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

Background: 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. Objectives: 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. Patients/methods: 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. Results: 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. Conclusion: 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.