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Electrocardiography and ambulatory monitoring

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Abstract

An electrocardiograph is a device for measuring electrical activity at the body surface. The electrical activity is generated by neuromuscular and cardiac tissue activity but, as the objective is to document cardiac muscle activity, precautions should be taken to minimize skeletal muscle movement. The electrocardiogram (ECG) produced is a record of the potential difference (in mV) between electrodes plotted on the vertical axis against time (in seconds) plotted on the horizontal axis. Electrical activity is generated in cardiac tissue by changes in the transmembrane electrical potentials as a result of ion movement. This electrical activity then triggers myocardial cell contraction and is therefore essential for normal cardiac function. In the heart, pacemaker cells are located in the sinoatrial (SA) node and also in the atrioventricular (AV) node, Bundle of His, bundle branches and Purkinje fibres. These pacemaker cells have the ability to depolarize spontaneously (automaticity). The SA node depolarizes spontaneously at the fastest rate and is therefore the dominant pacemaker under normal conditions. The rate of discharge of the SA node can be influenced by numerous external factors, primarily sympathetic and parasympathetic tone. Electrocardiography; and Ambulatory monitoring are discussed.

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Figures

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9.1 The electrical conduction system. Note that the tricuspid valve leaflets have been removed to more accurately show the position of the AV node in the right atrial endocardium. The AV node lies directly above the septal leaflet of the tricuspid valve. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission
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9.2 Normal ECG waveforms and intervals from a dog (lead II shown; paper speed 25 mm/s; gain 1 cm/mV).
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9.3 Lead systems.
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9.4 Patient positioning and restraint for a resting ECG. (Courtesy of S. Dennis.)
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9.6 Clips attached by adhesive pads to prevent skin trauma.
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9.7 Electrical interference artefact shown in leads I and III.
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9.9 Holter monitor set up. Lightweight digital Holter monitor. Monitor inside padded case positioned in dorsal midline using harness. Position of adhesive electrodes on the left side of the chest. The third electrode is placed over the apex beat on the right side of the thorax. A small T-shirt is used to prevent the hindlegs becoming tangled in the leads running from the monitor to the electrodes. A waterproof jacket is used to protect the monitor if the dog is exercising in the rain.
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9.10 Excerpt from a Holter recording from a dog during an episode of collapse. The trace shows sinus rhythm with a normal rate and baseline artefact, suggesting vigorous muscle activity. This implies that an arrhythmia is not the cause of the signs seen.
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9.11 Heart rate of a dog with atrial fibrillation during a 24-hour Holter recording. Heart rate is shown on the vertical axis and time on the horizontal axis. The mean heart rate is 175 beats/minute, suggesting suboptimal heart rate control.

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