Absorption of lidocaine following subarachnoid and epidural administration

The absorption of lidocaine into the vascular system from the epidural and subarachnoid spaces was determined in 20 patients. Seventy-five milligrams (1.5 ml of a 5% solution) of lidocaine was administered intrathecally to 10 patients for spinal anesthesia and 75 mg (3.75 ml of a 2% solution) was injected epidurally. Venous plasma levels of lidocaine were determined at 2, 5, 10, 15, and 30 minutes after subarachnoid or epidural injection. No significant differences existed between the maximum venous plasma concentrations of lidocaine following epidural injection (0.41 +/- 0.07 microgram/ml) and following subarachnoid injection (0.32 +/- 0.07 microgram/ml). However, the rate of vascular absorption of lidocaine from the subarachnoid space was significantly slower than from the epidural space. By 2, 5, and 10 minutes after injection, venous plasma concentrations of lidocaine were significantly higher following epidural injection than they were after subarachnoid injection. This variation was not attributable to differences in blood pressure folloiwng injection. The differential rate of absorption is probably related to the anatomical arrangement of the venous plexuses in the epidural and subarachnoid spaces.