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Cardiac arrhythmias are the most common cause of death in patients with a myocardial infarction or terminal heart failure. They increase in prevalence in the elderly and atrial fibrillation is particularly common in this group. Arrhythmias are also the most serious manifestation of digitalis toxicity and are often associated with anesthetic procedures, hyperthyroidism, and electrolyte disorders. The drugs used for arrhythmias fall into five major groups or classes, but most have very low therapeutic indices and when feasible, nondrug therapies (cardioversion, pacemakers, ablation, implanted defibrillators) are used. In addition, conditions associated with certain arrhythmias, especially increased risk of thromboembolism in atrial fibrillation, must be treated with appropriate drugs from other groups (eg, anticoagulants).
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A. Nature of Arrhythmias
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Normal electrical cardiac function (normal sinus rhythm, NSR) is dependent on generation of an impulse in the normal sinoatrial (SA) node pacemaker and its conduction through the atrial muscle, through the atrioventricular (AV) node, through the Purkinje conduction system, to the ventricular muscle (Figure 14–1) where it is finally extinguished after activating all the myocytes. A new impulse must arise in the SA node for the next conducted action potential. Normal pacemaking and conduction require normal action potentials (dependent on sodium, calcium, and potassium channel activity) under appropriate autonomic control. Arrhythmias (also called dysrhythmias) are therefore defined by exclusion, that is, an arrhythmia is any cardiac rhythm that is not normal sinus rhythm.
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Abnormal automaticity and abnormal conduction are the two major mechanisms for arrhythmias. Abnormalities of conduction include reentrant conduction and less commonly, complete block. A few of the clinically important arrhythmias are atrial flutter, atrial fibrillation (AFib), atrioventricular nodal reentry (a common type of supraventricular tachycardia [SVT]), premature ventricular beats (PVBs), ventricular tachycardia (VT), and ventricular fibrillation (VF). Examples of electrocardiographic (ECG) recordings of normal sinus rhythm and some of these common arrhythmias are shown in Figure 14–2. AFib is the most common serious arrhythmia and, as noted above, is particularly common in older patients. It is often symptomatic and ...