The 'knee' pattern in spirometry flow-volume curves in children: Does it relate to tracheomalacia?

There is little data on patterns of spirometry curves in children with tracheomalacia but convex inflection on flow-volume curves (identified as the 'knee') is thought to represent tracheomalacia.

To determine (a) the prevalence of tracheomalacia in children with the 'knee' pattern on spirometry, and (b) whether spirometry parameters and visual characteristics of the 'knee' can identify presence/absence or severity of tracheomalacia.

We reviewed the spirometry undertaken at Queensland Children's Hospital between 2016 and 2019 and retrieved spirometry with the 'knee' pattern in the flow-volume curves. Flexible bronchoscopy videos of these children were reviewed for tracheomalacia diagnosis and severity in a blinded manner. We also evaluated several 'knee' characteristics (onset of inflection, angle of inflection, a scoop before plateau, plateau progression), spirometry parameters and tracheomalacia severity.

Of the 78 children with the 'knee', 51 (65.4%) had tracheomalacia. Spirometry values were significantly lower in those with tracheomalacia, compared to those without (predicted FEV₁ = 86.1% vs 99.9%, FVC = 95.1% vs 104%, FEF_25-75% = 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). There was no significant difference in spirometry parameters or the 'knee' characteristics between children with mild versus moderate-to-severe tracheomalacia.

Most but not all children with the 'knee' pattern have flexible bronchoscopy-defined tracheomalacia. Those with tracheomalacia had lower spirometry values and the presence of a scoop before plateau was the most characteristic feature. A prospective longitudinal study is required to determine the diagnostic value of spirometry flow-volume curve characteristics in children.