Scope of linear estimators of tidal and occluded volumes using thoracoabdominal indications of breathing movement coordination

Abstract

The basic theory for respiratory inductive plethysmography (RIP) applications was re-examined, refined and tested. A realistic model of the RIP interpretation of respiratory mechanics related tidal volumes (VT) to a linear combination of ribcage and abdomen movements. Lissajous plots of asynchronous thoracoabdominal movements revealed their net effect equivalent to the superposition of synchronous and antipathetic respiration modes at right angles, along the principal axes specific to the combined motion. Predictors of relative changes in VT, degree of asynchrony and volume thus being occluded were developed via least squares estimation theory, with an optional validation facility. The approach enabled clinically adequate analysis of 452 h of RIP data from 29 postoperative patients. Correct identification of only seven complete apnoeas in 111 incidences of obstruction during periodic, variable, asynchronous or paradoxical natural breathing was substantiated via non-invasive airflow monitoring. The modelling helped clarify RIP limitations--the possibility of misleading indications from obese or abnormal physiques or movement artefacts degrading its otherwise nearly optimal performance. Nevertheless, our uncalibrated predictors had better theoretical basis, improved reliability and more convenient practical utility than the traditional approach of calibrating RIP by spirometry prior to non-invasive monitoring and identifying and classifying apnoeas.