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Background The Surgical Pleth Index ( SPI), derived from pulse amplitude and heartbeat interval, is proposed to monitor anti-nociception during anaesthesia. Its response to noxious stimulation can be affected by the intravascular volume status. This study investigated the effect of a fluid challenge ( FC) on SPI during steady-state conditions. Methods After Institutional Review Board approval, 33 consenting patients undergoing neurosurgery received a 4 ml/kg starch FC over less than 5 min under stable surgical stimulation conditions and stable propofol ( CePPF) and remifentanil ( CeREMI) effect-site concentrations as estimated by target-controlled infusion systems. Intravascular volume status was assessed using the Delta Down ( DD). We looked at the SPI response to FC according to DD, CePPF, and CeREMI. Results Following FC, SPI did not change in 16, increased in 12, and decreased in 3 patients. CeREMI poorly affected the SPI response to FC. In normovolaemic patients, the probability of an SPI change after FC was low under common CePPF (0.9 to 3.9 μg/ml). A decrease in SPI was more probable with worsening hypovolaemia and lowering CePPF, while an increase in SPI was more probable with increasing CePPF. SPI changes were only attributable to modifications in pulse wave amplitude and not in heart rate. Conclusions During stable anaesthesia and surgery, SPI may change in response to FC. The effect of FC on SPI is influenced by volaemia and CePPF through pulse wave amplitude modifications. These situations may confound the interpretation of SPI as a surrogate measure of the nociception-anti-nociception balance. [ABSTRACT FROM AUTHOR]

Background: No validated monitoring method is available for evaluating the nociception/antinociception balance. We assessed the surgical stress index (SSI), computed from finger photoplethysmographic waveform amplitudes and pulse-to-pulse intervals, in patients undergoing shoulder surgery under general anesthesia (GA) and interscalene plexus block and in patients with GA only. Methods: In this prospective, randomized study in 26 patients, increased blood pressure (BP) or heart rate, movement, and coughing were considered to be signs of intraoperative nociception and were treated with alfentanil. GA was maintained with desflurane aiming at a State Entropy level of 50. Photoplethysmographic waveforms were collected from the contra-lateral arm to the surgery and SSI values from 0 (no surgical stress) to 100 (maximal surgical stress) were calculated off-line. Results: Two minutes after skin incision, SSI had not increased in the plexus group and was lower in the plexus group (38 ± 13) compared with the controls (58 ± 13, P<0.005). Among the controls, 1 min before alfentanil administration, the SSI value was higher than during periods of adequate antinociception, 59 ± 11 vs. 39 ± 12 ( P<0.01). The total cumulative need for alfentanil was higher in controls (2.7 ± 1.2 mg) compared with the plexus group (1.6 ± 0.5 mg; P=0.008). Tetanic stimulation to the ulnar region of the hand increased SSI significantly only among the patients with plexus block not covering the site of the stimulation. Conclusion: SSI values were lower in patients with plexus block covering the sites of nociceptive stimuli. In detecting nociceptive stimuli, SSI had better performance than heart rate, BP, or response entropy. [ABSTRACT FROM AUTHOR]