Impedance Pneumography: Assessment of Dual-Frequency Calibration Approaches

Impedance pneumography (IP), a measure of the changes in the lung and thoracic bioimpedance, holds promise for non-invasive monitoring of pulmonary health. A key limitation of IP is the need for complex and frequent calibrations that require the subject to perform various maneuvers. In this work, we explore different calibration approaches to reduce the effects of inter-subject variability and postural changes on IP by utilizing a dual-frequency calibration approach. Dual-frequency IP was deployed for the first time in this work and its performance in estimating tidal volume (TV) was evaluated and compared to the conventional single frequency approaches. TV values obtained from a spirometer estimated with the subject- and posture specific IP calibration approach are shown to correlate highly with the ground truth TV (r > 0.9) in all postures, including supine, left/right lateral, and seated postures for both 5 kHz and 100 kHz IP signals. Eliminating posture specificity results in a correlation of r > 0.8. With the globalized calibration approach that does not require any subject or posture-specific calibration, a correlation of r = 0.75 was achieved with the dual-frequency approach, and this was higher than the corresponding correlation of around r = 0.68 using any single frequency. This result has implications for the feasibility of dual-frequency IP for mitigating inter-subject variability and posture-specific calibrations.