AUTHOR AND EDITOR INFORMATION

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• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
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INTRODUCTION

Section 2 of 11  

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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

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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.

DIFFERENTIALS

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Other Problems to be Considered

• Recording equipment problems (ie, intermittent recording of monitor leads)
• Blocked premature atrial contractions (PACs) that are concealed in the preceding T wave
• Concealed premature junctional extrasystoles
• First-degree AV block in which a prolonged conduction is present from the sinus node (SN) through the atrial tissue but the electrical impulse is not blocked (usually a benign and asymptomatic condition)
• Vasovagal syncope
• Other causes of syncope in children, such as a breath-holding spell, a hypoxic spell, aortic stenosis, cerebral and spinal chord disorders (eg, trauma, polyneuropathies, migraine, inner ear/vestibular dysfunction, micturition syncope), subclavian artery steal, hypoglycemia, drugs, hyperventilation, and hysterical reactions
• Drugs such as beta-blockers and calcium channel blockers, which can suppress SN function, causing marked SN bradycardia and prolonged SN recovery time (SNRT) and sinoatrial conduction time (SACT)

WORKUP

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Imaging Studies

• Echocardiography: This study is useful for the evaluation of ventricular function and in patients with coexistent CHD for the assessment of associated anatomic and hemodynamic abnormalities.

Other Tests

• Electrocardiography: Impulses originate from the sinus node (SN), with a P-wave axis between +0º and +90º. A surface ECG may reveal the following:
• • Moderate-to-severe bradycardia (for age) occurs.
  • Severe sinus arrhythmia is indicated by variation from the R-R interval by 100% or more. Bradycardia may alternate with paroxysmal atrial tachyarrhythmias (ie, bradycardia-tachycardia syndrome).
  • Sinus pause or arrest occurs when the SN fails to generate an impulse.
  • Second-degree sinoatrial (SA) block type I is known as Wenckebach SA block and occurs when prolonged conduction of the SN impulse through the atrial tissue is present without actual block in the AV node (all QRS complexes are preceded by P waves). The P-P intervals shorten until block occurs (ie, while the SN-to-SN interval is constant, the SN–to–high right atrium [HRA] interval lengthens until an SN impulse is not followed by a P wave). The surface ECG shows progressively shortening P-P intervals until the SN impulse is dropped; it also shows P-P intervals that are less than twice the preceding (normal) P-P intervals.
  • Long pauses follow PACs. Sinoatrial conduction time can be directly measured with EP testing when PACs are present. In sinus node dysfunction (SND), the prolonged P-P interval is not a multiple of the sinus (normal) P-P interval. Pauses in SND occur at the end of lengthening P-P intervals rather than at the end of shortening P-P intervals (which is observed in second-degree SN exit block type I).
  • In second-degree sinoatrial block type II, the long P-P interval is a multiple of the sinus P-P interval.
  • In third-degree SN exit block, impulse generation in the SN is normal; however, no conduction to the RA occurs. Diagnosis is confirmed only by EP (intracardiac) study. An atrial, junctional, or ventricular escape rhythm is present.
  • Escape rhythm at slow rates occurs after a prolonged sinus arrest.
  • • The escape rhythm may originate in the atria, such as an ectopic right or left atrial rhythm, or it may originate from multiple foci in the atria as is observed in wandering atrial pacemaker, in which the P-wave axis changes on the same ECG recording.
    • An escape rhythm may also originate below the atria at the His-Purkinje junction (ie, junctional escape rhythm, with a rate of 60-80 min in infants and 50-70 min in children) or lower if originating in the ventricles (ie, ventricular escape rhythm).
    • The further below the atria the escape rhythm originates, the slower the rate.
    • Escape rhythms, which are those that occur by default, should be distinguished from usurpation rhythms, which are those that occur because of increased automaticity from other pacemakers that fire at a faster rate than the SN.
  • Bradyarrhythmias-tachyarrhythmias occur when bradycardia and tachycardia alternate. The bradycardia may originate in the sinus, atria, AV junction, or ventricle, whereas the tachycardia is usually caused by atrial flutter or fibrillation and is less commonly caused by reentrant supraventricular tachycardia (SVT) in the SN or atrial muscle.
• Holter monitoring
• • Recording the ECG for 24-48 hours is useful to assess SND related to the previously explained ECG findings that may be present.
  • The specificity of a direct observation of spontaneous (ie, not provoked by EP study) SND is 100%, and an EP study is not required. Therefore, cardiac monitoring and not EP study is the method of choice to assess SND.
  • A 24-hour Holter study also has the advantage of revealing whether SND produces symptoms such as dizziness, presyncope, or syncope, which cannot be determined during an EP study because the patient is heavily sedated. Therefore, a 24-hour Holter study can help decide if pacemaker therapy is required.
• Exercise stress test
• • Because the SN usually responds to autonomic nervous system input, exercise increases the heart rate in response to increased sympathetic tone.
  • Patients with SND usually have a blunted response. Therefore, an exercise stress test can determine whether chronotropic incompetence is present.
• Autonomic tone assessment
• • The influence of autonomic tone on the SN can be studied by measuring heart rate and assessing heart rate variability responses to changes in autonomic tone. This is done by evaluating the response of the SN to atropine, isoproterenol, and propranolol.
  • Patients with SND usually exhibit a blunted response to atropine (ie, failure to increase the heart rate 20-50% above the control rate).
  • Patients with SND who are given isoproterenol usually fail to produce an SN acceleration of at least 25% more than the basal rate. However, their response to this medication is occasionally normal.
  • Patients with SND respond to propranolol in the same way as patients with normal SNs (ie, increase of 12-22% in sinus cycle length). This suggests that sympathetic response in SND is normal.
  • Pharmacologic denervation with atropine and propranolol is used to determine intrinsic heart rate (IHR). Patients with SND have no increase in IHR, similar to patients with intact SNs. IHR depends on the patient's age and is calculated using the following equation in patients aged 15-70 years: IHR = 117.2 - (0.57 X age) bpm.
• Transesophageal atrial pacing: Esophageal EP study constitutes a relatively safe and inexpensive method to detect SND by determining SN recovery time in patients who present with dizziness or syncope and palpitations.

Procedures

EP studies are indicated in patients with signs of bradyarrhythmias (mainly syncope) in whom bradycardia could not be documented during Holter monitoring.

• SN recovery time
• • EP studies can document SND when studying SN automaticity by directly recording its electrical activity.
  • One EP catheter, which has 2 proximal electrodes that record the HRA electrogram and 2 distal electrodes to pace the HRA near the SN, is positioned in the RA.
  • A second EP catheter, which is used to record low RA (LRA) electrical activity, is positioned across the tricuspid valve.
  • Measurement of SNRT is achieved by pacing the atrium. Pacing should be performed in the HRA near the SN at the junction of the SVC and the RA for 4 -6 trials of 30 seconds each. Each trial should use successively shorter pacing cycle lengths (eg, 600 ms, 550 ms, 500 ms), beginning with a cycle length just shorter than the resting sinus cycle length.
  • SNRT is the time interval between the last paced captured beat to the first spontaneous sinus beat.
  • Gradual return of SN to its baseline rate occurs over 5-6 beats. Prolonged pauses (ie, secondary pauses) can occur after the initial recovery interval in SND.
  • If the longest interval for the recovery interval or secondary pause exceeds 1500 milliseconds, the SNRT is prolonged.
  • To adjust for HR and before each pacing increase, the resting sinus cycle length (SCL) is measured. This resting SCL is subtracted from SNRT, and the CSNRT is obtained. Its upper reference range limit is 525 milliseconds; if the SNRT exceeds the SCL by more than 525 milliseconds, the SNRT is abnormal. The same occurs if the ratio of SNRT to SCL (ie, SNRT/SCL X 100) is more than 160%.
• Sinoatrial conduction time
• • SACT is another parameter to assess SN function. It is the time interval in milliseconds for an impulse that originates in the SN to conduct through the perinodal tissue to the adjacent RA tissue.
  • The tissue that surrounds the SN or perinodal tissue has similar characteristics as those of the AV node.
  • Eight PACs are fired in the HRA at 5-10 bpm faster than the SN rate before they are stopped abruptly.
  • SACT represents the time in milliseconds it takes for the PAC fired in the HRA to enter and reset the SN. It also represents the time for the new spontaneous SN impulse (ie, SCL) to reach the HRA.
  • SACT is measured as the time in milliseconds from the last PAC to the first spontaneous sinus beat.
  • When the time interval between the last spontaneous SN depolarization (ie, before the PAC) and the one that occurred after a PAC is less than twice the value of the 2 previous spontaneous SN depolarizations, reset of the SN by the PAC has occurred.
  • SACT can be calculated as the interval from the PAC to the next spontaneous SN beat, which includes conduction through the perinodal tissue into the SN, resetting the SN, and conduction through the same perinodal tissue back into the HRA (ie, [return interval - SCL]/2). The reference range is 50-125 milliseconds in children and 200-250 milliseconds in adults.
  • If the SN cannot produce a spontaneous impulse following PACs (ie, these have not reset the SN, and, therefore, SACT cannot be calculated), sinoatrial entrance block is present.
  • These could be caused by markedly prolonged sinoatrial conduction and/or increased refractory period of peri-SN or SN fibers, both of which indicate SND.
  • Alternating SN entrance block with reset responses also denotes SND.
  • SN reentry tachycardia occurs when activation of the atrium during the SVT is the same as sinus beats (ie, P-wave axis and morphology are the same as those in sinus rhythm). It is usually indicative of SND.
• Complications: Complications from a diagnostic EP study are rare but may include the following:
• • Hematoma at the puncture site in the groin and or neck
  • Hemorrhage
  • Infection caused by manipulation of catheters inside the heart (theoretical risk)
  • Perforation upon catheter manipulation inside the heart of small patients (most commonly involving the right atrial appendage and the right ventricular outflow tract)

TREATMENT

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Medical Care

No treatment is required for asymptomatic patients, even if they have abnormal SNRTs or SACTs. If the patient is receiving medications that can provoke sinus bradyarrhythmias (eg, beta-blockers, ACE inhibitors), the medications should be stopped if possible.

• Acute treatment consists of atropine (0.04 mg/kg intravenously [IV] every 2-4 h) and/or isoproterenol (0.05-0.5 mcg/kg/min IV).
• A transvenous temporary pacemaker sometimes is required despite medical therapy.
• In patients with bradyarrhythmias-tachyarrhythmias, the tachyarrhythmias may be controlled with digoxin, propranolol, or quinidine. However, these patients should be monitored closely with frequent Holter monitoring to ensure that the bradyarrhythmias are not exacerbated or causing symptoms (eg, dizziness, syncope, CHF); if this is the case, permanent pacemaker therapy is also required.

Surgical Care

Implantation of a permanent pacemaker is required if bradycardia is severe for the patient's age or if SND is accompanied by dizziness, fatigue, CHF, chest pain and palpitations (in patients with bradyarrhythmias or tachyarrhythmias), and, especially, syncope.

• Sinus node dsyfunction (SND) is frequently encountered in patients who have undergone Fontan palliation for single ventricle; such patients are often either in junctional rhythm or may have ventricular pacemaker systems for bradycardia. Because ineffective atrial kick with loss of AV synchrony is known to decrease cardiac output in patients with a single ventricle physiology, atrial pacing (if AV node function is adequate) or dual-chamber pacing has been advocated as a better option than ventricular pacing alone.
• Patients with CHD and postoperative SND who require a more physiologic rhythm can benefit from antitachycardia pacemakers capable of delivering atrial pacing and antitachycardia-pacing therapies.⁴ Interestingly, young patients show circadian variability of the atrial threshold, with higher thresholds during night time.⁵

Diet

Patients with vasovagal syncope may require increased dietary salt intake.

Activity

Patients may participate in activities as tolerated.

MEDICATION

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Pharmacotherapy may be warranted as acute treatment or adjunctively with implanted pacing devices. Patients with tachyarrhythmias may require treatment with antiarrhythmic agents. Those with increased parasympathetic tone may experience vasovagal syncope, requiring pharmacologic intervention.

Drug Category: Anticholinergic agents

Atropine depresses the vagus, thereby increasing the HR. This agent is used in various disorders or circumstances in which bradyarrhythmias occur. It is frequently used in sudden-onset bradyarrhythmias; although it may also be used for the initial treatment of chronic arrhythmias, cardiac pacing is preferred for long-term control.

Drug Category: Adrenergic agonists

When given systemically, isoproterenol stimulates beta receptors in the heart, which produces positive inotropic and chronotropic effects. This results in increased cardiac output.

Drug Category: Antiarrhythmic agents

These agents alter the EP mechanisms responsible for arrhythmia. In sinus node dysfunction (SND), they may be used when tachyarrhythmias occur. Patients must be carefully monitored to ascertain if bradyarrhythmia is exacerbated.

Drug Category: Mineralocorticoids

These agents are used to treat syncopal episodes caused by fluid or electrolyte imbalances. They restore fluid and electrolyte balance by enhancing sodium reabsorption in the kidney, which results in expanded extracellular fluid volume. They increase renal excretion of potassium and hydrogen ions.

FOLLOW-UP

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Complications

• Symptomatic patients with sinus node dysfunction (SND) may develop syncope, exercise intolerance, and cardiac dysfunction due to bradycardia and loss of AV synchrony.
• These patients can also develop atrial tachyarrhythmias, such as atrial flutter or fibrillation.
• Treatment of symptoms is achieved with the implant of an atrial pacemaker to provide atrial rate support. This prevents symptoms related to bradycardia from occurring. In patients with atrial tachyarrhythmias, it is a useful adjunct to antiarrhythmic therapy.
• After undergoing a Mustard procedure for transposition of the great arteries or a Fontan procedure for single ventricle, many pediatric patients who have SND and frequent episodes of atrial flutter or fibrillation currently receive an intracardiac defibrillator (ICD) device. The ICD lead is placed in the RA. When the atrial rate exceeds the programmed value, the ICD assumes that the patient has an atrial tachyarrhythmia and provides a shock of 10-30 J.

Prognosis

• Symptomatic patients with normal systemic ventricular function and SND have an overall good prognosis with atrial (rate-responsive) pacing.
• The overall prognosis in patients with SND and additional systemic ventricular dysfunction (eg, numerous postoperative Mustard and Fontan patients) depends on their underlying ventricular dysfunction or degree of CHF.
• A study has shown that in those patients who have undergone a Fontan surgery and developed SND, endocardial atrial leads can be implanted relatively safely and permit low-energy thresholds as long as 5 years after implant.⁶

Patient Education

• Because most of the pediatric patients with SND have already received surgery for CHD (eg, Mustard procedure, Fontan procedure), their education is focused on recognizing symptoms of CHF and tachyarrhythmias, such as atrial flutter/fibrillation, which are usually poorly tolerated.
• Patients who are on antiarrhythmic medication for atrial flutter or fibrillation should be instructed to take their medications regularly and to visit the cardiologist as scheduled. They should also be cognizant of adverse effects and toxicity of the medication.
• In patents who have already received a Mustard or Fontan procedure, undergoing yearly echocardiography to monitor cardiac function is advisable. If cardiac function is decreased, anti-CHF management should be started and close follow-ups with the cardiologist are advisable.
• Patients who have a pacemaker should be instructed about how to have regular checks. This is usually achieved from home with a transtelephonic monitor that transmits to a central monitoring station, which in turn, contacts the cardiologist in case a problem is detected (eg, device malfunction, arrhythmia).
• Patients who have an ICD device should receive the same instructions that are given to patients who have pacemakers. Because these patients often are placed on antiarrhythmic medication, they also should receive instruction regarding medication schedules and information about adverse effects and toxicity. In addition, in cases of frequent atrial flutter or fibrillation episodes, which are followed by a shock from the ICD, patients are instructed to avoid activities that may pose a risk to themselves and/or other people (eg, driving) and are instructed when to go to the cardiologist or the emergency department.
• For excellent patient education resources, please refer to eMedicine's Heart Center and, for information specific to arrhythmias, see the eMedicine article Heart Rhythm Disorders.

MISCELLANEOUS

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Medical/Legal Pitfalls

• Because sinus node dysfunction (SND) in the pediatric population mainly affects those who have undergone a Mustard or intracardiac Fontan operation for CHD, failure to recognize or treat SND may lead to CHF caused by bradycardia and/or atrial tachyarrhythmias. Both of these complications may cause sudden cardiac death caused by cardiac arrest in the case of bradycardia or by rapid ventricular response with development of ventricular fibrillation in the case of tachyarrhythmias.
• Patients with SND should be differentiated from patients with normal SN and vasovagal syncope who have an increased parasympathetic tone with periods of marked bradycardia and even asystole that lasts several seconds. In the latter group, the high parasympathetic tone is accompanied by marked vasodilatation with consequent hypotension, which causes the syncopal event. SACT and SNRT are usually within the reference range, as opposed to what is observed in patients with SND. Treatment for this condition consists of hydration and increasing the amount of salt in the diet; treatment for refractory cases may include fludrocortisone acetate (Florinef), beta-blockers, alpha-agonists, or selective serotonin reuptake inhibitors.

Special Concerns

• Patients with SND who become pregnant and take antiarrhythmic medications should have their levels measured because they frequently require adjustment. In addition, medication with teratogenic effects (eg, amiodarone, which is associated with fetal thyroid dysfunction) should be replaced, if possible.
• Patients with SND who become pregnant and have a pacemaker are advised to perform frequent pacemaker checks and make the appropriate adjustments (especially when an increase in the lead's threshold is noted).
• Patients with SND who become pregnant and have ventricular dysfunction due to cardiomyopathy or a Mustard or Fontan procedure should have regular and close medical follow-ups with the obstetrician and the cardiologist. This permits appropriate adjustment and implementation of anti-CHF medication. If the CHF progresses despite medical management and becomes intractable, the mother and fetus are at risk, for which early delivery may be scheduled.

MULTIMEDIA

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Media file 1:  This 12-lead ECG is from an asymptomatic 10 year-old girl, which was brought to our attention because of the irregularity of the P-P intervals. This ECG shows sinus arrhythmia at a rate of 65-75 beats per minute. The P waves all originate from the sinus node because they have a positive axis (upright) in leads I, II, and aVF. The PR interval is 104 milliseconds, and the QRS is narrow at 86 milliseconds, with a normal axis of 64°. The corrected QT (QTc) interval measures 402 milliseconds. Therefore, this is a normal ECG.
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Media type:  ECG

Media file 2:  Below is an ECG of a 2-year-old girl who was referred to the clinic by the pediatrician for evaluation of a heart murmur. This ECG shows atrial rhythm originating most likely from the lower left atrium (P waves are inverted in lead I and are positive in II and aVF, with a frontal axis of 124°). The PR interval measures 113 milliseconds, and the QRS is narrow at 90 milliseconds. Right ventricular conduction delay is shown, which is best seen in the precordial leads V1 and V2. The QRS frontal axis shows right axis deviation (reference range for a 2-year-old child is 0-110°). The patient does not have right ventricular hypertrophy by voltage criteria. The inverted T waves in V1 are a normal finding at this age. An echocardiogram showed a moderately sized atrial septal defect. Nonsinus atrial rhythm is not a synonym of sinus node dysfunction.
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Media type:  ECG

Media file 3:  This is a 12-lead ECG from a 12-year-old boy with history of syncope. This patient was healthy until 1 month earlier, when he started to experience episodes of lightheadedness. The ECG shows sinus arrhythmia (bradycardia) at a rate of 50-79 beats per minute with a PR interval of 136 milliseconds. Two junctional escape beats are present after a prolonged pause. The QRS is narrow at 85 milliseconds with a normal frontal axis of 70°. The corrected QT interval (QTc) is 411 milliseconds. A later electrophysiologic (EP) study showed prolonged sinus node recovery time (SNRT) and sinoatrial conduction time (SACT). Because of the patient's symptoms and his sinus node dysfunction, he received an atrial pacemaker. If this 12-lead ECG had been recorded from an asymptomatic patient, the findings would be considered within normal limits and no further workup would be indicated. In this case, the lightheadedness and, ultimately, the syncope define sick sinus syndrome, with the patient requiring pacemaker therapy.
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Media type:  ECG

REFERENCES

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1. Sumitomo N, Karasawa K, Taniguchi K, et al. Association of sinus node dysfunction, atrioventricular node conduction abnormality and ventricular arrhythmia in patients with Kawasaki disease and coronary involvement. Circ J. Feb 2008;72(2):274-80. [Medline].
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Article Last Updated: Jul 25, 2008