Anatomy of the right atrium relevant to atrial flutter ablation. The figure shows right anterior oblique view of a heart model of the right atrium and right ventricle with the anterolateral walls removed. The ablation catheter is positioned on the annulus of the tricuspid valve (TV) in the cavotricuspid isthmus (CTI), the starting point for ablation. Dashed line shows the path along which the catheter is withdrawn to create a line of block to the inferior vena cava (IVC). CS indicates the ostium of the coronary sinus.