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.

Hyperbaric dye solution distribution characteristics after pencil-point needle injection in a spinal cord model.

BACKGROUND: The flow-rate limiting and directional characteristics of caudally directed microcatheters, which lead to intrathecal maldistribution of hyperbaric 5% lidocaine, are believed to have contributed to at least 11 cases of cauda equina syndrome. The authors investigated the distribution characteristics of hyperbaric dye solutions via caudally directed side port needles at various rates of injection in a spinal cord model to determine the potential for maldistribution. METHODS: Using a digital video image processing technique, we injected a hyperbaric solution of phthalocyanine blue dye through caudally directed side-port needles into a supinely oriented transparent spinal canal model filled with simulated cerebrospinal fluid. Injections via commonly used spinal needles (24-gauge and 25-gauge Sprotte, and 25-gauge and 27-gauge Whitacre) were recorded using five injection rates (2, 4, 6, 8, and 16 ml/min). RESULTS: For all needles tested, injection rate had a significant effect on the peak dye concentration (P < 0.0001). Injection rates > or = 6 ml/min (2 ml/20 s) resulted in peak dye concentrations of less than 168 mg/1 (extrapolated concentration of 1% lidocaine). Injection via the 24-gauge Sprotte needle, which has a larger orifice area and internal diameter, resulted in significantly lower peak dye concentrations than via the smaller Whitacre needles tested (P < 0.05). CONCLUSIONS: Sacral maldistribution could be minimized by using injection rates > or = 6 ml/min (2 ml/20 s), for all of the side-port spinal needles used in this model study. When very slow injection rates (2 ml/min) are used, peak dye concentrations varied inversely and significantly with needle internal diameter and orifice area.