MEASUREMENT OF TISSUE OXYGEN-TENSION - COMPARISON BETWEEN 2 SUBCUTANEOUS OXYGEN TONOMETERS

We compared the 95% response time (95% RT) of two tissue oxygen tonometers under two sets of circumstances. We first evaluated the devices during normoxia, hyperoxia, and anoxia in vitro, using a transcutaneous PO2 electrode (PtcO2) as the reference. The responses to normoxia and to different grades of hyperoxia were examined in vivo in 8 healthy volunteers to assess the relationship between changes in subcutaneous PO2 and PtcO2, an estimate of arterial PO2 (PaO2). One subcutaneous method (ScA) used a technique based on a polarographic needle electrode in situ connected to an ammeter; the second method (ScB) was based on a blood gas analyzer system first described by Hunt (Lancet 164;2:1370). ScA and PtcO2 both responded to stepwise changes in ambient oxygen concentration (21-100%) in vitro within 10 seconds; the 95% RT of ScA was 1.39 +/- 0.5 to 2.39 +/- 0.8 minutes and that of PtcO2 was 0.32 +/- 0.1 to 0.49 +/- 0.1 minutes. ScB had a lag of 3 minutes, and the 95% RT was 6.75 +/- 0.5 to 8.2 +/- 0.8 minutes. In contrast to the results in vitro, the response of ScA to changes in FiO2 in vivo was delayed compared with the rapid response of PtcO2, reflecting the physiologic delay of tissue PO2 in response to increased PaO2. The time lag and the long 95% RT of ScB were even more evident in vivo. ScA reacted three to four times faster than ScB, both in vitro and in vivo, to changes in the oxygen environment. The in vitro 95% RT of ScA to changes in ambient oxygen varied from 2 to 3.5 minutes. In contrast, the 95% RT of ScB was 8 to 9 minutes. PtcO2 had the fastest 95% RT, from 0.4 to 0.5 minutes. The results suggest that the subcutaneous needle electrode method (ScA) provides close to real-time assessment of tissue PO2.