ObjectivesTo assess the accuracy of pulse oximetry under hypothermic conditions in neonates and infants undergoing cardiac surgery, and to assess the effect of probe site as well as probe site skin temperature on the reliability of pulse oximetry.DesignProspective study.SettingCardiac operating room and intensive care unit of a children's hospital.PatientsTwenty-five infants less than 3 months of age undergoing cardiac surgery with cardiopulmonary bypass.InterventionsPulse oximeter readings (SpO2) from probes placed on the hand and foot were recorded at various skin temperatures and compared with hemoximeter oxygen saturations (SaO2) obtained on simultaneously drawn arterial blood samples. Core temperature, arterial pressure and vasodilator use were recorded simultaneously.Measurements and Main ResultsPulse oximetry bias (SpO2-SaO2) increased to an unacceptable range (more than plus 3% or less than minus 3%) in 45.5% of the readings at foot probe site temperatures of less than equals 27 degrees C. Pulse oximetry bias was within an acceptable range in 94.7% of the readings at temperatures more than 29 degrees C. There was no significant difference between oximeter readings obtained from two probe sites (hand and foot). Administration of phenoxybenzamine improved the accuracy of pulse oximetry in ten infants at skin temperature of less than 27 degrees C.ConclusionsPulse oximetry readings in small infants are likely to be unreliable at skin temperatures of less than 27 degrees C, irrespective of probe site. Intravenous phenoxybenzamine appeared to improve the accuracy of pulse oximetry at low temperatures.(Crit Care Med 1996; 24:507-511)