Expiratory airflow limitation in subjects with obesity

Obesity is a cause of dyspnea due to mechanical impairment of pulmonary
ventilation. One of the causes of this impairment is expiratory flow
limitation, which is related to decreased lung volume. As a result, obesity can
cause an asthma-like symptoms. Therefore, some patients with obesity are
misdiagnosed as asthma-patients, and treated with asthma medication. The
effects of bronchodilators on the mechanical airway obstruction in obese
subjects have not been well established.

Posture also has effect on lung volumes: they are decreased in supine
position. Therfore, the interaction of obesity and supine posture might result
in a larger decrease in lung volumes, and thereby a more increased airflow
limitation. It has been suggested that both obesity and supine posture result
in an obstruction of peripheral airways. Such an obstruction can be measured by
spirometry, using the ratio of forced expiratory flow between 25 and 75% and
vital capacity. This measure is highly variable, however.
The forced oscillation technique (FOT) is a non-invasive method to measure the
resistance and reactance of the respiratory system. Particularly the reactance
has been shown useful in the measurement of airflow limitations.

We hypothesize that obesity causes a posture dependent end- expiratory airflow
limitation due to a mechanical compression of lung tissue, resulting in
increased resistance and reactance in the airways. Therefore, we expect no
protective effect of bronchodilation by salbutamol. We expect that reactance
measured by FOT detects differences in airflow limitation and correlates with
airflow limitation as measured by spirometry.

Objective: To investigate posture dependent small airway obstruction in
subjects with obesity. To study the capacity of FOT as a measurement tool for
small airways obstruction.

Observational research without invasive measurements (see flowchart pag 11)

Participating in this study is low risk. Spirometry, body plethysmography and
helium rebreathing are standard diagnostic tests performed in the pulmonary
function test department. No extra risk is foreseen in this study. Forced
oscillation technique is not (yet) a standard diagnostic test. It is a
non-invasive test. No adverse events are foreseen due to this test. Benefit is
an improvement in the assessment of bronchial obstruction with a technique that
does not require patient cooperation and can be applied in situations where the
patient really has complaints. Benefit could be improvement in diagnostics in
patients with obesity, saving unnecessary treatment with inhaled medicaments.