Obesity and Expiratory Flow Limitation (EFL)

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. Therefore, 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 reactant of the respiratory system. Particularly the reactance has been shown useful in the measurement of airflow limitations.

The investigators 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, the investigators expect no protective effect of bronchodilation by salbutamol. The investigators expect that reactance measured by FOT detects differences in airflow limitation and correlates with airflow limitation as measured by spirometry.