Arterial occlusive disease that does not respond favourably to or is not treatable by angioplasty and stenting is often treated by surgically bypassing the arterial occlusion with a bypass graft. These grafts can be synthetic or created from one of the patient’s superficial veins and can be in the neck, abdomen, arms or legs. The function of these bypass grafts can easily be assessed with duplex ultrasound to determine how well they are working and if there are any areas of narrowing (stenosis). They can also be assessed for aneurysmal dilatation and intraluminal thrombus.
After a bypass graft has been performed the Vascular Surgeon will often request regular routine duplex ultrasound surveillance of the bypass graft to ensure that any early complications in the graft will be detected. Regular monitoring of the graft complications, and if necessary early surgical intervention to correct those complications, leads to greater long term patency of the graft.
The frequency of ultrasound surveillance of bypass grafts is usually at 3months, 6months and 12 months post-surgery. If the bypass graft is working satisfactorily at 12 months post-operation, frequency of ongoing surveillance is annually.
PATIENT PREPARATION AND EXAMINATION TECHNIQUE:
Upper Limb & lower limb bypass grafts: no preparation required.
Abdominal grafts; fasting for 6 hours prior to the examination to minimize bowel gas interference. All medication should be taken with clear fluids.
The examination is non-invasive with no use of needles with patient privacy ensured.
The patient is examined in a dim, quiet room and is examined lying on the bed with the body area to be examined exposed. Gel is applied to the skin and the ultrasound probe moved over the skin of the area of interest. B-mode and Doppler ultrasound is used to assess the bypass graft as well as the arteries above and below the bypass graft to determine patency. Blood flow throughout these arteries and graft is measured with any area of stenosis identified and recorded. Any aneurysmal dilatation or the presence of thrombus is documented. A detailed worksheet is created and a formal report generated.
The examination takes approximately 20 – 30 minutes. Resting/exercise ABI studies are often performed at the same time.
B-Mode imaging of the bypass graft to assess for any aneurysmal dilatations and thrombus.
Velocities of blood flow throughout the graft and in the inflow and outflow arteries recorded.
The waveform and velocities within the graft will vary depending on inflow or outflow disease, whether the graft is synthetic or a vein graft and the diameters of the graft. Each graft needs to be assessed depending on its own characteristics.
As the severity of a stenosis increases the peak systolic velocities also increase at this point. Generally, the higher the PSV the greater the degree of stenosis. Comparing the highest PSV at a stenosis to the PSVs more proximally can help us generate a ratio which can help us grade the severity of the stenosis.
The stenosis grading criteria is <50% (ratio of 1.5-2), >50% (ratio of 2-4), >75% (ratio of >4) and occlusion.