I. Sick Sinus Syndrome: What every physician needs to know.

Sick sinus syndrome (SSS), more well-known as sinus node dysfunction (SND), is a term that includes a variety of cardiac arrhythmias that have as their basis an abnormality of initiation or propagation of the sinus node impulse. SND may be categorized as intrinsic or extrinsic.

Intrinsic SND is due to either abnormalities of the sinus node or contiguous atrial tissue. The etiology is usually degeneration with scarring of the sinus node and perinodal atrial tissue.

SND may also arise from other conditions associated with degeneration or destruction of these structures, including myocardial infarction, restrictive or congestive cardiomyopathies, and some cardiac surgeries most notably for correction of congenital heart disease. Extrinsic SND results from other causes (thyroid disorders, drugs or drug interactions, disorders of or affecting the autonomic nervous system, or extreme body temperatures).

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II. Diagnostic Confirmation: Are you sure your patient has Sick Sinus Syndrome?

The diagnosis is made electrocardiographically. SND may manifest with atrial bradyarrhythmias, tachyarrhythmias, or a combination of the two.

It is commonly accepted that ambulatory sinus pauses of greater than 3 seconds in duration are rare and may indicate SND. Sinus pauses may be due to either sinus arrest or sinus exit block.

Other rhythms associated with or indicative of SND are severe sinus bradycardia, paroxysmal atrial tachyarrhythmias (classically with associated bradyarrhythmias with or without posttachyarrhythmia conversion pauses, the so-called tach-brady syndrome), and chronic atrial fibrillation with slow ventricular rates in the absence of AV nodal blocking pharmacologic therapy).

Chronotropic incompetence is another manifestation of SND. It is the inability to increase heart rate with exercise.

There exist varying quantitative criteria for defining chronotropic incompetence. One is the inability to achieve a maximal exercise heart rate of 100 bpm. Another is the inability to achieve 80% of the predicted maximal heart rate when adjusted for age.

SND may first manifest after initiation of antiarrhythmic drugs. Sometimes the SND is solely due to the drug but other times may unmask or exacerbate intrinsic SND.

If the first manifestation of the SND is with pharmacologic therapy that may be discontinued or there exists an alternative drug, cessation of the drug and observation with prolonged monitoring may be appropriate to differentiate between intrinsic and extrinsic SND.

Usually there is underlying (intrinsic) physiologic substrate for SND if pharmacologic therapy causes SND, though it might not be clinically apparent or significant. Symptoms may or may not be present.

The absence or presence of symptoms is not necessary for the diagnosis of SND, though it is the determinant of therapeutic recommendations. Extrinsic causes are addressed with assessment of medications, blood tests (for thyroid abnormalities), and evaluation for associated autonomic dysfunction, if suspected.

A. History Part I: Pattern Recognition:

The typical patient with SND presents with arrhythmias detected via electrocardiogram (ECG), ambulatory monitoring 24-hour recording, prolonged electrocardiographic event monitoring, or outpatient telemetry recordings (see diagnostic tests below).

The recordings may have been prompted as a part of the routine exam (ECG), or in the case of monitoring, prompted by findings found on a 12-lead ECG or by symptoms. Symptoms are diverse, corresponding to a broad range of electrocardiographic manifestations.

They may include palpitations (due to PACs, paroxysmal atrial tachycardias, or atrial fibrillation), fatigue (from bradycardia or chronotropic incompetence), light-headedness, or syncope (from pauses). More unusual symptoms may include cognitive abnormalities, periodic polyuria (related to atrial distention with release of atrial natriuretic peptide experienced with or immediately after paroxysmal atrial tachycardia or atrial fibrillation), and thromboembolic phenomena (from bradycardia-induced hypoperfusion of a stenotic vessel or embolic related to atrial fibrillation).

As many times symptoms are present without electrocardiographic findings and nonspecific electrocardiographic findings present without symptoms, the correlation of electrocardiographic abnormalities with symptoms is paramount in establishing a causal relationship to best determine the most appropriate therapeutic approach.

B. History Part 2: Prevalence:

The prevalence of SND is not known because so many patients are asymptomatic. In addition, the criteria for defining a heart rate or pause as abnormally slow are more dependent on the association of symptoms than an arbitrary number.

SND is usually diagnosed in the seventh and eighth decades of life, with an average age of 65 in a few studies. There are rare familial cases of SND that may manifest in younger patients, some of which may be associated with long Q–T syndrome.

C. History Part 3: Competing diagnoses that can mimic Sick Sinus Syndrome.

The differential diagnosis of SND includes chronically high vagal tone [usually manifest as chronic sinus bradycardia, which may be marked, or sinus pauses during sleep, which may be accompanied by varying degrees of second-degree atrioventricular (AV) block (with or without AV dissociation)].

These patients are asymptomatic, are often young, and are usually in excellent physical condition and athletic. Thyroid disease must be ruled out because it requires specific treatment, which generally results in resolution of the arrhythmia problem.

D. Physical Examination Findings.

Physical examination is rarely helpful in establishing a diagnosis. A resting bradycardia on examination is usually not associated with symptoms in patients with SND.

E. What diagnostic tests should be performed?

Correlation of diagnostic electrocardiographic abnormalities is what is necessary to best guide management decisions. The resting electrocardiogram is usually unrevealing or demonstrates nondiagnostic abnormalities without symptoms.

The next step should be to obtain ambulatory monitoring. This may be a 24-hour test (which has a very low yield but may be mandated by the insurer prior to longer term monitoring).

Interestingly, up to 15% of patients will experience symptoms without arrhythmias, providing equally useful information. If the 24-hour monitoring is unrevealing, or if symptoms occur on a less frequent than daily basis, then a prolonged monitor with an event recorder or outpatient telemetry is warranted.

Cardiac event recorders in a well-defined population (recurrent syncope and/or presyncope within a 1-month period) may be very useful. The yield may be as high as 25%.

Symptoms may be associated with either tachyarrhythmias, bradyarrhythmias, or both. If symptoms are less frequent or the test unrevealing, an implantable loop recorder is a diagnostic option. The device is activated to store recordings either automatically for detected significant brady or tachyarrhythmias, or patient activated with symptoms.

If the presenting symptom is syncope in a patient who is suspected of having SND with an otherwise unrevealing workup, then consideration should be given to invasive testing with an intracardiac electrophysiology test. This is an insensitive but specific test.

The sinus node recovery time (SNRT) is usually determined by pacing the atrium at decremental cycle lengths from 600 to 350 msec and measuring the return interval, which is then corrected for the patient’s resting heart rate. Corrected values under 525 msec are regarded as normal and greater than 1 second as markedly abnormal.

A secondary pause (pause occurring after the first return sinus complex) is a more sensitive indicator of SND. The positive yield of EP testing is approximately 5%. The test is highly insensitive, and a negative test does not rule out sinus node disease. Rarely induction of clinically relevant tachyarrhythmias is seen.

1. What laboratory studies (if any) should be ordered to help establish the diagnosis? How should the results be interpreted?

Laboratory studies indicated in patients presenting with symptoms and suspected of possibly having SND (with or without electrocardiographic abnormalities at the time of initial visit) should include a complete blood count, electrolytes, renal function tests, liver function tests, and serum thyroid studies. These tests are performed to diagnose secondary causes of SND or causes of the presenting symptoms, rather than making a diagnosis of intrinsic SND.

2. What imaging studies (if any) should be ordered to help establish the diagnosis? How should the results be interpreted?

In patients with significant neurologic symptoms, a radiologic test of the head (CT or MRI) may be indicated, as intracranial processes resulting in increased intracranial pressure may rarely be a cause of bradycardia. An echocardiogram may be useful in determining if there is presence of structural heart disease.

This will be useful information if institution of antiarrhythmic drug therapy is considered. The presence of structural heart disease increases the risk of proarrhythmia and will be important in determining which drug is safest.

In addition, arrhythmias associated with SND, especially atrial fibrillation, may result in atrial remodeling, leading to dilatation, which may facilitate the formation of an arrhythmia itself. Left ventricular function should be assessed.

Rarely, extrinsic pressure on the heart by either an intracardiac or mediastinal mass found on echocardiography may result in arrhythmias thought to be due to SND.

III. Management.

Management of the patient is determined by the results of diagnostic testing and more importantly the correlation of results with the presence or absence of symptoms. Patients with symptomatic SND and bradyarrhythmias usually are indicated for permanent pacemaker therapy.

Dual-chamber pacing versus single ventricular pacing has been shown to result in less atrial fibrillation, stroke, and congestive heart failure, but no difference in mortality. Ventricular pacing even in the DDDR mode can promote heart failure, and newer pacemakers employ algorithms that favor maximizing atrial pacing, with back-up dual chamber pacing.

These algorithms should not be programmed in patients with preexisting AV nodal disease (approximately 15% of patients with SND). An exception to the requirement for symptoms is the patient with concomitant severe bradyarrhythmias, who has tachyarrhythmias requiring treatment with drugs that might worsen the bradyarrhythmias. Another exception to the requirement for symptoms is the patient with a history of myocardial infarction requiring beta-blocker therapy as part of post-MI care, who has sinus bradycardia.

Patients with atrial tachyarrhythmias and rapid ventricular responses should receive AV nodal blocking therapy with a beta-blocker, or if contraindicated, verapamil or diltiazem. Antiarrhythmic drug therapy for atrial fibrillation, if indicated, would be determined according to established clinical guidelines.

A. Immediate management.

Immediate management of SND is rarely required. Intravenous atropine or beta-agonist for symptomatic severe sinus node dysfunction may be warranted.

In a hospital or other health care facility setting, external pacing pads may be employed until more definitive treatment is administered (temporary or permanent transvenous pacing). Discontinuation of any possibly offensive cardioactive drug is warranted until pacing therapy is instituted.

B. Physical Examination Tips to Guide Management.

Patients who receive permanent pacemakers should be monitored for signs and symptoms of congestive heart failure. If heart failure develops, they should then have the pacemaker, if possible (no AV nodal disease present) programmed to minimize ventricular pacing.

If left ventricular dysfunction develops as a cause of necessary ventricular pacing, consideration should be given to upgrade the pacemaker to a cardiac resynchronization device. For patients with atrial tachyarrhythmias, one should monitor for signs or symptoms of congestive heart failure or coronary artery disease, which may be related to the tachyarrhythmias or inappropriate ventricular tracking of such arrhythmias in patients with AV block.

The pacemaker diagnostic data (giving duration and ventricular rates of tachyarrhythmias) should be used to guide antiarrhythmic drug therapy.

C. Laboratory Tests to Monitor Response To, and Adjustments in, Management.

If the SND is extrinsic and due to thyroid disease, periodic assessment of thyroid function tests is indicated. Regular periodic measurement of INR indicated for those patients with atrial fibrillation/flutter indicated warfarin for thromboemboli prophylaxis. INR should be 2.0 to 3.0 in patients without mechanical valves and 2.5 to 3.5 in those with mechanical valves.

D. Long-term management.

The patient with clinically significant bradyarrhythmias, tachyarrhythmias, or both, regardless of having received a permanent pacemaker requires long-term follow-up. If a pacemaker is not implanted, clinical reevaluation is warranted to check for progression of bradycardia and/or symptoms, progression of sinus node disease to involve AV nodal or infranodal disease (which might signal a change of medications or pacemaker prescriptive therapy).

In addition, patients with SND often have concomitant (or develop later) hypertension and/or coronary artery disease, and if present would be diagnoses that also require long-term follow-up. Pacemaker evaluation should include interrogation of arrhythmia logs (looking for atrial and ventricular tachyarrhythmias), percentage of ventricular pacing, rate histograms, and automatic threshold checks performed by the pacemaker.

Patients indicated for long-term antithrombotic therapy for atrial tachyarrhythmias should be evaluated for signs and symptoms of bleeding, and have regular INR checks if taking warfarin.

E. Common Pitfalls and Side-Effects of Management

The most common pitfall in patients being evaluated for SND is treating bradycardia during sleep, asymptomatic sinus bradycardia unassociated with atrial tachyarrhythmias not requiring antiarrhythmic drug therapy, bradycardia associated with high vagal tone, or vagally mediated events with pacemaker therapy. The correlation of symptoms with arrhythmias is a cornerstone of evaluation in the vast majority of patients.

Side effects of antiarrhythmic drug therapy may include worsening of sinus node dysfunction or conduction below the sinus node, ventricular arrhythmias in patients with structural heart disease or noncardiac side effects and toxicities related to the specific drug.

Pacemaker therapy may be complicated by heart failure with ventricular pacing (see above). Pacemaker function needs to be checked as per established guidelines.

IV. Management with Co-Morbidities

Patients with concomitant sinus node dysfunction and significant aortic valve stenosis who receive pacemakers should have their pacemakers programmed carefully to avoid elevated heart rates via the rate response mode to avoid heart failure, angina, syncope, or severe ventricular arrhythmias related to hemodynamic causes (personal experience).

Patients with coronary artery disease who require beta-blocker therapy may have had their drug discontinued when SND was diagnosed. If this was the case and a pacemaker is implanted, the drug should be reinstituted. This is almost always possible to do.

If a patient is on a class 1C antiarrhythmic drug and develops coronary artery disease, the drug should be discontinued. If the patient is on a type 1C drug or dronedarone and develops atrial fibrillation that is deemed to be chronic, or congestive heart failure, it should be discontinued. Patients on amiodarone should have liver and thyroid function tests, a chest radiograph, pulmonary function tests with diffusion capacity, and eye examinations as per established guidelines.

V. Patient Safety and Quality Measures

A. Appropriate Prophylaxis and Other Measures to Prevent Readmission.

Patients experiencing presyncope, syncope, chest pain, and/or shortness of breath should be instructed to alert their physician, as these symptoms may represent worsening arrhythmia status, the development of concomitant coronary artery disease, or congestive heart failure, or adverse effects from medication or pacemaker programmed parameters.

B. What's the Evidence for specific management and treatment recommendations?

“ACC/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities”. Circulation. vol. 117. 2008. pp. e350

“2011 ACCF/ACC/HRS focused update of the management of patients with atrial fibrillation”.

C. DRG Codes and Expected Length of Stay.

ICD-10 Codes

sick sinus syndrome: 149.5

other specified cardiac arrhythmias: 149.8

supraventricular tachycardia: 147.1

atrial fibrillation: 148.0

atrial flutter: 148.1

sinus bradycardia: R00.1

tachycardia unspecified: R00.0