THE CEREBRAL AND SYSTEMIC EFFECTS OF MOVEMENT IN RESPONSE TO A NOXIOUS STIMULUS IN LIGHTLY ANESTHETIZED DOGS - POSSIBLE MODULATION OF CEREBRAL FUNCTION BY MUSCLE AFFERENTS

Background: Afferentation theory predicts that agents or maneuvers tha t stimulate muscle stretch receptors (i.e., muscle afferents) will pro duce cerebral stimulation. From this theory it follows that, regardles s of the source (e.g., drug effect, active muscle movement), increases in stretch receptor activity should result in a similar effect on the brain. The present study tested the hypothesis that active muscle mov ement in lightly anesthetized subjects would result in cerebral stimul ation. Methods: Studies were conducted in six dogs who were lightly an esthetized with halothane (0.70% end-expired). The following physiolog ic variables were quantified before and for 6 min after the initiation of a standardized (1-min duration) noxious stimulus to the trachea an d the skin overlying the hind limb: cerebral blood flow, cerebral meta bolic rate for oxygen (CMIRO(2)), cerebral perfusion pressure, cerebra l vascular resistance, electroencephalogram activity, electromyogram a ctivity, arterial carbon dioxide partial pressure (PaCO2), central ven ous pressure, and serum epinephrine and norepinephrine concentrations. Response to stimulation was evaluated initially in unparalyzed dogs a nd later was evaluated in the same dogs after they were paralyzed with intravenous pancuronium (0.2 mg/kg). Results: In unparalyzed dogs, st imulation produced episodes of coughing plus head and limb movement du ring the 6-min study period. Accompanying the movement was activation of the electromyogram, an increase in electroencephalogram frequency, and a reduction in electroencephalogram amplitude. There also was a 35 % increase in cerebral blood flow, a 25% decrease in cerebral vascular resistance, and a 7% increase in CMRO(2) versus the baseline values f or each variable. There were no significant increases in either cerebr al perfusion pressure, central venous pressure, PaCO2, or serum norepi nephrine concentration to account for the cerebral effects; however, s erum epinephrine concentrations increased by 61%. In pancuronium-paral yzed dogs, noxious stimulation resulted in a 5% increase in cerebral b lood flow, a 7% decrease in cerebral vascular resistance, and an 5% In crease hn CMRO(2) versus baseline levels. Electroencephalogram frequen cy was increased, but amplitude was unchanged. Central venous pressure , electromyogram activity, and serum norepinephrine concentration were unaffected. The serum epinephrine response was similar to that observ ed when the dogs were not paralyzed. Conclusions: These data support t he hypothesis that active muscle movement in lightly anesthetized subj ects has an effect on the brain that is mediated in part by muscle aff erent receptors. This cerebral response was manifested as electroencep halogram activation, cerebral vasodilation unrelated to central venous pressure changes, and an increase in cerebral blood flow greater than that required to meet metabolic demands. Paralysis with pancuronium a bolished movement induced by stimulation (and, thus, the muscle affere nt response) and also attenuated the cerebral blood flow, cerebral. va scular resistance, and electroencephalogram responses.