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These results show that all four local anesthetics form molecular aggregations in the same manner as dodecyltrimethylammonium chloride, a common surfactant. At osmotic pressures insufficient to affect the membrane, local anesthetics caused membrane disruption at the same concentrations at which molecular aggregation occurred. This shows that disruption of the model membrane results from the detergent nature of local anesthetics. Nerve membrane solubilization by highly concentrated local anesthetics may cause irreversible neural injury.

We investigated whether cutaneous stimulus threshold (CST), as determined using a Neurometer, could be used for quantitative and differential nerve evaluation of reversible and irreversible nerve block following intrathecal lidocaine administration in rats. Rats with intrathecal catheters were randomly assigned to one of five groups (saline or 2, 5, 10, or 20% lidocaine). Prior to and 4 days after drug administration, CST was determined at 5, 250, and 2000 Hz. In the 2% lidocaine group, CST from end of lidocaine infusion to recovery from anesthesia was also monitored. Skin-clamp testing and gait observation were performed for comparison with CST findings. Behavioral examinations revealed persistent sensory or motor impairment lasting 4 days in groups receiving Ն5% lidocaine but not in the saline and 2% lidocaine groups. With 2% lidocaine, return to baseline CSTs at 5 and 250 Hz was delayed compared with thresholds at 2000 Hz. Although CSTs in the 5% group at 5 and 250 Hz increased significantly, thresholds at 2000 Hz did not differ from those in rats administered saline. CSTs with Ն10% lidocaine displayed no differences between frequencies. At each frequency, CSTs for rats with Ն5% lidocaine increased in a clearly concentration-dependent manner. These results suggest that CST testing enables evaluation of the different nerve functions for A␤, A␦, and C fibers in rats for lidocaine concentrations Յ5% and allows quantitative assessment of persistent neurological deficit induced by lidocaine in rats.

Lowered shoulder position increases both overlap and proximity between the clavicle and the subclavian vein, producing a more constant relation between the clavicle and the subclavian vein, without affecting vein diameter. Proper use of a lowered shoulder position should thus increase the safety and reliability of subclavian venipuncture compared with other shoulder positions.