Reproducibility of Thigh-Thigh Bioimpedance Measurements for Detecting Change in Respiratory Volume

Introduction:: Non-invasive monitoring of pulmonary health may be useful for tracking several conditions such as COVID-19 recovery and progression of pulmonary edema. Some of the current methods use impedance-based technologies to non-invasively measure the thorax impedance as a function of respiration. These methods comprise of attachment of wet electrodes to the chest and show a high correlation ( >0.9) of breathing to impedance, however, face challenges such as poor repeatability of electrode placement, high and variable skin-electrode contact impedance, inconvenience and discomfort caused by continuous wearing of electrodes under clothing. Thus, these challenges limit the feasibility, accuracy, and practicality of tracking daily changes with these methods. In our prior work [1], we designed a novel approach to monitor respiration by measuring changes in impedance from the back of the thigh. The integration of dry electrodes into the toilet seat ensured repeatable electrode placement on the skin and patient compliance. In our previous work [1], we reported the feasibility of using thigh-thigh bioimpedance measurements for measuring the respiration rate and demonstrated a linear relationship between thigh-thigh bioimpedance and lung volume. Here, we investigate the reproducibility of the thigh-thigh impedance measurements to understand the sensitivity of the technique for detecting the change in the respiratory volume. The inter-day and intra-day impedance measurements were conducted using dry electrodes (thigh), dry + gel electrodes (thigh), and wet electrodes (thorax) across the same normative subject along with simultaneous gold standard spirometer measurements.

Materials and Methods:: Thigh-thigh bioimpedance measurements were conducted using MAX30001 Analog Frond End (Maxim Integrated) at 80 kHz and 48μA in a 4-electrode configuration using dry Titanium electrodes integrated into a toilet seat. (Figure 1). The volumetric flow was measured simultaneously with a spirometer (MTL1000L, AD Instruments). The subject was asked to perform a 2 min sequence of respiratory tasks: shallow – normal – deep – maximal inhale-exhale. Following that, gel (Spectra 360, Parker Laboratories) was applied to the dry electrodes and the experiments repeated. For comparison, bioimpedance measurements were performed using wet electrodes across the thorax, again synchronized with the spirometer. To understand the inter-day variability of the bioimpedance measurements, experiments using the dry (thigh), dry + gel (thigh), and wet (thorax) electrodes were repeated for five consecutive days. Further, to evaluate the intra-day variability, five measurements with each of the dry (thigh), dry + gel (thigh), and wet (thorax) electrodes were obtained on the same day within the same session. The subject was asked to stand-sit between the thigh-thigh impedance measurements and the placement of the wet electrodes across the chest was not altered between the measurements. For comparison of the spirometer volume signal to the impedance signal (aligned using cross-correlation), mapping was performed between the peak-peak value of the spirometer volume signal (ground-truth tidal volume) and the peak-peak value of the impedance value. The relationship between the respiratory volume measured by the spirometer volume signal and the impedance signal was computed by using linear regression with y-intercept forced to zero.

Results, Conclusions, and Discussions:: The peak-peak bioimpedance measurements were found to be highly correlated (0.87 dry electrodes across thigh; 0.89 dry + gel electrodes across thigh; 0.90 wet electrodes across thorax) to the peak-peak spirometer tidal volume (Figure 2 and Figure 3). For the measurements across five days, the slopes of the linear regression depicted a change of 32.4%, 19.6%, and 57.2% for dry (thigh), dry + gel (thigh), and wet (thorax) electrodes respectively (Figure 2). The day-to-day variation is significantly higher for thorax impedance measurements as compared to the thigh-thigh impedance measurements, due to the challenges with consistent placement of the thorax electrodes. The high day-to-day variability in the thorax measurements obtained in this work is consistent with that of Grenvik et al. [2], where a change of 53% and 64% was observed in the slopes depicting the changes in chest impedance to that of changes in the lung volume of two subjects on two different days. For the intra-day/within-the-session measurements, there was a change of 25.3%, 21.3%, and 24.0% in the correlation slope for dry (thigh), dry + gel (thigh), and wet (thorax) electrodes respectively (Figure 3). The results indicate that the gel on electrodes helped to reduce the thigh-thigh impedance variability but is not a significant factor affecting the reproducibility of the bioimpedance measurements as a function of respiratory volume. For the wet electrodes across the thorax, the variability in measurements conducted within the same session (24%) reduced as compared to that of the inter-day (57%), as the electrodes were not moved between measurement sessions. We showed in this and earlier works that thigh-thigh bioimpedance depicts a high correlation to the respiration rate and volume signal and thus can be effectively used to measure respiratory rate and breathing patterns. Although this is preliminary data with a single subject, it indicates that the day to day variability of the relationship between impedance and volume will limit the accuracy and sensitivity of respiratory tidal volume. There is a need to understand the factors that lead to the variations, especially within the same session measurements and future work will help determine these limits.


Acknowledgements (Optional): : This research was funded by National Institute on Health, grant number R01NR018301.

References (Optional): : [1] K. Goyal, D. A. Borkholder and S. W. Day, "Unobtrusive In-Home Respiration Monitoring Using a Toilet Seat," 2022 IEEE-EMBS International Conference on Biomedical and Health Informatics (BHI), Ioannina, Greece pp. 01-05, 2022.

[2] A. Grenvik, S. Ballou, E. McGinley, J. E. Millen, W. L. Cooley, and P. Safar, “Impedance pneumography. Comparison between chest impedance changes and respiratory volumes in 11 healthy volunteers.,” Chest, vol. 62, no. 4, pp. 439–443, 1972.