Duplex Surveillance of Infrainguinal Venous Bypass Graft: Occlusion Threat Thresholds

Why Venous Grafts Are Evaluated Separately

A bypass graft, especially a venous one, has its own 'normal corridor' of velocities and a specific threat indicator — too low a velocity. Low volumetric velocity in the graft means the flow is about to stop (risk of thrombosis), even in the absence of local stenosis. Therefore, a separate set of duplex criteria is applied to grafts.

The Two-Pole Rule

Both velocity poles are dangerous in a graft:

— High Velocity: PSVR >3.5 (PSV >300 cm/s) — hemodynamically significant, graft-threatening stenosis;
— Low Velocity: approximately <45 cm/s (adjusted for diameter) — risk of thrombosis.

The graft is examined along its entire length: inflow, body, valve zones, and both anastomoses. The criterion for significant stenosis is a doubling/tripling of velocity relative to the adjacent segment.

Duplex Criteria for Arterial Bypass Grafts (Table 5.3)

Threat Thresholds: Minimal and Significant Criteria

Minimal criteria for graft stenosis are PSV >150–180 cm/s and PSVR >1.5–2.0 (zone for increased surveillance). The criterion for significant, graft-threatening stenosis is PSV >300 cm/s and PSVR >3.5 (consider correction). These thresholds correspond to data from Gahtan V., Mills J.L., Bandyk D.F. et al. (J Vasc Surg): PSV >300 and Vr >3.5 distinguish progressive stenoses, minimal criteria — PSV >150, Vr >1.5.

Caveat on Low Velocity

The low-velocity threshold (<45 cm/s) is not absolute. In large diameter grafts (>4–7 mm) and distal (pedal) grafts, normal velocity may be lower, so low velocity is interpreted along with the graft diameter and dynamics.

Signs of Thrombosis and AVF in In Situ Grafts

Signs of a thrombosed graft: absence of color signal and spectrum throughout the lumen; in B-mode — hyperechoic content of the lumen. Arteriovenous fistulas in in situ grafts on color Doppler appear as a local zone of turbulent overflow between the graft and adjacent vein with a characteristic high-velocity low-resistance signal. Hemodynamically significant AVFs reduce perfusion pressure distally and require closure.

Surveillance Schedule

Standard surveillance schedule (SVU 2024 / AVLS 2025): baseline study within 1 month after the procedure. Typical schedule post-revascularization: baseline study in the first 1–3 months, then at 3, 6, and 12 months, and annually thereafter (ESVS 2023 / AVLS 2025). The schedule is intensified upon detection of borderline stenosis, in grafts, and in the first year (highest risk).

Dynamics as a Warning Signal

A decrease in PSV by >30% from the baseline is a warning signal requiring more frequent surveillance. Even in the absence of a critical stenosis threshold, negative velocity dynamics serve as a basis for increased monitoring.