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Oxygenation Indexes and Degrees of Lung Injury

I read with great interest the article by Villar and colleagues (1), in which the authors concluded that the current American-European Consensus Conference (AECC) definitions of acute respiratory distress syndrome (ARDS) and acute lung injury (ALI), and in particular use of the Pa_O2/FI_O2 ratio, overestimate the incidence of ARDS and underestimate ARDS mortality. They speculated that this misclassification bias can result in the overestimation or underestimation of the beneficial effect of any therapeutic intervention. Villar and colleagues (1) applied different combinations of positive-end expiratory pressure (PEEP) levels and FI_O2 during the determination of the Pa_O2/FI_O2 ratio at Day 0 and Day 1 of inclusion in the study. They showed that the use of PEEP can significantly affect the value of the Pa_O2/FI_O2 ratio and subsequently change patients' classification among ARDS (Pa_O2/FI_O2 200 mm Hg), ALI (201 mm Hg < Pa_O2/FI_O2 < 300 mm Hg), and acute respiratory failure (ARF) (Pa_O2/FI_O2 > 300 mm Hg).

I strongly agree with the authors' findings because, currently, ARDS, ALI, and ARF are defined without reference to the PEEP and FI_O2 applied during ventilatory support. We have previously shown the superiority of a new oxygenation index (2) in reflecting intrapulmonary shunting and lung oxygenation status. The new oxygenation index is termed the oxygenation factor and is expressed as Pa_O2/(FI_O2 x MAP), where MAP is the mean airway pressure. By incorporating MAP into the new oxygenation index, changes in PEEP, inspiratory-to-expiratory time ratio (I:E ratio), and tidal volume, which can all affect the MAP, are reflected in the new oxygenation index.

We showed that the new oxygenation index is more sensitive than the traditional Pa_O2/FI_O2 ratio in assessing the oxygen-exchanging status and severity of the lung injury, because the new index additionally accounts for the functional status of the lung resulting from changes in end-expiratory lung volume due to manipulation of either PEEP or I:E ratio. It would have been interesting if Villar and colleagues (1) had used our oxygenation index in addition to the Pa_O2/FI_O2 ratio on their patients to see whether our index behaved similarly to the traditional ratio or better in classifying patients among different groups with ALI.

Mohamad F. El-Khatib

American University of Beirut
Beirut, Lebanon

FOOTNOTES

Conflict of Interest Statement: The author has no financial relationship with a commercial entity that has an interest in the subject of this manuscript.

REFERENCES

1. Villar J, Pérez-Méndez L, López J, Belda J, Blanco J, Saralegui I, Suárez-Sipmann F, López J, Lubillo S, Kacmarek R; HELP Network. An early PEEP/FI_O2 trial identifies different degrees of lung injury in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 2007;176:795–804.[Abstract/Free Full Text]
2. El-Khatib MF, Jamaleddine GW. A new oxygenation index for reflecting intrapulmonary shunting in patients undergoing open-heart surgery. Chest 2004;125:592–596.[CrossRef][Medline]

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