Type of solution and PCO₂ measurement errors during tonometry

Abstract

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.