Regional distribution of gas and tissue in ARDS. III. Consequences for the effects of positive end-expiratory pressure. Intensive Care Med 26: 1215-1227

ArticleinIntensive Care Medicine 26(9):1215-1227 · April 2012with19 Reads
Impact Factor: 7.21 · DOI: 10.1007/s001340051340

    Abstract

    Objective: To determine whether differences in lung morphology assessed by computed tomography (CT) affect the response to positive
    end-expiratory pressure (PEEP).¶Design: Prospective study over a 53-month period.¶Setting: Fourteen-bed surgical intensive care unit of a university hospital.¶Patients and participants: Seventy-one consecutive patients with early adult respiratory distress syndrome (ARDS).¶Measurements and results: Fast spiral thoracic CT was performed at zero end-expiratory pressure (ZEEP) and after implementation of PEEP 10 cmH2O. Hemodynamic and respiratory parameters were measured in both conditions. PEEP-induced overdistension and alveolar recruitment
    were quantified by specifically designed software (Lungview). Overdistension occurred only in the upper lobes and was significantly
    correlated with the volume of lung, characterized by a CT attenuation ranging between –900 and –800 HU in ZEEP conditions.
    Cardiorespiratory effects of PEEP were similar in patients with primary and secondary ARDS. PEEP-induced alveolar recruitment
    of the lower lobes was significantly correlated with their lung volume (gas + tissue) at functional residual capacity. PEEP-induced
    alveolar recruitment was greater in the lower lobes with “inflammatory atelectasis” than in the lower lobes with “mechanical
    atelectasis.” Lung morphology as assessed by CT markedly influenced the effects of PEEP: in patients with diffuse CT attenuations
    PEEP induced a marked alveolar recruitment without overdistension, whereas in patients with lobar CT attenuations PEEP induced
    a mild alveolar recruitment associated with overdistension of previously aerated lung areas. These results can be explained
    by the uneven distribution of regional compliance characterizing patients with lobar CT attenuations (compliant upper lobes
    and stiff lower lobes) contrasting with a more even distribution of regional compliances observed in patients with diffuse
    CT attenuations.¶Conclusions: In patients with ARDS, the cardiorespiratory effects of PEEP are affected by lung morphology rather than by the cause of
    the lung injury (primary versus secondary ARDS). The regional distribution of the loss of aeration and the type of atelectasis
    –“mechanical” with a massive loss of lung volume, or “inflammatory” with a preservation of lung volume – characterizing the
    lower lobes are the main determinants of the cardiorespiratory effects of PEEP.

    Key words Acute respiratory distress syndrome–Positive end-expiratory pressure–Computed tomography–Alveolar recruitment–Lung overdistension