4.3 Scenario Practice for Surgically Altered Anatomy/Pathology

4.3 Scenario Practice

Scenario stems give you a construct + a finding + a number and ask for interpretation or next step. Use a fixed reading order: name the construct, locate the lesion, read the value, then choose.

Expected hemodynamics of a functioning AV fistula

An arteriovenous (AV) fistula short-circuits high-pressure arterial flow into a low-pressure vein. This converts the feeding artery (e.g., radial artery of a radiocephalic fistula) into a low-resistance conduit: a monophasic waveform with markedly increased diastolic flow, sometimes with diastolic velocity approaching systolic. A persistent high-resistance triphasic waveform in the feeding artery after maturation suggests the fistula is not developing or there is an inflow problem. Distal to the anastomosis, retrograde radial flow can occur and, if symptomatic, indicates steal.

AV graft failure pattern

A prosthetic AV graft (typically a PTFE loop) most often fails at the venous outflow, specifically the graft-to-vein anastomosis or draining central veins, where neointimal hyperplasia narrows the lumen. The sequence is: outflow stenosis, rising venous pressures and recirculation on dialysis, falling access flow, then thrombosis. Surveillance therefore concentrates Doppler interrogation at the venous anastomosis, calling a greater than 50% stenosis when the focal PSV ratio exceeds 2.0.

Vein-graft lesion timeline

For lower-extremity vein bypass grafts, lesion timing is testable:

Most surveillance-detected, treatable stenoses appear in the 3-15 month window, which is why intensive follow-up clusters there.

Renal transplant and TIPS quick facts

• Renal transplant: donor renal artery is usually anastomosed end-to-side to the recipient external iliac artery, kidney in the iliac fossa. Transplant renal artery stenosis occurs in roughly 3-15% of cases; an intrarenal resistive index above 0.80 or tardus-parvus waveform raises concern.
• TIPS: connects a portal vein branch to a hepatic vein. A normal shunt PSV is roughly 90-190 cm/s; a value below 90 (or a greater than 50 cm/s drop) or a focal velocity above 190 cm/s suggests malfunction.

When two answers seem plausible, anchor on the construct's expected physiology: the fistula should be low-resistance, the AV graft fails at the venous side, and the vein-graft lesion clock points to hyperplasia at 3-15 months.

Steal syndrome scenario

A classic stem: a patient with a brachiocephalic fistula develops a cold, painful, pale hand during dialysis. This is dialysis access-related steal, where the low-resistance fistula diverts arterial flow away from the distal extremity. On duplex you may see retrograde flow in the artery distal to the anastomosis that reverses or augments with fistula compression. The correct interpretation is steal, not stenosis, and recognizing reversed distal arterial flow (which normalizes when the access is manually compressed) is the diagnostic key.

Pseudoaneurysm and infection scenario

Repeated cannulation of an AV graft can create a pseudoaneurysm: a pulsatile mass with the characteristic yin-yang color pattern and a to-and-fro waveform in the neck connecting it to the graft. Distinguish this from a true aneurysm (all wall layers) and from a hematoma (no internal flow). An infected graft may show perigraft fluid and irregular margins. The exam wants you to read the neck waveform to confirm a pseudoaneurysm rather than assume any pulsatile mass is an aneurysm.

Sequencing the scenario answer

When a scenario offers both an interpretation and an action, prefer the answer that both names the correct mechanism and specifies the matching next step. For a low-flow fistula, that is recognizing inflow stenosis and recommending fistulogram. For reversed distal flow with hand ischemia, it is recognizing steal and recommending vascular surgery evaluation. For a to-and-fro neck waveform, it is recognizing pseudoaneurysm. Mechanism plus matching action beats a half-answer that names only one.

Carotid stent scenario

A stem gives a stented internal carotid artery with PSV 250 cm/s and ICA/CCA ratio 2.9 at the 7-month follow-up, with a baseline of PSV 140 cm/s. Do not apply native criteria, where 250 cm/s would imply greater than 70%. For a stent, PSV roughly 220-240 cm/s with a ratio near 2.7 corresponds to greater than or equal to 50% in-stent restenosis, which prompts tighter follow-up rather than immediate reintervention; the greater than or equal to 80% level (roughly 325-340 cm/s) is the usual trigger to treat. The rise from the 140 cm/s baseline confirms evolving neointimal hyperplasia.

The correct read is moderate ISR with short-interval re-imaging, and the trap is over-treating based on native thresholds.

EVAR scenario

A stem describes an endograft 18 months out with a sac that has shrunk 4 mm and a faint area of color in the sac fed by a lumbar artery. The shrinking sac is reassuring, the lumbar source identifies a type II endoleak, and the standard action is continued observation, not reintervention. Contrast this with a stem where the sac grew 6 mm and color enters at the proximal attachment zone: that is a type I leak requiring prompt treatment. The decisive variables are the sac trend and the leak source, exactly the construct-specific reasoning this domain rewards.