Characterization of the silicone tonometer using a membrane-covered transcutaneous electrode

A new oxygen tonometry system consisting of a silicone tube, highly permeable to O2 and CO2 is described. The silicone tube was connected to a membrane-covered transcutaneous oxygen electrode (E5242 Radiometer A/S, Copenhagen, Denmark) via an airtight polycarbonate chamber, and flushed with isotonic saline equilibrated with atmospheric air. The present tonometer system offers certain advantages compared with other systems: continuous reading, minimal oxygen consumption, furthermore the system is thermostated and is insensitive to movement. The tonometry system was tested in vitro for characterization of a silicone tube (Coroplast, Fritz Müller KG, Wuppertal, FRG) 1.0 mm in inner diameter and 1.5 mm in outer diameter. The experiments showed that the oxygen tension measured at the electrode after passage of the tonometer approached the oxygen tension outside the tonometer when the length of the tonometer was increased and when the flushing rate of saline through the tonometer was decreased. The time taken for the flushing solution to reach an equilibrium with the oxygen tension outside the tonometer increased with increasing tonometer length, and decreased with increasing flushing rate. Changing the difference between oxygen partial pressure in the flushing solution and the oxygen partial pressure outside the tonometer tube did not influence the relative equilibration value and the equilibration time. When a pO2 value is measured by the electrode, the exact oxygen tension outside the tonometer tube, for every given length of the tonometer and flushing rate through the tonometer can be read from our calibration curves.