Expiratory pressure-volume curves in pulmonary and extrapulmonary ARDS

Objective

To assess the differences in lung mechanics during expiration between acute respiratory distress syndrome from pulmonary (ARDSp) and extrapulmonary (ARDSe) origin.

Methods

The expiratory pressure-volume (PV) curve was recorded after standardisation of volume history. The ventilator was switched to CPAP of 35 cmH₂O and was then reduced in 5 cmH₂O steps. Volume corresponding to static conditions was recorded. Esophagic pressure was recorded with a fluid-filled catheter [1]. The PV curves were fitted to a sigmoid model [2] for comparing volumes (absolute and percentage to estimated total lung capacity) at the same pleural and transpulmonary pressures. All data are expressed as mean ± SD. Differences between groups were performed using a Mann-Whitney U test.

Results

Patients

Ten patients with early ARDS (5 ARDSp/ 5 ARDSe). Mean age was 59 ± 15.5 years. APACHE-II score: 22.8 ± 6.8. Lung injury score: 2.9 ± 0.3. PaO₂/FiO₂: 124 ± 50.7. No differences were found between ARDSp and ARDSe in these results.

Compliance (C)

ARDSp and ARDSe show similar respiratory systemC (30 ± 8.7 ml/cmH₂O vs 45 ± 18.1 ml/cmH₂O respectively, n.s.), but ARDSp has lower lung C (35.9 ± 11.3 ml/cmH₂O vs 77.2 ± 50.6 ml/cmH₂O, P = 0.05) and higher chest wall C (199.6 ± 44.4 ml/cmH₂O vs 125.5 ± 16.5 ml/cmH₂O, P < 0.05).

PV curves (see Fig.)

Figure

Respiratory system PV curve from ARDSp is shifted down and right with respect to ARDSe. When fractional volumes are considered these differences are also significative. The lung PV curve of ARDSp is shifted in a similar way. However, when considering fractional volumes, the differences are only significative in the low pressure range (0–10cmH2O). The chest wall PV curve in the ARDSp group is, as expected, shifted to the left.

Respiratory system PV curve from ARDSp is shifted down and right with respect to ARDSe. When fractional volumes are considered these differences are also significative. The lung PV curve of ARDSp is shifted in a similar way. However, when considering fractional volumes, the differences are only significative in the low pressure range (0-10 cmH₂O). The chest wall PV curve in the ARDSp group is, as expected, shifted to the left.

Conclusions

The ARDSp has a respiratory system PV curve displaced downwards when compared with ARDSe, which suggests a small amount of reclutable tissue. When the lung PV curve is considered, these differences are higher. The fractional PV curves show similar differences for the respiratory system, but not for the lung. The difference in the latest affects only to the low pressure range, which suggest a greater airway closure in ARDSe.