SummaryResponse characteristics of a transcutaneous PO2electrode to step changes in PaO2were determined.In vitrolag time and 95% response time of a transcutaneous PO2monitor were compared toin vivoresponse. Release of arterial occlusion was used to produce rapid local Pao2changes in human infants and adults.In vitrolag time and 95% response time varied according to whether an increment or decrement in PO2was produced.In vivo95% response time of this electrode was two to six times slower than previous estimates, and it varied significantly with the magnitude of the step change, subject age, anatomic location, and local hemodynamic factors. Also, we found thatin vivolag time is two to three times faster than previously reported.SpeculationUnder steady-state conditions, transcutaneous PO2correlates well with PaO2, but when rapid change is encountered as during oxygen administration or apnea, transcutaneous PO2will significantly underestimate PaO2change. On the other hand, the relatively short lag time suggests that the transcutaneous PO2electrode may be useful in following rapid PaO2changes qualitatively.