Atrial tachycardias

Atrial premature beats

An atrial premature beat (APB) is characterized by a nonsinus P wave that occurs before the next anticipated sinus beat. Several electrocardiographic leads may be needed to distinguish P waves originating in the sinus node versus an ectopic focus. In the rare case of sinus nodal premature beats, the P wave will be identical. The post-extrasystolic cycle length is typically less than compensatory because of penetration and resetting of the sinus node by the premature depolarization, but suppression of sinus node automaticity may also occur and result in pauses equal to or longer than the sinus cycle. Intraventricular conduction may be normal or aberrant, whereas the PR interval can be longer than that of the normal sinus beat because of atrioventricular nodal delay. This especially occurs in the rare case of interpolated APBs, which do not depolarize the sinus node and do not affect the sinus rate. Blocked premature beats may masquerade as pauses or bradycardias when P waves are not seen. The T wave of those beats that precede a pause should be carefully inspected to detect any “hidden” premature P wave. Nonconducted APBs are a common cause of unexpected pauses. They can arise anywhere in the atria, including the sinus node.

Physiologic sinus tachycardia

Sinus tachycardia is defined as a nonparoxysmal increase in sinus rate to more than 100 beats per minute (bpm) in keeping with the level of physical, emotional, pathologic, or pharmacologic stress. ^,

It is due to physiologic influences on individual pacemaker cells and from an anatomic shift in the site of origin of atrial depolarization superiorly within the sinus node.

In normal sinus rhythm, the P wave on a 12-lead electrocardiogram (ECG) in adults is positive in leads I, II, and aVF and V ₃ to V ₆ . It is negative in aVR and V ₁ and V ₂ . In sinus tachycardia P waves have a normal contour, but a larger amplitude may develop and the wave may become peaked.

Inappropriate sinus tachycardia

Inappropriate sinus tachycardia (IST) is a fast sinus rhythm (>100 bpm) at rest or minimal activity that is out of proportion with the level of physical, emotional, pathologic, or pharmacologic stress. The syndrome of inappropriate sinus tachycardia is also defined as a sinus heart rate greater than 100 bpm at rest (with a mean 24-hour heart rate >90 bpm not a result of primary causes) and is associated with distressing symptoms of palpitations. ^,

The underlying mechanism of IST remains poorly understood and is likely to be multifactorial (dysautonomia, neurohormonal dysregulation, intrinsic sinus node hyperactivity). A gain-of-function mutation of the pacemaker hyperpolarization-activated cyclic nucleotide-gated (HCN) 4 channel has been reported in a familial form of IST. There is also evolving evidence that immunoglobulin G anti-β receptor antibodies are found in IST.

Sinus nodal reentrant tachycardia

Sinus nodal reentrant tachycardia arises from a reentry circuit involving the sinus node and, in contrast to IST, is characterized by paroxysmal episodes of tachycardia. On the ECG, the polarity and configuration of the P waves are similar to the configuration of sinus P waves.

It is unclear whether the reentrant circuit is completely confined to the sinus node or if surrounding atrial tissue is involved.