Measurement of a Spinal Reflex Response (H-Reflex) During General Anesthesia in Man

Open Access
Publisher Website
Google Scholar
Abstract
An electrically evoked monosynaptic reflex (the H-reflex) was studied during general anesthesia, in 25 surgical patients. Different levels of muscular relaxation were obtained by varying inspired anesthetic concentrations of halothane or methoxyflurane. At each level, the ratio of maximum reflex response during anesthesia to maximum response when awake (the H-ratio), was calculated. Poor relaxation, during light anesthesia, was accompanied by a large reflex response and a high H-ratio. As relaxation improved, reflex amplitude fell, and H-ratio decreased. With profound relaxation of jaw and abdominal muscles, the H-ratio approached zero. The relation between H-ratio and muscular relaxation was statistically significant. Thus, muscular relaxation during general anesthesia is associated with decreased reflex excitability of the spinal motoneuron pool. This presumably is the result of direct or indirect anesthetic action at the spinal synapse. With increasing anesthetic concentrations, synaptic transmission is depressed, the reflex decreases in amplitude and muscular relaxation becomes more profound.