Abstract

Lumbar peridural anesthesia with 20 ml of 2% lidocaine hydrochloride (HCI) or lidocaine hydrocarbonate (CO₂) and epinephrine 1:200,000 was administered to 50 men ranging in age from 22 to 78 years. Venous blood samples were obtained over an 8-hour period to determine clearance (CI), half-life (t 1/2), volume of distribution (Vd), and peak concentration. There were no significant pharmacokinetic differences between the two lidocaine salts. Subgroups of patients younger than 40 (n = 9) and older than 55 (n = 33) years were compared. Lidocaine clearance was 33% lower in the older group (0.81 ± 0.30 vs. 0.54 ± 0.15 L/min, p < 0.0005). Volume of distribution was 20% less (1.68 ± 0.52 vs. 2.06 ± 0.48 L/kg, p < 0.05) and half-life was 10% greater (167 ± 60 vs. 144 ± 34, p > 0.05) in older patients. There was a weak but statistically significant negative correlation between clearance and age for the entire group of 50 patients r = −0.41 (p < 0.01). Peak lidocaine concentration was not affected by age, and no toxicity was found. The findings suggest that a single 400-mg dose of lidocaine is safe in older patients, but during continuous peridural block, lidocaine accumulation and toxicity might occur in some older patients due to reduced clearance. The pharmacokinetic technique of superposition was used to simulate lidocaine concentrations in a 72-year-old patient with a lidocaine clearance of 0.22 L/min during a continuous peridural block (400 mg initial dose, 200 mg every 45 minutes thereafter). Toxic concentrations, greater than 10 μg/ml, would occur after the third reinjection.