Assessment of capped-end pulsation method for measuring pulse wave velocity

Abstract

Cardiovascular disease is a growing epidemic in the U.S. and worldwide. As more and more individuals suffer from complications from poor cardiovascular function, there is a growing need for early diagnostic modalities. Vessel stiffness is a common biomarker to assess vascular health. For example, atherosclerosis caused by plaque formation can cause vessels to stiffen. For obvious reasons, it is not reasonable to take out an arterial sample to mechanically test for stiffness. Thus, there is a need for a non-invasive way to measure stiffness. Described by the Moens-Korteweg equation, pulse wave velocity (PWV) is non-linearly proportional to the stiffness of the vessel. Therefore, this equation provides a mechanism in which stiffness can be measured non-invasively. Because of its ease of measurement as well as efficacy, PWV has become a gold standard in assessing vascular health. Commonly, the time delay in pulse pressures taken at the carotid and femoral arteries is used as quick way to assess aortic health. Unfortunately, this method is not standardized and variations between populations could affect this measurement. Additionally, in clinical practice, PWV is only a measure of the average wave speed through a person’s aorta. As such, it doesn’t provide any local measurements along the arterial tree. Current measuring techniques, such as tonometry and ultrasound, struggle to achieve the resolution needed to make localized measurements. Additionally, different geometries along the vessels, such as curvature and bifurcations, can cause reflections and affect wave speed. All these variables are difficult to control for in vivo. As such, ex vivo experimentation with the aim of studying how wave speed is affected by these variables, is an attractive alternative. In an ex vivo setting, there are more tools available to achieve the resolution needed to make local measurements. Just like in vivo, ex vivo experiments need to overcome two main challenges: 1) achieve the resolution needed capture the speed of a wave and 2) control wave reflections. To address both issues, our group proposes that capping the end of a vessel while sending periodic pulse waves can solve both problems simultaneously. By embracing wave reflections off the capped end, we reduce that issue down to a problem that is well-behaved and understood. Additionally, the interference pattern caused by the incoming and reflected waves has the potential to create a standing wave. Because the shape of a standing wave is time-invariant, temporal resolution becomes less of an issue. By knowing the wavelength of the standing wave as well as the forcing frequency, one could calculate the wave speed.