Objective: To evaluate the paediatric 5-French (Fr) saline-filled gastric tonometer.
Design: (a) In vitro comparison of saline bath reference pCO2 with tonometric pCO2 measured by normal saline-filled and phosphate-buffered saline-filled 5-Fr tonometers, and by a recirculating gas tonometer. ( b) In vivo comparison of gastric intramucosal pCO2i, measured by normal saline-filled 5-Fr tonometer (NST) and simultaneously by recirculating gas tonometer (RGT) in ten paediatric intensive care patients. (c) In vivo comparison of pCO2i measured simultaneously by 2 NST 5-Fr tonometers, before and after enteral feeding, in ten paediatric intensive care patients.
Measurements and main results: (a) Twenty consecutive measurements of pCO2 were made at constant reference pCO2 of 19, 38, 56, and 75 mmHg (2.5, 5.0, 7.5, and 10.0 kPa), respectively. The NST tonometer underestimated reference pCO2 by mean bias (limits of agreement) of 58% (20%), and the phosphate-buffered saline-filled tonometer by 6% (26%). The RGT showed mean bias 5.7% with narrow limits of agreement (1.5%). (b) In 50 paired (NST vs. RGT) in vivo measurements over pCO2i range 23-73 mmHg (3.0-9.7 kPa), the NST underestimated RGT pCO2i by a mean bias of 10 mmHg (1.3 kPa), with limits of agreement +/-10 mmHg (1.5 kPa). This resulted in NST consistently overestimating pHi and underestimating pCO2 gap (both P < 0.001). (c) One hundred simultaneous paired NST measurements were assessed (50 without, and 50 with enteral feeding). The mean biases (limits of agreement) were identical in the fasted and fed states 0.4+/-6 mmHg, with no difference between the fed and fasting states (P = 0.7).
Conclusions: There are inherent problems in the methodology of saline tonometry, which adversely affect the accuracy and reliability of the 5-Fr paediatric gastric tonometer in comparison to recirculating gas tonometry.