Agreement between analgesia nociception index (ANI) and surgical pleth index (SPI) in assessing nociception under varying systemic vascular resistance conditions

Analgesia nociception index (ANI) is derived from heart rate variability, while surgical pleth index (SPI) is a combined measurement of pulse wave amplitude and heartbeat interval. Given that systemic vascular resistance (SVR) may affect the pulse wave amplitude, the reliability of SPI may be affected during dynamic changes in SVR. We aimed to measure the agreement between ANI and SPI in assessing nociception across varying SVR. In this prospective clinical study, we collected ANI and SPI data simultaneously at 5-minute intervals in each patient undergoing living donor liver transplantation. Also, the predictability of noxious stimuli was assessed by measuring changes in ANI and SPI in response to standardized noxious stimulus. The primary outcome was the agreement between ANI and SPI in determining analgesic status under different SVR conditions. And the predictabilities of ANI and SPI on noxious stimulation were tested using receiver-operating characteristic curve analysis. We analyzed a total of 1,318 ANI and SPI data pairs extracted from the 47 patients included in the study. The agreement between ANI and SPI was 32.4%, reflecting minimal agreement on analgesic status (Cohen’s kappa coefficient [95% Confidence interval]: 0.03 [0-0.05]). Although, the agreement did not change significantly across varying SVR, the kappa coefficient values were slightly higher in cases with higher SVR. Also, delta ANI showed stronger predictability for nociception than delta SPI (Areas under the curves (AUC) [95% confidence interval]: 0.860 [0.727–0.944] vs. 0.636 [0.483–0.771], respectively). However, under abnormal SVR, the predictability decreased for both delta ANI and delta SPI (0.814 [0.618–0.936] vs. 0.514 [0.315–0.709], respectively). The overall agreement regarding analgesic status between ANI and SPI was minimal. However, ANI predicted standard noxious stimulation during surgery more successfully and may be a more appropriate index compared to SPI under abnormal SVR. This study was registered in Clinical Research Information Service on May 26, 2023 (https://cris.nih.go.kr; Identifier: KCT0008473, Principal investigator: Gaab Soo Kim).