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| Dialysis Access Graft: B Mode and Color Doppler Ultrasound Imaging Before and After Vascular Access Graft Placement by Katrina Roberson |
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Abstract
Ultrasound is an excellent diagnostic imaging modality before and after the placement of vascular access grafts. Hemodialysis grafts are currently the predominant form of permanent access for patients with end stage renal disease (ESRD).1 Although grafts have an important role in delivering hemodialysis, graft rates have poor duration due high infection rates.2 The aim of this article is to focus on ultrasound imaging and how it can provide a great detailed evaluation of venous and arterial anatomy preoperatively and postoperatively. This article will also discuss the basics of hemodialysis, the preoperative examination before graft placement, types of grafts, a standard vascular protocol for imaging the graft, and some complications that occur after graft placement. |
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Keywords: synthetic polytetrafluoroethylene (PTFE) grafts, vascular access
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Introduction
Ultrasound is the most accurate, non-invasive technique that can show more vascular detail than physical examination, without the risk of conventional IV contrast agents.2 A detailed evaluation of arterial and venous anatomy can increase visualization of vessels suitable for graft placement, particularly in the patient with a history of failed access/or central line placement.2 A high resolution linear ultrasound transducer is used to evaluate the upper and lower extremities (arteries and veins) and to evaluate their diameter and wall thickness.3 The veins are assessed for competency, distal augmentation and sequential vein compression is used to verify patency of the venous system. The arteries are assessed for intimal thickness and stenosis.3 |
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A thorough preoperative evaluation with Color Doppler ultrasound (CDU) and venous mapping allows for successful graft placement due to direct visualization of the vessels under investigation.1, 2 A thorough preoperative evaluation with Color Doppler ultrasound (CDU) and venous mapping allows for successful graft placement due to direct visualization of the vessels under investigation.1, 2 Vascular mapping prior to hemodialysis access also may change surgical management with an increase in the number of grafts placed.2 When compared to arteriovenous fistulas, grafts are considered undesirable for hemodialysis access because of poor duration and higher infection rates.2 In elderly patients with co-morbid medical conditions (e.g. diabetes, vascular disease), maintaining the patency duration of vascular access grafts has become a challenge to both vascular surgeons and nephrologists. However, in the United States, polytetrafluoroethylene (PTFE) grafts remain the predominant form of permanent vascular access for patients who receive hemodialysis.1 Postoperative ultrasound investigation after vascular access placement is also important because it can detect many complications that occur from grafts such as of stenosis, thrombosis, and arterial insufficiency.2 Although several surveillance methods have been suggested for the detection of graft complications, no definite consensus regarding which is the best method have been established. 1, 2 |
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Basic Concepts of Hemodialysis
"Hemodialysis is the lifeline by which the patient with end stage renal disease eliminates fluids and various wastes from the blood. An artificial graft may be tunneled in superficial soft tissues of the forearm, upper arm, or upper thigh with arterial and venous anastomosis." 2 Hemodialysis uses a dialyzer, or a special filter to clean the blood and rid the body of harmful wastes.4 Two 15 gauze needles are place in the access graft, with a distal needle carrying blood from the patient to the dialyzer. The other needle is placed in a more proximal location in the graft and blood is then returned to the patient’s blood circulation.2 "Access can be internal (usually under the skin) or external (outside the body)." 4 |
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Preoperative Upper Extremity Vascular Evaluation
Duplex Ultrasound is an excellent diagnostic imaging tool for evaluating vessels of the upper extremity. An important aspect of planning for graft creation is vessel size assessment.2 Vein mapping of the upper and lower extremity is performed to assess the suitability of a vein prior to creation of permanent dialysis access in the patient with chronic renal failure.5 An experienced vascular sonographer performing the exam will provide the surgeon a more detailed identification of suitable upper extremity vessels. A High- resolution B mode duplex with color flow imaging scanner with a 7.0 MHZ/5 MHz Doppler is used to assess the patency of the vessels under investigation.1 In transverse view, the basilic vein is evaluated for a basilic vein transposition or graft and the brachial veins are measured for possible graft placement. If a suitable vein is found, its continuity should be confirmed with the deep venous system. A tourniquet may be used to dilate the veins.2 The vessels are measured along their entire length 1-2 cm paying special attention to the area of focus proximal, in the area of, the antecubital space, and distal to the antecubital space.5 The basilic vein should extend at least 4 mm for adequate graft placement. Dialysis grafts may be anastomosed to either the cephalic or basilic vein.2 Using B-mode imaging precise diameter calculation is measured from inner edge to inner edge.1 |
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Venous outflow tract
In addition to color Doppler, B mode imaging also valuable for investigation the venous system. An evaluation of the entire venous outflow tract is needed to determine the patency of the basilic, cephalic, brachial, axillary, subclavian, innominate or any veins that may be a recipient of the graft (IJV, GSV, etc).3 Studies are usually unilateral studying the non-dominant arm.5 An experienced vascular sonographer will evaluate the vessels to see if the veins are available, if they are free of defects (such as residual thrombus) and for patency.3 The internal jugular and subclavian veins should be evaluated routinely for respiratory phasicity and for any transmitted cardiac pulsality.2 Non-visualized portions of the central veins (axillary or distal subclavian) can only be assessed indirectly by Doppler waveform analysis.1 Disturbances in respiration and a missing increase of venous flow during deep inspiration may point at a stenosis of the central venous system.3 B- mode imaging data interpretation includes determination that the vein walls are compressible and free of any blood clots.5 A non-compressible or partially compressible vein indicates the presence of an occluding thrombus within the lumen of the vessels. The vein will not be acceptable as a conduit if a blood clot or thrombus is present. Spectral and Color Doppler is used to document any indirect evidence of superficial or deep vein thrombus and for localizing and assessing venous insufficiency.5
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Arterial Inflow Evaluation
An arterial inflow assessment is also important for graft placement. Bilateral brachial pressures will be assessed for the absence of significant pressure differences > 20 mm Hg.1 Using Spectral and Color Doppler ultrasound scan the proximal and distal brachial and radial arteries and look for occlusion, stenosis, and anomalous anatomy.3 If the graft is placed in the upper arm or forearm, a sonographic assessment of the feeding brachial artery is critical. Identify diseased arteries and document patency of the arterial system. In longitudinal plane, Doppler spectral analysis is performed with samples being performed at 60 degrees or less in respect to blood flow. Cursor gate alignment is highly recommended parallel to the vessel walls.5
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Types of Access Grafts
The four most common grafts include a forearm loop graft, upper arm straight graft, axillary loop graft, and a thigh graft. The graft may be synthetic or it is usually made of polytetrafluoroethylene (PTFE), which is commercially known as Gore-Tex.3 The arterial end of the graft may be tapered off at the end to reduce the volume of flow.3 A forearm graft is preferred to placement of an upper arm graft.2 If no upper extremity graft is available, a thigh graft is preferred.2 Vascular surgeons prefer to create vascular access in the non-dominant arm so the patient can continue to carry out his/her daily activities with the dominant arm.2
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| A Common straight graft may begin at the distal radial artery and connect to the cephalic, median cubital or basilic vein in or near the antecubital fossa distal to the brachial access feeding artery and connect to the proximal basilic or axillary vein.3 A forearm graft is preferred to placement of an upper arm graft.2 |
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A Common loop or thigh graft may begin at the distal brachial artery and connect to the cephalic, median cubital or basilic veins, the proximal brachial artery to axillary vein, or the superficial femoral artery to the greater saphenous vein. After graft placement, color Doppler can extensively investigate the graft through its entire length. Other sites may be selected if the patient has a history of previously used graft sites. Graft placement can be very creative for patients on long term dialysis.3
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Protocol for Duplex Imaging of Graft
Grafts can be investigated by using Doppler ultrasound along the entire length of the graft. “The graft is readily identified sonographically by the presence of two echogenic parallel lines representing the graft wall.”2
One should explain the procedure to the patient and obtain any history pertinent to vascular disease. Determine type of graft, configuration of graft, date of the initial surgery and any surgical revisions. Position the patient comfortably, externally rotating the arm to access graft site. Note any conditions that may limit or contraindicate Duplex examination such as incisional tenderness, hematoma, and signs of infection, obesity, and metal staples. Palpate any lumps. Do not take a blood pressure over a synthetic graft. If the access graft is bleeding, use plastic wrap, Tegaderm, or a transducer cover with sterile gel.
Begin by examining the inflow artery with the probe in a short axis plane just proximal to the graft. Scan from the proximal end through the distal end to quickly determine the course of the graft and observe for extravascular abnormalities. Document and tape any characteristic segment, describing normal anatomy as well as any abnormal findings.Scan the entire length of graft in longitudinal plane moving from proximal (inflow artery), through the graft itself, and distal (run off vein).
Identify the proximal anastomosis in long axis. With pulsed Doppler, interrogate in the inflow artery, beginning approximately 2 cm prior to the anastomosis through approximately 2-cm beyond the arterial anastomosis. Digitally capture images of any abnormal findings.Utilizing color Doppler imaging, slowly move along the graft in increments equal to the length of the probe face, observing any flow disturbances, lumen reduction, soft tissue bruits, or other abnormality from inflow to outflow points.
Identify the distal anastomosis. A change in transducer may be necessary in order to visualize a deeply placed distal anastomosis. With pulsed Doppler, interrogate beginning approximately 2 cm prior to the anastomosis through approximately 2-cm beyond the distal anastomosis. Carefully examine the entire length of the outflow artery for any abnormality. Digitally capture images of any abnormal findings.
The technologist performing the exam reviews and signs the report. Any results of urgent nature are called as a preliminary report to the ordering physician. The reading physician will review the results either the same day or the next day of the examination. Any additional comments will be made at this time. The original report becomes part of the patient’s hospital chart. Copies are sent to the attending physician and to any consulting physicians. Before releasing any patient information, compare results with previously recorded values, if available, in order to detect changes that may indicate new graft lesions or progression in the degree of stenosis in the existing lesion. 2, 3
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Normal Appearance of Access graft
Grafts have very high systolic velocities (100-400 cm/s), high diastolic velocity (60-200), high flow, and low resistance. The flow pattern may be disorganized with spectral broadening. Normal high flow volumes may be maintained in the presence of venous stenosis, if there is retrograde flow in the vein distal to the efferent anastomosis. Recently created access grafts may not penetrate the graft wall of the synthetic material for a few days. The Doppler signal may not penetrate the graft wall and this could lead to a false determination of occlusion. In this case, rely on information from the inflow and outflow vessels.3 |
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Occlusion of the Access Graft
No flow is detected in graft with color or spectral Doppler
No flow in efferent vein distal to graft.
A high resistance waveforms at the proximal anastomosis of the graft.3
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Common Complications of Graft Failure
Graft failure is intimal hyperplasia caused by stress at the venous anastomosis site.1, 2 Graft stenosis usually occurs in the venous outflow tract between the graft anastomosis and the vein. Frictional force is generated by blood flow at sites of changing luminal diameter at the site anastomosis.1 Color Doppler can be estimated by mosaic color aliasing. Ultrasound graft surveillance is reserved for patients whom there is intermediate likelihood of graft stenosis.2 Aliasing artifact lowers the frequency components when the pulse repetition frequency is less than twice the highest frequency of a Doppler signal.1 A peak systolic ratio (PSV) equaling or exceeding 3.0cm/sec. is usually a conformation of a stenosis at the arterial anastomosis resulting in 75% reduction in the diameter lumen.3 A PSV ratio is calculated at the anastomosis and at any visible stenosis site. When the PSV exceeds 2.0cm/sec. or greater, the stenosis is classified as equaling or exceeding a 50% luminal reduction.2, 3 For calculation of stenosis, the minimal intraluminal cross-sectional area is compared with the diameter of a nearby normal segment using the formula 1
(orginal lumen-residual lumen/original lumen)x 100% = stenosis
Recently created grafts have the ability to contain air in the wall of the synthetic material for a few days. Doppler signals may not penetrate the graft wall of the synthetic material and lead to a false occlusion. One should rely on any information from the inflow artery for criteria of a graft stenosis.3
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Thrombosis in the Hemodialysis Graft
Thrombus is the most common form of hemodialysis graft failure.1 Color Doppler ultrasound can be extremely useful in detecting thrombosis.1 PTFE graft thrombosis is primarily the result of progressive venous outflow stenosis with access blood flow decreasing progressively after creation of access.1 CDU is the safest, accurate, and non-invasive method for direct detection of thrombus formation within grafts. For postoperative diagnosis of thrombosis, indirect color Doppler parameters include triphasic waveforms and low flow values of the access feeding artery.1 In addition, direct visualization of thrombosis formation should be the focus, due to indirect ultrasound parameters at the feeding artery that may be similar in association with severe graft stenosis and thrombosis. Localizing the extent of acute or chronic clot is assessed easily with color Doppler versus B mode imaging, because fresh acute thrombus may have similar echogencity as blood. Chronic thrombus will show up more echogenic as blood. B mode imaging is better for diagnosis of chronic clot due to it’s brightly echogenic nature. Manual compression of the veins with a transducer is not effective enough to diagnosis vascular access thrombosis using B mode imaging because the access draining veins react like arteries when compressed.1 |
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Arterial Insufficiency “The Steal”
Elderly patients usually females age 60 and over are at greater risk of arterial insufficiency.2 Patients usually have diabetes, vascular disease, and end-stage renal disease. These patients have an increased risk of developing a ‘steal’ syndrome. Steal phenomenon is particularly frequent in patients with prosthetic straight or loop grafts Because of low resistance in the venous outflow, the graft sucks not only the antegrade flow into the feeding artery but also ‘steals’ retrograde flow from the hand via the palmar arch and jeopardizes adequate perfusion of the hand. Some ‘steal’ occurs in 75–90% of patients after creation of the vascular access. Usually the steal phenomenon is clinically silent and the patient remains asymptomatic.1 To assess for arterial steal, perfusion distally in the hands and fingers is necessary. Indirect testing measures as an adjunct to color Doppler. Obtain pulse volume recordings, or photoplethsmography (PPG waveforms on all digits) and compare to the asymptomatic hand.3 Doppler assessments is used to predict low flow steal or low diastolic flow in correlation of the palmer arch arteries vasodilating.1 |
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Conclusion
B mode imaging and color Doppler ultrasound are both excellent diagnostic imaging modality for assessing vascular assesses grafts. Graft placement is increasing due to the high population of patients on hemodialysis. Although color Doppler and ultrasound imaging are valuable modalities for surveillance of the vascular access graft, many surveillance methods have been suggested for detection of graft failure. Grafts require constant investigation to determine patency rates, but current no definite consensus has been established regarding the best method of surveillance at this time. However, research in this important area continues because all other methods of surveillance are less invasive than regular angiography.2 |
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References:
1.Patrick Wiese and Barbara Nonnast-Daniel. Colour Doppler ultrasound in dialysis access. Nephrol Dial Transplant (2004) 19:1956-1963. http://ndt.oxfordjournals.org/cgi/content/full/19/8/1956
2. Zwiebel, William J., Introduction of Vascular Ultrasonography. 5th Edition. 325-339 Philadelphia: W.B. Saunders Company., 2004© 2005 National Kidney Foundation of
33. Daigle, Robert. Tecbniques in Noninvasive Vascular Diagnosis. 2nd Editon. 205-212. Littleton, CO: Sumner Publishing, 2002
4. National Kidney and Urologic Diseases. “That's Right For You," Clearinghouse, 3 Information Way, Bethesda, Maryland 20892-3580.
5. Vascular Professional Performance Guidelines. Upper Extremity Vein Mapping. Society for Vascular Ultrasound. 2005. www.svunet.org
Miscellaneous
1.Figure1.http://www.vasca.com/patients_families/pat_2.asp
2.Figue .http://www.vasca.com/patients_families/pat_2.asp
3.Figure .http://www.iame.com/learning/upExtVen/upextven.html
4.Figure4.http://venousacess.com/ANATOMY.HTML
5.Figure5.http://ndt.oxfordjournals.org/cgi/content/full/19/8/1956#BIB21
6.Figue6.http://ndt.oxfordjournals.org/cgi/content/full/19/8/1956#BIB21
7.Figure7.http://ndt.oxfordjournals.org/cgi/content/full/19/8/1956#BIB21
8.Besarab,Lubowski T, Frinak S, Ramanathan S, Escobar F. Detecting vascular access dysfunction. ASAIO J 1997; 43: M539–M543[ISI][Medline]
9. Strauch BS, O’Connell RS, Geoly KL et al. Forecasting thrombosis of vascular access with Doppler color flow imaging. Am J Kidney Dis 1992; 19: 554–557[ISI][Medline]
10. Kwun KB, Schanzer H, Finkler N, Haimov M, Burrows L. Hemodynamic evaluation of angioaccess procedures for hemodialysis. Vasc Surg 1979; 13: 170–177[ISI]
11. Wiese P, Eras J, Koch C, Nonnast-Daniel B. Colour Doppler ultrasound imaging of radial and ulnar artery development and flow contribution in arteriovenous fistulas. Nephrol Dial Transplant 2003; 18: S729
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