When the atria, the two upper chambers, contract at an excessively high rate, and in an irregular way, the patient has AFib. AFib is very similar to superventricular tachycardia (SVT). Both conditions affect the atria of the heart. The difference rests in the regularity of the heartbeat. SVT is a fast, regular heartbeat; AFib is a fast, irregular heartbeat. When the two lower chambers beat irregularly and flutter, the patient has ventricular fibrillation (VFib). Both produce an irregular heart rhythm.