Accelerated idioventricular rhythm (AIVR)
A ventricular rhythm that runs faster than a ventricular escape rhythm but slower than ventricular tachycardia. A focus in the ventricles speeds up until it is quick enough to take over from the sinus node, and the two then compete for control of the ventricles.
Key ECG features
- ✓Wide, bizarre QRS complexes
- ✓No consistent preceding P wave
- ✓Rate only modestly above the sinus rate
- ±AV dissociation: sinus P waves drift through
- ±Fusion beats at the start and end of runs
✓ required · ± may be present
ECG characteristics
Wide, bizarre QRS complexes at a rate only modestly above the sinus rate, with no consistent preceding P wave. Sinus P waves continue at their own rate and drift through the run. Because the two rates are so close, fusion beats - hybrids of a sinus and a ventricular beat - are common at the onset and offset of each run, and periods of the ventricular rhythm alternate with periods of sinus rhythm.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Most often severe extra-cardiac disease rather than primary cardiac damage: gastric dilatation-volvulus, blunt trauma, splenic disease, and other causes of systemic illness or myocardial hypoxia.
Why it matters
Rate is usually the most important feature distinguishing this from ventricular tachycardia, and the distinction changes management. AIVR runs close to the sinus rate: faster than an escape rhythm, which only appears when nothing else is pacing, and below the rate at which a ventricular rhythm is called tachycardia, a threshold set by species and convention. Because the rate is only modestly elevated, it is usually well tolerated and cardiac output is preserved, so treatment targets the underlying condition rather than the rhythm.
Differentiate from
- Ventricular tachycardiaBoth are wide, ventricular in origin, and dissociated from the sinus P waves. Rate is usually what separates them, so count it rather than judging by eye.
- Ventricular escape rhythmAn escape rhythm is slower still and appears because nothing faster is pacing. This rhythm is faster than escape and competes with a sinus node that is still working.