigned to receive general (n = 15) or spinal anaesthesia with (n = 15) or without sedation (n = 15), as well as 15 awake volunteers. Mean (SD) values of the surgical pleth index after adoption of the lithotomy position were reduced from 57 (22) to 21 (6) under general anaesthesia, 63 (15) to 31 (9) under spinal anaesthesia alone, and 52 (14) to 22 (8) under spinal anaesthesia with sedation (all p < 0.01). In healthy volunteers, the surgical pleth index increased from 37 (13) to 57 (11) (p < 0.01) after 30° head-up tilt and was reduced from 35 (11) to 25 (11) after head-down tilt (p < 0.05). Change in posture has a marked effect on the surgical pleth index which lasts for at least 45 min, and this must be considered when interpreting the displayed values.

Evaluation of the surgical stress index during spinal and general anaesthesia.

BACKGROUND: Although in modern anaesthesia, monitoring depth of anaesthesia and quality of neuromuscular block are routine, monitoring of analgesia still remains challenging. Recently, the surgical stress index (SSI), derived from finger photoplethysmographic signal, was introduced as a surrogate variable reflecting the nociception-antinociception balance. This study aimed at evaluating the SSI in patients undergoing regional anaesthesia either alone or combined with sedation compared with patients undergoing general anaesthesia (GA). METHODS: Seventy-one patients undergoing general (n=24) or spinal anaesthesia with (n=24) or without sedation (n=23) were included. SSI was measured the day before surgery and at defined time points during anaesthesia and surgery and also in the recovery room. SSI was compared with haemodynamic variables like heart rate and systolic arterial pressure. RESULTS: The SSI was higher in patients undergoing spinal anaesthesia [mean 65, CI (59.3-70.5)] compared with GA [48 (39.9-56.4), P<0.01], and baseline [41 (37.3-44.2), P<0.001]. During spinal anaesthesia with sedation [44 (36.2-50.9)], it was comparable with the baseline level (P>0.05). In comparison with baseline, SSI in the recovery room was higher in patients after GA [59 (48.4-67.9), P<0.025] but not after spinal anaesthesia [53 (47.6-60.1), P>0.05] or after spinal anaesthesia with sedation [54 (45.8-65.1), P>0.05]. Changes of the SSI were not reflected by changes of haemodynamic variables. CONCLUSIONS: In fully awake patients under spinal anaesthesia, the SSI does not reflect the nociception-antinociception balance. This may be due to the influence of mental stress on the sympathetic nervous system. Even light sedation attenuates these influences.