Article

Regional distribution of gas and tissue in acute respiratory distress syndrome. III. Consequences for the effects of positive end-expiratory pressure

Intensive Care Medicine (Impact Factor: 5.54). 04/2012; 26(9):1215-1227. 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

0 Followers
 · 
133 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Modeling the respiratory system can aid clinical decision making during mechanical ventilation (MV) in the intensive care. However, most lung models only use airway pressure and flow data during inspiration phase of the breathing, and the data from expiration are often neglected. In this study, we hypothesized that the airway pressure and flow data during expiration have equal important information as the inspiratory data for respiratory mechanics modeling. In particular, expiratory time constant parameter, K, can provide unique information and relation with respiratory system elastance in MV patients. Two different respiratory models were investigated, with one model focusing on data from inspiratory cycle (Single compartment model) and the other focusing on expiratory cycle (Time constant model). The expiratory time constant model and single compartment model were evaluated based on 22 retrospective datasets of acute respiratory distress syndrome (ARDS) patients from two different cohorts. The expiration time constant model captured a moderate correlation with the lung elastance with R2= 0.568 in Cohort 1 and weak correlation with R2= 0.184 in Cohort 2, resulting in an overall of R2 = 0.435. Significant variations in lung resistance may lead to a poor correlation between K and lung elastance, Elung. Thus, the application of K as a surrogate to the respiratory elastance warrants further investigation.
    19th World Congress The International Federation of Automatic Control, Cape Town, South Africa; 08/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Modeling the respiratory system can aid clinical decision making during mechanical ventilation (MV) in the intensive care. However, most lung models only use airway pressure and flow data during inspiration phase of the breathing, and the data from expiration are often neglected. In this study, we hypothesized that the airway pressure and flow data during expiration have equal important information as the inspiratory data for respiratory mechanics modeling. In particular, expiratory time constant parameter, K, can provide unique information and relation with respiratory system elastance in MV patients. Two different respiratory models were investigated, with one model focusing on data from inspiratory cycle (Single compartment model) and the other focusing on expiratory cycle (Time constant model). The expiratory time constant model and single compartment model were evaluated based on 22 retrospective datasets of acute respiratory distress syndrome (ARDS) patients from two different cohorts. The expiration time constant model captured a moderate correlation with the lung elastance with R 2 = 0.568 in Cohort 1 and weak correlation with R 2 = 0.184 in Cohort 2, resulting in an overall of R2 = 0.435. Significant variations in lung resistance may lead to a poor correlation between K and lung elastance, E lung. Thus, the application of K as a surrogate to the respiratory elastance warrants further investigation.
    19th World Congress The International Federation of Automatic Control, Cape Town, South Africa; 08/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Lung-protective ventilator strategies are considered standard practice in the care of patients with the acute respiratory distress syndrome (ARDS). To minimize ventilator-induced lung injury, attention is directed at avoidance of alveolar overdistention and cyclical opening and closing. The lowest possible plateau pressure and tidal volume (VT) should be selected. A reasonable target VT in all mechanically ventilated patients is 6 mL/kg. A topic of much controversy is the optimal setting of positive end-expiratory pressure (PEEP). Results of a meta-analysis using individual patient data from three randomized controlled trials suggest that higher PEEP should be used for moderate and severe ARDS, whereas lower PEEP may be more appropriate in patients with mild ARDS. PEEP should be set to maximize alveolar recruitment while avoiding overdistention. Volume and pressure limitation during mechanical ventilation can be described in terms of stress and strain. Fraction of inspired oxygen (Fio 2) and PEEP are typically titrated to maintain arterial oxygen saturation (Spo 2) of 88 to 95% (Pao 2 55-80 mm Hg). There is currently no clear proven benefit for advanced modes.
    Seminars in Respiratory and Critical Care Medicine 08/2014; 35(4):418-430. DOI:10.1055/s-0034-1382154 · 3.02 Impact Factor