Obesity reduces airway caliber by unloading the airway wall often resulting in sleep disordered breathing and asthma-like symptoms. These respiratory conditions often improve following weight loss however the mechanisms are not well understood.
We investigated the role of unloading the airway smooth muscle in 8 obese patients and the change in lung mechanics before and 5 weeks after bariatric surgery by measuring respiratory system resistance (Rrs) and reactance (Xrs) with the forced oscillation technique (TremoFloTM) at 6, 11, 19 Hz in the upright and supine position, and pre- and post-bronchodilation (BD, 200 mcg salbutamol).
Results and Discussion
Before surgery and pre-BD, the change from upright to supine increased average Rrs from 3.61±0.6 to 4.19±0.54 cm H20 l-1s-1 (p<0.05) while Xrs decreased at each frequency (by 1.01±0.28 cm H20 l-1s-1 at 11 Hz). These differences disappeared following a mean weight loss of 11.3±0.7 kg after surgery. Although posture had no effect on Rrs post-BD, it caused a decrease in Xrs both before and after surgery. Similarly, BD in the upright posture caused pre-surgery Xrs at 6 Hz to increase by 0.8±0.33 cm H20 l-1s-1 but no effect post-surgery while BD in the supine posture decreased average Rrs by 20±9% before surgery but not after.
The reductions in supine induced changes in Rrs and Xrs following surgery suggest a decrease in unloading of the airway wall from upright to supine due to weight loss. This is consistent with the loss of the supine BD effect on Rrs and Xrs post-surgery which also implies an increase in peripheral airway baseline diameter and improved lung function.