Sympathetic and mesenteric venous responses to baroreceptor or chemoreceptor stimulation during epidural anesthesia in rabbits.

BACKGROUND

Baroreceptor and chemoreceptor reflexes maintain homeostasis through mechanisms that involve sympathetic activation. Because sympathetic control of the mesenteric veins plays a central role in hemodynamic responses to stress, the effects of epidural blockade on reflex responses to hypoxia and bilateral carotid occlusion (BCO) were examined by monitoring direct measures of splanchnic sympathetic neural traffic and mesenteric venous capacitance.

METHODS

Rabbits were studied during alpha-chloralose anesthesia and mechanical ventilation. Sympathetic efferent nerve activity to the mesenteric vessels was measured by surgically placed electrodes, and mesenteric venous diameter was measured by videomicroscopy. Heart rate and mean arterial pressure were monitored by intraarterial cannulation. Intraluminal venous pressure was monitored by a servo-null micropressure technique. Responses were recorded during repeated administration of three different stresses, F1O2 = 0% for 40 s, F1O2 = 11% for 2.5 min, and BCO for 60 s. Animals received either thoracolumbar epidural blockade (0.4 ml/kg lidocaine 1.5%; n = 7) or 15 mg/kg intramuscular lidocaine (n = 7).

RESULTS

Hypoxia and BCO produced sympathetic stimulation and active constriction of mesenteric veins. Epidural anesthesia accentuated the mean arterial pressure decrease from F1O2 of 0%, caused the 11% response to F1O2 to become depressor instead of pressor, and decreased the pressor effect BCO. Sympathetic efferent nerve activity and venous diameter responses to hypoxia and BCO were attenuated or eliminated.

CONCLUSIONS

The hemodynamic effects of hypoxia result from a combination of direct depression and reflex activation. Thoracolumbar epidural anesthesia in rabbits impairs compensatory reflexes invoked by chemoreceptor stimulation and eliminates response to baroreceptor stimulation. Loss of splanchnic control of mesenteric capacitance contributes to the inhibition of the hemodynamic response to hypoxia or BCO during epidural anesthesia in rabbits.