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

UNLABELLED: 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 II 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 O2 was maintained at 0.8%. Ventilation was controlled to maintain a steady-state ETCO2 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 H-reflex amplitude (6.8 +/- 2.7 mV) decreased to 4.0 +/- 2.0 mV (P < 0.01) at an ETCO2 of 25 mm Hg and to 2.0 +/- 2.2 mV (P < 0.01) at an ETCO2 of 45 mm Hg. The F-wave persistence (100%) decreased to 77% +/- 24% (P < 0.05) at an ETCO2 of 25 mm Hg and to 61% +/- 19% at an ETCO2 of 45 mm Hg (P < 0.01). Changing ETCO2 values affected H-reflex amplitude 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.