Am. J. Respir. Crit. Care Med., Volume 165, Number 8, April 2002, 1107-1112

Pressure-Volume Curves in Acute Respiratory Distress Syndrome
Clinical Demonstration of the Influence of Expiratory Flow Limitation on the Initial Slope

Antoine Vieillard-Baron, Sebastien Prin, Jean-Marie Schmitt, Roch Augarde, Bernard Page, Alain Beauchet, and François Jardin

Medical Intensive Care Unit, and Department of Biostatistics, University Hospital Ambroise Paré, Assistance Publique Hôpitaux de Paris, Boulogne Cedex, France

The presence of an initial segment with a low compliance on the static pressure-volume (PV) curve in patients with acute respiratory distress syndrome (ARDS) indicates that some lung compartments do not initially receive insufflated gas. We tested the hypothesis that an uneven distribution of time constants, producing a "slow compartment," was in part responsible for the change in compliance between the initial and the intermediate segment of the PV curve. In 16 patients with ARDS submitted to mechanical ventilation in volume-controlled mode with a supportive respiratory rate of 15 breaths/minute, we constructed the static PV curve on the first day of respiratory support and determined the intrinsic positive end-expiratory pressure (PEEPi₄) during a prolonged end-expiratory pause (4 seconds). We also measured the volume of a "slow compartment" during a prolonged expiration (> 6 seconds), and determined an external PEEP (PEEPe) suppressing PEEPi₄. Among the 16 patients studied, 11 exhibited a low inflection point, associated with a "slow compartment" of 172 ± 83 ml, responsible for a PEEPi₄ of 3 ± 2 cm H₂O. Conversely, the five remaining patients had a linear PV curve, associated with a minimal "slow compartment" of 28 ± 10 ml, responsible for a negligible PEEPi₄. We observed that individual slopes of the initial segment of the PV curve were inversely and significantly correlated with the proportion of the "slow compartment" (r = 0.85). We concluded that the shape of the inspiratory PV curve in ARDS might be dependent on the presence of a "slow compartment," and demonstrated that a low external PEEP appeared sufficient to achieve a substantial mechanical improvement in clinical practice.

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