Wearable ultrasound patch accurately detects hypovolaemia

A wearable Doppler ultrasound patch can accurately detect decreases in stroke volume (SV) triggered by moderate-to-severe central hypovolaemia, according to a recent study. This device may prove clinically valuable for monitoring haemorrhage.

The study included 11 healthy volunteers (mean age 29.5 years, 39 percent women) who were made to wear a Doppler ultrasound patch as they underwent simulated haemorrhage and resuscitation during graded lower body negative pressure (LBNP) and release. During the procedure, stroke volume (SV), common carotid artery velocity time integral (VTI), and corrected time flow (FTc) were measured and their correlations with each other were assessed.

The wearable Doppler device introduced a novel metric called the Doppler shock index (DSI), whose two variations, DSI-VTI and DSI-FTc, were evaluated for their diagnostic accuracy for moderate-to-severe central hypovolaemia.

Overall, the 11 participants underwent a total of 22 LBNP protocols, yielding 17,822 cardiac cycles available for analysis. From resting baseline to the lowest achieved LBNP, SV fell by an average of 37.5 mL (38.8 percent; p≤0.0001). Meanwhile, mean arterial pressure increased from 97.2 to 98.8 mm Hg (p≤0.0001) and heart rate from 63 to 110 bpm (p≤0.0001). [J Am Coll Emerg Physicians Open 2021;2:e12533]

During the simulated moderate-to-severe haemorrhage, carotid Doppler VTI dropped significantly by 55.5 percent, as did FTc by 15.5 percent (p≤0.0001 for both). Under the same circumstances, DSI-VTI and DSI-FTc increased by 317 percent and 102 percent, respectively (p<0.0001 for both).

During simulated blood transfusion, SV increased by 38.9 mL (66 percent) from the period of lowest LBNP to recovery. Similarly, carotid Doppler VTI and FTc rose from 15.8 to 36.6 cm, and from 270.0 to 320.67 ms, respectively (p≤0.0001 for both). In contrast, recovery was marked by significant decreases in both DSI-VTI (–76.9 percent) and DSI-FTc (–53.2 percent; p≤0.0001 for both).

Changes in SV were strongly correlated with the common carotid artery VTI and FTc, with an R² value of 0.87.

Moderate-to-severe hypovolaemia was defined as a 30-percent reduction in SV. For detecting this hallmark, a 23.5-percent decrease in VTI yielded an area under the receiver operator curve (AUROC) of 0.95, with sensitivity and specificity values of 1.0 and 0.92, respectively. The corresponding values for a 3.0-percent drop in FTc were 0.97, 1.0, and 0.92.

Notably, both novel wearable device metrics yielded comparable predictive capabilities. A 46.5-percent increase in DSI-VTI had an AUROC of 0.96, sensitivity of 1.0, and specificity of 0.92. Meanwhile, a 15.9-percent increase in DSI-FTc yielded respective values of 0.97, 1.0, and 0.92.

“Two variations of the DSI measured by a wearable Doppler ultrasound accurately detected diminished SV in a model of moderate-to-severe central hypovolemia,” the researchers said. “Although this suggests that a wearable Doppler monitor is useful for identifying and monitoring cryptic haemorrhage, other clinical scenarios typified by diminished SV may also benefit from this technology.”

“Future studies in patients at risk for cryptic haemorrhagic shock and in the peri-intubation period are planned,” they added.