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The effect of hypothermia on calculated values using saline and automated air tonometry
M V Chapman1, R L Woolf, E Bennett-Guerrero
1Centre for Anaesthesia, UCL, London, UK. miriam.chapman@virgin.net
Objective:
To quantify in vitro the effect of hypothermia on results obtained when performing automated air tonometry (Tonocap, Datex-Ohmeda, Instrumentarium Corp, Helsinki, Finland) and saline tonometry.
Design:
In vitro validation study.
Setting:
University hospital research laboratory.
Interventions:
Two TRIP sigmoid catheters, one connected to the Tonocap device and the other instilled with 2.5 mL of 0.9% saline, were placed in a saline bath at 30.3 degrees C (mean) through which 5% carbon dioxide (CO(2)) was bubbled.
Measurements And Main Results:
A total of 50 paired measurements were taken at 30-minute equilibration times of saline bath CO(2) tension and saline tonometry and air tonometry readings. Saline samples were analyzed at 37 degrees C and corrected for temperature. Bias and precision of each technique as a percentage of predicted CO(2) values were calculated. The Tonocap device had bias and precision values of -2.6% and +/-1.4%. Measurement of CO(2) is in the gaseous phase so that temperature correction is not required. Saline tonometry readings processed at 37 degrees C exhibited a large positive bias of 23.06% (precision +/- 7.02%). Correction for temperature improved bias to -10.98 % with a similar precision profile of +/-5.78%.
Conclusion:
When using gastrointestinal tonometry during hypothermic cardiopulmonary bypass, saline tonometry samples should be temperature corrected. The Tonocap device proved the most accurate and precise measurement technique independent of the need for temperature correction.