Type of solution and PCO2 measurement errors during tonometry

Objective: The choice of solution for gastrointestinal tonometry influences the PCO₂ measurement bias, precision and the time required for equilibration. We compared saline with buffered solutions during in vitro tonometry, with respect to systematic and accidental measurement errors and equilibration time. Design: A prospective laboratory study. Measurements: Saline, phosphate, phosphate bicarbonate and succinylated gelatin solutions were equilibrated in a specialized blood gas tonometer at PCO₂s of 2.7, 3.6, 4.5, 6.2 and 9.0 kPa, using calibration gases. Accidental errors were determined: the within-syringe decline of PCO₂ and the effects of handling errors (five up and down movements of the plunger). The PCO₂ build up in gastrointestinal tonometers was determined in 5000 ml saline baths with fixed PCO₂ levels of 2.7 and 9.0 kPa. Results: The build up of PCO₂ in phosphate bicarbonate and gelatin was about 4 and 2 times slower than in saline and phosphate, respectively, both for gas and gastrointestinal tonometers. The bias of the measured PCO₂ at equilibrium was −15 % for saline, and between −1 and 3 % for phosphate, phosphate bicarbonate and gelatin. The precision was comparable among the solutions: 2 ± 1 % for saline, 2 ± 1 % for phosphate, 1 ± 0 % for phosphate bicarbonate and 1 ± 1 % for gelatin. The accidental errors were virtually absent with phosphate bicarbonate, intermediate with gelatin and largest with saline and phosphate. Conclusion: Phosphate bicarbonate buffer and succinylated gelatin allow accurate PCO₂ measurements, but their equilibration is too slow for clinical application. The advantage of phosphate over saline solution is a smaller bias only. Thus, both saline and phosphate are currently the tonometer solutions of choice, provided that strictly anaerobic conditions are applied and the bias by the blood gas analyzer is known. Received: 5 July 1996 Accepted: 4 March 1997     

Usefulness of gastric intramucosal pH for monitoring hemodynamic complications in critically ill children

Objective: To assess the efficacy of gastric intramucosal pH for the evaluation of tissue perfusion and prediction of hemodynamic complications in critically ill children. Design: Open prospective study without controls Received: 28 January 1997 Accepted: 19 September 1997     

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 tonometry and to assess the effects of variation of temperature and carbon dioxide concentration within the range encountered in clinical use. Design: a) A prospective, observational study in ICU patients b) A comparative laboratory study. Setting: The general Intensive Care Unit (ICU) and the laboratory at Leeds General Infirmary. Patients and participants: 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 & Altman analysis showed a mean bias in the measurement of gastric PCO₂ of 1.88 kPa with a precision of 1.22 kPa, with saline giving the lower result. In vitro, saline, air (Tonocap), gelatin and heparinised blood were used, at temperatures of 33–42 °C and at carbon dioxide concentrations of 4–8 kPa. While gelatin and blood gave unpredictable results, dependent on temperature and carbon dioxide concentration, air tonometry gave highly reproducible results. A consistent bias between the results with saline and air tonometry was seen over the range of temperatures and carbon dioxide (Co₂) concentrations studied. The mean bias was 0.85 kPa with a precision of 0.40 kPa, saline consistently giving lower results. Conclusions: There are clinically significant differences in values for gastric mucosal PCO₂ measured by air tonometry and saline tonometry both in vivo and in vitro. Received: 19 December 1997 Accepted: 22 May 1998