Worst Oxygenation Index during the first 24 hours of ventilation predicts mortality

Introduction

The ratio of PaO₂ to FiO₂ (P/F ratio) is often used to classify patients with hypoxic respiratory failure, and is recommended in guidelines from a UK expert group [1] but does not take airway pressures into account. A study found that adjusting for PEEP did not affect the predictive ability of the P/F ratio [2]; however, the mean airway pressure (MAP) may be a better indicator of lung recruitment. The Oxygenation Index (OI = (FiO₂×MAP)/PaO₂)) includes an adjustment for MAP.

Methods

We retrospectively assessed a computerised record (from 2008 to 2010) of ventilator parameters and identified the highest OI for all ventilated patients from a general adult university teaching hospital ICU, during the first 24 hours of ventilation. Patients were grouped according to highest OI, and mortality was calculated for subgroups.

Results

Data were available for 815 patients (see Figure 1). Increasing OI was associated with increasing mortality (P < 0.0001 chi-squared test for trend). Each step increase in OI was associated with approximately a 6% absolute increase in mortality. The OI was also associated with increasing Standardised Mortality Ratio (ICNARC model).

Figure 1.

Mortality and number of patients by Oxygenation Index.

Conclusion

The highest OI occurring in the first 24 hours of ventilation is an independent predictor of mortality. Collection of OI data may allow better prediction of outcome than P/F ratio data alone.