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.