An in vitro characterization of a silicone tonometer system for synchronous measurement of tissue oxygen- and carbon dioxide tension

Abstract

A new tonometry system for continuous and synchronous measurement of tissue oxygen- and carbon dioxide tension is described and characterized in vitro. The tonometer system consists of an O₂ and CO₂ permeable silicone tube continuously flushed with isotonic saline by an injection pump. When the saline passes through the tonometer tube it equilibrates with O₂ and CO₂ outside the tube. The oxygen- and carbon dioxide tension 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 tonometer tube via an airtight polycarbonate chamber. In order to characterize the tonometer system in vitro the tonometer tube was submerged in a test chamber containing isotonic saline, 33°C to 41°C, with varying partial pressures of O₂ and CO₂. For various lengths of the tonometer and flushing rates through the tonometer the partial pressures of oxygen and carbon dioxide in the flushing solution (pO₂eq and pCO₂eq), after passage through the tonometer were recorded and compared to the known partial pressures of oxygen and carbon dioxide in the test chamber solution (pO₂ test and pCO₂test). PO_zeq and pCO₂eq approached pO₂test and pCO₂test, when the length of the tonometer was increased, and the flushing rate through the tonometer was decreased. The relative differences (D) between pO₂eq and pCO₂eq at the one hand and pO₂test and pCO₂test at the other hand were calculated, and equilibration curves were constructed. The time taken for the flushing solution to equilibrate with the oxygen and carbondioxide in the test chamber decreased with increasing flushing rate, but was independent of the length of the tonometer, when the length was less than 10cm. A flushing rate of 4mlh⁻¹ typically used in the clinic, gave a 90% equilibration time for O_z and CO₂ of 73 s and 130 s respectively.

When the tonometer system is used in vivo, the equilibration curves enables conversion of the measured pO_zeq 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.

Key Words:

• oxygen
• carbon dioxide
• tonometry