The neurophysiologic mechanisms of tourniquet pain. The activity of neurons in the rostroventral medulla in the rat

Abstract

Background: Tourniquet pain frequently complicates the use of pneumatic tourniquets during surgical procedures involving the extremities. The mechanisms of tourniquet pain are not understood and therefore the treatment is nonspecific. An animal model was sought to provide an ethical means to study the neurophysiologic and neuropharmacologic mechanisms of tourniquet pain. Neurons in the rostral ventromedial medulla are involved in the nociceptive-and circulatory-responsive neuronal networks. The goal of this study was to determine the activity of neurons in the rostral ventromedial medulla in response to the maintenance of tourniquet inflation in the rat as a component of an investigation of the neurophysiologic mechanisms of tourniquet-induced pain.

Methods: In 18 male, pentobarbital-anesthetized rats, heart rate, systolic blood pressure (SBP), and cell firing rates (CFR) of nociceptive-responsive rostral ventromedial medulla neurons, characterized as having an OFF or ON cell response to noxious heat, were monitored. CFR was monitored continuously at baseline, during an IV infusion of phenylephrine sufficient to increase SBP by 50%, during the application of a pneumatic tourniquet to the hind limb thigh and inflated to 300 mmHg for a period of 60 min, and during a 30-min recovery period.

Results: Phenylephrine-induced hypertension resulted in an increase in OFF CFR. Maintenance of tourniquet inflation resulted in a progressive decrease in OFF CFR and a progressive increase in ON CFR. An increase in SBP in response to tourniquet pain paralleled the changes in CFR. Mean SBP at 5 min preinflation, 5 min postinflation, 55 min postinflation, and 10 min postdeflation were 101 +/- 11, 103 +/- 9, 118 +/- 14, and 103 +/- 12 mmHg, respectively.

Conclusions: The changes in SBP and CFR observed during tourniquet inflation were consistent with previously reported responses to nociceptive stimuli. Phenylephrine-induced hypertension caused an opposite effect on the CFR of rostral ventromedial medulla neurons as compared with a noxious stimulus such as heat or maintenance of tourniquet inflation. This experimental design is presented as an animal model to study the neurophysiologic and neuropharmacologic aspects of tourniquet pain.