Last updated: Apr 14, 2008
Bradycardiaan unusually slow heartbeat of less than 60 bpm (beats per minute)also starts in the atria. In most cases the heart's "natural pacemaker"a cluster of cells in the upper right chamber that regulates the heart's electrical activitysimply isn't firing correctly. Fit people have low heart rates, too, because their hearts are especially strong and efficient. In other people, bradycardia can cause dizziness and fainting.
Arrhythmias that start in the ventricles are more dangerous. In ventricular fibrillation, if the heart isn't getting enough oxygenperhaps because a heart attack or heart failure has cut off the supplythe lower chambers can start quivering instead of beating regularly. Very little blood will get where it needs to go, and the heart can go into cardiac arrest. Ventricular fibrillation kills 300,000 Americans each year.
More about arrythmia
Ventricular tachycardia occurs when the ventricles start pounding away at up to 200 bpma disruption that can cause dizziness and breathlessness. The normal resting heart rate for adults is between 60 bpm and 100 bpm. In many cases, a person with ventricular tachycardia will briefly lose consciousness and faint.