Hysteresis and Lung Recruitment in Acute Respiratory Distress Syndrome Patients: A CT Scan Study*

<jats:sec> <jats:title>Objectives:</jats:title> <jats:p>Hysteresis of the respiratory system pressure-volume curve is related to alveolar surface forces, lung stress relaxation, and tidal reexpansion/collapse. Hysteresis has been suggested as a means of assessing lung recruitment. The objective of this study was to determine the relationship between hysteresis, mechanical characteristics of the respiratory system, and lung recruitment assessed by a CT scan in mechanically ventilated acute respiratory distress syndrome patients.</jats:p> </jats:sec> <jats:sec> <jats:title>Design:</jats:title> <jats:p>Prospective observational study.</jats:p> </jats:sec> <jats:sec> <jats:title>Setting:</jats:title> <jats:p>General ICU of a university hospital.</jats:p> </jats:sec> <jats:sec> <jats:title>Patients:</jats:title> <jats:p>Twenty-five consecutive sedated and paralyzed patients with acute respiratory distress syndrome (age 64 ± 15 yr, body mass index 26 ± 6 kg/m<jats:sup>2</jats:sup>, Pa<jats:sc>o</jats:sc> <jats:sub>2</jats:sub>/F<jats:sc>io</jats:sc> <jats:sub>2</jats:sub> 147 ± 42, and positive end-expiratory pressure 9.3 ± 1.4 cm H<jats:sub>2</jats:sub>O) were enrolled.</jats:p> </jats:sec> <jats:sec> <jats:title>Interventions:</jats:title> <jats:p>A low-flow inflation and deflation pressure-volume curve (5–45 cm H<jats:sub>2</jats:sub>O) and a sustained inflation recruitment maneuver (45 cm H<jats:sub>2</jats:sub>O for 30 s) were performed. A lung CT scan was performed during breath-holding pressure at 5 cm H<jats:sub>2</jats:sub>O and during the recruitment maneuver at 45 cm H<jats:sub>2</jats:sub>O.</jats:p> </jats:sec> <jats:sec> <jats:title>Measurements and Main Results:</jats:title> <jats:p>Lung recruitment was computed as the difference in noninflated tissue and in gas volume measured at 5 and at 45 cm H<jats:sub>2</jats:sub>O. Hysteresis was calculated as the ratio of the area enclosed by the pressure-volume curve and expressed as the hysteresis ratio. Hysteresis was correlated with respiratory system compliance computed at 5 cm H<jats:sub>2</jats:sub>O and the lung gas volume entering the lung during inflation of the pressure-volume curve (<jats:italic toggle="yes">R</jats:italic> <jats:sup>2</jats:sup> = 0.749, <jats:italic toggle="yes">p</jats:italic> < 0.001 and <jats:italic toggle="yes">R</jats:italic> <jats:sup>2</jats:sup> = 0.851, <jats:italic toggle="yes">p</jats:italic> < 0.001). The hysteresis ratio was related to both lung tissue and gas recruitment (<jats:italic toggle="yes">R</jats:italic> <jats:sup>2</jats:sup> = 0.266, <jats:italic toggle="yes">p</jats:italic> = 0.008, <jats:italic toggle="yes">R</jats:italic> <jats:sup>2</jats:sup> = 0.357, <jats:italic toggle="yes">p</jats:italic> = 0.002, respectively). Receiver operating characteristic analysis showed that the optimal cutoff value to predict lung tissue recruitment for the hysteresis ratio was 28% (area under the receiver operating characteristic curve, 0.80; 95% CI, 0.62–0.98), with sensitivity and specificity of 0.75 and 0.77, respectively.</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions:</jats:title> <jats:p>Hysteresis of the respiratory system computed by low-flow pressure-volume curve is related to the anatomical lung characteristics and has an acceptable accuracy to predict lung recruitment.</jats:p> </jats:sec>