An Accelerated Idioventricular Rhythm is defined as an ectopic ventricular rhythm at a rate between 40 bpm and 100 to 120 bpm. This rate is faster than the intrinsic rhythm (40 bpm) but slower than that of ventricular tachycardia (which would be > 100 to 120 bpm). The ventricular origin of this rhythm can be demonstrated by the usual EKG criteria which include AV dissociation, fusion beats, and capture complexes.

The rate of cardiac contraction is determined by the intrinsic rate of depolarisation of the cardiac cells. In normal hearts the sinoatrial node in the atria depolarises at a rate of 70 beats per minute. This suppresses the intrinsic depolarisation of the other parts of the heart.

The accelerated idioventricular rhythm occurs when depolarisation rate of a normally suppressed focus increases to above that of the “higher order” focuses (the sinoatrial node and the atrioventricular node). This most commonly occurs in the setting of a sinus bradycardia.

Cardiology

Thomas Lewis first described accelerated idioventricular rhythm (AIVR) in 1910.^[1]

An accelerated idioventricular rhythm is often caused by reperfusion injury in acute MI, sinus bradycardia or digitalis.

• Acute MI both anterior MI and inferior MI. The incidence is 8% to 36%.
• Anesthetic agents
• Cocaine
• Digitalis
• Heart surgery
• Reperfusion injury
• Sinus bradycardia

AIVR must be distinguished from normal sinus rhythm, junctional tachycardia, and slow VT.

Because of its slower rate, AIVR may resemble normal sinus rhythm (NSR). AIVR is disnguished by the presence of numerous fusion beats to distinguish it from NSR. Given the difficulty in distinguishing these rhythms, the term accelerated isorhythmic ventricular rhythm has been suggested.

AIVR should also be distinguished from junctional tachycardia with preexisting intraventricular conduction delays (IVCD)s. In patients with junctional tachycardia, there are no fusion beats or capture beats.

In general, the rate of AIVR is slower (<100-120 bpm) than ventricular tachycardia (at least 100-120 bpm). There can, however, be overlap in the rate associated with AIVR and slow VT. The distinction is critical as misdiagnosis as slow VT can results in inappropriate therapy such as cardioversion and anti-arrhythmic administration.

The incidence of Accelerated Idioventricular Rhythms following acute MI is reported to be between 8 and 36%.