Enantiomer-specific effects of an intravenously administered arrhythmogenic dose of bupivacaine on neurons of the nucleus tractus solitarius and the cardiovascular system in the anesthetized rat

Abstract

Background and objective: This investigation was designed to determine whether the effects on the cell firing rate (CFR) at the nucleus tractus solitarius (NTS) and on the cardiovascular system, which are associated with a toxic dose of bupivacaine, have an enantiomer-specific component.

Methods: Adult Sprague-Dawley rats were anesthetized with chloral hydrate, and a femoral artery and vein were cannulated. After the cranial surface was exposed, a 3-mm hole was drilled 2 mm caudal and 1.5 mm lateral with respect to lambda for placement of a 1-microns tungsten microelectrode. Cells of the NTS were located 6-6.5 mm from the brain surface, and CFR was continuously recorded. Lead II electrocardiogram and arterial blood pressure were also recorded. Twenty-four animals received either d- or l-bupivacaine (2 mg/kg) in random order.

Results: Cell firing rates decreased from 21 +/- 13 to 0 +/- 0 impulses/second (p < 0.001) at 34 +/- 15 seconds after the injection of d-bupivacaine. Cell firing rates decreased from 22 +/- 17 to 2 +/- 4 impulses/second (p < 0.01) at 68 +/- 45 seconds after injection of l-bupivacaine. In addition to the decreases in blood pressure and heart rate that were found, all animals exhibited an inversion in electrical axis beginning within 2-3 seconds after bupivacaine administration. Mild bradycardia was noted in four of the animals receiving the l-bupivacaine, whereas severe bradycardia was observed in all animals receiving d-bupivacaine. Most important, this severe bradycardia was accompanied by progressive hypotension. In addition, all animals receiving d-bupivacaine became apneic and died, whereas all animals receiving l-bupivacaine continued to breathe and all but two of the animals survived.

Conclusions: Data in the current report support the hypothesis that effects of bupivacaine on neurons of the NTS and on the cardiovascular system have an enantiomer-specific component.