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 clear consensus on when should the sinus pause be considered as pathological. Sinus pauses longer than 2 seconds in duration are frequently found on holters EKG without associated symptoms, not necessarily meaning sick sinus syndrome. Correlation of symptoms with EKG findings is important.

Generally speaking, sinus pauses longer than 3 seconds in duration are considered suggestive of sick sinus syndrome. Diagnosis should be verified with other medical analysis.

Sinoatrial Block

In sinoatrial block the sinus node generates the stimulus; however, it is not transmitted to the atria or is delayed: P waves are not generated, creating thus a pause between two P waves.²

Sinoatrial block classification is similar to that of the atrioventricular blocks, but it is more difficult to diagnose.

• First degree SA block: The stimulus is delayed on its way to the atria. It can not be detected on a normal EKG.
• Second degree SA block, type I: The P-P interval progressively shortens — similar to R-R intervals in 2nd-degree AV block type I (Wenckebach phenomenon) — until a longer P-P interval appears (which contains the blocked impulse). The longest P-P interval is shorter than the sum of two consecutive intervals.
• Second degree SA block, type II: Interval without P waves which approximately equals twice the previous PP interval - although it could also be 3 or 4 times greater than the interval before the pause.
• Third degree SA block: It could be described as the absence of P waves, but it can’t be confirmed without a previous EKG tracing of the sinus rhythm or an electrophysiology study.

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.

Diferences between Second Degree Sinoatrial Block Type II and Sinus Pause

If previous P-P intervals shorten and pause duration is less than the two preceding PP intervals, diagnose is second degree sinoatrial block, type I.

If previous P-P intervals are similar and pause duration is less than the two preceding P-P intervals, diagnose is sinus pause.

If pause duration is equal to the two preceding P-P intervals, diagnose is second degree sinoatrial block, type II.

Bradycardia-Tachycardia Syndrome

Another form of sick sinus syndrome is the bradycardia-tachycardia syndrome where episodes of sinus bradycardia followed by tachyarrhythmias — mainly atrial fibrillation, but also atrial flutter or atrial tachycardia — are observed.^1 2

Bradycardia-Tachycardia Syndrome:
Sinus bradycardia alternating with fast atrial fibrillation ending in a long pause.

These tachyarrhythmias tend to cease spontaneously causing a long sinus pause.

Sick Sinus Syndrome and Pacemakers

Permanent pacemaker implantation is recommended for patients with relevant, documented symptomatic sinus bradycardia (heart rate below 40 bpm).

It is also recommended when symptoms cannot be associated with SSS but there is a high clinical suspicion or when there is no adequate chronotropic response to exercise performance