Influence of FIO2 on the PaO2/FIO2 ratio

Abstract

The acute respiratory distress syndrome (ARDS) is characterized by severe hypoxemia, a cornerstone element in its definition. Numerous indices have been used to describe this hypoxemia, such as the arterial to alveolar O2 difference, the intrapulmonary shunt fraction, the oxygen index and the PaO2/FIO2 ratio. Of these different indices the PaO2/FIO2 ratio has been adopted for routine use because of its simplicity. This ratio is included in most ARDS definitions, such as the Lung Injury Score [1] and in the American–European Consensus Conference Definition [2]. Ferguson et al. recently proposed a new definition including static respiratory system compliance and PaO2/FIO2 measurement with PEEP set above 10 cmH2O, but FIO2 was still not fixed [3]. Important for this discussion, the PaO2/FIO2 ratio is influenced not only by ventilator settings and PEEP but also by FIO2. First, changes in FIO2 influence the intrapulmonary shunt fraction, which equals the true shunt plus ventilation–perfusion mismatching. At FIO2 1.0, the effects of ventilationȓperfusion mismatch are eliminated and true intrapulmonary shunt is measured. Thus, the estimated shunt fraction may decrease as FIO2 increases if V/Q mismatch is a major component in inducing hypoxemia (e.g., chronic obstructive lung disease and asthma). Second, at an FIO2 of 1.0 absorption atelectasis may occur, increasing true shunt [4]. Thus, at high FIO2 levels (> 0.6) true shunt may progressively increase but be reversible by recruitment maneuvers. Third, because of the complex mathematical relationship between the oxy-hemoglobin dissociation curve, the arterio-venous O2 difference, the PaCO2 level and the hemoglobin level, the relation between PaO2/FIO2 ratio and FIO2 is neither constant nor linear, even when shunt remains constant.