Reliability of analgesia nociception index (ANI) and surgical pleth index (SPI) during episodes of bleeding – A pilot study

BACKGROUND AND AIMS: Analgesia Nociception Index (ANI) and Surgical Pleth Index (SPI) are measures of nociception-antinociception balance during general anaesthesia. The validity of these two monitors during autonomic changes brought about by intraoperative blood loss and noradrenaline infusion has not been studied earlier. The primary objective of the study was to determine the effect of bleeding on ANI and SPI. The secondary objective was to observe the effect of noradrenaline infusion on ANI and SPI and the correlation between ANI and SPI. METHODS: In this prospective observational study, 43 patients undergoing surgery for excision of a meningioma or for traumatic brain injury were included. Heart rate, mean arterial pressure, ANI, SPI, response and state entropy were recorded every 5 min from anaesthetic induction till skin closure. ANI and SPI values before bleeding were compared with the values following bleeding. The effect of bleeding and noradrenaline on ANI and SPI were studied using linear mixed effect model. RESULTS: Bleeding increased the values of ANI and the increase was higher in the presence of noradrenaline and it was statistically significant (P = 0.028 and 0.023). SPI was not affected by bleeding or with noradrenaline infusion. ANI and SPI had a poor negative correlation (r = - 0.01). CONCLUSION: Values of ANI increased significantly during episodes of intraoperative acute blood loss and with co-administration of noradrenaline. SPI seems to be more dependable when compared to ANI under such conditions.