Changes of the nociceptive flexion reflex threshold in patients undergoing cardiac surgery-a prospective clinical pilot study

Abstract

Opioid dosage for general anaesthesia and sedation relies on surrogate parameters such as heartrate and blood pressure. This implies the risk of both under- and overdosing. A promising tool to provide target-oriented opioid dosing is measuring the nociceptive flexion reflex threshold (NFRT). The aim of this study was to investigate the individual trajectories and to determine this methods' clinical practicability in the perioperative setting of cardiac surgery. NFRT was measured preoperatively (twice as baseline), immediately after surgery and later in the general ward (primary outcomes). No intraoperative measurements were performed since neuromuscular blockade hinders NFRT assessment. Administered analgesics and pain scores were also recorded (secondary outcomes). Data were collected from August 2019 to March 2020. 264 patients scheduled for cardiac surgery were screened for eligibility. 55 patients were included, 30 rendered datasets for analysis. Thresholds after conclusion of surgery [T_ICU: median (IQR), 31.1 mA (21.5-50.0 mA)] were significantly higher than preoperatively [T_pre: 9.2 mA (5.4-13.4 mA); P < 0.001]. In 11 patients (36.7%), no immediate postoperative reflex response was elicited. Later, all reflexes returned, but thresholds remained significantly higher than preoperatively [T_post: 11.9 mA (9.2-16.6 mA); P = 0.043]. NFRT values after surgery were higher compared to baseline measurements. Subsequently they decreased but did not reach their baseline levels. There was no corresponding dose-dependency, suggesting multimodal effects on the nociceptive system. Unless measurements are not prevented by technical issues NFRT-assessment appears to be a future tool to target analgesics in patients not able to self-report pain. Trial registration Study registration: DRKS00021617. https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00021617 (registered retrospectively).

Keywords: Analgesiometry; Cardiac anaesthesia; Nociceptive flexion reflex; Opioids.