eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Cardiology

Sinus Node Dysfunction

Author: M Silvana Horenstein, MD, Associate in Pediatric and Fetal Cardiac Diagnostic, Diagnostico Gineco-Obstetrico, PC; Associate Director, Legacy Department, Best Doctors, Inc
Coauthor(s): Peter P Karpawich, MD, Professor of Pediatric Medicine, Department of Pediatrics (Cardiology), Wayne State University School of Medicine; Director, Cardiac Electrophysiology and Pacemaker Services, Children's Hospital of Michigan
Contributor Information and Disclosures

Updated: Jul 25, 2008

Introduction

Background

The sinus node (SN) is a subepicardial structure normally located in the right atrial wall near the superior vena cava entrance on the upper end of the sulcus terminalis. It is formed by a cluster of cells capable of spontaneous depolarization. Normally, these pacemaker cells depolarize at faster rates than any other latent cardiac pacemaker cell inside the heart. Therefore, a healthy SN directs the rate at which the heart beats. Electrical impulses generated in the SN must then be conducted outside the SN in order to depolarize the rest of the heart. SN activity is regulated by the autonomic nervous system. For example, parasympathetic stimulation causes sinus bradycardia, sinus pauses, or sinoatrial exit block. These actions decrease SN automaticity, thereby decreasing the heart rate.

Sympathetic stimulation on the other hand, increases the slope of phase 4 spontaneous depolarizations. This increases the automaticity of the SN, thereby increasing the heart rate. Blood supply to the SN is provided by the right coronary artery in most cases.

SN dysfunction (SND) constitutes an important cause of morbidity in patients who have undergone surgery for congenital heart disease (CHD). It is also commonly seen in elderly persons who have normal cardiac anatomy.

SND may manifest as abnormal SN impulse formation and/or propagation, which leads to rhythms that are slow (ie, bradyarrhythmias) or fast (ie, tachyarrhythmias) for the person's age. SND is referred to as "sick sinus syndrome" when the SND is accompanied by symptoms such as dizziness or syncope.

ECG criteria for SND apply to the presence of one or more of the following:

• Sinus bradycardia below the heart rate expected for age (ie, <100 beats per minute [bpm] in an infant, <80 bpm in a preschool child, <60 bpm in a school child, <50 bpm in an adolescent)
• Sinus pause or absence of an expected P wave for more than 3 seconds, which may be due to sinus arrest (failure of the SN pacemaker cells to depolarize) or due to sinoatrial exit block (depolarization of the SN but failure to conduct to the atria)
• Slow escape rhythms that originate within the atria, His bundle, or ventricles
• Marked sinus arrhythmia with constant variation in the P-P interval, which is likely to be accompanied by sinus bradycardia
• Presence of both bradyarrhythmias and tachyarrhythmias (ie, SN reentry tachycardia, atrial tachycardias from an ectopic focus, atrial flutter, atrial fibrillation)

Classic electrophysiologic (EP) criteria for SND apply to the presence of one or more of the following:

• Corrected SN recovery time (CSNRT) greater than 275 milliseconds
• Sinoatrial conduction time greater than 200 milliseconds
• Sinoatrial node arrest
• Sinoatrial exit block
• SN reentry tachycardia

However, the current recommendation is that the diagnosis should rely on noninvasive methods rather than measuring SN recovery time or sinoatrial conduction time in the EP laboratory because results can be normal despite the patient having symptoms of SND or vice versa. The most reliable noninvasive methods to diagnose SND are 24-hour Holter monitoring (which may show one or more of the ECG criteria already mentioned) and exercise testing (which may reveal chronotropic incompetence). 

Pathophysiology

The acquired form of SND may occur after damage to the SN artery during cardiac surgery or may be due to occlusion, such as after myocardial infarction. In the pediatric population SND and atrioventricular (AV) block have been found to occur more frequently in patients with Kawasaki disease with moderate to severe coronary artery disease than in the general population. This is believed to be secondary to myocarditis or abnormal microcirculation in the SN artery and the AV-node artery.¹

The idiopathic form of SND is degenerative, with fibrosis and fatty infiltration of the SN and consequent decrease of functional nodal cells.

Clinical

History

In children with no structural heart anomalies, sinus node dysfunction (SND) is less common. These children are usually asymptomatic. Otherwise, they may report dizziness, fatigue, exercise intolerance, syncope, or shortness of breath with or without palpitations. Infants may present with poor feeding or easy fatigability, which may also be evident in toddlers and older children.

Physical

Older patients with SND and no structural congenital heart defects may be asymptomatic in general, despite being bradycardic for their age. Conversely, they may present with signs and symptoms of congestive heart failure (CHF), especially those who have undergone a Mustard operation for transposition of the great arteries or a Fontan operation for a single ventricle. Infants with CHF secondary to SND may be tachypneic and may have signs of pulmonary congestion on auscultation (ie, wheezing and rales) and hepatomegaly. In addition, they may show evidence of failure to thrive, with weight below the fifth percentile.

Causes

The etiology for most cases is unknown. Causes can be categorized as follows:

• Nonsurgical causes
  • SND may be familial. An autosomal dominant pattern of inheritance has been described.
  • Emery-Dreifuss muscular dystrophy is an X-linked muscle disorder associated with SND and AV conduction defects. If AV conduction defects are present, sudden cardiac death may result unless treated with permanent pacing. Males and females may be affected with equal frequency.
  • SND may be idiopathic. However, this form is rarely seen in children, especially in those without CHD or previous heart surgery.
  • SND may be caused by CHD, independent of the effects of surgical procedures. Examples include sinus venosus atrial septal defect (ASD), Ebstein anomaly, and heterotaxy syndromes, particularly left atrial isomerism.
  • Inflammatory diseases (eg, myocarditis, pericarditis, rheumatic fever) can result in SND.
  • Animal models of congenital complete AV block have shown that maternal immunoglobulin G (IgG) inhibits SN pacemaker cells, as well as AV node cells.
  • SND results from inactivation of genes that code for units of channels that play a key role in the regulation of cardiac pacemaker activity:
    • Mutations in the cardiac ion channel gene SCN5A causes SND as well as other cardiac arrhythmias, including long QT syndrome, Brugada syndrome, idiopathic ventricular fibrillation, and progressive cardiac conduction disturbances.
    • Loss of function of a pacemaker channel that plays a key role in the automaticity of the SN has been associated with mutations in the HCN4 gene.
    • Animal studies have shown that increased expression of A(3)AR adenosine receptors in mice resulted in AV block and pronounced SND in vivo.² Also, homozygous mutant mice with a defect of klotho gene expression (kl/kl) developed sudden death under stress, associated with SND (conduction block or arrest).
  • SND may be caused by CNS disease, which is usually secondary to increased intracranial pressure with subsequent increase in the parasympathetic tone.
  • SND may be secondary to antiarrhythmic drugs (eg, digitalis [because of SN exit block], propranolol, verapamil, quinidine, procainamide, lidocaine, disopyramide, reserpine).
  • Hypothyroidism may cause SND.
  • Hypothermia may cause temporary SND.
• Surgical causes, especially with operations involving the right atrium (RA)
  • Gradual loss of sinus rhythm occurs after the Mustard, Senning, and all varieties of the Fontan operation. This is thought to be secondary to direct injury to the SN during surgery and also due to later chronic hemodynamic abnormalities. Paroxysmal atrial tachycardias are frequently associated with SND, and loss of sinus rhythm appears to increase the risk of sudden death. Patients with transposition of the great arteries now undergo the arterial switch operation, which avoids the extensive atrial suture lines that lead to SN damage.
  • SND was described in 15% of patients who had undergone the Ross operation for aortic valve disease or complex left-sided heart disease 2.6-11 years earlier.³ Other arrhythmias, such as complete AV block and ventricular tachycardia, were present as well after the Ross operation.
  • When repairing ASDs, especially sinus venosus ASDs, SND frequently occurs because of the proximity of the defect with SN tissue.
  • Patients who have undergone surgery for endocardial cushion defects (ECDs) may also have SND later.
  • SND may be caused by a Blalock-Hanlon atrial septectomy.
  • SND may occur after repair of partial anomalous pulmonary venous return (PAPVR) or total anomalous pulmonary venous return (TAPVR).
  • Cannulation of the superior vena cava (SVC), usually performed for cardiopulmonary bypass or extracorporeal membrane oxygenation (ECMO), may damage SN tissue.
  • Ischemic cardiac arrest may cause SND.

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