Effects of an intravenously administered arrhythmogenic dose of bupivacaine at the nucleus tractus solitarius in the conscious rat.

This study was designed to further investigate the CNS-mediated cardiovascular toxicity of local anesthetics and to determine any effect of anesthesia on the firing rate of cells in the nucleus tractus solitarius (NTS) of conscious animals. Adult Sprague-Dawley rats were anesthetized with chloral hydrate. A femoral artery and vein were cannulated. A 3-mm hole was then drilled for placement of a unidirectional microdrive. The animal was removed from the stereotactic instrument, and EKG electrodes, pressure transducer and the microdrive containing a 1-micron tungsten electrode were attached. Animals were then placed in a canvas sling for recovery from the anesthetic. Cells of the NTS were located and cell firing rate (CFR) was continuously recorded. None of the animals exhibited any sign of discomfort upon recovery from anesthesia. CFRs decreased from 19 +/- 13 to 4 +/- 4 impulses/second at 20 +/- 16 seconds after the IV injection of bupivacaine (p less than 0.01). Maximum decreases in blood pressures were 26 +/- 15% of control and occurred at 32 +/- 14 seconds after the injection of bupivacaine. The maximum decrease in heart rate was 38 +/- 16% and occurred at 13 +/- 10 seconds after the injection of bupivacaine. Maximum heart rate changes occurred significantly earlier than maximum decreases in CFR (p less than 0.01). On the other hand, maximum blood pressure changes occurred significantly later than maximum decreases in CFR (p less than 0.01). The data reported here demonstrate that conscious animals respond similarly to animals anesthetized with chloral hydrate with respect to the medullary effect of local anesthetics.