Oscillometry is highly sensitive to changes in respiratory system mechanics that occur with weight loss and induced bronchodilation (BD). However, the contribution of central and peripheral lung mechanics to the measured changes in total respiratory mechanics is poorly understood. We used a series, two-compartment model of the lung to estimate central and peripheral mechanics of the lung and analyzed how these parameters changed in response to weight loss, bronchodilation and moving from upright to supine position.
Respiratory impedance was measured using airwave oscillometry (tremoFto) at 6, 11, and 19 Hz oscillation frequency in 15 severely obese female patients (mean BMI 48.2t SD 6.0 kg/m2). Measurements were conducted pm- and post-BD in the upright and supine position, before and six months after bariatric surgery. The series two-compartment model of the lung was fit to respiratory impedance spectra and parameters of the central and peripheral lung compartments obtained. The model consisted of a series arrangement of an upper airway compartment (central resistance, Rc, and central inertance, Ic, upper and central airway elastance, Ec) connected to the peripheral compartment through a single peripheral airway (peripheral resistance, Rp). The peripheral compartment reflected the peripheral elastance (Ep) from the alveolar tissues while total static elastance, Etotal was the parallel sum of Ec and Ep.
A mean weight loss of 21.41 7.1kg was recorded after bariatric surgery (p<0.001) but this had no significant effect on Ec estimated in the upright position. However, upright Ep reduced by 44.7±11.5% contributing to a significant reduction in Etotal. Weight loss also induced a 27.8±5.8% and 463±10.7% reduction in upright Rc and Rp, respectively but Ic was unaffected. In the supine position, weight loss had no significant effect on model parameters except for Rc which reduced by 11.3±5.2%. Before bariatric surgery, BD had no significant effect on supine model parameters, except for Rc and Rp which reduced by 15.7±4.0% and 17.4±15.3%, respectively; however, after surgery, significant reductions were observed in Ep. Etotal, Rc and Rp.
Weight loss decompresses the lung and this lowers the resistance of the central compartment of the lung in both the upright and supine positions. While large reductions in resistance and elastance of the peripheral compartments were observed in the upright position after weight loss, no significant changes were observed in these parameters in the supine position, implying that the beneficial effects of weight loss on breathing mechanics are attenuated in the supine position.