Originally Posted by MoAlpha

In AF the ventricle keeps contracting, otherwise there would be cardiac arrest and death. However, it is no longer under the control of the of the sinus node and is driven by ectopic activity from elsewhere in the atrium or associated structures.. The resulting ventricular rate is usually high, but can be “normal” or even slow. An abnormally high ventricular rate prevents the ventricle from filling completely and reduces cardiac output, sometimes drastically. That’s why people experiencing pathological tachycardia feel faint or collapse, or die.

The big danger in AF, however, is not from tachycardia, but from clot forming in the non-contracting atrium and causing strokes, etc.