Wearable patch continuously tracks Doppler blood flow metrics in carotid artery

A novel, hands-free ultrasound patch for continuous monitoring of quantitative Doppler accurately measures flow velocity, is easy to use, and can constantly record Doppler spectrograms over many cardiac and respiratory cycles, a study has shown.

“[T]he CW Doppler patch was able to continuously record and track instantaneous changes in carotid velocity–time integral (VTI) during a passive leg raise (PLR) in a healthy volunteer and during quiet respiration in a patient with congestive heart failure,” the researchers said. “This Doppler patch may be useful for functional haemodynamic monitoring in the emergency department, operating room, and intensive care unit; further investigation in these settings is planned.”

The accuracy of measuring changes in flow velocities with the continuous-wave (CW) Doppler patch was ascertained through a series of in vitro experiments. The researchers used a motorized string in a water tank to establish absolute velocity accuracy. They also conducted a small usability study in 22 volunteers (five physicians, 11 nurses, one technician, and five lay users) to identify potential difficulties or hazards that might arise among users.

Finally, a proof-of-concept in vivo study was carried out in a healthy 35-year-old adult volunteer (female, 37-cm neck circumference, body mass index [BMI] 25.1 kg/m²) with no known cardiovascular history and on no regular cardiovascular medications. A stable 94-year-old patient on the general medical floor admitted for congestive heart failure (male, 44-cm neck circumference, BMI 24.8 kg/m²) was also recruited. Written informed consent was obtained from the participants.

The CW Doppler patch adheres to the neck and tracks Doppler blood flow metrics in the common carotid artery using an automated algorithm.

In string and blood-mimicking test objects, both manually and automatically traced Doppler velocity waveforms accurately measured the changes in velocity. In the usability study with 22 volunteers, all users successfully located the carotid Doppler signal on a volunteer participant. They also agreed in a subsequent survey that the device was easy to use. [Sci Rep 2021;11:7780]

Result of the in vivo study to demonstrate system capabilities showed that the hands-free device easily adhered to the neck, captured the audible Doppler shift, and continuously captured Doppler spectra from the common carotid artery. The automated maximum velocity tracing algorithm qualitatively tracked the Doppler spectrograms and calculated VTI for each beat. Furthermore, beat-to-beat variability could be seen in the spectrogram, the automated trace, and the calculated VTI values.

Earlier studies have shown the potential of carotid Doppler in functional hemodynamic monitoring. [Chest 2013;143:364-370; Crit Care Med 2018;46:e1040-e1046]

“We have previously shown in healthy volunteers performing squat manoeuvres that a 10-percent rise in stroke volume corresponds to a 15-percent VTI threshold,” the researchers said. “A rise was also observed during PLR in a healthy volunteer in the present study … [but] not all clinical studies have replicated these findings.” [Crit Care Explor 2020;2:e0072; J Ultrasound Med 2020;39:1965-1976]

The small number of beats sampled (eg, 1–3) used to calculate blood flow metrics coupled with normal, inherent VTI variation introduced by the respiratory cycle could potentially explain these discrepancies. [Crit Ultrasound J 2017;9:10; J Intensive Care Med 2020;35:650-655; Intensive Care Med Exp 2020;8:54]

“The hands-free CW Doppler patch is well-suited to overcome this potential source of variability by acquiring continuous measurements over many beats,” the researchers said. “The preclinical phantom, healthy volunteer and congestive heart failure patient data shown here demonstrate that the device can accurately measure changes in flow velocities as well as continuously track changes in carotid VTI during a PLR manoeuvre.”

The present study was limited by the following: stroke distance (VTI) was measured but not the absolute flow rate; a healthy participant with normal blood pressure was examined instead of a critically-ill patient; finally, the utility of the device have yet to be characterized in abnormal anatomy such as stenotic and tortuous vessels.