The nonallergic asthma of obesity. A matter of distal lung compliance

Abstract

Rationale: The pathogenesis of asthma in obesity is poorly understood, but may be related to breathing at low lung volumes.

Objectives: To determine if lung function in obese patients with asthma and control subjects would respond differently to weight loss.

Methods: Lung function was evaluated by conventional clinical tests and by impulse oscillometry in female late-onset, nonallergic patients with asthma and control subjects before, and 12 months after, bariatric surgery.

Measurements and main results: Patients with asthma (n = 10) had significantly lower FEV1 (79.8 ± 10.6 vs. 95.5 ± 7.0%) and FVC (82.4 ± 13.2 vs. 93.7 ± 8.9%) compared with control subjects (n = 13). There were no significant differences in FRC or TLC at baseline. Twelve months after surgery, control subjects had significant increases in FEV1 (95.5 ± 7.0 to 100.7 ± 5.9), FVC (93.6 ± 8.9 to 98.6 ± 8.3%), FRC (45.4 ± 18.5 to 62.1 ± 15.3%), and TLC (84.8 ± 15.0 to 103.1 ± 15.3%), whereas patients with asthma had improvement only in FEV1 (79.8 ± 10.6 to 87.2 ± 11.5). Control subjects and patients with asthma had a significantly different change in respiratory system resistance with weight loss: control subjects exhibited a uniform decrease in respiratory system resistance at all frequencies, whereas patients with asthma exhibited a decrease in frequency dependence of resistance. Fits of a mathematical model of lung mechanics to these impedance spectra suggest that the lung periphery was more collapsed by obesity in patients with asthma compared with control subjects.

Conclusions: Weight loss decompresses the lung in both obese control subjects and patients with asthma, but the more pronounced effects of weight loss on lung elastance suggest that the distal lung is inherently more collapsible in people with asthma.

Keywords: bariatric surgery; forced oscillation technique; impedance; lung volume.

Figure 1.

Electric circuit representation of lung mechanics in which the proximal airways (resistance [Rc]; gas inertance [Ic]) connect to a central elastic compartment representing the stiffness (Ec) of the conducting airways. The central compartment in turn connects to a distal compartment representing the elastance of the alveolar tissue (Ep) via a conduit representing the resistance (Rp) of the peripheral airways.

Figure 2.

Respiratory system resistance (A) and reactance (B) measured (mean ± SE) in control subjects and patients with asthma both at baseline and at 12 months after bariatric surgery. Mixed models repeated measures analysis of variance showed a significant group × visit interaction for change in resistance in response to weight loss between patients with asthma and control subjects (P = 0.03 both unadjusted and adjusted for body mass index), but not for change in reactance (P = 0.68 unadjusted and P = 0.48 adjusted for body mass index).

Figure 3.

Change in airway reactivity to methacholine (MCh) versus change in FRC with weight loss in subjects with asthma. PC₂₀ = provocative concentration of methacholine causing a 20% drop in FEV₁.

Figure 4.

Respiratory system resistance (A) and reactance (B) obtained by fitting the model illustrated in Figure 3 (solid lines) to the means of the experimental measurements (symbols). The best-fit model parameter values are shown in Figure 5.

Figure 5.

Parameter values determined by fitting the model shown in Figure 1 to the impedance data shown in Figure 2. The model parameter values were obtained by fitting the model to the mean impedance data, whereas the standard deviations shown by the error bars were obtained using a Monte Carlo approach described in the online supplement. The only significant difference among the individual model parameters is Rc in the control group after weight loss, which is different than the other three groups. Etotal, however, was significantly reduced by weight loss in both the control subjects and the patients with asthma, and was significantly greater in the patients with asthma compared with the control subjects before weight loss (*P < 0.05). Ec = central elastance; Ep = peripheral elastance; Etotal = total elastance; Ic = gas inertance; Rc = central resistance; Rp = peripheral resistance.