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Pressure/volume curves and lung computed tomography in acute respiratory distress syndrome

¹ Réanimation Chirurgicale Pierre Viars, Dept of Anesthesiology, Hospital Pitié-Salpétrière, University Paris VI, Paris, France, ² Dept of Internal Medicine, Faculty of Medicine Federal University from Rio Grande Do Sul, Intensive Care Unit, Hospital de Clinicas de Porto Alegre, Porto Alegre, Brazil

CORRESPONDENCE: J-J. Rouby, Réanimation Chirurgicale Pierre Viars, Hôpital Pitié-Salpétrière, 83 boulevard de l'hôpital, 75013 Paris, France. Fax: 33 142177326. E-mail: jjrouby.pitie@invivo.edu

Keywords: acute respiratory distress syndrome, alveolar recruitment, computed tomography scan, lung morphology, pressure/volume curve

Pressure/volume (P/V) curves can be measured by static methods, constant or sinusoidal flow methods and the dynostatic method that allows a breath-to-breath determination of P/V curves. Recent ventilators are equipped with specific flow generators and software aimed at obtaining P/V curves without disconnecting the patient from the ventilator.

The most recent generation of computed tomography scanners allows the quantitative determination of lung aeration, lung volumes (gas and tissue), alveolar recruitment and lung overinflation of the whole lung. In the supine position, the acute respiratory distress syndrome (ARDS) lung is characterised by an increase in lung tissue that predominates in upper lobes and a massive loss of aeration that predominates in lower lobes.

In a minority of ARDS patients, the loss of aeration is homogeneously distributed. The overall lung volume of upper lobes is preserved suggesting an alveolar flooding-induced loss of aeration. In contrast, the overall lung volume of lower lobes is reduced because the heart and the abdomen exert an external compression that contributes to the loss of aeration. The P/V curve is a lung recruitment curve and the chord compliance indicates the potential for recruitment. In such patients, alveolar recruitment resulting from positive end-expiratory pressure is not accompanied by lung overinflation.

In a majority of acute respiratory distress syndrome patients, upper lobes remain partially or totally aerated despite a marked regional increase in lung tissue. The upper lobes' overall lung volume is either normal or increased, suggesting that the lung does not collapse under its own weight as generally believed. In lower lobes, the overall lung volume is reduced because the heart and the abdomen exert an external compression that contributes to the loss of aeration. The pressure/volume curve is influenced by the recruitment of poorly and nonaerated lung regions and by the mechanical properties of the part of the lung remaining aerated. In such patients, alveolar recruitment resulting from positive end-expiratory pressure >10 cmH₂O is preceded and accompanied by lung overinflation.