Prediction of arterial oxygen partial pressure after changes in FI O2: Validation and clinical application of a novel formula

Abstract

Background. Existing methods allow prediction of PaO2 during adjustment of FIO2. However, these are cumbersome and lack sufficient accuracy for use in the clinical setting. The present studies aim to extend the validity of a novel formula designed to predict PaO2 during adjustment of FIO2 and to compare it with the current methods. Methods. Sixty-seven new data sets were collected from 46 randomly selected, mechanically ventilated patients. Each data set consisted of two subsets (before and 20 min after FIO2 adjustment) and contained ventilator settings, pH, and arterial blood gas values. We compared the accuracy of PaO2 prediction using a new formula (which utilizes only the pre-adjustment Pa O2 and pre- and post-adjustment FIO2) with prediction using assumptions of constant PaO2/FIO2 or constant PaO2/PAO2. Subsequently, 20 clinicians predicted Pa O2 using the new formula and using Nunns isoshunt diagram. The accuracy of the clinicians predictions was examined. Results. The 95% limits of agreement (LA95%) between predicted and measured PaO2 in the patient group were: new formula 0.11 (2.0) kPa, PaO2/FI O2 -1.9 (4.4) kPa, and PaO2/PAO2 -1.0 (3.6) kPa. The LA95% of clinicians' predictions of PaO2 were 0.56 (3.6) kPa (new formula) and -2.7 (6.4) kPa (isoshunt diagram). Conclusions. The new formulas prediction of changes in PaO2 is acceptably accurate and reliable and better than any other existing method. Its use by clinicians appears to improve accuracy over the most popular existing method. The simplicity of the new method may allow its regular use in the critical care setting. © The Author [2011]. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved.