Cardiac arrhythmias are the most common cause of death in patients with a myocardial infarction or terminal heart failure. They are also the most serious manifestation of digitalis toxicity and are often associated with anesthesia, 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.
|Abnormal automaticity||Pacemaker activity that originates anywhere other than in the sinoatrial node|
|Abnormal conduction||Conduction of an impulse that does not follow the path defined in Figure 14–1 or reenters tissue previously excited|
|Atrial, ventricular fibrillation (AFib, VF)||Arrhythmias involving rapid reentry and chaotic movement of impulses through the tissue of the atria or ventricles. Ventricular, but not atrial, fibrillation is fatal if not terminated within a few minutes|
|Group (class) 1, 2, 3, and 4 drugs||A method for classifying antiarrhythmic drugs, sometimes called the Singh-Vaughan Williams classification; based loosely on the channel or receptor affected|
|Reentrant arrhythmias||Arrhythmias of abnormal conduction; they involve the repetitive movement of an impulse through tissue previously excited by the same impulse|
|Effective refractory period||The time that must pass after the upstroke of a conducted impulse in a part of the heart before a new action potential can be propagated in that cell or tissue|
|Selective depression||The ability of certain drugs to selectively depress areas of excitable membrane that are most susceptible, leaving other areas relatively unaffected|
|Supraventricular tachycardia (SVT)||A reentrant arrhythmia that travels through the AV node; it may also be conducted through atrial tissue as part of the reentrant circuit|
|Ventricular tachycardia (VT)||A very common arrhythmia, often associated with myocardial infarction; ventricular tachycardia may involve abnormal automaticity or abnormal conduction, usually impairs cardiac output, and may deteriorate into ventricular fibrillation; for these reasons it requires prompt management|
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). 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, any rhythm that is not normal sinus rhythm is an arrhythmia.
Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG). The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart. The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization, and the T wave by ventricular repolarization. The PR ...