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 auto mated 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 t o 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 mathematic al 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 signifi cantly faster with the automated gas system than with traditional saline to nometry, The mathematical time constant for a 5% change in CO2 in a gas env ironment was 2.8 mins (95% confidence interval, 2.6-3.0 mins) for the gas a nd 6.3 mins (95% confidence interval, 5.8-7.3 mins) for the saline techniqu e. 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 sa line tonometry, There was a significantly lower final equilibration value f or the CO2 measurement with saline tonometry, There was essentially no diff erence 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 tonome try 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.