"In comparison to existing non-invasive devices that use peripheral arteries, the aim of our STAbp device is to estimate the central pressure more accurately and reliably," explained Kong Chin from the University Hospitals of Leicester NHS Trust, UK. "Continuous recording of ABP is an essential requirement in many areas of physiology and clinical investigation. One of our long-term goals is to commercialize the STAbp device."

Seeking non-invasive alternatives

When it comes to acquiring a continuous ABP waveform, the gold standard approach is intra-cannula catheterization in the ascending aorta. However, as this is an invasive technique, it does carry an increased risk of infection and requires specialized single-use catheters that inflate running costs. As a consequence, non-invasive alternatives are of particular interest.

Today, the most common approaches to obtaining non-invasive continuous measurements of ABP rely either on arterial volume clamping of the digital artery (such as the commercially available Finapres device) or the use of arterial tonometry of the radial artery. The main drawback in these cases is that the ABP waveform varies downstream the arterial tree. As a result, measuring the central pressure using peripheral arterial pressure can lead to poorer diagnostic performance.

"It is known that ABP pulse wave propagation and reflection occur downstream the arterial tree, which can lead to a change in the waveform morphology from central to peripheral arteries," commented Chin. "This is likely to produce inaccurate estimation of central ABP, especially the systolic pressure."

Studying the STAbp

The STAbp device developed by Chin and his Leicester-based colleague Ronney Panerai uses a variety of optical sensors and pneumatic components to perform arterial volume clamping and photoplethysmography. This system is mounted on a head frame, which is placed on the patient, and is controlled from a desktop computer.

To measure the ABP, the STAbp device first goes through what the authors have called an open-loop procedure. Here, a suitable photoplethysmogram of the STA is selected and monitored through a range of compressing pressures. Once this procedure has been completed, a closed-loop procedure is activated to monitor the ABP continuously. The real-time ABP waveform can also be displayed on a screen.

Chin and Panerai carried out a test on 19 healthy volunteers to compare the performance of the STAbp against the Finapres device. The five parameters assessed were: resting ABP; drift; ABP bandwidth; frequency response and magnitude squared coherence; and isometric exercise. Finally, statistical analysis was performed to evaluate the relative performance of the STAbp compared with the Finapres.

"We were pleased to see that the STAbp provided ABP waveforms in good agreement with the Finapres, and that it also responded similarly to dynamic manoeuvres like a hand-grip, without showing any additional signal drift," noted Chin. "Our motivation is not to replace the Finapres in all its applications, but rather to be a better alternative in specific areas. STAbp could be particularly advantageous for dynamic measurements of ABP during changes in head height, like tilting or sit-to-stand manoeuvres in conjunction with studies of cerebral haemodynamics."

Moving forward

Thanks to these encouraging results, the researchers are keen to validate the performance of the STAbp against the gold standard ascending aortic ABP measurement.

"We would also like to evaluate the accuracy and reliability of the STAbp in different age groups and patients with specific cardiovascular pathologies such as hypertension, cerebral autoregulation abnormality and autonomic dysfunction," said Chin. "Ultimately, we would like to commercialize the STAbp device for applications which require continuous ABP waveforms."