Abstract
This is a feasibility study in using radar for estimating blood pressure and which could allow for continuous, non-invasive measurements both inside and outside the clinic. Blood pressure has extensive use in emergency care for monitoring the state of a patient, in detection and treatment of hypertension as an important factor of cardiovascular diseases, in emerging applications and more. The invasive cannula for central, continuous, measurements and the sphygmomanometer for peripheral, punctual measurements are currently the reference tools for measuring blood pressure in the operating room and in the physician’s office respectively. However, new blood pressure estimation devices could increase mobility in the hospital and reduce white-coat hypertension in the office. Moreover, such accessible and low-cost devices could extend measurements outside the clinic.
The main work was focused on the estimation of the time-varying aortic radius as a prerequisite for the estimation of blood pressure. This work was conducted primarily based on theoretic considerations and simulations using realistic models of the human thorax. In addition, one article analyzed radar signatures from a phantom model and from an in vivo experiment to test findings. Radar echoes were analyzed for information regarding changes in the aorta radius. A magnetic resonance imaging study was conducted to map tissue movements as an aid to radar analysis.
Theoretical considerations proposed an optimal frequency band for aorta radius estimation. Given sufficient dynamic range in the radar system, and shown in both simulations and measurements on a phantom model, interference phenomena determined radius estimation success. In the optimistic scenario, radius changes could be estimated. In the more realistic scenario, only a proxy on aorta radius could be tracked. The magnetic resonance imaging study identified a mechanical coupling between the aorta and the heart which would complicate radius estimation. Despite this coupling, the study confirmed findings in the literature that aorta radius is related to pressure. The in vivo experiment showed evidence of an echo from the aorta, however the study did not find evidence of two distinct reflectors.
Although successful in identifying a signature from the aorta, the mechanical coupling between the heart and the aorta implies that this signature contains information on both location and radius. Because two distinct echoes could not be identified, direct estimates of radius was not considered feasible. An indirect approach based on blood pressure propagation speed may still be possible, although the coupling would need to be compensated for. As a feasibility study, certain simplifications were imposed and in particular the effects of respiration were disregarded.