Original article
Functional respiratory imaging of the airways in the acute respiratory distress syndrome

https://doi.org/10.1016/j.accpm.2019.10.017Get rights and content

Abstract

Background

Alveolar flooding and airway obstruction are present in the acute respiratory distress syndrome. The impact of positive end-expiratory pressure on regional airway aeration has not been described.

Aim

To assess bronchial and lung recruitment and distension during an incremental positive end-expiratory pressure trial in patients with acute respiratory distress syndrome.

Methods

Six patients underwent lung and airway imaging at four positive end-expiratory pressure levels in a cohort trial. Images were post-processed by means of Functional Respiratory Imaging. This technique offers 3-dimensional visualisation and quantification of patients’ airway and lung geometry on a regional level.

Results

With increasing positive end-expiratory pressure from 0 to 20 cmH2O, the median bronchial recruitment was 151% and the median bronchial distension 43%. Non-aerated lower lobes bronchi had more bronchial volume increase at high positive end-expiratory pressure than partially aerated upper lobes bronchi. Lung recruitment tended to be higher in patients with non-focal acute respiratory distress syndrome. In two patients, bronchial volume increase at high positive end-expiratory pressure largely exceeded bronchial volume increase observed in matched healthy control subjects at total lung capacity, suggesting severe bronchial over-distension.

Conclusions

In early acute respiratory distress syndrome, Functional Respiratory Imaging gives an innovative insight into the relationship between positive end-expiratory pressure-induced bronchial distension and recruitment, positive end-expiratory pressure-induced lung recruitment and hyperinflation and lung morphology.

Introduction

The acute respiratory distress syndrome (ARDS) is a severe and heterogeneous type of lung injury, which still carries a high mortality rate in the critically ill patient [1]. In the initial exudative phase, loss of capillary membrane integrity leads to non-cardiogenic pulmonary oedema, leakage of proteins and alveolar flooding [2]. The combination of interstitial and intrapulmonary oedema results not only in the partial or complete loss of alveoli aeration but also in small airways obstruction and increased airway resistance [3]. Tidal airway opening and closure increases the extent of small airways injury, intrinsic Positive End-Expiratory Pressure (PEEP) and hyperinflation [4].

In further proliferative fibrotic phases, airway wall thickening occurs combined with a profound decrease in lung compliance if initial lesions do not resolve [5]. In all phases, partial or complete obstruction of the airways leads to increased airway resistance and airway pressures [6]. Long lasting distension and distortion of small airways (bronchiectasis) result from high airway pressure, possibly secondary to reduction of compliance of the adjacent parenchyma. It can be demonstrated at autopsy in up to 25% of ARDS patients [9], and is also visible on Computed Tomography (CT) imaging [7]. Air cysts and bronchiectasis prevail in non-dependent areas in severe ARDS [8] and seem to be a permanent change, persisting even 6 months after the initial radiological diagnosis [7].

The detection of lung aeration loss, the assessment of lung morphology [9] and lung recruitability [10], and the quantification of alveolar oedema and inflammation by CT are still at the forefront of the imaging modalities [11]. Functional Respiratory Imaging [12] allows visualising the 3D structure of airways. Our primary objective was to use Functional Respiratory Imaging to assess PEEP-induced airway distension and recruitment (bronchial volume increase, bronchial aeration) and their relationship with lung morphology and recruitability in ventilated patients with ARDS. Our second objective was to identify whether lung morphology (focal versus non-focal loss of lung aeration) [13], [14] and lung parenchyma recruitability influence bronchial aeration.

Section snippets

Study population and design

A prospective single-centre cohort trial was designed to compare regional bronchial aeration and lung recruitment/hyperinflation in six patients with ARDS using Functional Respiratory Imaging. Institutional Review Board approval was granted and written informed consent was obtained and signed by the closest relative before inclusion in the trial. The study was performed in accordance with the ethical standards of the Declaration of Helsinki. Trial registration: clinicaltrials.gov, NCT03309085.

Patient characteristics

Seven patients completed the study without complications. One patient was excluded because the presumed diagnosis of ARDS could not be confirmed on CT imaging. ARDS was caused by pneumonia in four patients. A summary of patient characteristics is provided in Table 1.

All patients were scanned while they were in the early phase of the ARDS disease process. The patients were exposed to a total estimated effective radiation dose of 8.53 mSv (IQR 8.08–11.26), with a dose length product of 609.52

Discussion

In this study, we propose a novel airway-imaging tool based on the technique of functional regional imaging to visualise bronchi in ventilated patients with ARDS and quantify PEEP-induced bronchial distension and recruitment. We report the possibility of bronchial over-distension in PEEP-related lung recruitment in ARDS after an incremental PEEP trial.

As opposed to structural imaging, functional imaging has been designed to reveal physiological activities within a certain tissue or organ and

Conclusions

In this study, high PEEP levels do not only recruit and hyperinflate the lung, but also distend the bronchi. Furthermore, we demonstrate the possibility of bronchial over-distension in PEEP-related lung recruitment in ARDS after an incremental PEEP trial. By the use of functional respiratory imaging, we can further expand our knowledge on bronchial behaviour in ARDS by assessing airway distension and recruitment at different PEEP levels. Further studies may consider including functional

Human and animal rights

The authors declare that the work described has been carried out in accordance with the Declaration of Helsinki of the World Medical Association revised in 2013 for experiments involving humans as well as in accordance with the EU Directive 2010/63/EU for animal experiments.

Informed consent and patient details

The authors declare that this report does not contain any personal information that could lead to the identification of the patient(s).

The authors declare that they obtained a written informed consent from the patients and/or volunteers included in the article. The authors also confirm that the personal details of the patients and/or volunteers have been removed.

Disclosure of interest

CH, WV and JDB are employees of FLUIDDA, a company that develops and markets part of the technology described in this paper. The other authors have no financial relationships with any organisation or company that might have an interest in the submitted work and received no direct funding from FLUIDDA.

Funding

Dr. Lu Xiao received financial support from the Department of Emergency Medicine, 2nd Affiliated Hospital, Zhejiang University School of Medicine, Institute of Emergency Medicine, Zhejiang University, Hangzhou 310009, P.R China for his one-year stay in Paris (third year of PhD) in the Research Unit of the Multidisciplinary Intensive Care Unit, Department of Anesthesiology and Critical Care, Pitié-Salpêtrière Hospital, Assistance Publique Hôpitaux de Paris, Sorbonne University of Paris, France.

Author contributions

All authors attest that they meet the current International Committee of Medical Journal Editors (ICMJE) criteria for Authorship.

Acknowledgements

We thank Michel Geldof and his team of radiologic technologists for their enthusiasm and dedication during the scanning sequences.

References (30)

  • M.M. Treggiari et al.

    Air cysts and bronchiectasis prevail in nondependent areas in severe acute respiratory distress syndrome: a computed tomographic study of ventilator-associated changes

    Crit Care Med

    (2002)
  • J.J. Rouby et al.

    Acute respiratory distress syndrome: lessons from computed tomography of the whole lung

    Crit Care Med

    (2003)
  • L. Gattinoni et al.

    Lung recruitment in patients with the acute respiratory distress syndrome

    N Engl J Med

    (2006)
  • G. Bellani et al.

    Looking closer at acute respiratory distress syndrome: the role of advanced imaging techniques

    Current opinion in critical care

    (2017)
  • J.W. De Backer et al.

    Validation of computational fluid dynamics in CT-based airway models with SPECT/CT

    Radiology

    (2010)
  • 1

    JJR and PGJ share senior authorship.

    View full text