Effects of ephedrine and phenylephrine on the Analgesia Nociception Index (ANI) and Nociception Level (NOL)

Purpose: Nociceptive monitor values is thought to be influenced by vasopressors; however, no study has investigated this in detail. Therefore, this study aimed to investigate the effect of vasopressors on nociceptive monitors using Analgesia Nociception Index (ANI) and Nociception Level (NOL). Methods: This prospective observational study was conducted between February and April, 2023. Data were obtained during anesthesia induction only to exclude surgical invasion. Blood pressure, heart rate, ANI, and NOL were measured 5 min after ephedrine or phenylephrine administration during hypotension. Low ANI and high NOL values indicate severe pain. Statistical analyses were performed on the values before and after vasopressor administration for each parameter. Results: Twelve patients (male, 10; female, 2) with a mean age of 71.3 ± 6.7 years were included. ANI and NOL, before versus after ephedrine administration were 76.3 ± 11.8 versus 33.6 ± 8.1 (p < 0.001) and 2.1 ± 1.5 versus 27.6 ± 10.5 (p < 0.001), respectively. ANI and NOL, before versus after phenylephrine administration were 75.7 ± 9.2 versus 53.0 ± 15.3 (p = 0.010) and 4.0 ± 3.5 versus 21.5 ± 4.3 (p < 0.001), respectively. The mean time beyond the pain threshold (ANI < 50 and NOL > 25) for ephedrine and phenylephrine was 58.0 s and 21.0 s for ANI, and 29.3 s and 1.0 s for NOL, respectively. Conclusion: The use of vasopressors, albeit a short time, significantly altered the nociceptive monitor values, and in some cases, exceeded the values considered to be the threshold for pain.