Abstract
Background. Existing methods allow prediction of Pa(O2) during adjustment of F(IO2). 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 Pa(O2) during adjustment of F(IO2) 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 F(IO2) adjustment) and contained ventilator settings, pH, and arterial blood gas values. We compared the accuracy of Pa(O2) prediction using a new formula (which utilizes only the pre-adjustment Pa(O2) and pre-and post-adjustment F(IO2)) with prediction using assumptions of constant Pa(O2) /F(IO2) or constant Pa(O2) /PA(O2). Subsequently, 20 clinicians predicted Pa(O2) using the new formula and using Nunn's isoshunt diagram. The accuracy of the clinician's predictions was examined.
Results. The 95% limits of agreement (LA95%) between predicted and measured Pa(O2) in the patient group were: new formula 0.11 (2.0) kPa, Pa(O2) /F(IO2) 21.9 (4.4) kPa, and Pa(O2) /PA(O2) 21.0 (3.6) kPa. The LA95% of clinicians' predictions of Pa(O2) were 0.56 (3.6) kPa (new formula) and 22.7 (6.4) kPa (isoshunt diagram).
Conclusions. The new formula's 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.