Carbon dioxide depresses the F wave by a central, not peripheral, mechanism during isoflurane anesthesia

Carbon dioxide (CO(2)) has anesthetic properties and has been reported to depress the F wave of the evoked electromyogram; the F wave is thought to reflect motoneuron excitability. Anesthetics such as isoflurane also depress the F wave. Because CO(2) can depress muscle contractile function, as well as spinal cord neurons, it is unclear whether CO(2) depresses the F wave via a central or peripheral mechanism. We anesthetized rabbits with isoflurane (1.4%) and prepared for hindlimb bypass (with a membrane oxygenator) whereby the partial pressures of CO(2) in the hindlimb muscle and torso could be independently adjusted. The F wave was recorded from the hindlimb plantar muscles when the CO(2) was normal to the hindlimb and torso, and when it was increased (to approximately 90 mm Hg) in the hindlimb, the torso, or both. Increasing the CO(2) to just the hindlimb had no significant effect on the F-wave amplitude, but increasing the CO(2) to the torso depressed the F wave to 52% +/- 32% of control; adding CO(2) to the hindlimb during torso hypercarbia did not result in any additional depression of the F wave. CO(2) depressed the F wave via a central, not peripheral, mechanism, although the precise mechanism is unknown.