Thoracoabdominal asynchrony in small children with lung disease--methodological aspects and the relationship to lung mechanics.

Thoracoabdominal asynchrony (TAA) has been regarded as a clinical sign of lung disease. A measure of TAA is the phase angle (phi) between ribcage (RC) and abdominal (ABD) respiratory motion. The aim of this study was to assess the effect of the points chosen for phi calculation. The influence of correct respiratory timing was assessed by calculating TAA indices using a pneumotachometer (PTM) as timing reference and using the calibrated respiratory inductive plethysmograph (RIP) signal for respiratory timing. The relationship between TAA and lung mechanics was studied in 15 young children 9 months to 2.5 years of age with a wide span of restrictive and/or obstructive lung disease. phi as calculated from mid-RC points was poorly related to phi as calculated from the top RC and ABD positions, indicating non-sinusoidal respiratory motions. The estimation of the TAA indices depended on correct respiratory timing, which in the case of severe asynchrony cannot be inferred from the RIP signals alone. An external source for respiratory timing, such as the airway flow measured by a PTM, is needed. The degree of asynchronous chest wall movement was only a weak indicator of pathological lung mechanics. We conclude that the usefulness of TAA indices as indicators of impaired lung mechanics is limited by the sensitivity to the points used for their calculation (phi) and the need of an external source for respiratory timing. It was therefore not surprising that a rather weak relationship was seen between TAA indices and lung mechanics.