7.3 Bypass Graft Surveillance Protocols & Criteria

7.3 Bypass Graft Surveillance Protocols & Criteria

Surgical lower extremity bypass grafting bypasses extensive, non-crossable native arterial occlusive disease to restore direct pulsatile arterial perfusion to ischemic limbs. Duplex ultrasound surveillance of lower extremity bypass grafts is an established standard of care. Because vein grafts frequently develop asymptomatic, progressive focal stenoses during the first 12 to 24 months postoperatively, routine surveillance identifies impending failure, allowing elective revision before catastrophic graft thrombosis occurs. Elective repair of a failing graft (assisted primary patency) preserves limb salvage at rates dramatically exceeding revision after acute thrombosis (secondary patency).

Bypass Graft Types & Structural Characteristics

Lower extremity bypass conduits are categorized into autogenous vein grafts and prosthetic (synthetic) grafts:

Surgical Bypass Configurations:

1. Reversed Vein Graft (GSV):       2. In-Situ Vein Graft (GSV):         3. Synthetic PTFE Graft:
   - Harvested, reversed end-for-end   - Remains in native saphenous bed   - Double-line echogenic wall
   - Large end anastomosed distally    - Valves excised with valvulotome   - Uniform caliber throughout
   - Small end anastomosed proximally  - Tributary branches ligated        - Frequent ring reinforcement
   - High velocity prox, low distally  - Natural anatomical taper

1. Autogenous Vein Grafts

Autogenous conduits utilize the patient's own veins and offer superior long-term patency, particularly for infrapopliteal (tibial and pedal) targets:

• Reversed Great Saphenous Vein (GSV): The GSV is completely excised from the limb, reversed $180^\circ$ end-for-end so that its internal valves do not impede forward arterial flow, and all tributary branches are ligated. The small distal caliber of the saphenous vein is anastomosed to the large proximal inflow artery (e.g., common femoral artery), and the large proximal caliber of the vein is anastomosed to the small distal runoff artery (e.g., popliteal or tibial artery).
  • Hemodynamic Consequence: Because cross-sectional area increases from proximal to distal, normal peak systolic velocities are naturally higher at the narrow proximal graft body and lower at the wider distal graft body.
• In-Situ GSV: The GSV is left within its native subcutaneous bed. The proximal and distal ends are mobilized and anastomosed to the inflow and outflow arteries. Because the vein is not reversed, its valves point against arterial flow; an instrument called a valvulotome is passed through the lumen to mechanically disrupt/excise the valve cusps. Furthermore, all perforating veins and tributary branches connecting the saphenous vein to the deep venous network must be surgically ligated.
  • Hemodynamic Consequence: The graft maintains its natural anatomical taper (wide proximally, narrow distally), matching native arterial geometry.
• Non-Reversed Translocated Vein: Vein harvested from the contralateral leg or upper extremity (cephalic/basilic vein), valves excised with a valvulotome, and translocated into the arterial position without reversing.

2. Prosthetic (Synthetic) Grafts

Prosthetic conduits are selected when autogenous vein is absent, diseased, or of inadequate caliber ($< 3.0\text{ mm}$):

• Polytetrafluoroethylene (PTFE / Gore-Tex): Features a characteristic bright, double-line echogenic wall appearance on B-mode grayscale. Ringed PTFE incorporates external rigid plastic rings across the knee joint to prevent mechanical compression and kinking during leg flexion.
• Dacron (Knitted or Woven Polyester): Demonstrates a characteristic corrugated, undulating, or accordion-like wall appearance on longitudinal B-mode images.

Standardized Duplex Surveillance Protocol

1. Surveillance Timeline

Routine duplex surveillance is scheduled at:

• Intraoperative baseline (completion duplex) or discharge baseline
• 1 month postoperatively
• 3 months postoperatively
• 6 months postoperatively
• 9 months postoperatively
• 12 months postoperatively
• Every 6 to 12 months indefinitely thereafter

2. Systematic 5-Zone Interrogation Sequence

A complete duplex graft evaluation requires systematic interrogation across five distinct anatomical zones:

Five-Zone Graft Interrogation Map:
┌──────────────────────┬─────────────────────────────────────────────────┐
│ Zone 1: Inflow       │ Native artery 2-3 cm proximal to anastomosis    │
├──────────────────────┼─────────────────────────────────────────────────┤
│ Zone 2: Proximal Jxn │ Inflow artery, heel, toe, and graft hood        │
├──────────────────────┼─────────────────────────────────────────────────┤
│ Zone 3: Graft Body   │ Continuous sweep: Proximal, Mid, Distal body    │
├──────────────────────┼─────────────────────────────────────────────────┤
│ Zone 4: Distal Jxn   │ Graft hood, heel, toe, and outflow artery       │
├──────────────────────┼─────────────────────────────────────────────────┤
│ Zone 5: Outflow      │ Native runoff artery 2-3 cm distal & down to DPA│
└──────────────────────┴─────────────────────────────────────────────────┘

1. Zone 1: Inflow Artery: Interrogate the native inflow vessel (e.g., external iliac or CFA) $2 - 3\text{ cm}$ proximal to the graft to ensure unimpeded inflow. Document PSV and waveform morphology.
2. Zone 2: Proximal Anastomosis: Interrogate the native artery, the proximal anastomotic heel, the toe, and the proximal graft hood. Document maximum PSVs. A minor degree of flow disturbance and boundary layer separation is normal at anastomotic sites due to caliber discrepancy.
3. Zone 3: Entire Graft Body: The transducer is walked along the entire length of the conduit in longitudinal plane. Systematic spectral Doppler samples and PSV recordings are captured at the proximal, mid, and distal segments, as well as at any site of visual narrowing, color aliasing, or surgical scar tissue.
4. Zone 4: Distal Anastomosis: Interrogate the distal graft body, the distal anastomotic heel, toe, and the adjacent native outflow artery. The distal anastomosis is the single most common site for postoperative failure between 1 and 24 months due to myointimal (neointimal) hyperplasia.
5. Zone 5: Outflow Runoff: Interrogate the native runoff artery $2 - 3\text{ cm}$ distal to the anastomosis, continuing down to the ankle to document patent tibial runoff vessels and pedal perfusion.

Interpreting graft velocity and interval change

A graft examination integrates visible narrowing, focal acceleration, distal waveforms, and the clinical/ABI trend. Use validated criteria specific to the conduit and reconstruction. Published autogenous vein-graft surveillance work commonly uses a focal PSV above 300 cm/s with a ratio above about 3.5 as a high-risk pattern, and a diffusely low graft PSV around 45 cm/s as another concern. These are screening/management criteria in defined populations, not a guarantee of imminent thrombosis in every graft.

Measure a representative jet and an appropriate upstream reference using consistent angles at or below 60 degrees. The ratio is jet PSV divided by reference PSV. A diseased or very low-flow reference can distort it. Anastomotic geometry, caliber transitions, cardiac output, rhythm, and inflow/outflow disease can also alter velocities.

An ABI decline of more than about 0.15 from a reliable baseline is a commonly used prompt for further graft assessment, but noncompressibility, pressure variability, and symptoms matter. Do not assign a universal failure percentage or mandatory operation from a single low velocity. Verify acquisition, inspect the full circuit, and communicate concerning findings for timely clinical assessment.

Worked graft concern

A patient develops recurrent symptoms with diffuse graft PSV below 45 cm/s and a substantial ABI decline. This is a concerning low-flow pattern. Evaluate inflow, anastomoses, graft body, and outflow and notify the treating service. Whether the solution is surveillance, angiography, endovascular treatment, or surgery depends on the complete assessment.

The 2024 PAD guideline supports scheduled ABI/duplex follow-up after suitable autogenous infrainguinal bypass, including early postoperative, 6-month, 12-month, and annual assessment. The evidence and schedule differ by procedure, conduit, symptoms, and the treating plan. A new symptom warrants assessment outside the routine schedule.

Complications Specific to In-Situ Vein Grafts

In-situ bypass procedures present unique technical failure modes that vascular sonographers must recognize:

In-Situ GSV Failure Modes:

1. Retained Valve Cusp:                2. Unligated Arteriovenous Fistula (AVF):
   - Fluttering echogenic web             - Arterial blood siphoned into deep vein
   - Focal velocity doubling (Vr >= 2.0)  - Proximal: High volume, low resistance (elevated diastolic)
   - Post-stenotic turbulence             - Distal: Dampened, low velocity, high resistance

1. Retained / Incompletely Lysed Valve Cusps

• Mechanism: During surgery, the valvulotome may miss or only partially lacerate a venous valve leaflet. The intact or torn valve cusp flutters within the lumen, creating a mechanical obstruction.
• Sonographic Appearance:
  • B-Mode: An echogenic, thin, linear membrane or web protruding into the lumen, often fluttering with pulsatile flow.
  • Color Doppler: Focal jet aliasing around the leaflet.
  • Spectral Doppler: Localized velocity acceleration fulfilling $\ge 50\%$ stenosis criteria ($V_r \ge 2.0$), followed immediately by post-stenotic flow disturbance.

2. Unligated Arteriovenous Fistulas (AVFs)

• Mechanism: The great saphenous vein naturally possesses numerous perforating and tributary branches communicating with the deep venous system. If a branch is overlooked and left unligated during surgery, high-pressure arterial bypass blood is shunted directly into the low-pressure deep venous circulation.
• Hemodynamic Signs Along the Graft:
  • Proximal to the AVF: The bypass graft supplies both the fistula and the distal limb. The shunt creates a low-resistance run-off, transforming the spectral waveform into a hyperdynamic, low-resistance signal with continuous, elevated diastolic forward flow and elevated PSV.
  • At the Fistula Branch: Color Doppler reveals a rapid, chaotic color jet exiting the graft into the tributary vein, accompanied by a perivascular color bruit (tissue vibration). Spectral Doppler confirms a continuous, high-velocity, turbulent, low-resistance jet.
  • Distal to the AVF: Because arterial volume has been siphoned away into the venous circulation (arterial steal), flow entering the distal graft is markedly depleted. The spectral waveform distal to the AVF displays abruptly decreased PSV and conversion into a high-resistance, dampened pattern.

Anastomotic Intimal Hyperplasia & Structural Pathology

1. Anastomotic Neointimal Hyperplasia

• Pathophysiology: Between 1 and 24 months postoperatively, mechanical compliance mismatch (rigid synthetic or vein graft vs. native artery), cyclic shear stress, and surgical suture injury induce vascular smooth muscle cell proliferation and extracellular matrix deposition in the subendothelial layer.
• Primary Site: The distal anastomosis (especially the anastomotic floor and toe) is most vulnerable.
• Sonographic Appearance: Hypoechoic or moderately echogenic concentric/eccentric tissue encroaching upon the anastomotic lumen, producing progressive velocity acceleration ($V_r \ge 2.0$).

2. Other Structural Complications

• Graft Entrapment / Kinking: Occurs when grafts are routed through improper subcutaneous or subfascial tunnels, particularly when crossing joint creases.
• Seroma: A perigraft collection of sterile, clear transudative fluid surrounding a prosthetic graft. Appears on B-mode as an anechoic, non-pulsatile fluid collection without color filling or internal flow.
• Perigraft Infection / Abscess: Complex fluid collection containing mobile echoes, debris, or gas bubbles (bright punctate echoes with dirty shadowing), accompanied by clinical erythema and leukocytosis.