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6 Applications and Methods in Biosignal Processing

symptomatic patients lead to a categorical underdiagnosed clinical situation. Existing

diagnostic techniques like transesophageal echocardiography, doppler sonography or

computed tomography either lack sensitivity or require specialised expertise, are in-

vasive and costly which hampers effective screening. Therefore it is of great interest to

establish a new diagnostic method that is suitable for effective screening on the family

physician level.

One idea for such a new diagnostic method is based on the analysis of biosignals.

As described in previous sections, the heart initiates pulse waves of blood pressure

and flow that propagate from the heart through the aorta and the other arteries to the

periphery of the body. It is observed that the shape and properties of the wave change

if it meets irregularities in the structure of the arterial system such as aneuryms, sten-

oses (constrictions) or bifurcations (branching). Thus the forms of the pressure and

flow pulse wave depend on the structure of the cardiovascular system and can be mod-

elled if the structure is known.

The attempt for an early diagnostic method of aortic aneurysms is to solve the

inverse problem. Let us assume that the pulse pressure or flow waves are given at some

peripheral measuring locations. Is it possible to deduce information on the structure

of the cardiovascular system from these measurements? This question is called the

inverse hemodynamic problem. The general version has infinitely many solutions [63],

but the solution to the constraint case, that we will consider here, remains unclear. All

methods and results reported here are originally published in [24] and [25].

In [24], the authors simulated pressure waveforms at arteria brachialis and ar-

teria femoralis respectively and could show that one can deduce information on the

structure of the cardiovascular system from these two simultaneous measurements.

Therefore, coefficients of the pressure-pressure transfer functions between these two

measuring locations were evaluated. The use of transfer function parameters bears

the hope to eliminate the strong influence of different boundary conditions between

patients (heart, peripheral resistance due to auto-regulation) and isolate the arterial

channel properties.

These results based on in-silico data and the functionality of the proposed dia-

gnostic method was checked in [25] on in-vivo data from a clinical study performed

at the University Hospital Tübingen, Germany. This study consists of 55 patients

thereof 28 with aneurysms at different locations (thoracial, abdominal and thoracic-

abdominal aorta) and 27 vascular healthy patients. The results were compared with

the diagnoses of the gold standard computed tomography angiography (CTA). As it is

much easier to collect PPG-signals than pressure signals, PPG-signals at six different

locations were measured simultaneously.

The 55 patients were divided into two study groups based on the available CTA

data sets, with each group containing 27 (Group a)) and 28 (Group b)) respectively

patients: