Characteristics and effects of depth of anaesthesia on late motor responses

OBJECTIVE: Late motor responses, such as F-waves and H-reflexes, may reflect excitability of specific spinal cord segments and motor pathway integrity, offering potential value during intraoperative neurophysiological monitoring (IONM). However, these responses are not routinely utilized. A better understanding of the effect of general anaesthesia on their characteristics, particularly across varying depths, is essential for integration into IONM. METHODS: Participants received propofol/remifentanil-based anaesthesia, targeted at BIS values of 30, 40, and 50. F-waves, H-reflexes and Tc-mMEPs were recorded every 2-4 min. Anaesthetic effect was evaluated using linear mixed-effects models. RESULTS: 23 of 25 patients exhibited consistent H-reflexes. F-waves were elicitable in all patients, though muscle-dependant variability was observed. H-reflex amplitude decreased by 10% per 10-point BIS increase (p = 0.014), while F-wave amplitude increased by 6-9% per 10 points BIS increase in abductor hallucis and hand muscles (p < 0.01), with no significant changes in the tibialis anterior and gastrocnemius muscles. F-wave persistence varied < 5% per 10 BIS-points. CONCLUSION: Late responses can be reliably recorded during propofol/remifentanil anaesthesia. F-waves exhibit greater variability than H-reflexes, potentially limiting their utility in certain muscle groups. They were modestly influenced by deeper anaesthesia but were still elicitable and interpretable. SIGNIFICANCE: Changing depth of anaesthesia has differential effects on late responses of different muscles.