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Gastric mucosal tonometry: should we adjust PiCO2 and CO2-gap to temperature?

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It has been under discussion for many years, whether acid-base-equilibrium during hypothermia should be managed using the alpha-stat or the pH-stat concept. Several studies in cardiac patients undergoing hypothermic cardiopulmonary bypass (CPB) led to controversial results. Until now there are only few data available, on whether to adjust PiCO2 and CO2-gap to temperature or not. Thus we calculated PiCO2 and CO2-gap in cardiac surgery patients with and without temperature correction looking for differences in the prediction of postoperative complications.


After IRB approval, we studied 69 patients undergoing elective aortocoronary bypass surgery, ASA class II-III. In addition to standard monitoring each patient received a pulmonary artery catheter and an air-filled nasogastric tonometry catheter connected to a Tonocap. Documentation of gastric-intramucosal PCO2 (PiCO2) followed at 15-min-intervals, documentation of hemodynamics, arterial and mixed-venous blood gas analyses and lactate followed seven times until extubation (T1-T7). We used mild hypothermic CPB and the alpha-stat concept. PaCO2 and PiCO2 were adjusted to temperature by the following algorithm: PCO2t = PCO2 × 10 0.0185× (t-37) [mmHg] (t=body temperature; PCO2t = temperature adjusted PCO2).

Statistical analyses were done using the paired t-test, P<0.05 was regarded as significant.


There was a significant difference of about 3-4 mmHg in temperature corrected PiCO2 and PaCO2 compared to uncorrected values. The CO2-gap ([Pi-Pa]CO2) differed significantly at T3, T6 and T7 in all patients and at T3 and T4 in patients with postoperative cardiopulmonary resuscitation (n=5). However, the time course of measurements was equal.


For clinical applications, it is not necessary to adjust PiCO2 and CO2-gap to body temperature during mild hypothermic CPB with alpha-stat concept since no further information could be gained from the correction. In laboratory settings or deep hypothermic CPB with pH-stat concept temperature adjustment of tonometric parameters may be helpful. In addition, the correction may also be useful in hyperthermia (septic patients). However, this has to be investigated in further studies.

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Vogelsang, H., Uhlig, T., Seifert, C. et al. Gastric mucosal tonometry: should we adjust PiCO2 and CO2-gap to temperature?. Crit Care 4 (Suppl 1), P150 (2000).

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