Published in:
01-10-2006 | Physiological Note
Relation between PaO2/FIO2 ratio and FIO2: a mathematical description
Authors:
Jerome Aboab, Bruno Louis, Bjorn Jonson, Laurent Brochard
Published in:
Intensive Care Medicine
|
Issue 10/2006
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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 O
2 difference, the intrapulmonary shunt fraction, the oxygen index and the PaO
2/F
IO
2 ratio. Of these different indices the PaO
2/F
IO
2 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 PaO
2/F
IO
2 measurement with PEEP set above 10 cmH
2O, but F
IO
2 was still not fixed [
3]. Important for this discussion, the PaO
2/F
IO
2 ratio is influenced not only by ventilator settings and PEEP but also by F
IO
2. First, changes in F
IO
2 influence the intrapulmonary shunt fraction, which equals the true shunt plus ventilation–perfusion mismatching. At F
IO
2 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
IO
2 increases if V/Q mismatch is a major component in inducing hypoxemia (e.g., chronic obstructive lung disease and asthma). Second, at an F
IO
2 of 1.0 absorption atelectasis may occur, increasing true shunt [
4]. Thus, at high F
IO
2 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 O
2 difference, the PaCO
2 level and the hemoglobin level, the relation between PaO
2/F
IO
2 ratio and F
IO
2 is neither constant nor linear, even when shunt remains constant. …