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

Carbon dioxide (CO₂) 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 CO₂ can depress muscle contractile function, as well as spinal cord neurons, it is unclear whether CO₂ 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 CO₂ in the hindlimb muscle and torso could be independently adjusted. The F wave was recorded from the hindlimb plantar muscles when the CO₂ was normal to the hindlimb and torso, and when it was increased (to ≈90 mm Hg) in the hindlimb, the torso, or both. Increasing the CO₂ to just the hindlimb had no significant effect on the F-wave amplitude, but increasing the CO₂ 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. CO₂ depressed the F wave via a central, not peripheral, mechanism, although the precise mechanism is unknown.