VALIDATION OF AIR TONOMETRIC MEASUREMENT OF GASTRIC REGIONAL CONCENTRATIONS OF CO2 IN CRITICALLY-ILL SEPTIC PATIENTS

Objective: To evaluate the accuracy of continuous air tonometry (Tonoc ap, Tonometric Division, Instrumentarium, Helsinki, Finland). Design: The accuracy of air tonometry was tested by comparing it with conventi onal saline tonometry in mechanically ventilated, critically ill septi c patients and in vitro determining the partial pressure of carbon dio xide (PCO2) of humidified gases with known concentrations of CO2. Sett ing: A mixed intensive care unit in a university hospital. Patients: 1 6 mechanically ventilated patients with sepsis. Measurements and resul ts: Two gastric tonometer catheters (TRIP NGS catheter, Tonometric Div ision, Instrumentarium, Helsinki, Finland) were introduced into the pa tients' stomachs. The control catheter was used as a conventional sali ne tonometer and the other catheter was used with the Tonocap monitori ng device. A total of 153 paired measurements was made and analysed ac cording to Bland and Altman. The mean difference between air PCO2 and saline PCO2 values (bias), the standard deviation of the differences ( precision), and the Pearson correlation coefficient between air PCO2 a nd saline PCO2 were calculated. The data on patients were pooled and c alculated for different cycle times. The mean bias (kPa) was -0.02 wit h a 10-min cycle time, 0.31 with 15 min, 0.56 with 30 min and 0.21 wit h 60-min. The precisions were 0.39, 0.54, 0.44 and 0.76, respectively. Pearson correlation coefficients were 0.93, 0.97, 0.95 and 0.82, resp ectively (p < 0.0001). In vitro tonometry with the Tonocap was perform ed in a gas chamber fully saturated with known CO2 concentrations. The clinically important 10-min cycle time was tested with 5 Tonocap moni tors. Except for the first 10-min cycle time, PCO2 values determined b y the Tonocap monitoring systems were comparable to known CO2 concentr ations. Conclusions: The accuracy of Tonocap continuous air tonometry is close to that of conventional saline tonometry. Moreover, the clini cally important 10-min cycle time with air tonometry correlated very w ell with saline tonometry and the time response with air tonometry was short.