Osmotic swelling effects on neural conduction

Local anesthetics administered intrathecally seen more effective when in hypobaric solution than when in hyperbaric solution. To test whether an unrecognized osmotic effect might be playing a part in this, sheathed vagus nerves of rabbit were incubated in electrolyte-deficient or electrolyte-free media of various degrees of hypo-osmolarity. The nerves gained weight over a period of 15 min. They lost nearly half their sodium, but very little potassium, within 5 min. Electrolyte depletion by incubation in sucrose solutions depressed the amplitude of the C-fiber component of the compound action potential more rapidly in hypo-osmotic than in iso-osmotic solutions. In iso-osmotic sucrose, 50 per cent depression developed in 61 +/- 12 min (mean +/- SD, n = 5), but in 0.6 iso-osmotic sucrose, 50 per cent depression was reached in 17 +/- 3 min (n = 5). Lidocaine, 100 microM (approximately 0.003 g/100 ml) in iso-osmotic sucrose was without observed effect; lidocaine, 100 microM in 0.6 iso-osmotic sucrose produced 50 per cent depression in 7 +/- 2 min (n = 4). Thus, osmotic swelling plus electrolyte depletion, but not electrolyte depletion alone, markedly intensified inhibition of conduction by lidocaine. All effects were reversible by returning the nerves to isotonic physiologic incubation medium. The results suggest that intrathecal osmotic swelling of neural tissue may contribute to the conduction block in hypobaric spinal anesthesia.