Monitoring of immobility to noxious stimulation during sevoflurane anesthesia using the spinal H-reflex.

Background: The spinal H-reflex has been shown to correlate with surgical immobility, i.e., the absence of motor responses to noxious stimulation, during isoflurane anesthesia. Here, the authors established individual concentration-response functions for H-reflex amplitude and tested the predictive power of the H-reflex for movement responses during sevoflurane anesthesia in comparison to electroencephalographic parameters. In addition, they investigated the effect of noxious stimulation on the H-reflex itself.

Methods: The authors studied 12 female patients during sevoflurane anesthesia before surgery. The sevoflurane concentration was increased, a laryngeal mask was inserted, and then the sevoflurane concentration was decreased until H-reflex amplitude (recorded over the soleus muscle) recovered. Thereafter, the end-tidal sevoflurane concentration was kept at a constant value close to the minimum alveolar concentration for suppression of movement responses after tetanic stimulation (MACtetanus), determined by the Dixon up-down method. Pharmacodynamic modeling of H-reflex amplitude and of the Bispectral Index was performed, and predictive values for motor responses to noxious electrical stimulation (50 Hz, 60 mA tetanus, volar forearm) were compared using the prediction probability.

Results: Concentration-dependent depression of H-reflex amplitude by sevoflurane was well modeled (median r2 = 0.97) by a sigmoid function with a median EC50 of 1.5 vol% and a median slope parameter of 3.7, much steeper than the slope for the Bispectral Index. MACtetanus calculated by logistic regression was 1.6 vol%. H-reflex amplitude predicted motor responses to noxious stimulation with a prediction probability of 0.76, whereas the prediction probability for Bispectral Index and spectral edge frequency (SEF95) were not different from chance alone. Noxious stimulation was followed by a substantial increase of H-reflex amplitude for several minutes, whereas the Bispectral Index and SEF95 exhibited no significant changes.

Conclusions: Suppression of movement to noxious stimulation and suppression of H-reflex amplitude by sevoflurane follow similar concentration-response functions. Although this does not imply a causal relation, it explains the high predictive value of H-reflex amplitude for motor responses to noxious stimuli, even in a narrow concentration range around the MACtetanus.