Significance of the lower inflection point detected on the lung pressure-volume curve

an assessment by computed tomography

Abstract

OBJECTIVE: The goal of this study was to assess lung morphology in patients with acute lung injury according to the presence or the absence of a lower inflection point on the lung pressure-volume curve and to compare the effects of positive endexpiratory pressure (PEEP).
MATERIALS AND METHODS: Eight patients with and six without a lower inflection point (LIP) underwent a computed tomography performed at zero end-expiratory pressure (ZEEP) and at two levels of PEEP: PEEP1 = LIP + 2 cmH2O e PEEP2 = LIP + 7 cmH2O, or PEEP1 = 10 cmH2O and PEEP2 = 15 cmH2O in the absence of LIP and, based on the analysis of the lung density histograms, the gas-tissue ratio and the lung areas volumes were calculated (nonaerated, poorly aerated, normally aerated and overdistended volumes).
RESULTS: In the ZEEP condition, patients with and without LIP presented similar total lung volume, volume of gas, and volume of tissue, although the percentage of
normally aerated lung was lower and the percentage of poorly aerated lung was greater in patients with LIP than in patients without it. Lung density histograms of
patients with LIP showed an unimodal distribution with a peak at 7 Housenfield units (HU), while histograms of patients without LIP had a bimodal distribution, with a first peak at -727 HU, and a second at 27 HU. Lung compliances were lower in patients with LIP whereas all other cardiorespiratory parameters were similar in the two groups. In both groups, PEEP induced an alveolar recruitment that was associated with lung overdistension only in patients without LIP.
CONCLUSIONS: The evaluation of the pressure-volume curve in patients with acute lung injury allows us to divide them into two groups according to the presence or absence of LIP. This division is associated with the differences in lung morphology and in the responses to PEEP application in terms of alveolar recruitment and
overdistention, the latter being defined as the occurrence of pulmonary parenchyma under -900 HU. In patients with LIP, gas and tissue are more homogeneously distributed within the lungs and increasing levels of PEEP result in additional alveolar
recruitment without lung overdistention. In patients without LIP, normally aerated areas coexist with nonareted lung areas and increasing levels of PEEP result in lung overdistention rather than in additional alveolar recruitment.