PULSE OXIMETRY DURING LUMBAR EPIDURAL-ANESTHESIA - RELIABILITY OF VALUES MEASURED AT THE HAND AND THE FOOT

Pulse oximetry is dependent upon the presence of a pulsating vascular bed. The signal detection will be impaired in the presence of vasocons triction or venous congestion, conditions which may occur readily in c linical practice. We compared the oximetric measurements (Spo(2)) at t he hand and the foot with arterial hemoglobin saturation (Sao(2)) duri ng lumbar epidural anesthesia. After administration of a crystalloid s olution (20 mL/kg body weight lactated Ringer's solution), 40 adult ma le patients, scheduled for inguinal hernioplasty, received 15 mL of 0. 50% plain bupivacaine into the lumbar epidural space. Two pulse oximet er probes were applied to the index finger and toe of the patients, an d the Spo(2) values were recorded continuously. Arterial hemoglobin sa turation (Sao(2)) was measured using a co-oximeter 5 min before and 30 min after the onset of sensory block. No significant differences were detected between Sao, (97.7%, SD 0.4%) and Spo(2) basal values record ed from the hand (97.8%, SD 0.8%) and the foot (98.1%, SD 0.4%). After the onset of epidural anesthesia, a progressive decrease of Spo(2) va lue recorded from the hand was observed: at 30 min it was 92.3% +/- 1. 3% (P < 0.01 compared with baseline). At the same time, the Sao, value was 97.5% +/- 0.9% (P < 0.01 compared with Spo(2) from the hand). On the contrary, no significant difference from both basal value and Sao( 2) was detected in Spo(2) measurements from the toe during the epidura l block. In all patients intraoperative decrease of heart rate and art erial blood pressure was less than or equal to 15% from baseline. The decrease of Spo(2) values recorded from sympathetically unaffected are as during epidural anesthesia could be related to vasoconstriction, co unterbalancing lumbar preganglionic sympathetic block, which occurred despite perioperative crystalloid volume loading. Because of this comp ensatory vasoconstriction, pulse oximetry during lumbar epidural anest hesia gives a falsely low reading when the oximetric sensor is placed at the upper limbs.