Measurement of thoracoabdominal asynchrony: importance of sensor sensitivity to cross section deformations

Discrepancies in the assessment of thoracoabdominal asynchrony are observed depending on the choice of respiratory movement sensors. We test the hypot hesis that these discrepancies are due to a different dependence of the sen sors on cross-sectional perimeter and area variations of the chest wall. Fi rst, we study the phase shift between perimeter and area (Phi(PA)) for an e lliptical model, which is deformed by sinusoidal changes of its principal a xes. We show that perimeter and area vary sinusoidally in the physiological range of deformations, and we discuss how Phi(PA) depends on the elliptici ty of the cross section, on the ratio of transverse and dorsoventral moveme nt amplitudes, and on their phase difference. Second, we compute the relati onship between perimeter, area, and the output of the inductive sensor, and we proceed by comparing inductive plethysmography with strain gauges for s everal cross section deformations. We demonstrate that both sensors can pro vide different phase information for identical cross section deformations a nd, hence, can estimate thoracoabdominal asynchrony differently. Furthermor e, the complex dependence of the inductive sensor on perimeter and area war ns against this sensor for the evaluation of thoracoabdominal asynchrony.