Lung Function in Cystic Fibrosis Affected by Sputum Viscoelasticity and Poor Clearance

Sputum viscoelastic properties were associated with lung function and disease status in patients with cystic fibrosis (CF), according to a study published in CHEST.

CF airway secretions are abnormal, which contributes to decreased clearance and a cycle of infection and inflammation. Jonathan T. Ma, MD, from the Department of Pediatrics, Children’s Hospital of Richmond at Virginia Commonwealth University in Richmond, and colleagues hypothesized that sputum viscoelasticity and clearance abnormalities would inversely correlate with pulmonary function during exacerbations and that sputum properties would return to baseline after therapy.

The investigators collected sputa longitudinally from 13 patients with CF who had moderate to severe lung disease during both periods of clinical stability and exacerbations. They analyzed dynamic rheology using cone and plate rheometer and measured mucociliary clearability on mucus-depleted frog palate, cough clearability in a simulated cough machine, and sputum hydration as percent solids following lyophilization.

The investigators found that elastic modulus G’ and viscous modulus G” increased during exacerbations but returned to baseline levels with recovery (P <.05 for both). Solid content did not change, but sputum mucociliary clearability decreased during exacerbations (P <.01) but not cough clearance. Forced expiratory volume in 1 second (FEV₁) percent predicted correlated inversely with G’ and G” (P <.01 for both). The regression slope of the natural log (ln) transformed of G’ and G” vs FEV₁ percent predicted was statistically homogenous in participants (estimated common slope m = –3.84, P <.001 and m = –8.53, P <.0001, respectively).

The authors concluded that there is a similar slope that defines the relationship between ln G’ or ln G” and FEV₁ across all individuals with CF, which suggests sputum viscoelasticity is an important factor capable of affecting airflow obstruction. Some of the study’s limitation included the use of a single center and the exclusion of nonsputum expectorators, who represent the majority of patients with CF. The investigators surmised that once patients become sputum expectorators, there is an accelerated decline in the results of pulmonary function tests.

Reference

Ma JT, Tang C, Kang L, Voynow JA, Rubin BK. Cystic fibrosis sputum rheology correlates with both acute and longitudinal changes in lung function [published online March 17, 2018]. Chest. doi:10.1016/j.chest.2018.03.005