Does Severity of Airflow Obstruction Correlate to Static Lung Volumes Obtained by Nitrogen Washout and Body Plethysmography?

Obstructive lung disease is defined by limitations in expiratory airflow, caused by excess mucus, loss of muscle tone, and structural changes. Over time airflow reduction can lead to gas trapping in the lungs (hyperinflation). Hyperinflation is linked to diminished exercise tolerance, shortness of breath, and a poor quality of life. Early treatment options include inhalers and pulmonary rehabilitation; however, surgical intervention and oxygen therapy may be required in the later stages. More prompt, accurate diagnosis will help to improve patient outcomes and optimise their treatment pathways.

Two methodologies used to determine lung volumes and hyperinflation, are nitrogen washout and body plethysmography. The accuracy of each in defining lung volumes in patients with obstructive lung disease is debated in literature. Plethysmography requires the patient to sit in an enclosed box and perform a panting manoeuvre and uses measured changes in volume and pressure to derive lung volumes. Plethysmography has been suggested to overestimate lung volumes in patients with obstructive lung disease. On the other hand, nitrogen washout relies on 'washing out' all the nitrogen from the lungs to calculate lung volumes. Gas trapping and poor airflow circulation that occurs in patients with airflow obstruction may lead to underestimated lung volumes.

This study will aim to investigate if there is a significant difference between lung volumes obtained by both nitrogen washout and body plethysmography in patients with obstructive lung disease. Subjects with mild, moderate, severe, and very severe obstruction, including those with no obstruction for comparison will be included, with approximately 10 from each group. They will be asked if they consent to undergo an extra test during their routine hospital appointment, which will add ~15 minutes to their visit.

Expiratory airflow limitation is the hallmark of obstructive pulmonary disease; Parenchymal remodelling, mucous impaction, oedema, and a decrease of smooth muscle tone all contribute to the structural and anatomical changes that occur (O'Donnell, 2006). Individuals with bronchiectasis and chronic obstructive pulmonary disease often have inflammation in their airways (COPD). Impaired mucociliary clearance and increased mucus production are caused by inflammation that occurs inside the airway epithelium. Because of increased connective tissue deposition, the bronchial walls thicken, and the lumen of the airways decrease over time (Hogg, 2004). Airflow obstruction is defined as a reduction in expiratory airflow when compared to the total volume of air exhaled and is investigated by a test called spirometry. The two measurements required to identify obstructive lung disease are FEV1 (forced expiratory volume in the first second of expiration following a maximal inspiration) and FVC (forced vital capacity - the maximal amount of air that can be exhaled forcibly following maximal inhalation). For airflow obstruction to be diagnosed the FEV1/FVC ratio of must be <70%. The FEV1 percent predicted is then used to determine the severity of obstruction following this. Spirometry is commonly used to identify the presence of COPD (chronic obstruction pulmonary disease) or the severity/absence of obstruction in diseases such as asthma (Eschenbacher, 2016). TLC (total lung capacity) describes the amount of air in the lungs at maximal inspiration, the expected amount is determined by height, age, weight, ethnicity and gender. Chest wall deformities, tumours, level of physical activity and the presence of respiratory disease can all alter an individual's TLC.

Often, as a result of chronic obstructive lung disease, hyperinflation occurs which is the abnormal increase in FRC. This is caused by the imbalance between the reduction in airflow from the lungs compared to the total volume of the lungs. Changes in elastic properties of the lungs and impaired inspiratory muscle function also contribute to the extent of hyperinflation over time (Gibson, 1996). Hyperinflation is associated with reduced exercise capacity, dyspnoea and reduced quality of life. Individuals with COPD were found to spend 1/3 of the day standing/walking compared to healthy individuals of the same age who spent around ½ of the day doing so; such physical deconditioning significantly accelerates disease progression (Cooper, 2009). Treatment and intervention that targets hyperinflation can improve not only respiratory symptoms but also metabolic parameters and chronic inflammation. Pulmonary rehabilitation, bronchodilators and oxygen therapy are often utilised, however, surgical intervention such as lung volume reduction surgery (LVRS) is believed to provide the most benefit (Criner, 2017). The accuracy of lung volume measurements is vital to determine if interventional treatment (e.g., bronchodilators, LVRS, supplemental oxygen) have been successful or if a patient needs surgical intervention.