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A comparison of the response of the tonocap and saline tonometry to a change in CO2


Gastrointestinal tonometry has proven to be a useful technique to evaluate gut mucosal perfusion. The most commonly used method is saline tonometry which does not allow rapid assessment of gut luminal CO2 and is subject to sampling errors. The Tonocap (Tonometrics Division, Instrumentarium Corp, Helsinki, Finland) is a new device which can be used with the established tonometric catheters. The Tonocap fills the tonometer balloon with air rather than saline. Following an equilibration period, the gas is automatically sampled, measured with an infra-red sensor and PCO2 and calculated intramucuosal pH (pHi) are displayed. We studied the in vitro response of the Tonocap and traditional saline tonometry to a step change in PCO2.


Two TRIP sigmoid tonometers (Tonometrics Division, Instrumentation Corp, Helsinki, Finland) were placed in an air tight test chamber. In one set of experiments, the chamber was perfused with saline in a closed circuit with a roller pump and a membrane oxygenator. The oxygenator allowed for rapid change of PCO2 in the fluid. In the other set of experiments, the chamber was flushed with calibration gas. The two systems mimic the clinical situations of a fluid filled viscus or a gas filled viscus. The CO2 in the chamber could be fully changed from 0 to 5 or 10% within 60 s in the saline system, and 30 s in the gas system. One catheter was connected to the Tonocap, the other was filled with 2.5 cc saline that was analysed with an IL1610 blood gas analyser. CO2 readings were taken at different times of equilibration after a step change in CO2 from 0. Between each reading, each catheter was flushed to eliminate CO2 in the dead space. CO2 versus equilibration time was fit to an exponential function and a time constant was derived.


A plot of CO2 equilibration time is shown for the gas circuit. The time to reach half of the final CO2 value (T1/2) and the exponential time constants (τ) are shown in the table. The CO2 response curves were significantly different between saline and Tonocap methods by linear regression in each CO2 level (P = 0.0001).

figure 1



The Tonocap responds approximately twice as fast as saline tonometry to a step change in CO2. This should translate to a quicker detection of changes in intraluminal CO2 and incracellular acidosis in patients, thereby leading to more expedient intervention to correct the underlying perfusion deficit.


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Noone, R., Mythen, M. & Vaslef, S. A comparison of the response of the tonocap and saline tonometry to a change in CO2. Crit Care 1 (Suppl 1), P088 (1997).

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