Relation between PaO₂/F_IO₂ ratio and F_IO₂: a mathematical description

Excerpt

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/F_IO₂ ratio. Of these different indices the PaO₂/F_IO₂ 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/F_IO₂ measurement with PEEP set above 10 cmH₂O, but F_IO₂ was still not fixed [3]. Important for this discussion, the PaO₂/F_IO₂ ratio is influenced not only by ventilator settings and PEEP but also by F_IO₂. First, changes in F_IO₂ influence the intrapulmonary shunt fraction, which equals the true shunt plus ventilation–perfusion mismatching. At F_IO₂ 1.0, the effects of ventilation–perfusion mismatch are eliminated and true intrapulmonary shunt is measured. Thus, the estimated shunt fraction may decrease as F_IO₂ increases if V/Q mismatch is a major component in inducing hypoxemia (e.g., chronic obstructive lung disease and asthma). Second, at an F_IO₂ of 1.0 absorption atelectasis may occur, increasing true shunt [4]. Thus, at high F_IO₂ 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/F_IO₂ ratio and F_IO₂ is neither constant nor linear, even when shunt remains constant. …