Sick Sinus Syndrome

The sinoatrial (SA) node, or the Keith-Flack node, is the primary structure of the cardiac conduction system where the electrical impulse that initiates a heartbeat normally originates (see sinus rhythm). It is frequently referred to as the heart's "natural pacemaker."

It possesses a fusiform morphology and is located within the superior posterolateral wall of the right atrium, adjacent to the ostium of the superior vena cava (see conduction system).

When an alteration in its automaticity or in the conduction of the stimulus to the atria results in a decrease in heart rate, it is termed sinus node dysfunction (SND) or sick sinus syndrome (SSS).

Sinus node dysfunction, along with high-grade atrioventricular (AV) blocks, represents the most frequent indications for permanent pacemaker therapy.¹

Sinus Arrhythmia

Related article: Sinus arrhythmia.

Although sinus arrhythmia is included in this section, it is not a primary pathology of the SA node, as it is considered a normal variant of sinus rhythm.

It is a common finding in children and young adults, typically asymptomatic, and not associated with organic heart disease.

Sinus Arrhythmia:
Variation in P-P intervals in relation to the respiratory cycle.

Key Features on EKG:

• Variation in P-P intervals in relation to the respiratory cycle (increasing during inspiration and decreasing during expiration).
• The P wave morphology is sinus-derived and must be followed by a QRS complex with a normal PR interval.

More information: Sinus arrhythmia.

Sinus Pause

Sinus pause is categorized under sinus node diseases and is caused by a failure of the SA node to generate an electrical impulse.

On the electrocardiogram, the hallmark of a sinus pause is a prolonged interval between two P waves exceeding 3 seconds, giving the impression of a "dropped" QRS complex.²

Unlike sinoatrial blocks, in a sinus pause, the preceding P-P intervals are constant, and the P-P interval containing the pause is less than double the preceding intervals (the subsequent P wave appears outside the regular cycle).

Non Pathological Sinus Pause:
Previous P-P intervals are regulars (1.24 sec), pause (red) of 2.12 sec, less than twice the previous intervals.

There is no universal consensus on when a sinus pause is considered pathological. It is common to find pauses >2 seconds on 24-hour Holter monitors in asymptomatic patients without it signifying clinical SND. Clinical correlation (dizziness, syncope) is essential.

Generally, pauses >3 seconds are considered suggestive of SND and warrant further evaluation.

Sinoatrial Block

In sinoatrial blocks, there is a conduction delay or failure between the SA node and the surrounding atrial tissue.²

The node generates the stimulus, but it either fails to reach the atria or is delayed; consequently, P waves are absent, resulting in a pause.

SA blocks are classified similarly to atrioventricular blocks but are more challenging to diagnose:

• First degree SA block: A delay in conduction to the atria. It cannot be identified on a standard surface EKG.
• Second-degree SA block, Type I (Wenckebach): Progressive shortening of the P-P interval until a longer P-P interval (containing the blocked impulse) occurs. This longer interval is less than the sum of two consecutive cycles.
• Second-degree SA block, Type II: A pause without P waves that is an exact multiple (2x, 3x, or 4x) of the preceding P-P interval.
• Third-degree SA block: Characterized by a total absence of P waves and the presence of an escape rhythm. This requires an electrophysiological study (EPS) for definitive confirmation.

Second Degree Sinoatrial Block Type I:
The P-P interval progressively shortens until a longer P-P interval appears (red). The longest interval is shorter than the sum of two consecutive intervals.

Differences Between Second-Degree Sinoatrial Block and Sinus Pause

• Second-degree sinoatrial (SA) block type I (Wenckebach): If the preceding P-P intervals progressively shorten and the pause is less than the sum of the two preceding intervals, it is a type I second-degree SA block.
• Sinus pause (or sinus arrest): If the P-P intervals preceding the pause are constant (similar) and the duration of the pause is not a multiple of the baseline P-P intervals, it is a sinus pause.
• Second-degree sinoatrial (SA) block type II: If the pause is an exact multiple (equal to two or more) of the preceding P-P intervals, it is a type II second-degree SA block.

Tachycardia-Bradycardia Syndrome

Bradycardia-tachycardia syndrome is the most common variant of sinus node dysfunction.¹

This disorder is characterized by episodes of sinus bradycardia, ectopic atrial bradycardia, or sinus pause followed by tachyarrhythmias—predominantly atrial fibrillation, although atrial flutter or atrial tachycardia may also present.^1 2

Bradycardia-Tachycardia Syndrome:
Sinus bradycardia alternating with rapid atrial fibrillation that terminates with a prolonged pause.

These tachyarrhythmias tend to terminate spontaneously, resulting in a prolonged sinus pause.

The tachycardia may be associated with suppression of sinus node automaticity, leading to a sinus pause of variable duration upon tachycardia termination.²

Chronotropic Incompetence

Broadly defined as the heart's inability to increase its rate in proportion to increased activity or demand, in many studies this translates to an inability to achieve 80% of the predicted heart rate reserve during exercise.

Stress testing can be utilized to diagnose symptomatic chronotropic incompetence, defined as the inability to increase the heart rate proportionally to the increased metabolic demands of physical activity.

Sick Sinus Syndrome and Pacemakers

In general, for sick sinus syndrome in asymptomatic patients, permanent cardiac pacing has not been shown to affect prognosis.¹

Therefore, in sinus node dysfunction, a permanent cardiac pacemaker is only indicated in patients whose bradycardia is symptomatic and directly attributable to the sinus node dysfunction.^1 2

In patients presenting with exercise intolerance in whom chronotropic incompetence has been identified, the clinical utility of cardiac pacing is uncertain, and the decision to implant a pacemaker in these patients should be made on a case-by-case basis.¹

In patients with symptomatic sick sinus syndrome secondary to potentially reversible or treatable causes, permanent pacemaker implantation is not indicated, and correction of these underlying disturbances is required.^1 2

In patients undergoing evaluation for syncope in whom asymptomatic pauses greater than 6 seconds due to sinus arrest are documented, pacing may be indicated.¹