Two thirds of children with the ‘knee’ pattern of convex inflection on the expiratory flow-volume curves on spirometry have flexible bronchoscopy-defined tracheomalacia, Australian researchers have shown.
The ‘knee’ pattern is described by four visual characteristics – the onset of inflection (before or at/after 50% of vital capacity had been expired), angle of inflection (sharp or very wide with a cut-off at approximately 150◦), a scoop before plateau (presence or absence) and plateau progression (horizontal or decreasing).
A study, published in Respiratory Medicine [insert link] reviewed a database of spirometry tests undertaken at the Queensland Children’s Hospital between 2016 and 2019.
Two respiratory paediatricians, blinded to all medical data including the respiratory diagnosis and flexible bronchoscopy findings, independently reviewed the spirometry with close inspection of the flow-volume curve to identify presence or absence of the ‘knee’ pattern.
The study found 5.9% of 1,950 children in the database had the ‘knee’ pattern on flow-volume curves.
Of 78 children with available flexible bronchoscopy records, 51 (65%) had both the ‘knee’ pattern and flexible bronchoscopy-defined tracheomalacia.
“In their ‘knee’ flow-volume curves, visual characteristics revealed that mostly, onset of inflection was at/after FEF50 (70.5%); angle of inflection was within 150◦ (79.5%); a scoop before plateau was present (57.7%); and plateau progression was a decreasing slope (70.5%).”
“Among the four characteristics of the ‘knee’ studied, the only component that was statistically significant between the groups was that a scoop before plateau was more likely present in children with tracheomalacia compared to those without (p = 0.03).”
The study found spirometry values of children with tracheomalacia were significantly lower than those without tracheomalacia, although still within population normal range consistent with other studies.
Of the children who did not have tracheomalacia, 81.5% had other airway conditions including suppurative disease (protracted bacterial bronchitis, bronchiectasis) and fixed airway obstruction.
The investigators, including paediatric respiratory physician Professor Anne Chang, said the study findings were “novel and important.”
“Our findings suggest the ‘knee’ pattern, determined on spirometry is not a common flow-volume pattern in children and should not be considered as a normal variant,” they said.
Instead, a finding of ‘knee’ pattern on spirometry was highly suggestive of tracheomalacia.
“In addition, identifying the ‘knee’ is relatively easy and highly reliable, making this tool useful to all clinicians. Hence, using spirometry as an initial investigation in children with suspected tracheomalacia should be considered, as it may lead to earlier management and optimisation of respiratory health.”