Lung Elastrosonography in Diagnosis and Stratification of COPD and Fibrosis

Introduction

Chronic Obstructive Pulmonary Disease (COPD) is a common respiratory disease characterized by irreversible expiratory airflow limitation with bronchial hyperinflation and air trapping, causing a reduction of pulmonary and thoracic compliance, characterized by chronic inflammation with emphysematous destruction of the lungs (GOLD Guidelines 2019; Singh P et al 2019; Bagdonas E et al 2015).

In the last decade, bedside lung ultrasonography (LUS) has rapidly gained popularity for the differential diagnosis of respiratory diseases, especially in the acute setting, for its accuracy, repeatability, rapidity and versatility that make it the ideal diagnostic tool in the internal medicine and geriatric setting (Ticinesi A et al 2019).

COPD is not associated with any specific sign at LUS, so that, in a context of acute exacerbation with respiratory failure, it remains a diagnosis of exclusion (Lichtenstein D 2015). Instead, lung function tests (i.e., spirometry) is considered the gold standard test for COPD detection (GOLD Guidelines 2019).

However, spirometry suffers from some limits, the major being its dependence on the cooperation of the subject, because during the test it is necessary to perform a correct expiratory forced manoeuvre with restricted acceptability standards (American Thoracic Society 1991; American Toracic Society 1995). As such, it can not be perfomed in patients with severe cognitive and physical impairment. Furthermore, spirometry is contraindicated in many conditions which may be aggravated by the effort (i.e. unstable cardiovascular status, recent myocardial infarction, pulmonary embolism and other) (American Association for Respiratory Care 1996). Alternative methods of identification of COPD and stratification of its risk in clinical practice are thus highly desirable, but unavailable to date.

Interstitial lung disease (ILD) represents a wide range of lung pathologies characterized by connective tissue deposition and fibrous remodeling of the lungs, causing a severe and progressive loss of respiratory function (Meyer KC 2017). High-resolution computed tomography (HRCT) is currently considered the gold standard in diagnosing and monitoring ILD, but, in its use, it is necessary to consider the ionizing radiation exposure (the cumulative dose for each exam is estimated around 7mSv, equivalent to two years of sunlight exposure), which may affect the repeatability of the exam, the high cost and the unavailability in many hospitals.

Moreover, because of main alveolar involvement in ILD, LUS can show a variable number of B-lines, even if this is a non-specific artifact common to other pathologic lung conditions (i.e. heart failure with pulmonary oedema and acute respiratory distress syndrome), and a thickening of pleural line greater than 3 mm (Wohlgenannt S et al 2001).

Pathophysiological basis of COPD and ILD allow to assume that lung parenchyma of these patients is stiffer than the normal one, but easy tools for measuring lung parenchyma stiffness in vivo are still lacking in clinical practice.

Elastography is a noninvasive diagnostic tool, complementary to ultrasonography, that assess in real-time the parenchymal elasticity degree, evaluating tissue mechanical properties (Gennisson JL et al 2013). Currently, this technique is widely used to study liver fibrosis and characterize suspect nodules and lesions of breast, thyroid, kidney and prostate (Gennisson JL et al 2013).

At present, only one study (Zhang X et al 2019) has considered the role of surface wave elastography in measuring the elastic properties of superficial lung tissue in patients with ILD, but there are not studies concerning its application in COPD for diagnostic purposes or for stratifying the risk of onset of the disease in smokers.

The aim of this study is to evaluate the applicability of strain elastography, integrated with LUS, in the assessment of the risk and the diagnosis of COPD and ILD.

Study hypothesis

The main hypothesis of the study is that the use of surface wave strain elastography during execution of a chest ultrasound examination is able to differentiate subjects suffering by COPD or ILD and subjects who have not respiratory diseases, documenting different stiffness properties of lungs. Another hypothesis is that the lungs of active smokers or ex-smokers with a long exposure to smoke exhibit a different stiffness pattern than that of the lungs of healthy non-smokers at elastosonographic examinations. Finally, another hyothesis is that the anatomical damage detected by lung ultrasonography is significantly correlated with parameters of lung function tests.

Objectives

1. To compare the stiffness properties of lung parenchyma, measured in vivo by lung ultrasound integrated with surface wave strain elastography, across three different groups of subjects: one group with established diagnosis of COPD or fibrosis, one group with no chronic lung diseases but high risk of developing COPD (i.e., active smokers or ex-smokers), and one group of healthy non-smokers.
2. To verify the possible correlations of stiffness properties of lung parenchyma, as measured by surface wave strain elastography, with lung function tests.

Study design

Single-center experimental diagnostic study, no profit and not involving drugs.

Setting

The study will be performed and completed at the Internal Medicine and Critical Subacute Care Unit of Parma University-Hospital, Italy. The unit is a large internal medicine area (100 beds for inpatients) dedicated to the care of older patients. It also includes an outpatient area where ultrasound and spirometry diagnostic resources are available. Both inpatients and outpatients will be involved in the study.

Study population

One-hundred thirty-eight participants will be enrolled (46 per group). The COPD/fibrosis group must have anamnestic record of chronic respiratory disease (COPD or ILD), confirmed by recent imaging and/or spirometric findings. The "high risk" group must be composed of active smokers or ex-smokers with significant exposure dating back to no more than 5 years and high risk of COPD according to the GOLD Guidelines 2019. The "healthy" group must have never smoked and have no record of chronic respiratory diseases. Excluded will be all subjects with acute respiratory failure not allowing to undergo study procedures, cognitive impairment or dementia, severe mobility disability, severe neuromuscular diseases, severe heart failure, severe kidney or liver disease, cancer or lack of compliance to study procedures (exclusion criteria common to all groups).

The sample size is set at 138 participants (46 per group) basing on a temptative calculation supposing the non-inferiority of lung elastography as compared with spirometry (sensitivity not lower than 10%) in detecting COPD or ILD (alpha=0.05, precision 5%). However, a precise sample size calculation is not possible due to the complete absence of clinical data on the diagnostic relevance of elastography in COPD.

Study procedures

Participants will be enrolled either on an inpatient or outpatient basis, provided that they fulfil all inclusion criteria for the respective group and that they do not have any of the exclusion criteria. The enrolment of volunteers will be also supported by the use of flyers and public notices.

After obtaining written informed consent and consent to personal data management (according to the European Union GDPR Regulation 2016/679), all participants will be evaluated by a staff physician collecting personal history and basic clinical data, if available. Collected data will include age, presence or absence of respiratory diseases (especially COPD and ILD), comorbidities, chronic drug treatments, most recent laboratory tests, reports of most recent chest imaging examinations, reports of most recent lung function tests. Smoking habits will be carefully assessed, with special attention on timing of the habit and quantity of cigarettes.

Lung function tests will be then performed following standard and internationally recognized protocols (American Association for Respiratory Care 1996), by using a Carefusion MSC Body spirometer. FEV1 and Tiffeneau Index (FEV1/FVC) will be particularly assessed.

Lung ultrasound will then be performed by a staff physician, by means of a Esaote Mylab Seven ultrasound system (Esaote, Genova, Italy), equipped with the strain elastography module ElaXto©. The examinations will be all performed using a convex 3.5-5 mHz probe, with the participant in sitting position and the examiner behind him or her.

First, the physician will perform a routine B-mode lung ultrasound, exploring both lungs in order to rule out other diseases which could influence pulmonary and thoracic compliance and lung parenchyma elasticity. Then, the physician will activate the elastography module and performe little compressions on the convex probe with the wrist, according to ultrasound detector, to obtain elastography images suitable for color analysis. Elastography images are in fact composed of a mix of red, green and blue colors, whose combination is an expression of the stiffness of the examined parenchyma. The examiner will perform a systematic bilateral scan, beginning from posterior lung areas, two intercostal spaces above diaphragm, and acquiring two images for each side, a basal one and an apical one. The examiner will then move to lung lateral regions, to the base of safety triangle, acquiring other two images, a basal one and an apical one, and finally to lung anterior regions, at the third intercostal space on the mid-clavicular line on the right side and at the second intercostal space on the mid-clavicular line on the left side (to avoid the cardiac area), acquiring other two images, a basal one and an apical one, for a total of 12 scans per participant.

Elastography images will be archived on ultrasound system and subsequently analysed in post-processing on the specific ultrasound software ElaXto©. Through this software, it is possible to measure qualitatively the percentage of tissue elasticity and stiffness.

For each image, the percentage of stiffness, with a stiffness threshold set at 20%, will measured in three areas of interest, by using the manual trace:

• The total area of lung parenchyma explored (excluding subcutaneous and muscular tissue located above the pleural line);
• The superficial half of lung parenchyma explored area (until approximately 3-4 cm under the pleural line);
• The deep half of lung parenchyma explored area (from 3-4 cm of depth until the lower limit of the image).

The use of three different stiffness measures for each image assures to highlight the overall stiffness of the explored lung parenchyma, as well as potential regional differences between the superficial and the deep lung plans. In fact, COPD is expected to mainly involve deeper portions of lung parenchyma, being basically a disease of small airways, while ILD is expected to mainly involve superficial plans of lung parenchyma (subpleural fibrosis). For each image, a qualitative index of elasticity and a quantitative index of stiffness will be obtained.

All the examinations on participants will be performed during the same day.

Statistical analysis

Data will be expressed as median and interquartile range or mean plus/minus standard deviation, according to variable distribution. Elastography parameters will be compared across the three study groups by means of ANCOVA and Bonferroni test. The correlations between spirometric and elastographic parameters will be assessed by Pearson correlation index.

Ethical issues

Spirometry and elastosonography are two safe examinations not implying any specific risk for participants. All procedures will follow the Good Clinical Practice and Declaration of Helsinki principles. Data will be handled in compliance with the current European Union legislation.