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 response to epidural local anesthetic test and bolus doses in 11 patients. Sympathetic activity was measured by monitoring cutaneous perfusion in the foot and the reflex vasoconstrictive response to deep inspiration (IGVR) using laser Doppler flowmetry. Sensory tests included the detection of touch, cold and painful stimuli. Following the 3-mL test dose 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 to 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. Five control patients who received only 60 mg of lidocaine intravenously and the two patients with failed epidurals did not show any perfusion or IGVR changes. This study confirms that sympathetic block precedes sensory block in sacral dermatomes after epidural anesthesia. Perfusion and IGVR changes are sensitive measures of sympathetic blockade and may predict successful epidural catheter placement.