Comparison of neural blockade and pharmacokinetics after subarachnoid lidocaine in the rhesus monkey. II: Effects of volume, osmolality, and baricity.

The effects of volume, osmolality, and baricity on lidocaine spinal anesthesia in the rhesus monkey were studied. Changes in neural blockade, physical properties of cerebrospinal fluid, and arterial pharmacokinetics associated with variations in injectate composition were assessed. Wide ranges of volume, baricity, and osmolality were studied using 1, 2, and 5% lidocaine prepared in either sterile water or 7.5% dextrose. Minimal changes in neural blockade were found in the ranges of osmolality and baricity studied, although 5% lidocaine in sterile water resulted in significantly shorter complete recovery times for both sensory and motor block when compared to other solutions. Samples of cerebrospinal fluid obtained after injection of lidocaine showed increases or decreases in specific gravity and osmolality depending on the physical properties of the solution injected. No differences in elimination phase pharmacokinetics were found with any of the lidocaine solutions. Rates of systemic absorption increased with decreasing osmolality. Osmotic potentiation of lidocaine spinal anesthesia could not be demonstrated.