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 (Tonocap, Tonometric Division, Instrumentarium, Helsinki, Finland). Design: The accuracy of air tonometry was tested by comparing it with conventional saline tonometry in mechanically ventilated, critically ill septic patients and in vitro determining the partial pressure of carbon dioxide (PCO2) of humidified gases with known concentrations of CO2. Setting: A mixed intensive care unit in a university hospital. Patients: 16 mechanically ventilated patients with sepsis. Measurements and results: Two gastric tonometer catheters (TRIP NGS catheter, Tonometric Division, Instrumentarium, Helsinki, Finland) were introduced into the patients' stomachs. The control catheter was used as a conventional saline tonometer and the other catheter was used with the Tonocap monitoring device. A total of 153 paired measurements was made and analysed according 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 and saline PCO2 were calculated. The data on patients were pooled and calculated for different cycle times. The mean bias (kPa) was -0.02 with a 10-min cycle time, 0.31 with 15 min, 0.56 with 30 min and 0.21 with 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, respectively (p < 0.0001). In vitro tonometry with the Tonocap was performed in a gas chamber fully saturated with known CO2 concentrations. The clinically important 10-min cycle time was tested with 5 Tonocap monitors. Except for the first 10-min cycle time, PCO2 values determined by the Tonocap monitoring systems were comparable to known CO2 concentrations. Conclusions: The accuracy of Tonocap continuous air tonometry is close to that of conventional saline tonometry. Moreover, the clinically important 10-min cycle time with air tonometry correlated very well with saline tonometry and the time response with air tonometry was short.