Radiofrequency sensing enables non-invasive cardiac hemodynamic monitoring

Introduction

In patients with heart failure, accurate monitoring of cardiac function provides key information for disease management. Currently available non-invasive monitoring techniques provide limited information on cardiac function and invasive sensors are only used for a selected patient group. Therefore, in this study, we introduce a novel approach to monitor cardiac mechanical function: radiofrequency sensing (RFS), a fully non-invasive technique.

Methods

RFS measures scattering of radio waves transmitted and received by a lightweight, flexible, on-body radio antenna. The received RFS signals are modulated in time by cardiac and respiratory motion. Figure 1A shows a schematic overview of the RFS set-up and an example of ECG aligned with RFS signal during multiple cardiac cycles (referred to as RFS waveforms).

To study the effect of intrapatient hemodynamic changes on the RFS signals, the first part of the study involved RFS measurements in patients during a right heart catheterization (RHC). In the second part of the study, RFS measurements were performed in patients after echocardiography, to determine interpatient correlation between SV determined by echocardiography and RFS determined SV. Lastly, to evaluate the measurement of respiratory function, in separate subjects ground-truth breathing rate and depth were collected using a respiratory belt sensor during RFS measurements.

Results

In three patients RFS measurements were performed during RHC, yielding a total of five measurements across patients. The pacemaker rate was decreased as part of the diagnostic procedure in four out of five measurements, and epoprostenol was administered during one measurement. The RFS waveform showed an increase in amplitude during epoprostenol infusion (upper panel of Figure 1B). This increase in amplitude remained consistent within individual patients with paired measurements (lower panel of Figure 1B). A total of 32 patients (50% female, mean age: 59 (SD=16) years, mean left ventricular ejection fraction: 57 (SD=9)%) were included after echocardiography. A 2nd order polynomial regression model was fitted for RFS derived SV, showing a good correlation with echocardiographic SV during training (R = 0.71) (Figure 2C). In six separate subjects, a total of 23 combined RFS and respiratory belt measurements were acquired. Respiratory rate and tidal depth determined with RFS showed excellent linear correlations, respectively R = 0.98 and R = 0.84 (Figure 2A and B).

Conclusion

Intra- and interpatient measurements show that RFS can serve as a non-invasive modality for remote cardiac monitoring. A good correlation was demonstrated between changes in the RFS waveform amplitude and changes in stroke volume within subjects. Future studies will focus on evaluating RFS in a large, diverse populations for monitoring cardiac function.For image description, please refer to the figure legend and surrounding text.For image description, please refer to the figure legend and surrounding text.