The convex inflection on flow-volume curves, sometimes known as the “knee,” is assumed to signify tracheomalacia. However, there was limited information on the patterns of spirometry curves in children with tracheomalacia. For a study, researchers sought to ascertain the prevalence of tracheomalacia in kids with the spirometry pattern known as the “knee” and if the existence, absence, or severity of tracheomalacia may be detected using spirometry parameters and visual features of the “knee.”

They examined the spirometry performed between 2016 and 2019 at Queensland Children’s Hospital, and we found spirometry with the “knee” pattern in the flow-volume curves. Blinded reviewers identified the presence and extent of tracheomalacia in these kids’ flexible bronchoscopy footage. In addition, they assessed the severity of the tracheomalacia, spirometry parameters, and a number of ‘knee’ features (beginning of inflection, angle of inflection, scoop before plateau, plateau progression).

About 51 (65.4%) of the 78 kids with the “knee” (the condition) had tracheomalacia. When tracheomalacia was present, spirometry results were considerably lower than those in individuals without it (predicted FEV1 = 86.1% vs 99.9%, FVC = 95.1% vs 104%, and FEF25-75%= 68.6% vs. 89.6%, all P< 0.02). A scoop before plateau was significantly associated with tracheomalacia (66.7% vs. 40.7%, P = 0.03). Compared to moderate-to-severe tracheomalacia, children with mild tracheomalacia had similar spirometry values and “knee” features.

The flexible bronchoscopy-defined tracheomalacia was present in most, but not all, children with the “knee” pattern. Spirometry readings were lower in those with tracheomalacia, and the most distinguishing sign was the existence of a scoop before the plateau. To ascertain the diagnostic relevance of children’s spirometry flow-volume curve features, prospective longitudinal research was necessary.

Reference: resmedjournal.com/article/S0954-6111(22)00294-3/fulltext