TYPE OF SOLUTION AND PCO2 MEASUREMENT ERRORS DURING TONOMETRY

Objective: The choice of solution for gastrointestinal tonometry influ ences the PCO2 measurement bias, precision and the time required for e quilibration. We compared saline with buffered solutions during in vit ro tonometry, with respect to systematic and accidental measurement er rors and equilibration time. Design: A prospective laboratory study. M easurements: Saline, phosphate. phosphate bicarbonate and succinylated gelatin solutions were equilibrated in a specialized blood gas tonome ter at PCO(2)s of 2.7, 3.6, 4.5, 6.2 and 9.0 kPa, using calibration ga ses. 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 de termined 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 gelat in was about 4 and 2 times slower than in saline and phosphate, respec tively, 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 wa s comparable among the solutions: 2 +/- 1% for saline, 2 +/- 1 % for p hosphate. 1 +/- 0 % for phosphate bicarbonate and 1 +/- 1 % for gelati n. The accidental errors were virtually absent with phosphate bicarbon ate, intermediate with gelatin and largest with saline and phosphate. Conclusion: Phosphate bicarbonate buffer and succinylated gelatin allo w 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 co nditions are applied and the bias by the blood gas analyzer is known.