AN IN-VITRO CHARACTERIZATION OF A SILICONE TONOMETER SYSTEM FOR SYNCHRONOUS MEASUREMENT OF TISSUE OXYGEN-TENSION AND CARBON-DIOXIDE TENSION

A new tonometry system for continuous and synchronous measurement of t issue oxygen- and carbon dioxide tension is described and characterize d in vitro. The tonometer system consists of an O2 and CO2 permeable s ilicone tube continuously flushed with isotonic saline by an injection pump. When the saline passes through the tonometer tube it equilibrat es with O2 and CO2 outside the tube. The oxygen- and carbon dioxide te nsion of the flushing solution after passage of the tonometer tube are measured by a transcutaneous combined oxygen/carbon dioxide electrode (E5280 Radiometer A/S, Copenhagen, Denmark), connected to the tonomet er tube via an airtight polycarbonate chamber. In order to characteriz e the tonometer system in vitro the tonometer tube was submerged in a test chamber containing isotonic saline, 33-degrees-C to 41-degrees-C, with varying partial pressures Of O2 and CO2. For various lengths of the tonometer and flushing rates through the tonometer the partial pre ssures of oxygen and carbon dioxide in the flushing solution (pO2eq an d pCO2eq), after passage through the tonometer were recorded and compa red to the known partial pressures of oxygen and carbon dioxide in the test chamber solution (pO2 test and PCO2test). PO2eq and PCO2eq appro ached PO2test and PCO2test, when the length of the tonometer was incre ased, and the flushing rate through the tonometer was decreased. The r elative differences (D) between PO2eq and PCO2eq at the one hand and P O2test and pCO2test at the other hand were calculated, and equilibrati on curves were constructed. The time taken for the flushing solution t o equilibrate with the oxygen and carbondioxide in the test chamber de creased with increasing flushing rate, but was independent of the leng th of the tonometer, when the length was less than 10 cm. A flushing r ate of 4 ml h-1 typically used in the clinic, gave a 90% equilibration time for O2 and CO2 of 73 s and 130 s respectively. When the tonomete r system is used in vivo, the equilibration curves enables conversion of the measured pO2eq and PCO2eq values to PO2test and PCO2test values , which in vivo corresponds to the oxygen- and carbon dioxide tensions of the tissue surrounding the silicone tube.