Background and Goal of Study. The effectiveness of surgical pleth index (SPI) for managing the nociception-antinociception balance during general anaesthesia has been demonstrated in many clinical settings (1-2). Especially, SPI could differentiate haemodynamic action of a β-blocking agent (esmolol) from analgesic action of remifentanil (3). However, its effectiveness has not yet been proven in the clinical settings with the use of a vasodilator including nicardipine. We hypothesized that while keeping bispectral index (BIS) of 40-60, SPI would be higher in patients receiving nicardipine than in those receiving remifentanil during thyroidectomy under general anaesthesia.

Materials and Methods. 
Forty patients undergoing thyroidectomy were assigned randomly to receive nicardipine (group N; n=19) or remifentanil (group R; n=21). Anaesthesia was induced with propofol 2 mg/kg, fentanyl 1 μg/kg and rocuronium 0.6 mg/kg and maintained with desflurane and nitrous oxide 50% in oxygen to keep BIS at around 50. The infusion of nicardipine or remifentanil was started before first incision and adjusted to keep mean blood pressure (MBP) within ±20% from the preoperative value. SPI and BIS, end-tidal desflurane concentration, MBP and heart rate were recorded at 2.5 min intervals during surgery. Extubation time, recovery time (time to reach a modified Aldrete score of 10) and adverse events at post-anaesthesia care unit (PACU) were recorded.

Results and Discussion. There was no significant difference between the groups in patients’ characteristics.
The trend of SPI was comparable between the groups during surgery, although heart rate was significantly higher in group N than in group R (P=0.03, Fig. 1). The trends of intraoperative BIS, end-tidal desflurane concentration and MBP were comparable between the groups. Extubation time, recovery time and incidence of nausea/vomiting at PACU were comparable between the groups. 
Unlike our expectation, SPI was not different between the patients receiving nicardipine and those receiving remifentanil during thyroidectomy under general anaesthesia. Therefore, SPI does not seem to reflect the level of surgical stress and may not help guide the use of opioids in the clinical settings with the administration of nicardipine during general anaesthesia.

Conclusion. The administration of nicardipine may confound the interpretation of SPI as a surrogate measure of the nociception-anti-nociception balance during general anaesthesia.

References
1. Bergmann I, Göhner A, Crozier TA, et al. Surgical pleth index-guided remifentanil administration reduces remifentanil and propofol consumption and shortens recovery times in outpatient anaesthesia. Br J Anaesth 2013; 110: 622-8
2. Struys MM, Vanpeteghem C, Huiku M, Uutela K, Blyaert NB, Mortier EP. Changes in a surgical stress index in response to standardized pain stimuli during propofol-remifentanil infusion. Br J Anaesth 2007; 99: 359-67
3. Ahonen J, Jokela R, Uutela K, Huiku M. Surgical stress index reflects surgical stress in gynaecological laparoscopic day-case surgery. Br J Anaesth 2007; 98: 456-61

The effect of nicardipine on surgical pleth index during thyroidectomy under desflurane anaesthesia: a prospective randomized controlled trial