4.2 Core Workflows and Decision Points

4.2 Core Workflows and Decision Points

Each reconstruction has a step-by-step scan and a numeric decision point. Memorize the sequence and the trigger value.

Infrainguinal bypass graft surveillance

Scan the entire graft in sequence: native inflow artery, proximal anastomosis, graft body, distal anastomosis, and outflow runoff. Record PSV at each segment plus any focal velocity rise.

• Normal graft flow: PSV roughly 45-180 cm/s with a uniform waveform.
• Significant stenosis: focal PSV ratio greater than 2.0 (some labs use 3.5 for greater than 70%).
• Low-flow / failing graft: graft PSV below 45 cm/s anywhere, or a greater than 30 cm/s drop between studies; this is an ominous sign of impending thrombosis.
• Surveillance interval: classically at 1, 3, 6, and 12 months, then annually. Vein grafts are surveilled more aggressively than prosthetic because their stenoses are treatable.

AV access (fistula and graft) surveillance

Combine flow volume with anatomic stenosis detection.

For an AV graft, the most common failure site is the venous outflow / venous anastomosis. For an AV fistula, juxta-anastomotic inflow stenosis is common. A PSV ratio greater than 2.0 at a focal site defines a greater than 50% stenosis warranting referral.

EVAR endograft surveillance

Evaluate the aneurysm sac diameter, attachment zones, graft limbs, and look for endoleak.

• Sac change: an increase of 5 mm or more is abnormal and suggests an active endoleak; a shrinking or stable sac is reassuring.
• Endoleak types: I (attachment-zone leak, proximal/distal), II (retrograde branch flow, lumbar/IMA, the most common), III (component disconnection or fabric defect), IV (graft porosity), V (endotension, sac growth without visible leak).
• Tools: color and spectral duplex; contrast-enhanced ultrasound (CEUS) improves endoleak detection; CT angiography remains a common reference.

Carotid stent surveillance

In-stent restenosis (ISR) uses higher PSV thresholds than native ICA criteria. Representative validated cutoffs: PSV roughly 150 cm/s suggests greater than or equal to 20-30% (suboptimal result), PSV roughly 220-240 cm/s suggests greater than or equal to 50-60% ISR, and PSV roughly 325-340 cm/s suggests greater than or equal to 80% high-grade ISR warranting reintervention. The ICA/CCA ratio also rises (about 2.7 at the greater than or equal to 50% level). Surveillance is typically within 30 days, at 6 months, then annually.

Why the sequence matters

In each workflow the order of interrogation is itself testable. For a bypass graft, missing the proximal anastomosis or the outflow runoff can leave a critical lesion undetected even when the body looks pristine; lesions cluster at anastomoses and at retained valve sites in vein grafts. For AV access, you must obtain a true flow volume (vessel diameter and time-averaged velocity in a straight segment) before declaring the access adequate, because a focal stenosis can coexist with borderline flow. For EVAR, you measure the sac diameter in the same plane as prior studies so the trend is valid.

Sloppy sequence produces false-negative surveillance.

Decision-point summary

• Bypass: focal PSV ratio greater than 2.0 OR graft PSV below 45 cm/s triggers referral; a greater than 30 cm/s interval drop is a warning even if absolute values are not yet critical.
• AV access: flow below 500-600 mL/min, a greater than 25% flow drop between studies, or a focal PSV ratio greater than 2.0 triggers fistulogram or referral.
• EVAR: sac growth of 5 mm or more, or any type I or III endoleak, triggers reintervention; isolated type II leaks are often observed if the sac is stable.
• Carotid stent: PSV at the greater than or equal to 80% threshold (roughly 325-340 cm/s) is the usual reintervention trigger, while the greater than or equal to 50% level prompts tighter follow-up.

Each trigger is a number you should be able to recite; the exam frequently hands you the velocity and asks you to apply the matching rule.

Differentiating stenosis from occlusion

A frequently tested branch point: is the graft or access stenotic or occluded? A stenosis shows a focal velocity rise with post-stenotic turbulence and preserved distal flow; an occlusion shows absent color and spectral flow within the segment, often with a stump waveform proximally and reconstituted, dampened flow distally via collaterals. The action differs sharply: a stenosis is referred for elective angioplasty, while an acutely occluded bypass or thrombosed access is an urgent salvage problem with a narrow time window. Reading whether flow is merely accelerated versus truly absent is the decisive step.

Documentation and reporting standards

A complete surveillance report records PSV at each graft or access segment, the highest focal PSV and the computed ratio, the access flow volume where applicable, the EVAR sac diameter compared to the prior study, and any waveform abnormality such as tardus-parvus or monophasic flow. Comparison to the prior and baseline studies must be explicit, because every threshold in this domain is interpreted relative to either a fixed cutoff or the patient's own baseline. An answer that reports a single velocity without the ratio, the trend, or the baseline comparison is incomplete by the standards this domain enforces.