Cardiac Rhythms and Arrhythmias Chapter 12 Sinus Node Dysfunction
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SECTION II Cardiac Rhythms and Arrhythmias Chapter 12 Sinus Node Dysfunction SAROJA BHARATI, NORA GOLDSCHLAGER, FRED KUSUMOTO, RALPH LAZZARA, RABIH AZAR, STEPHEN HAMMILL, and GERALD NACCARELLI Epidemiology: Nora Goldschlager, Fred Kusumoto, and Rabih Azar Anatomy and Pathology: Saroja Bharati Basic Electrophysiology: Ralph Lazzara Etiology: Nora Goldschlager, Fred Kusumoto, and Rabih Azar Diagnostic Evaluation: Gerald Naccarelli Clinical Electrophysiology: Stephen Hammill Evidence-Based Therapy: Nora Goldschlager, Fred Kusumoto, and Rabih Azar In the early 1900s Keith and Flack identified the sinus due to the decrease in vagal tone that occurs with node as the region responsible for activation of the increasing age. heart.1 Laslett first suggested sinus node dysfunction as Sinus node dysfunction should be suspected when a a cause of bradycardia in 1909, and during the 1950s patient describes symptoms of fatigue, syncope or pre- and 1960s Short, Ferrer, Lown, and others described syncope, or exercise intolerance and is noted to have the clinical spectrum of sinus node dysfunction that is sinus bradycardia or pauses on the 12-lead ECG or dur- commonly called the “sick sinus syndrome resting sinus ing Holter monitoring. In general, symptomatic sinus bradycardia.”2-5 Sinus node dysfunction can have multi- node dysfunction increases with age, with the incidence ple electrocardiographic manifestations including sinus doubling between the fifth and sixth decades of life. pauses, the bradycardia-tachycardia syndrome, and inap- In one study of approximately 9000 patients visiting propriate sinus node response to exercise (chronotropic a regional cardiac center in Belgium, Kulbertus et al. incompetence). estimated that the incidence of sinus node dysfunction is less than 5 per 3000 people older than 50 years of age.9 However, sinus node dysfunction is more com- Epidemiology monly identified today because of an increased elderly population and increased physician awareness. In a It can be difficult to differentiate sinus node dysfunc- 1997 survey, sinus node dysfunction accounted for tion from physiologic sinus bradycardia in a specific approximately 50% of the 150,000 new pacemakers population. In a study of 50 young adult males, 24-hour implanted in the United States and 20% to 30% of the ambulatory electrocardiographic monitoring revealed 150,000 new pacemakers implanted in Europe.10,11 that 24% of the study group had transient heart rates Although more common in elderly patients, it is less than 40 beats per minute (bpm), pauses up to important to note that several specific younger patient 1.7 seconds while awake, and 2.1-second pauses while groups can also have sinus node dysfunction. First, asleep.6 Similarly, in a study of 50 young adult women, patients with congenital heart disease can have sinus pauses from 1.6 to 1.9 seconds were observed.7 In older node dysfunction. In 39 patients younger than 40 years asymptomatic individuals, transient heart rates less than of age who underwent pacemaker implant for sinus node 40 bpm and pauses of 1.5 to 2.0 seconds were observed dysfunction at the Mayo Clinic, 64% had associated in less than 2%.8 The decreased incidence of nocturnal congenital heart disease.12 The most common condition bradycardia in this normal elderly population is probably was transposition of the great arteries corrected by a 207 208 Cardiac Rhythms and Arrhythmias Mustard operation, since this procedure requires exten- surrounding atrial myocardial cells, and the striations sive atriotomies. Second, several familial forms of sinus are scarce but do increase with age. There are no inter- node dysfunction have been identified and account calated discs at the light microscopic level. The cells lie for approximately 2% of patients who present with in a massive amount of thick collagenous and thinner sinus node dysfunction.13-15 Bharati et al. described a elastic fibers.18-22 family with congenital absence of sinus rhythm. Several There are fewer myofibrils at the electron micro- other investigators have described different forms of scopic level as compared with the atrial cells. The sinus node dysfunction that appeared to be geneti- myofibrils are arranged in a disorganized pattern with a cally transmitted.13 Finally, sinus node dysfunction is lesser number of mitochondria, which are also disor- observed in approximately 4% of patients after cardiac ganized. The sarcoplasmic reticulum is poorly devel- transplant.16,17 Some, but not all, studies have found oped, and there is no transverse tubular system. There that a heart from a donor older than 40 years of age are very few fascia adherents and scant gap junctions, is associated with a higher incidence of sinus node and toward the periphery there are transitional cells, dysfunction requiring permanent cardiac pacing.16,17 which finally end in atrial cells.18-20 BLOOD SUPPLY Anatomy The SA node is supplied by way of ramus ostii cavae The sinoatrial (SA) node is situated in the sulcus ter- superioris (SA nodal artery) and reinforced by other minalis between the superior vena cava and the right atrial branches, and branches from the bronchial arter- atrial appendage in a somewhat curved fashion and ies. In addition, Kugel’s artery, a branch from the left extends from the hump of the atrial appendage, taper- coronary artery, also supplies the SA node. The ramus ing toward the inferior vena cava (Fig. 12-1A). The SA ostii cavae superioris originates in about 55% of cases node is a sizable structure, and although it is situated from the right coronary artery and in 45% of cases from epicardially, some of the fibers may extend deep the left coronary artery.18-22 There are nerve cells and intramyocardially. At the gross level, the SA node can- fibers evident in the periphery of the node. Nerve fibers not be dissected. It can be identified at the light micro- are present in the midst of the head of the SA node.18-23 scopic level; it consists of small fusiform cells, distinctly smaller than the surrounding atrial myocardial cells, PATHOLOGY arranged to a considerable extent along the line of the sulcus terminalis but also serpiginously surrounding The anatomic base of supraventricular arrhythmias may the SA nodal artery. The cytoplasm of the cells stain lie in the SA node and its approaches, as well as in the lighter than do the atrial myocardial cells (see Fig. 12-1B). atrial preferential pathways, the approaches to the atrio- The myofibrils are distinctly less prominent than the ventricular (AV) node, and the AV node. Although at SVC SA node R. atrium Intercaval band AB FIGURE 12-1 A, Diagrammatic sketch of location of sinoatrial (SA) node. R. atrium, right atrium; SA node, sinoatrial node; SVC, superior vena cava. B, Transverse section through the SA node and its approaches. Hematoxylin-eosin stain ×50. Arrows point to the SA nodal artery. A, atrial myocardium; SA, sinoatrial node. (From Lev M: The conduction system. In Gould SE (ed): Pathology of the Heart and Blood Vessels, 3rd ed. Springfield, Ill, Charles C Thomas, 1968.) Sinus Node Dysfunction 209 the moment there is no pathologic specificity for any one of the atrial arrhythmias, it would appear that the same type of pathology may be responsible for both bradyarrhythmias and tachyarrhythmias on different occasions.23 Rarely, sinus node dysfunction can be familial13 (Fig. 12-2). SICK SINUS SYNDROME IN THE YOUNG Although sick sinus syndrome is usually seen in the elderly, it may be seen in the young as well. Pathologically, a marked increase in connective tissue is found, with fat in the approaches to the SA node, the atrial preferen- R tial pathways, and approaches to the AV node. The pen- etrating AV bundle may be septated. In other cases an M increased amount of fat in other parts of the atria, as F well as around the SA and AV nodes, is seen.24 SICK SINUS SYNDROME IN THE ELDERLY-AGING In older patients with sick sinus syndrome, significant coronary artery disease is usually present; however, the basic pathology is variable. An abnormal formation of the atrial septum, resulting in fibrosis in the approaches to the SA and AV nodes and atrial preferential pathways, may be seen, or there may be similar findings within these structures along with fat. With advancing age, FIGURE 12-2 Histologic characteristics of the sinoatrial (SA) nodal area in a 72-year-old male member of a family with there may be loss of cells to a varying extent in the congenital absence of sinus rhythm and a tendency to SA and AV nodes and their approaches and in the develop atrial fibrillation at an early age who collapsed rising surrounding atrial myocardium, with replacement by from a chair and could not be resuscitated. He had a history fat and fibrosis, associated with varying degrees of focal of atrial fibrillation, at first paroxysmal and later chronic for hypertrophy or atrophy of myocardium (Fig. 12-3). In 18 years, but was only mildly incapacitated. Note the frag- addition, arteriolosclerosis, necrosis, or chronic inflam- mented SA node. Weigert-van Gieson stain ×30. F, fat; mation may be present in the atria.25-27 M, atrial myocardium; R, remnant of SA node. (From Bharati S, Surawicz B, Vidaillet, HJ, Lev M: Familial congenital sinus rhythm anomalies, clinical and pathological correlations. SENILE AMYLOIDOSIS Pacing Clin Electrophysiol 1993;15:1720-9). This is a type of primary amyloidosis seen in patients older than 80 years of age. It affects the atrial FIGURE 12-3 Fat accumulation around the SA node and replacement of SA node by fat with almost total separation of SA node from its approaches in an elderly patient with sick sinus syndrome. Weigert-van Gieson stain ×45. F, fat; N, SA node; RA, right atrium. Arrows point to the isolated SA node with marked fat in and around the node.