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Electrical signals arising from the heart can be picked up through skin electrodes and presented on a relatively slow-moving chart as the electrocardiogram (ECG). The origin of each part of the ECG waveform is very well understood and electrocardiographic records now represent one of the commonest measurement procedures performed on patients in hospital. Abnormalities in the waveform are indicative of various types of heart disease and of cardiac distress arising from other illnesses or biochemical disturbances.
Electrocardiograms are performed in the cardiac department, and also in the wards of the hospital. The electrocardiogram is also monitored on CRT (cardioscope) screens in many wards, and in the operating theatre, but not always for the purpose of making diagnoses, more the monitoring of the condition of the patient as reflected by the strength and rate of the signals.
An electrocardiograph consists of a set of electrodes and leads which are placed on the body according to a conventional plan. The most commonly used of these electrodes are placed on the legs and arms, and others are placed in a line roughly over the heart. The electrocardiograph has a lead selector switch to configure these electrodes into different groupings to produce differential signals which represent different planes through the heart. The waveform seen at each of the switch positions is slightly different.
Electrocardiographs are normally portable or mounted on light trolleys so that they can be moved around the hospital, and are often battery operated. The most common instruments record one lead configuration at a time, although three channel versions are becoming common. The ECG signal is normally about 1 mV in amplitude, and with frequency components from less than 1 Hz to 100Hz. These are amplified in a differential amplifier having a frequency response of typically 0.02 to 100 Hz, and very high common-mode rejection ratio (CMRR) to cut down the a.c. interference picked up from the mains supply by the long electrode leads. A standard paper speed of 25 mm/s and sensitivity of 10 mm/mV is used for most recordings and the amplitudes of the various parts of the waveform are usually quoted in millimetres.
A typical ECG recording procedure would consist of cleaning the skin, abraiding, attaching electrodes and applying the conductive electrode jelly, and with the patient relaxed, recording a few centimetres of the ECG at each of the standard lead positions. These are then sent for diagnostic reporting by a cardiac physician. The foetal ECG signal is also sometimes monitored during pregnancy and labour to record the foetal heart rate.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details