Pressure–Volume Curves and Compliance in Acute Lung Injury

TLDR

Measuring elastic pressure–volume curves and the volume recruited by PEEP allows the pressure range over which recruitment occurs in acute lung injury to be studied and its effect on compliance to be explained. Pel‑V curves were obtained with a low‑flow inflation technique in 11 mechanically ventilated ALI patients, recording Curve I during inflation after a prolonged expiration at PEEP and Curve II after expiration to zero end‑expiratory pressure, and aligning the curves on a common volume axis to assess the effect of a single complete expiration. In ALI patients the ZEEP‑derived curve was shifted to lower volumes by 205 ml at 15 cm H₂O and 78 ml at 30 cm H₂O, and showed 10 ml cm⁻¹ higher compliance than the PEEP curve, indicating that a single expiration to ZEEP causes derecruitment and that recruitment occurs above the lower inflection point.

Abstract

Measuring elastic pressure–volume (Pel-V) curves of the respiratory system and the volume recruited by a positive end-expiratory pressure (PEEP) allows one to study the pressure range over which recruitment occurs in acute lung injury (ALI), and to explain how recruitment affects the compliance. Pel-V curves were measured with the low flow inflation technique in 11 patients mechanically ventilated for ALI. Curve I was recorded during inflation from the volume attained after a prolonged expiration (6 s) at PEEP (9.0 ± 2.2 cm H2O), and Curve II after expiration to the elastic equilibrium volume at zero end-expiratory pressure (ZEEP). By using the end-expiratory volume of the breaths, the curves were aligned on a common volume axis to determine the effect of a single complete expiration. In each patient, Curve II (from ZEEP) was shifted toward lower volumes than Curve I. The volume shift, probably due to derecruitment, was 205 ± 100 ml at 15 cm H2O (p < 0.01) and 78 ± 93 ml at 30 cm H2O (p < 0.01); thus, during inflation from ZEEP, the volume deficit was successively regained over a pressure range up to at least 30 cm H2O. At any pressure, compliance was higher on the curve from ZEEP than from PEEP, by 10.0 ± 8.7 ml/cm H2O at 15 cm H2O (p < 0.01), and by 5.4 ± 5.5 at 30 cm H2O (p < 0.01). It is concluded that in ALI, a single expiration to ZEEP leads to lung collapse. High compliance during insufflation from ZEEP indicates that lung recruitment happens far above the lower inflection point of the Pel-V curve.