SUPPRESSION OF SPINAL-CORD MOTONEURON EXCITABILITY CORRELATES WITH SURGICAL IMMOBILITY DURING ISOFLURANE ANESTHESIA

Background: Recent evidence suggests that the spinal cord is an import ant site of anesthetic action that produces surgical immobility. Inhal ation anesthetics depress the Hoffmann's reflex (H reflex) and F wave, indicating spinal motoneuron suppression. The aim of this study was t o assess the correlation between isoflurane-induced immobility and H- and F-wave suppression. Methods: The baseline H reflex and F wave were measured before anesthesia in 15 adult patients. After induction, end otidal isoflurane was maintained for 20 min before the H and F waves w ere reelicited. Using an electric stimulus applied to the forearm and grading the response as movement or no movement, the authors increased or decreased the isoflurane concentration in 0.1% steps, depending on the movement responses. The H and F waves were recorded 20 min after each change of isoflurane concentration. The correlation between H- an d F-wave suppression and surgical immobility was analyzed using a pair ed t test with Bonferroni correction. Results: H-reflex amplitude (2.7 4 +/- 1.63 mV) and F-wave persistence (70.69 +/- 26.19%) at the highes t isoflurane concentration that allowed movement response to a stimulu s are different (P < 0.01) from these (1.97 +/- 1.46 mV; 43.16 +/- 22. 91%) at the lowest isoflurane concentration that suppressed response. At 0.8% isoflurane, the H-reflex amplitude was 3.69 +/- 1.83 mV with m ovement and 1.01 +/- 1.14 mV without movement (P < 0.01); F-wave ampli tude was 0.29 +/- 0.15 mV with movement and 0.11 +/- 0.06 mV without m ovement (P < 0.01); F-wave persistence was 80 +/- 22.36% with movement and 34.9 +/- 25.75% without movement (P < 0.01). Conclusions: The deg ree of H- and F-wave amplitude and F-wave persistence suppression corr elates with movement response, suggesting that isoflurane-suppressive action in the spinal cord plays a significant role in producing surgic al immobility.