The flow pattern of a vessel communicating with the residual aneurysm sac is typically characterized by which directionality?

• A. Unidirectional flow into the sac
• B. To-and-fro directionality similar to a pseudoaneurysm neck
• C. No flow entering the sac
• D. Continuous forward flow out of the sac

Answer: (B)

Flow through a vessel that remains in communication with an aneurysm sac tends to be bidirectional over the cardiac cycle. The systolic pressure pushes blood into the sac, then, because the neck is narrow and pressure equilibrates differently during diastole, some blood reverses direction back toward the parent vessel. This creates a characteristic to-and-fro Doppler pattern at the communication site, similar to what you’d see in a pseudoaneurysm neck. That bidirectional exchange is the hallmark you’d expect with a sac that remains connected to the feeding vessel.

If the flow were only entering the sac without backflow, or only moving forward out of the sac, you wouldn’t see that bidirectional waveform. No flow would suggest thrombosis or no hemodynamic connection.