Effect of Electrode Temperature and in Vivo Calibration on Accuracy of Transcutaneous Estimation of Arterial Oxygen Tension in Infants

¹ Departments of Paediatrics and Medical Physics and Bioengineering, University College Hospital and Medical School, London

Previous studies showed that a skin oxygen electrode with a macrocathode, when heated to 43 C, underestimated arterial oxygen tension (Pao₂). At 44 C the skin was damaged. The purpose of the present study was to assess the accuracy of the macrocathode electrode when set at 43.5 C. Transcutaneous oxygen tension (tcPo₂) recorded by the macrocathode electrode at 43.5 C was compared with Pao₂ measured continuously with an intravascular oxygen electrode, and with tcPo₂ recorded by a microcathode electrode which has been shown earlier to work well at a temperature of 44 C. Both the skin electrodes were calibrated in vitro and in vivo. Particular attention was given to the details of calibration. Twelve newborn infants with respiratory illnesses were studied, each for six hours. Transcutaneous Po₂ recorded by both skin electrodes was found to estimate Pao₂ resonably accurately for the entire six-hour duration of the study, with the exception of a large and unexplained overestimation of Pao₂ by the macrocathode electrode in one infant. This overestimation was corrected by in vivo calibration. Serious skin lesions were not seen after the skin electrodes were removed. We conclude that (1) The temperature setting of skin electrodes is crucial to their satisfactory performance. (2) For use on newborn infants, 43.5 C is the optimal temperature for the macrocathode electrode. (3) The optimal temperature for the microcathode electrode was confirmed as 44 C. (4) At these temperatures, both electrodes could be left on the same site on the skin for six hours. (5) Periodic in vivo calibration of skin electrodes is advisable.