title: Comparison of the response of saline tonometry and an automated gas tonometry device to a change in CO2.
abstract: OBJECTIVE
  To examine the speed of response of saline tonometry and an automated gas tonometry system by using standard tonometry catheters.
  
  
  METHODS
  In vitro validation study.
  
  
  METHODS
  Experimental research laboratory.
  
  
  METHODS
  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.
  
  
  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.
authors: Department of Surgery, Duke University Medical Center, Durham, NC 27710, USA.
authors: R B
authors: RB
authors: Noone
authors: J E
authors: JE
authors: Bolden
authors: M G
authors: MG
authors: Mythen
authors: S N
authors: SN
authors: Vaslef
mesh_terms: D002245:Carbon Dioxide / Q000032:analysis*; D003422:Critical Care; D005753:Gastric Mucosa / Q000098:blood supply; D006801:Humans; D007413:Intestinal Mucosa / Q000098:blood supply; D008365:Manometry / Q000295:instrumentation*; D008962:Models, Theoretical; D012016:Reference Values; D012039:Regional Blood Flow / Q000502:physiology; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*
chemical_list: D002245:Carbon Dioxide