What constitutes an effective but safe initial dose of lidocaine to test a thoracic epidural catheter?

To investigate the effects of age and dose on the spread of thoracic epidural anesthesia, we placed thoracic epidural catheters in 50 surgical patients divided into groups by age (Group I [young], 18-51 yr; Group II [old], 56-80 yr) and randomly assigned patients to receive either 5 mL (A) or 9 mL (B) of 2% lidocaine (plain) injected via the epidural catheter. Hemodynamic variables were measured (heart rate, mean arterial blood pressure, noninvasive impedance cardiac index) at baseline and every 5 min for 30 min. Detectable blockade occurred within 8 min after injection of 3 + 2 mL or 3 + 6 mL in 48 of 50 patients. Maximum spread of analgesia to pinprick occurred 15-23 min after completion of local anesthetic injection and was significantly different between age and volume groups by two-way analysis of variance (Group IA [young 5], 10.9 +/- 4.0 dermatomes; Group IIB [young 9], 13.9 +/- 4.5 dermatomes; Group IIA [old 5], 14.1 +/- 5.6 dermatomes; and Group IIB [old 9], 17.4 +/- 5.1 dermatomes). Minor decreases in mean arterial blood pressure (8%-17%) and heart rate (4%-11%) were noted. Two patients in the Old 9 group required IV ephedrine or ephedrine/atropine to treat hypotension and bradycardia. We conclude that given the rapid onset (3-8 min), extensive spread (11-14 dermatomal segments), and consistent hemodynamic stability, thoracic epidural anesthesia should be initiated with lidocaine 100 mg (5 mL 2% lidocaine) to establish proper location of the catheter in the epidural space in both younger and older patients.