Biomedical Engineering Reference
In-Depth Information
percutaneously in conscious patients led to the development of techniques for the
detailed study of cardiac dysrhythmias (Denes
et al.
, 1973). This in turn was instru-
mental in the application of radio frequency ablation therapy for the permanent cure
of many forms of dysrhythmia (Kay
et al.
, 1992). Similarly, it was ECG analysis
of rhythm abnormalities that led to the concept and introduction of cardiac pacing
(Bigelow
et al.
, 1950) for bradydysrhythmias and of defibrillation for ventricular
fibrillation (Beck
et al.
, 1947). Thus, a discovery originally based on the use of a
simple capillary electrometer has with successive innovations become the bedrock
for the clinical investigation of almost all forms of heart disease. Waller initially
did not imagine that “electrocardiography would play any very extensive use in
the hospital” and only with further research and innovation came to realise the full
potential of his initial recordings.
Cardiovascular Imaging
Ultrasound imaging
While the ECG provides indirect information about cardiac function based on
its electrical activity, direct imaging of the heart adds great diagnostic potential.
Echocardiography utilises the fact that high-frequency sound waves travel safely
through tissues at a known velocity and are reflected by tissue boundaries of dif-
fering acoustic density. In practice, a piezoelectric crystal is used to generate high-
frequency sound waves and to detect “echoes” from reflecting surfaces. Time and
frequency analysis of reflected ultrasound provides valuable information about the
structure and function of the cardiac chambers and valves (Keidel, 1950). From
this initial invention a host of innovations have followed. Examples of this have
been the introduction of phased array or rocking transducers, creating a fan-shaped
imaging field that results in a two-dimensional anatomical view of the heart in any
chosen plane. This advance greatly improved diagnostic power (Edler and Hertz,
1954), providing valuable diagnostic information about cardiac chamber size and
contractile function. A further refinement was the analysis of frequency shifts in
the reflected ultrasound which provides information about blood flow velocity and
direction within the heart and blood vessels (Feigenbaum
et al.
, 2004). Colour cod-
ing of displayed images based on the frequency shift of reflected sound allows the
direction and velocity of blood flow to be displayed. Valvular motion and orifice
areas can be determined and estimates of blood pressure gradients across valves can
also be derived. Complex congenital heart diseases including intra-cardiac shunts
can be examined. In addition, by using transducers with different depths of view,
these techniques can be applied to the study of blood vessels at different depths
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