Comparison of the response of saline tonometry and an automated gas tonometry device to a change in CO₂

Objective: To examine the speed of response of saline tonometry and an automated gas tonometry system by using standard tonometry catheters. Design: In vitro validation study. Setting: Experimental research laboratory. Interventions: Tonometry catheters were placed in a test chamber designed to simulate the lumen of a hollow viscus and were exposed to a rapid change in CO₂ from 0% to 5% or 10%. Measured CO₂ over time was fit to a mathematical model to determine the response time constant (the time to reach 63% of the final value) for each system. Measurements and Main Results: Response time to a change in CO₂ was significantly faster with the automated gas system than with traditional saline tonometry. The mathematical time constant for a 5% change in CO₂ in a gas environment was 2.8 mins (95% confidence interval, 2.6 - 3.0 mins) for the gas and 6.3 mins (95% confidence interval, 5.8 - 7.3 mins) for the saline technique. These times were longer for the CO₂ change in a liquid environment: The time constant was 4.6 mins (95% confidence interval, 4.5 - 4.7 mins) for the gas system and 7.8 mins (95% confidence interval, 7.15 - 8.6 mins) for the saline tonometry. There was a significantly lower final equilibration value for the CO₂ measurement with saline tonometry. There was essentially no difference in time constants for each system for a 5% change compared with a 10% CO₂ change, except for a slightly faster time constant for the gas tonometry system with a 5% change in the gas environment (5%: 2.8 mins vs. 10%: 3.3 mins). Conclusions: The automated gas tonometry system has a significantly faster response to a change in CO₂ than conventional saline tonometry.