Characteristics and effects of depth of anaesthesia on late motor responses

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.

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.

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.

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.

Changing depth of anaesthesia has differential effects on late responses of different muscles.