RVR is caused by the same electrical impulse, the extra ones, that cause the atria to fibrillate. However, the signal enters the ventricle either via the Bundle of His' branches, low in the heart, called the bundle branches and then via the Purkinje fibers. Or, it can enter the ventricle in a focal point, also called a re-entrant. This is my understanding...it might not be gospel. The point is that, while supraventricular ('above he ventricles') tachycardia (SVT) is only atrial tachyarrhythmia, RVR means the two lower large ventricles are trying to match the beat and rhythm of the atria...so the entire heart is thumping along at whatever the AF rate currently is. This is serious, more serious than AF by itself, and the patient needs urgent medical care.