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

To investigate the effects of age and dose on the spread of thoracic epidur al anesthesia, we placed thoracic epidural catheters in 50 surgical patient s 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, noninvas ive impedance cardiac index) at baseline and every 5 min for 30 min. Detect able 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 (G roup 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 HB [old 9], 17.4 +/- 5.1 dermatomes). Minor decreases in mean arterial blood press ure (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 sp read (11-14 dermatomal segments), and consistent hemodynamic stability, tho racic epidural anesthesia should be initiated with lidocaine 100 mg (5 mL 2 % lidocaine) to establish proper location of the catheter in the epidural s pace in both younger and older patients.