TIME COURSE OF SYMPATHETIC BLOCKADE DURING EPIDURAL-ANESTHESIA - LASER DOPPLER FLOWMETRY STUDIES OF REGIONAL SKIN PERFUSION

We studied the time course of sensory and sympathetic blockade in resp onse to epidural local anesthetic test and bolus doses in 11 patients. Sympathetic activity was measured by monitoring cutaneous perfusion i n the foot and the reflex vasoconstrictive response to deep inspiratio n (IGVR) using laser Doppler flowmetry. Sensory tests included the det ection of touch, cold and painful stimuli. Following the 3-mL test dos e perfusion increased to 192 +/- 38% (mean +/- SEM) of baseline (P < 0 .05) in the patients with successful epidural anesthesia (9 of 11). In 8 of these patients, IGVR decreased to 73 +/- 10% of baseline (P < 0. 05) within 6 min of the test dose, and preceded changes in sensation t o cold, pin-prick, and light touch by 3.8 +/- 3.5 min (P < 0.2), 9.6 /- 3.1 min (P < 0.01), and 11.6 +/- 2.7 min (P < 0.01), respectively F ive control patients who received only 60 mg of lidocaine intravenousl y and the two patients with failed epidurals did not show any perfusio n or IGVR changes. This study confirms that sympathetic block precedes sensory block in sacral dermatomes after epidural anesthesia. Perfusi on and IGVR changes are sensitive measures of sympathetic blockade and may predict successful epidural catheter placement.