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The heart electrical signals are initiated in its natural pacemaker – the sinoatrial node, or SA node, and travel through the atria to reach the atrioventricular node, or AV node. The AV node passes the signals onto the bundle of His, which then splits into two branches that conduct the impulses to the two ventricles.

The SA node is composed of two major cell types: P, for pacemaker, cells generate electrical impulses; and T, for transitional, cells transmit these signals to the right atrium and subsequently to the rest of the cardiac conduction system. Sinus node dysfunction occurs when any of these cells cease to function properly: failure to produce electrical impulses by P cells leads to sinus pause or sinus arrest, while delay or failure of signal transmission by T cells results in SA exit blocks.

On an ECG, sinus pause or arrest can be seen as a brief absence of P waves, which can last for seconds to minutes. In most cases, a downstream pacemaker in the atria, atrioventricular junction, or ventricles, will take over the pacing function, producing so-called escape beats or rhythms, and thus preserving heart rate and function until the SA node recovers and fires again; but long pauses may cause dizziness, fainting and possibly cardiac arrest.

In SA exit blocks, the SA node discharges normally, but the impulses are slow to reach the atrium, or completely interrupted before reaching the atrium. There are three degrees of SA blocks, similar to the 3 degrees observed with AV blocks.

• In first-degree SA block, there is an abnormal delay between the firing of the SA node and transmission to the atrium. Because SA node firing is not significant enough to be seen on a standard 12-lead ECG, this type of block cannot be detected on a surface ECG.
• In second-degree SA block type I, the electrical signals are delayed further and further with each heartbeat until a P wave is missing altogether. Due to the diminution in the increment of the delay, the P-P intervals are progressively shortened before the dropped P wave. This pattern results in pauses and the appearance of grouped beats. The duration of each pause is less than two P-P cycles.
• In second-degree SA block type II, some of the electrical signals do not reach the atrium. On an ECG, this is seen as intermittent dropped P waves; the pauses are multiple, usually twice, of the P-P interval.
• Third-degree SA block is a complete block; P waves are absent. This type of SA exit block is indistinguishable from a sinus arrest.

Sinus node dysfunction is most commonly due to degeneration or scarring of the SA node tissue, which can result from aging, other heart diseases or diabetes. Other causes may include certain medications or an excessive vagal tone.