Introduction

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Part 4

Basic Cardiology...(cont)

The functional unit of the heart is the cardiac muscle cell or cardiomyocyte. Each cardiomyocyte maintains an electrical charge or potential across its cell membrane, and contracts when this potential is discharged. In order for all of the cardiomyocytes to contract at the same time and thus produce an effective muscular contraction, the heart also maintains its own electrical conducting system which co-ordinates the electrical activity of the heart.

The sum total of the simultaneous electrical discharging and re-charging of all the cardiomyocytes in the heart is sufficient to be detected by sensing probes placed on the exterior of the body at various positions around the heart. This is the principle behind the electrocardiograph, or ECG, which can be used to monitor the rhythm of the heart. Also, since injured cardiomyocytes such as those suffering from lack of oxygen during a heart attack leak electrical current rather than discharge it in a co-ordinated manner, the altered electrical signal of the injured heart results in a characteristic ECG pattern which can lead to the diagnosis of acute myocardial infarction. In contrast, dead cardiomyocytes or scarred cardiac muscle does not carry or maintain an electrical charge, and this absence of electrical activity is also detectable by ECG. Thus, a previously unrecognised or "silent" heart attack can be diagnosed by electrocardiogram, and even localised to a particular area of the heart by using multiple sensing probes or ECG leads.

Whereas the electrical activity of the heart provides a powerful indicator of cardiac function and health, it is frequently desirable to directly monitor cardiac mechanical activity, that is the muscular contractions and relaxation's. Technological advances over the past ten to twenty years have provided a variety of sophisticated methods to achieve this without injury, or even significant discomfort to patients. The most powerful diagnostic tool is echocardiography or "cardiac ultrasound", in which high frequency sound waves sufficiently intense to penetrate bodily tissues are directed into the chest and the pattern of reflected sound provides a real time image of the contracting heart and its component structures. Echocardiography can provide accurate measurements of heart chamber size and shape, contractile function, and the status of the four heart valves. Furthermore, by using the related technique of Doppler echocardiography the Cardiologist can directly visualise blood flow through the heart, and therefore identify regurgitant or stenosed valves by reversed or highly turbulent blood flow, respectively.

Fluoroscopic cardiography
Cardiac function can also be assessed non-invasively by imaging a radioactively labelled blood pool within the cardiac ventricles during contraction and relaxation. This is the most accurate method for measuring cardiac contractile function because it is easy to precisely quantify the amount of radiation in the ventricle when it is most full and most nearly empty. The percent of blood expelled during each contraction, termed the ejection fraction, can then be mathematically calculated from these values; the typical ejection fraction of the left ventricle is 50 to 70% at rest, but can increase to 90% with strenuous exercise.

Cardiac catheterisation
The oldest method of objectively evaluating cardiac function is still regarded by many as the "gold standard", the cardiac catheterisation. During this procedure plastic tubes or catheters are harmlessly placed via arm or leg vessels into specific cardiac chambers or into the openings of arteries that feed the cardiac muscle, termed coronary arteries. Blood pressure and oxygen content can be measured at distinct points in the heart or great vessels to diagnose heart failure or to detect abnormal communications between cardiac structures. Dye is injected into the coronary arteries for visualisation by X-ray motion pictures to detect arteriosclerotic blockages which may cause cardiac pain (angina) or lead to heart attacks. A cardiac catheterisation procedure is also used to therapeutically restore coronary blood flow by balloon angioplasty or stenting of blocked arteries.

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