In vitro and clinical demonstration of relative velocity measurements with the Flopatch™: A wearable Doppler ultrasound patch

A hands-free continuous wave (CW) Doppler ultrasound patch has been developed for noninvasive assessment of carotid stroke volume. The patch enables easy, continuous monitoring, which is particularly useful in a fast-paced setting such as the intensive care unit. In vitro validation of the measurement of relative change in velocity was performed with a flow phantom for a maximum velocity range of 10–150 cm/s, showing good agreement, with average errors of 2.1% and 3.9% for constant and pulsatile flow, respectively. In a pilot clinical demonstration, a recording was made during on-pump coronary artery bypass surgery. The recorded maximum velocity trace was compared with the peristaltic pulsation frequency, showing that the patch tracked the large changes in pump flow rate. Notably, in the seconds following the changes in pump flow, clear differences were observed, consistent with cerebral autoregulation. Further investigations are ongoing into other metrics of carotid flow and stroke volume, and into the clinical utility of the device for assessing fluid responsiveness.