The ECG displayed on the monitor screen must be clear and well defined to avoid misdiagnosis or inappropriate treatment. A cardiac trace can be obtained through three or five colour-coded ECG leads that are attached to electrodes placed on the anterior surface of a patient's chest. Signals emitted from the heart are sensed from the skin surface by the electrodes and are subsequently transmitted, by means of a cable, to the cardiac monitor, which in turn magnifies them into recognisable waves. The displayed image reflects one view and while single-lead monitoring remains widely used, it has limited value in the diagnosis of other clinical conditions. However, modern monitors are multipurpose and permit a range of functions including continuous transmission of leads I, II, III (see Figure 8.1), as well as displaying non-invasive blood pressure and oxygen saturation readings. It is also possible to configure the technology to display 12-lead ECG and ST segment monitoring options.
Defibrillator paddles and gel pads can also be used to monitor a cardiac rhythm, particularly when the cardiac arrest is not witnessed. It is the quickest way to assess whether the casualty is in a shockable rhythm. Automated external defibrillators require application of large adhesive multipurpose electrodes to the casualty's chest, which allow for monitoring of cardiac rhythm and defibrillation.
Fig. 8.1 Monitor displays three leads and ST segment monitoring.
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