Smartphone-Enabled Flow-Monitoring Device for Peripheral Artery Disease1

Blood flow in a clinical metric with a variety of uses, namely, the diagnosing and longitudinal monitoring of patients with or at risk of cardiovascular disease. Currently, blood flow is able to be quantified through techniques including Doppler ultrasound (DUS), computer tomography (CT), and magnetic resonance imaging (MRI). The majority of these methods require an expensive, immobile device (MRI and CT) and a skilled technician to operate (DUS, MRI, and CT), thus are accessible to very few patients in need of the technology.One of the diseases mainly associated with weak blood flow in the extremities is peripheral artery disease (PAD). With around 202 × 106 people afflicted with the disease worldwide, PAD presents a need for more accessible flow monitoring for patients and at risk individuals, as many living with the disease are asymptomatic [1].A new scheme for estimating blood flow is explored to make blood flow a more accessible metric. Magnetohydrodynamic voltage (VMHD) overlays have been observed in physiological recordings taken near arterial blood flow in the presence of an externally applied magnetic field. The magnetic field interacts with the blood plasma electrolytes, inducing a voltage (VMHD) that is observable through electrocardiograms (ECG) (Fig. 1).Observed VMHD has been shown to be correlated with stroke volume and blood flow in recent studies [2–4]. The VMHD signal can be extracted from the ECG measurements infected with MHD noise by subtracting the noisy ECG with a baseline ECG taken outside of the magnetic field [3].We hypothesized that this effect can be reproduced outside the MRI with the use of a static neodymium magnet and that this MHD effect can be harnessed to produce a portable monitor capable of obtaining blood flow data. The developed device would help increase accessibility of the metric to the general population.A benchtop experiment was conducted to try and recreate the induced VMHD outside the MRI bore, using a static magnet and a commercial ECG monitor with associated software. A neodymium magnet with a magnetic field strength of 0.4 T at the surface was placed over a vessel of interest, and two electrodes from the ECG were placed on either side of the vessel to measure the potential difference, in this case the induced VMHD (Fig. 2).Carotid artery VMHD measurements were taken from a healthy human subject and an athletic subject (n = 2). A baseline measurement was taken of pure ECG from each subject, and then, ECG + VMHD measurements were taken at rest and then at an elevated heart rate after exercise stress.A prototype smartphone-enabled VMHD-based flow-monitoring device was devised. The base of the device was a three-lead ECG monitor, allowing the device to have both ECG and VMHD flow-monitoring capabilities. The setup consisted of an instrumentation amplifier and band-pass filtering with DC-offset correction (Table 1).The circuit fed the measured and treated biopotential signals to a microcontroller, which in turn transmitted the data over bluetooth to a smartphone, which acted as a display for plotting and giving readouts of the measured results (Fig. 3). The circuitry and smartphone required a custom enclosure, which was designed and 3D printed. Metal electrodes were attached to the case to allow for easy ECG measurement. An input for wired leads was added to allow for measurement using external electrodes for placement in nonconventional locations.The measurements were processed to show VMHD extracted from the benchtop test dataset obtained using a conventional ECG monitor. Using the method outlined in Ref. [3], the VMHD extraction was conducted through the following equation:(1)VMHD=ECGMHD−ECGBaselinewhere ECGMHD is the ECG + VMHD signal measured with the neodymium magnet, and ECGBaseline is the signal measured without a magnetic field present.The resulting VMHD measurements were compared between the healthy subject and the athlete. The healthy subject exhibited an increase of 59% in VMHD, while the athlete exhibited an increase of 106% (Fig. 4).In the development of the prototype device, some field testing was done in order to ensure optimal parameters for the general population. A group (n = 7) was asked to use the device to measure their ECG through the device's built in electrodes to assess ease of use.The device was packaged in a custom 3D printed case with three surface leads (Fig. 5). Lead placement was decided on to isolate the ground from the measurement area as well as for easy usage.The athlete, with a higher expected blood flow rate at elevated heart rates, showed a much more significant increase in VMHD measured between exercise stress and rest, when compared with a healthy subject.The main advantage of this device comes from its accessibility. The device circuitry is composed of readily available parts, and with the ubiquitous nature of smartphones, the device can be readily assembled and used for very low cost. The aforementioned current methods of obtaining blood flow metrics are limited to well-equipped medical centers. In the cases of cardiovascular disease like PAD, constant monitoring is essential for preventing ischemic events, like thrombosis and embolism. The device would allow for inexpensive and accessible way of implementing longitudinal monitoring for at-risk patients, especially those afflicted with PAD.The creation of the prototype opens the door for many potential studies. To allow for adoption, further studies should be conducted by both lay persons and clinicians alike to provide features desired by both parties. Since the device is bluetooth based, any bluetooth-enabled device could be used as a data receiver. Currently, only an Android application has been developed, but the software can be ported to other operating system. Further tests will also be conducted to compare the flow measurements made with device with the current clinical methods of measuring flow (DUS and MRI).This study examined the feasibility of a VMHD based flow measurement being made outside of the MRI and the ability of that technology to be put in a portable package. The developed device prototype meets all of these qualifications.