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.

AIVR appears similar to ventricular tachycardia but is benign and doesn't need any treatment. It can most easily be distinguished from VT in that the rate is less than 120 and usually less than 100 bpm.

References

1. http://www.emedicine.com/med/topic12.htm

2. Hampton, John (1997). The ECG Made Easy.