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EDNE MARCH 2013

When the heart’s electrical subsystem malfunctions The heart, a biochemical-electromechanical system, develops an electrical impulse that travels from the sinoatrial (SA) node in the upper right atrium to the atrioventricular (AV) node. The SA node acts as the pacemaker for the system (Figure 1). This electrical impulse generates the P wave, which can be seen on the ECG capture in Figure 2. From the AV node, the electrical signal propagates, via the His-Purkinje system, to the ventricles, causing the ventricle muscles to contract. Their contraction (the R wave) moves oxygenated blood from the left ventricle into and through the body, and moves deoxygenated blood from the right ventricle to the lungs. When the electrical system doesn’t work perfectly, different heart conditions can occur. For example, bradycardia occurs when the heart beats too slowly or misses beats. A typical surgical intervention for this condition would be to implant a pacemaker device (pulse generator) just under the skin of the patient’s chest, with endocardial leads routed through the veins directly to the heart, as shown in Figure 3. In another class of arrhythmias, called tachycardia, the heart beats too fast. This very serious condition is treated with implantable cardiac defibrillators (ICDs). Modern ICDs can also treat many bradycardia arrhythmias. Heart failure can occur when the heart becomes enlarged, lengthening its conduction paths and upsetting the timing of the ventricular contractions. This forms a positive feedback system, further aggravating the heart. Implantable cardiac resynchronization (ICR) devices retime the ventricles by pacing both ventricles and usually one atrium. These devices improve cardiac output, allowing the heart to recover to a certain degree. Cardiac resynchronization therapy (CRT) devices include an ICD as part of the system. References 1. Jaffe, C Carl, MD, and Patrick J Lynch, medical illustrator, Reizleitungssystem, http://en.wikipedia.org/wiki/ File:Reizleitungssystem_1.png 2. Sinus Rhythm Labels, http://en.wikipedia.org/wiki/ File:SinusRhythmLabels.svg 3. (a) Cardiac resynchronisation therapy, http://en.wikipedia.org/ wiki/File:Cardiac_resynchronisation_therapy.png (b) Fluoroscopy pacemaker leads right atrium ventricle, http:// en.wikipedia.org/wiki/File:Fluoroscopy_pacemaker_leads_right_ atrium_ventricle.png Figure 1 The heart muscle showing sinoatrial (1) and atrioventricular (2) nodes. (Reference 1). Figure 2 Electrical action during heart-muscle contractions (Reference 2). Figure 3 Pacemaker-lead insertions for the various pacemaker types. (RA is right atrium, RV is the right ventricle; LV is the left ventricle). 32 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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