Lung Injury Severity Scoring in the Era of Lung Protective Mechanical Ventilation: The Pao2/Fio2 Ratio

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Abstract
Lung protective ventilatory strategies using low tidal volume and high positive end-expiratory pressure (PEEP) have become standard practice. Such strategies, however, may invalidate measurement of lung injury severity by traditional methods that are based on plain chest radiograph findings, oxygenation, minute ventilation, lung compliance, and PEEP level, such as the Murray lung injury score (LIS). Many of these criteria are potentially therapy dependent and may change with different ventilatory strategies. The purpose of this study was to determine whether measurement of lung injury severity based simply on oxygenation criteria (Pao2/Fio2) was as accurate as the Murray LIS currently used in multiple organ failure (MOF) scoring. Since 1992, trauma patients at high risk for developing MOF have been prospectively identified and MOF scores calculated daily. Pulmonary dysfunction is graded from 0 to 3 on the basis of a modified Murray LIS incorporating the aforementioned parameters. Lung injury severity was redefined using the Pao2/Fio2 (P/F score): Grade 0 = >250; 1 = 175 to 250; 2 = 100 to 174; and 3 = <100. The maximum (worst) score using each was compared using logistic regression and receiver operating characteristic curve analysis. Five hundred thirty-nine trauma patients had lung injury severity assessed using both LIS and P/F score. The mean P/F score was over twice the mean LIS (1.9 ±.04 vs. 0.9±.04, p < 0.0001). In 28% of patients, the LIS and P/F score were identical, whereas in 71%, the P/F score was greater than the LIS. Both scores were significant predictors of mortality; however, receiver operating characteristic curve analysis showed that the P/F score was superior in predicting mortality (area under the curve, 0.74±.03 vs. 0.67±.04). The P/F score is a simple method of quantifying lung injury severity in trauma patients that better predicts mortality compared with the more complicated modified Murray lung injury score currently in use. The P/F score should replace more complex and potentially therapy-dependent scores.