HYPER-VENTILATION AND HYPOVENTILATION AFFECTS SPINAL MOTOR-NEURON EXCITABILITY DURING ISOFLURANE ANESTHESIA

Increasing evidence indicates that the spinal cord is an important sit e of anesthetic action necessary for surgical immobility. Whether clin ical hyper- or hypoventilation affects motor neuron excitability durin g general anesthesia is unknown. To clarify this issue, we studied sev en adult ASA physical status I or Il patients undergoing elective surg ery. Spinal motor neuron excitability was determined by measuring the posterior tibial nerve H reflex and F wave. The baseline H reflex and F wave were recorded before anesthesia using electrodes placed over th e soleus and abductor hallucis muscles. After inhaled induction, the e nd-tidal isoflurane concentration in O-2 was maintained at 0.8%. Venti lation was controlled to maintain a steady-state ETco(2) of 25 +/- 1 a nd 45 +/- 1 mm Hg randomly for 20 min. Then the H-reflex and F wave we re recorded. The difference in H reflex and F wave were analyzed using Student's paired t-test. The baseline I-I-reflex amplitude (6.8 +/- 2 .7 mV) decreased to 4.0 +/- 2.0 mV (P < 0.01) at an ETco(2) of 25 mm H g and to 2.0 +/- 2.2 mV (P < 0.01) at an ETco(2) of 45 mm Hg. The F-wa ve persistence (100%) decreased to 77% +/- 24% (P < 0.05) at an ETco(2 ) of 25 mm Hg and to 61% +/- 19% at an ETco(2) of 45 mm Hg (P < 0.01). Changing ETco(2) values affected H-reflex am plitude and F-wave persi stence (P < 0.05), which suggests a change of spinal cord motor neuron excitability, which may affect surgical immobility. Implications: The spinal cord is important for preventing patient movement during surge ry. The likelihood of movement may be predicted by measuring the spina l motor neuron excitability by using the H reflex and F wave. Our resu lts show that intraoperative hyper- and hypoventilation can change the H reflex and F wave, which may affect the probability of patient move ment during surgery.