Source: Annual Congress 2010 - Acute respiratory failure
Disease area: Respiratory critical care

Abstract

Introduction: We compared accuracy of predicting PaO₂ following F_IO₂ adjustment using an established method (_newPaO₂=_newP_AO₂×_oldPaO₂/_oldP_AO₂; P_AO₂=calculated alveolar partial pressure of oxygen) and our newly developed formula
Method: 106 datasets were collected from 64 randomly-selected, mechanically ventilated patients. Each dataset consisted of two subsets (before, and 20 min. after, F_IO₂ adjustment). Each set consisted of ventilator settings and arterial blood gases. The resulting PaO₂ was predicted using a newly derived formula: _newPaO₂=_newF_IO₂×_oldPaO₂ /_oldF_IO₂×k (k=1-(_oldF_IO₂-_newF_IO₂)/2) and the arterial/alveolar formula (see above), where P_AO₂=[(Pb-SVP_H2O)×F_IO₂]×(1.25×PaCO₂); Pb=atmospheric pressure (101.3 kPa), SVP_H2O=saturated vapour pressure of water (6.3 kPa), PaCO₂ (kPa)=partial pressure of carbon dioxide prior to F_IO₂ adjustment
Results: F_IO₂ decreased, on average (±SD), from 0.6 (0.21) to 0.5 (0.16), PaO₂ decreased from 22.9 (14.3) to 17.5 (7.9) kPa. 95% limits of agreement (LA_95%) between measured and predicted PaO₂ show a bias (±LA_95%) of 0.08 (1.95) kPa for the new formula and -0.67 (3.2) kPa for the arterial/alveolar formula. Predicted vs. measured changes in PaO₂ are shown in the figure

Conclusion: It seems that arterial/alveolar formula shows worsening inaccuracy in predicting PaO₂ when the magnitude of change of F_IO₂ is large. Our new formula improves upon this and is sufficiently accurate to be used in the clinical setting

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Citations should be made in the following way:
H. Al-Otaibi, W. Batobara, J. Hardman (United Kingdom; Makkah, Saudi Arabia). Predicting arterial oxygen tension following inspired oxygen adjustment. Eur Respir J 2010; 36: Suppl. 54, 2298

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