Hyper- and hypoventilation affects spinal motor neuron excitability during isoflurane anesthesia

Increasing evidence indicates that the spinal cord is an important site of anesthetic action necessary for surgical immobility. Whether clinical hyper- or hypoventilation affects motor neuron excitability during general anesthesia is unknown. To clarify this issue, we studied seven adult ASA physical status I or Il patients undergoing elective surgery. 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 the soleus and abductor hallucis muscles. After inhaled induction, the end-tidal isoflurane concentration in O-2 was maintained at 0.8%. Ventilation was controlled to maintain a steady-state ETco(2) of 25 +/- 1 and 45 +/- 1 mm Hg randomly for 20 min. Then the H-reflex and F wave were 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 Hg and to 2.0 +/- 2.2 mV (P < 0.01) at an ETco(2) of 45 mm Hg. The F-wave 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 persistence (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 surgery. The likelihood of movement may be predicted by measuring the spinal motor neuron excitability by using the H reflex and F wave. Our results show that intraoperative hyper- and hypoventilation can change the H reflex and F wave, which may affect the probability of patient movement during surgery.