A switched-gain nonlinear observer for estimation of thoracoabdominal displacements and detection of asynchrony

This paper develops and evaluates a wearable sensor-based system for the estimation of three-dimensional thoracoabdominal displacements and the detection of respiratory asynchrony between chest and abdomen. Such estimation is useful for a number of respiratory diagnosis applications, including detection of paradoxical breathing, quantification of tidal volume, and decision-support for initiation of, or weaning from, mechanical ventilation. The use of inertial sensors on the body to estimate respiratory displacements is challenging due to the tilting of the body that occurs with breathing, causing continuous changes in the gravity component at the same frequency as the breathing frequency. A method to estimate the front-to-back and side-to-side tilt angles is developed using a nonlinear observer. Due to the complex multivariable nonlinear measurement functions involved, a switched gain nonlinear observer is found to be necessary. The global stability of the observer is analyzed using Lyapunov analysis. Respiratory asynchrony is evaluated using Lissajous curves and a cross correlation phase angle calculation method. Example experimental results are presented using data from three subjects at various breathing frequencies, tidal volumes, and inspiratory resistances. Displacement estimates are found to be typically within ± 1 mm compared to a gold standard reference for most data sets. Phase angles are found to increase monotonically with inspiratory resistive or elastic loading.