Cardiac Arrhythmias
DEFINITION
The most common example of a relatively benign arrhythmia is atrial fibrillation (see the chapter “Atrial Fibrillation”). Similarly common are PACs and PVCs, which, although a nuisance, generally are benign in the absence of structural heart disease. In contrast, the presence of nonsustained ventricular tachycardia (VT) or syncope in patients with coronary artery disease (CAD) or severe left ventricular (LV) dysfunction may be a harbinger of subsequent sudden cardiac death and must not be ignored.
PREVALENCE
Cardiac arrhythmias are common. Symptoms such as dizziness, palpitations, and syncope are frequent complaints encountered by family physicians, internists, and cardiologists. In contrast to these ubiquitous complaints, which are generally benign, sudden cardiac death remains an important public health concern. Statistics from the Centers for Disease Control and Prevention (CDC) have estimated sudden cardiac death rates at more than 600,000 per year (Fig. 1).1 Up to 50% of patients have sudden death as the first manifestation of cardiac disease. Efforts at decreasing this alarming number have obviously focused on primary prevention, such as reducing cardiac risk factors, but have also led to the proliferation of automatic external defibrillators (AEDs). These devices have been shown to reduce mortality when used quickly in the first few minutes after an arrest.
PATHOPHYSIOLOGY
Regardless of the specific arrhythmia, the pathogenesis of the arrhythmias falls into one of three basic mechanisms: enhanced or suppressed automaticity, triggered activity, or re-entry. Automaticity is a natural property of all myocytes. Ischemia, scarring, electrolyte disturbances, medications, advancing age, and other factors may suppress or enhance automaticity in various areas. Suppression of automaticity of the sinoatrial (SA) node can result in sinus node dysfunction and in sick sinus syndrome (SSS), which is still the most common indication for permanent pacemaker implantation (Fig. 2). In contrast to suppressed automaticity, enhanced automaticity can result in multiple arrhythmias, both atrial and ventricular. Triggered activity occurs when early afterdepolarizations and delayed afterdepolarizations initiate spontaneous multiple depolarizations, precipitating ventricular arrhythmias. Examples include torsades de pointes (Fig. 3) and ventricular arrhythmias caused by digitalis toxicity. Probably the most common mechanism of arrhythmogenesis results from re-entry. Requisites for re-entry include bidirectional conduction and unidirectional block. Micro level re-entry occurs with VT from conduction around the scar of myocardial infarction (MI), and macro level re-entry occurs via conduction through (Wolff-Parkinson-White [WPW] syndrome) concealed accessory pathways.
SIGNS AND SYMPTOMS
Syncope in the setting of noxious stimuli such as pain, prolonged standing, or venipuncture, particularly when preceded by vagal-type symptoms (e.g., diaphoresis, nausea, vomiting) suggests neurocardiogenic (vasovagal) syncope. Occasionally, patients report abrupt syncope without prodromal symptoms, suggesting the possibility of the malignant variety of neurocardiogenic syncope. Malignant neurocardiogenic syncope denotes syncope in the absence of a precipitating stimulus, with a short or absent prodrome, often resulting in injuries, and is associated with marked cardioinhibitory and bradycardic responses spontaneously or provoked by head-up tilt-table testing.2 Sustained or paroxysmal sinus tachycardia, frequently associated with chronic fatigue syndrome and fibromyalgia, suggest the possibility of postural orthostatic tachycardia syndrome (POTS). This syndrome, which may be a form of autonomic dysfunction, currently is unexplained. It is characterized by a markedly exaggerated increased chronotropic response to head-up tilt-table testing and stress testing. POTS often has associated systemic signs, such as muscle aches (fibromyalgia), cognitive dysfunction, and weight loss. Inappropriate sinus tachycardia (IST) syndrome is similar in presentation, but it probably represents a separate disorder with another cause—possibly atrial tachycardias in the sinus node area or dysregulation of sinus node automaticity.
DIAGNOSIS
Assessment of Structural Heart Disease
Frequently, patients present with a wide complex tachycardia, possibly VT versus SVT with aberrancy. Various algorithms have been described to facilitate the differentiation of wide complex tachycardias. Brugada and colleagues have synthesized the various schemes into one convenient and simple protocol (Fig. 4). The general rule, however, is that sustained or nonsustained wide complex tachycardia in patients with known CAD or previous MI is VT until proven otherwise.3 Obviously, the initial approach to sustained wide complex tachycardia is to carry out cardioversion if the patient is hemodynamically unstable. In stable patients, assume VT and treat empirically with intravenous medications (e.g., amiodarone, procainamide, lidocaine). If SVT with aberrancy is strongly suspected, diagnostic maneuvers, such as administering adenosine, may be cautiously used.
Holter Monitoring
Ambulatory Holter monitoring has been available for several decades and has proved invaluable in identifying underlying rhythm abnormalities.4 Generally, 24- to 48-hour baseline Holter monitoring is useful in quantitating and qualifying arrhythmias in patients with frequent symptoms (Fig. 5).
Event Recording
For patients who have symptoms occurring on a weekly or monthly basis, Holter monitoring may not establish the diagnosis unless the patient fortuitously experiences an event during recording. Event recording monitoring systems, also called loop recorders (e.g., King of Hearts, Instromedix, Rosemont, Ill) can be worn for longer intervals (usually a month) and can document infrequent arrhythmia episodes and provide symptom-to-arrhythmia correlation. These devices are automatically activated or patient-activated and use telephone modem technology to transmit the electrocardiographic rhythm strips. They use continuous loop technology (retrograde memory) so that in the event of a symptom, the patient activates the device by pushing a button and records an electrocardiographic rhythm strip several minutes before the event. When prolonged external ambulatory event monitors fail to document an arrhythmia, an implantable device (Reveal, Medtronic, Minneapolis, Minn) can be used in patients with recurrent enigmatic syncope or arrhythmias, in whom conventional testing has not yielded a diagnosis. This device, with a battery life of 14 to 22 months, is implanted subcutaneously and continuously scans for arrhythmias (Fig. 6). The device automatically records and stores tachycardia or bradycardia events and can be patient-activated. Insurance reimbursement for the Reveal device requires extensive conventional diagnostic testing, including negative event monitors, tilt-table testing, and an electrophysiologic study (EPS). Preliminary reports of implantable event monitor studies have shown a significant reduction in time to diagnosis and decreased overall costs when used in patients with syncope and no structural heart disease.
Signal-Averaged Electrocardiogram and T Wave Alternans
Although initially touted as an important screening test for patients with syncope or ventricular arrhythmia risk, the signal-averaged ECG (SAECG) now has a limited role.5 The presence of low-amplitude late potentials, indicating a positive signal-averaged ECG, suggests an underlying abnormality in ventricular repolarization seen with a discrete scar and can be associated with ventricular ectopy and spontaneous VT (Fig. 7). However, the SAECG may be abnormal in patients with no evidence of structural heart disease and in patients with conduction disturbances (e.g., right bundle branch block [RBBB]) and therefore, a positive study has an uncertain specificity and sensitivity. In contrast, the SAECG can be helpful in screening patients or family members for arrhythmogenic right ventricular dysplasia (ARVD). Similarly, T wave alternans may have an important role for risk stratification in patients with LV dysfunction and complex ventricular arrhythmias. It has long been recognized that abnormalities in the ST segment and T wave may precede the onset of ventricular arrhythmias. Presumably, changes in autonomic activity, as well as repolarization, may facilitate the provocation of lethal ventricular arrhythmias in susceptible patients. Rosenbaum and colleagues6 have reported that abnormal T wave alternans may be an important marker for assessing patients and determining their risk for sudden cardiac death (SCD). T wave alternans can be measured by stress testing and ambulatory monitors (Fig. 8).
Electrophysiologic Testing
Electrophysiologic testing has become an important standard for identifying high-risk patients who have nonsustained VT, such as those with previous MI and LV dysfunction (Fig. 9).5,7 Inducible, sustained, monomorphic VT predicts substantial risk for subsequent, spontaneous, clinically sustained VT and ventricular fibrillation (VF). Electrophysiologic testing is the gold standard for evaluating patients with recurrent syncope and can help identify underlying His-Purkinje disease, inducible VT, SVT, and sinus node dysfunction (Box 1).8
Box 1 Indications for Electrophysiologic Testing for Syncope
From Zipes DP, DiMarco JP, Jackman WM, et al: Guidelines for clinical intracardiac electrophysiological and catheter ablation procedures. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Clinical Intracardiac Electrophysiologic and Catheter Ablation Procedures), developed in collaboration with the North American Society of Pacing and Electrophysiology. J Am Coll Cardiol 1995;26:555-573.
