COMPARISON OF AIR TONOMETRY WITH GASTRIC TONOMETRY USING SALINE AND OTHER EQUILIBRATING FLUIDS - AN IN-VIVO AND IN-VITRO STUDY

Objective: 1) To compare saline gastric tonometry monitoring with air tonometry (Tonocap) in a group of general ICU patients. 2) An in vitro investigation of the performance of other fluids used in gastric tono metry and to assess the effects of variation of temperature and carbon dioxide concentration within the range encountered ill clinical use. Design: a) A prospective, observational study in ICU patients b) A com parative laboratory study. Setting: The general Intensive Care Unit (I CU) and the laboratory at Leeds General Infirmary. Patients and partic ipants: Nine patients in the general ICU with severe sepsis or septic shock. Measurements and results: In vivo comparison of saline and air tonometry demonstrated a difference between the two techniques. Bland st Altman analysis showed a mean bias in the measurement of gastric PC O2 of 1.88 kPa with a precision of 1.22 kPa, with saline giving the lo wer result. In vitro, saline, air (Tonocap), gelatin and heparinised b lood were used, at temperatures of 33-42 degrees C and at carbon dioxi de concentrations of 4-8 kPa. While gelatin and blood gave unpredictab le results, dependent on temperature and carbon dioxide concentration, air tonometry gave highly reproducible results. A consistent bias bet ween the results with saline and air tonometry was seen over the range of temperatures and carbon dioxide (Co-2) concentrations studied. The mean bias was 0.85 kPa with a precision of 0.40 kPa, saline consisten tly giving lower results.Conclusions: There are clinically significant differences in values for gastric mucosal PCO2 measured by air tonome try and saline tonometry both in vivo and in vitro.