Error analysis of a natural breathing calibration method for respiratory inductive plethysmography

Respiratory volumes are measured non-invasively by recording rib cage and abdominal motions using respiratory inductive plethysmography (RIP). Qualitative diagnostic calibration (QDC) of RIP is based on the natural variability in the relative rib-cage-to-abdomen contribution during tidal breathing. QDC does not require subject cooperation but it has previously been shown that accuracy may deteriorate when breathing pattern changes. The aim of this study was to investigate the causes and situations where QDC accuracy deteriorates. The QDC method was compared to PRA (calibration during voluntarily preferential rib cage or abdomen breathing) in ten adults. A reference RIP calibration was obtained from all validation data (REF). The PRA method had better accuracy than the QDC method (p<0.01). The volumetric error ranged between 10% and 136% with QDC and between 5% and 33% with PRA. The PRA calibration factors were within 6% of those from REF, while the QDC rib-cage factor was underestimated by 15% and the abdominal factor was overestimated by 38%. Small natural variability in the relative rib-cage-to-abdomen contribution was related to poor accuracy. Each compartment's variability depended on its magnitude, which is a violation of the QDC assumptions.