Biomechanical Systems Technology

The following sections are included:

• PREFACE

• CONTENTS

This chapter gives an overview of various ventricular assist devices with a particular focus on axial flow pumps. Using the Hemopump as an example, we developed a model of a canine circulatory system assisted by an axial flow pump, and used computer simulation to predict the effects of the assist device under various hemodynamic conditions. In general, the results from the simulation are in good agreement with that observed in clinical and animal experiments. The same model is further used to explore the techniques and strategy for optimum control of the assist device by introducing an objective function, and choosing suitable membership functions with associated weighting factors, the model offers great flexibility in choosing the targeted hemodynamic variables, in specifying the particular way that each of these variables is to be optimized, and in assigning the relative importance of each targeted variable. The methods used and the results obtained in this study can be incorporated into the design of an advanced physiological controller for a long-term operation of the axial flow pump-based assist device as well as other types of continuous-flow LVAD.

Microcirculation plays a direct role in the accomplishment of the principal purpose of the circulation system. The functions of the microcirculation incorporates oxygen supply, transport, diffusion, and exchange of nutrients and metabolites between blood and tissue, maintenance of body temperature, regulation of blood pressure, tissue defense and repair. These complex functions are carried out in the microcirculation by a number of dynamic changes in the blood within the vessels, the vessels themselves or the tissues surrounding the vessels. Intravital microscopic approaches have greatly contributed to the advancement of research in the fields of microcirculation. The techniques represent the only method that allows direct visualization and quantitative analysis of the microcirculation. In this chapter in vivo microscopic techniques with useful experimental models, which provide significant information regarding the microcirculation, are introduced.

Impedance cardiography is a branch of bioimpedance primarily concerned with the determination of left ventricular stroke volume. The technique involves applying a current field across a segment of the body, usually the thorax, by means of a constant magnitude, high frequency, low amplitude alternating current. By Ohm's Law, the voltage measured within the current field is proportional to the electrical impedance Z. Upon ventricular ejection, a characteristic pulsatile impedance change occurs ΔZ(t), which, in early systole, represents both the radially oriented volumetric expansion of and longitudinal forward blood flow within the great thoracic arteries. Past methods assumed a volumetric origin for the peak systolic upslope of ΔZ(t), that is, dZ / dt_max. A new method assumes the rapid ejection of forward flowing blood in early systole causes significant changes in blood resistivity, and it is the peak rate of change of blood resistivity that is the origin of dZ /dt_max. As proposed, dZ /dt_max is an ohmic mean acceleration equivalent, and, as necessary for stroke volume calculation, must undergo square root transformation to yield an ohmic mean velocity equivalent. Further changes over older methods include a variable magnitude volume conductor based on body mass, rather than on current field segment length or body height.

The injection of indicators in the blood stream is used for several diagnostic measurements of the cardiovascular system functionality. An indicator is a non-toxic substance that can be detected by a specific sensor. The detection often implies a contact between indicator and sensor. As a result, the measurement is invasive, i.e. a catheter must be inserted through the blood vessels in the measurement site. The invasiveness issue can be overcome by means of contrast imaging techniques. Contrast agents are substances that enhance the signal detected by specific imaging modalities. The measurement of the contrast (or indicator) concentration versus time results in an indicator dilution curve. Several hemodynamic models can be adopted for the curve interpretation and for the estimation of the cardiovascular parameters of interest. This chapter provides an overview on the indicator dilution theory and the clinical techniques that can be adopted for the measurement of dilution curves. Both invasive and minimally invasive techniques are discussed. Particular attention is dedicated to the use of medical imaging techniques such as ultrasound and magnetic resonance imaging. These methods promise to open new possibilities for minimally invasive cardiovascular diagnostics based on indicator dilution.

Diseases of the heart have become the Number One cause of death in the industrialized nations of the world. Every heart disease affects the biomechanics of the heart in a direct or indirect way. These effects can primarily be analyzed by the signals of the heart sounds and cardiac murmurs using techniques such as auscultation and phonocardiography. But these methods are very sophisticated and require a high degree of specialization. During the last years electronic stethoscopes and commercial PC techniques have improved essentially so an automatic analysis of heart sound has become a potential supporting tool for physicians in particular as a screening method for heart diseases by the general practitioner. This paper introduces a new automatic system to diagnose heart valve diseases based on time and frequency analyzing methods and feature extraction. It also describes an multivariate approach for an enhanced risk stratification in patients with heart failure considering the time interval between the ECG signal, especially the R-wave, and the first heart sound.

In conclusion we could demonstrate, that analysis of the heart sound is a suitable method to evaluate the state of the heart and to detect changes on the biomechanics at an early stage.

Simple one-dimensional models of blood flow are widely used in simulating the transport of blood around the human vasculature. However, the effects of gravity have only been previously examined briefly and the effects of changes in wall properties and their interaction with gravitational forces have not been investigated. Here the effects of both gravitational forces and local changes in wall stiffness on the one-dimensional flow through axisymmetric vessels are studied. The governing fluid dynamic equations are derived from the Navier-Stokes equations for an incompressible fluid and linked to a simple model of the vessel wall, derived here from an exponential stress-strain relationship. A closed form of the hyperbolic partial differential equations is found. The flow behavior is examined in both the steady state and for wave reflection at a junction between two sections of different wall stiffness. A significant change in wave reflection coefficient is found under the influence of gravity, particularly at low values of baseline non-dimensional wall stiffness.

Cardiovascular disease is a major cause of death in Western society. Complications arise throughout the cardiovascular system with the arterial sub-system being prone to atherosclerosis and aneurysm formation. Atherosclerosis is a disease characterized by the deposition of lipoproteins in the arterial wall while an aneurysm is characterized by an abnormal swelling of the wall itself. Medical therapy, comprising life-style changes and drug administration is the mainstay of treatment for the majority of people with arterial vascular disease. However, a variety of surgical interventions are available to treat diseases associated with the arterial system and the primary objective of such interventions is to restore normal arterial function. These treatments may be performed using open surgery or minimally invasive techniques and are biomechanical in nature.

A range of numerical and experimental techniques has been developed to quantify and qualify disease-influencing biomechanical factors in the arterial system. These techniques may be used to perform basic research on disease forming mechanisms, to diagnose disease in vivo and to assess or develop new biomedical treatments. The techniques presented in this chapter are applied for various purposes at different locations in the arterial system using an integrated research and development approach. The chapter concludes with a discussion on new medical devices currently under development in order to demonstrate how numerical and experimental methods may be applied in the search for new and improved treatments for arterial disease.