What is the narrow band of muscle between the IVC and the tricuspid valve called?

• A. Fossa ovalis
• B. Eustachian ridge
• C. Cavotricuspid isthmus
• D. Chiari network

Answer: (C)

The narrow band of muscle between the opening of the inferior vena cava and the tricuspid valve is the cavotricuspid isthmus. This small, fiber-rich corridor in the inferior right atrium forms a distinct conduit that often serves as a critical pathway in reentrant circuits responsible for typical atrial flutter; disrupting conduction here with ablation can terminate the flutter by blocking the circuit.

Other structures mentioned are located in different parts of the right atrium: the fossa ovalis is in the interatrial septum, the Eustachian ridge marks the region near the IVC ostium but is not the narrow isthmus, and the Chiari network is a web-like remnant within the right atrium.