الفهرس 
PHYSICS OF COLOR DOPPLER ULTRASONOGRAPHY
US MAPPING TECHNIQUEOnly 14 pages are availabe for public view
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Abstract
S
tandard techniques for assessing AVF ana¬tomy and flow have major limitations. Clinical assessment, indicator-dilution techniques, and pulsed-wave Doppler without ultrasonographic imaging do not provide sufficient anatomical information. Angiography is an invasive tech-nique, so it cannot be frequently repeated and can be associated with discomfort and poten¬tial fistula injury (Abigail and Timothy, 2005). Furthermore, in patients with total or near complete fistula occlusion due to thrombosis, puncture of the vessel may be difficult or impossible. The length of a thrombosed segment is not easy to assess on angiography, and sometimes non-thrombotic stenoses cannot be differentiated angiographi-cally from partial AVF thrombosis; this diffe¬rentiation, however, is important for decisions on fistula repair. In addition, aneurysms filled with thrombotic material can be missed (Andrew etal., 2007), so duplex ultrasound is superior since it provides information both on the morphology and on the function of vascular access. In addition, CDU offers the advantage of a non-invasive bedside procedure with lower costs and with no need for radiocontrast (Weise and Nonnast, 2004).
Duplex sonography can be used to detect stenoses: the accuracy of the technique is estimated 90%. The loss in specificity can be explained by turbulent flow patterns set up within venous access fistulas. The diagnostic accuracy is improved in straight segment grafts to the efferent veins, where the accuracy increases to 95%. Peak-systolic velocities in well functioning dialysis access grafts are typically above 100 to 200 cm/sec, tending to be higher in the first 6 months after graft placement or shunt creation. Superimposed stenosis can therefore be difficult to detect given the high baseline velocities. The length of the graft or fistula must be examined with color Doppler imaging in order to insure detection of any significant stenoses. An area of increased blood flow velocity of 100% (velocity ratio of 2 or more) is considered to be consistent with high grade stenoses (Weise and Nonnast, 2004)
The diagnostic criteria on color Doppler and duplex ultrasonography for stenosis are visible narrowing of the lumen, increased velocity of flow greater than 100% compared with that in adjacent normal segment or a region of focal aliasing in color Doppler study (Azman A etal., 2006)
In our study, venous outflow stenosis accounted for %40 of access malfunctions, while access thrombosis was the second frequent access complication (30%). This percentage is not consistent with that reported in other screening programs, in which a lower percentage of stenoses were found in the groups studied.
In the study of Culp etal., 1995 access thrombosis was the predominant complication reaching 70% and stenosis was noted in only 20% of the patients. This may be explained by the higher sensitivity of measuring flow rate that we adopted in our study, which can diagnose any re¬duction in the flow rate associated with access stenosis
Asymptomatic venous stenosis with inadequate flow rate, was found in 3 patients (6%) with the percentage of stenosis ranging from 30% to 60%, while stenoses with edema¬tous upper limb was found in 7 patients (14%) with significant stenosis ranging from 55% to 85%. Schwab etal., 1989 found hemodinamically significant stenosis greater than 50% in 86% of patients with low AVF flow rate.
The location of the stenotic segment differs from one study to another. In our study, nine stenotic segments were found at the re¬gion of AVF, six in the venous limb distally and five stenotic segments in the subclavian vein. In the study of Sullivan etal., 1993 among patients with limb edema, the dominant stenosis was in the cen¬tral vein in seven cases and at the venous anas¬tomosis in one case.
Two false negative results (4%) were found in our study, missed due to overlying bony structures on subclavian stenosis. In other words CDS may be intermediate (i.e. not highly sensitive) in few cases because incom¬plete visualization of the subclavian or bra¬chiocephalic veins caused by artifact from bony structure (Sullivan etal., 1993).
Turbulent blood flow causes extensive vessel wall and perivascular tissue vibration. This lo¬calized tissue vibration causes artifactual color assignment of the perivascular soft tissues, which precludes adequate visualization of the venous anastomosis (Middleton etal., 2004), but the later cause may be overcome by mild compression of the afferent feeding artery decreasing flow to vein for better localization of stenotic area(Weise and Nonnast, 2004)
In our study, in 15 patients, the presence and location of stenoses were confirmed during the operation. This finding is consistent with that of Zibari etal., 1988. A new anastomosis was created in six patients for these fifteen patients. In eight patients, the stenotic segment was resected. A new access site had to be chosen in only one patient.
Thrombosis remains the most serious cause of graft failure. Thrombosis of a vascular access is diagnosed at physical examination and can easily be confirmed sonographically; typically anechoeic or hypoechoeic clot is seen within the vessel with absence of blood flow noted during pulsed or color Doppler study(Weise and Nonnast, 2004).
In our study, only 4 cases with PTFE graft were met; 3 of them showed mild thrombosis and were accompanied with peripheral ischemic changes of the hand (steal). It has been reported that early access failure is more common in AV fistulae than in prosthetic PTFE grafts (Middleton etal, 2004) However, other studies found that established AV fistulas had better long-term survival. The later results correlated to much extent with our study results.
In the study of Hill etal.1991, thrombosis of the graft occurred in 32% of patients, and many procedures were required to maintain patency of the original shunt or to place a new one. On the other hand, Green et al., 2002 reported that thrombectomy of the AVF alone did not provide satisfactory results in most cases. Thrombectomy was initially successful in almost every case, but the graft remained patent for only several hours to days in most patients. They confirmed that traditional treatment of thrombosed vascular access has been thrombectomy and surgical revision; however, thrombolysis, percutaneous angioplasty, and placement of venous stents have all been used to prolong the life of thrombosed fistulas.
True aneurysmal dilatation does not require treatment (often) but false aneurysms are always managed surgically. Although aneurysms or pseudo-aneurysm can be detected clinically, color Doppler flow imaging allows better esti¬mation of their size, of the degree of mural thrombosis, and of the size of their neck (Sinnamon and Mullan, 2007)
In our study 14 cases with venous aneurysmal dilatation were met; 12 0f them (i.e. vast majority) were true aneurysmal dilatations. This means those true aneurysmal dilatations are common from arterialization of the veins, which is aggravated by repeated puncture of the arterialized vein. In our study, the vast majority of these cases was of Brescia Cimino type (10 cases) and was static with intact over lying skin.
The remaining 2 cases were false aneurysmal dilatations; one was small (micro-pseudoaneurysm: diameter <5) which was stable and clinically non significant while the other was macro pseudoaneurysm (diameter >5) which is clinically significant (tend to enlarge) and was treated surgically.
Arterial steal is defined as retrograde flow in the native artery distal to the anastomosis. This phenomenon is most easily demonstrated in Brescia-Cimino fistulas, in which blood flow from the ulnar artery passes through the palmar arches into the distal radial artery and subsequently courses retrograde into the low resistance fistula. Such retrograde flow can also occur with brachial artery anastomosis, as smaller vessels can act as collateral path¬ways (Cornelis etal., 2005)
Increased flow rates, more than 1500 mL/min, were associated with either steal syndrome or venous hypertension. The Doppler findings noted in steal syndrome was marked reduction of flow in distal artery evidenced by low PSV with even end diastolic reversal, while high flow rate reaching over 900 mL/min in distal venous limb of the AVF is associated with venous hyper-tension. Legating the distal cephalic vein suc¬cessfully treated peripheral venous hyper¬tension and stiffness limited to the hand and distal forearm.
In our study we have 7 cases showing steal phenomenon; 5 of them were Radio cephalic ( Brescia Cimino shunt; i.e. majority of cases) which correlate to much greater extent to the study done by Cornelis etal., 2005. While the other two cases were one brachio cephalic and one brachiobasilic. In our study only 2 cases (out of 7) developed ischemic changes (coldness of the hand with pain, parasthesia and motor weakness). Because of the adequate efficient collateral circulation of the other cases, these two cases were one Brescia Cimino and the other was Brachiobasilic.
Concerning our cases developing steal phenomenon and ischemic symptoms, we noted that symptomatic improvement occurred on manual compression of the fistulae.
On follow up of the two cases, one was treated by surgical banding of the fistula (increasing its resistance) while the other is closed.
Considering the angiography as the gold standered, a study done by Middleton etal., 1989 aimed to define the color Doppler sonographic appearance of normal and abnormal hemodialysis vascular access and to compare color Doppler sonography with angiography. Twenty-eight patients were imaged with digital subtraction angiography and color Doppler sonography. The examinations were interpreted independently and then interpreted together to determine the accuracy of the two methods. Middleton etal.,1989 founded-with angiography as the gold standard-that color Doppler sonography correctly identified 20 of 23 stenosed vessels, three of four occluded vessels, four of four thrombosed vessels, and 18 of 19 pseudoaneurysms. Overall, color Doppler sonography correctly identified all lesions that were seen angiographically in 16 of 19 patients with synthetic grafts and in four of nine patients with Brescia-Cimino fistulas. Five asymptomatic arterial steals were detected sonographically. One of these was identified with angiography. They concluded that color Doppler sonography is an adequate means of imaging suspected complications of hemodialysis vascular access, and it should be used in selected patients.
Although subjective assessment of the AVF for maturation as detailed earlier is described (pag38), we found little in the literature about the objective evaluation of AVF maturity.
Wong etal., 1996 found a mean diameter of 0.58 cm ± 0.12 in the successful radiocephalic fistula at 6 weeks. However, little detail regarding diameters of unsuccessful fistulas was given. Therefore, in our study we sought to determine whether the minimum diameter of the outflow vein was predictive of eventual fistula maturity. An adequate fistula diameter is important, as a 15-gauge hemodialysis needle is placed into the vein by means of palpation and visual inspection.
In our study, our initial hypothesis based on ultrasound findings in correlation of clinical experience was that a minimum venous size of 0.4 cm is needed for easy cannulation.
The draining vein needs to be of a certain size not only for ease in needle placement but also for carrying adequate blood flow. It is important to correctly identify fistulas that are likely to mature, as well as those that will not (Weise and Nonnast, 2004).
In our study a minimum venous diameter of 0.4 cm was accompanied with adequate fistulae. Women are less likely to achieve a minimum venous diameter of 0.4 cm, a finding consistent with Prior observations of inferior fistula outcomes among women. This may be in part due to smaller vessels in women. The ability to maintain adequate blood flow during hemodialysis is another crucial determining factor in AVF maturity.
Generally, hemodialysis is typically performed at a dialysis blood flow rate of 350 mL/min, for 3.5–4 hours three times per week. A fistula blood flow rate less than 200 mL/min cannot sustain the desired dialysis blood flow rate and therefore results in inadequate dialysis.
In our study, blood flow rate has been measured to be of average 500 mL/min in normally functioning AVFs. Fistula adequacy according to blood flow volume and venous diameter was found to be more frequent among men and those with good stature. In contrast, inadequate fistulae were notable among women, poor statured, young girls and underweighted patients.
Lin etal., 2005 found a mean blood flow rate greater than 634 mL/min in the 2nd postoperative week in 152 patients with successful radiocephalic fistulas. Mean blood flow rate in successful AVFs was reported by Wong etal., 1996 to be approximately 650 mL/min at 12 weeks. Hence average blood flow rate in our randomized study was to much greater extent similar to those results found by Lin etal and Wong etal.
In our study, The number of the patients with preoperative CDU assessment (from archive, NO SELECTION) was 34 cases (68%) from the total number of cases of the study, showing appreciable results with 32 cases showing favorable outcome and 2 cases failed while those not underwent preoperative CDU assessment was 16 cases (32%) with 9 cases showing favorable outcome and 7 cases failed, which is alarming for the great value of CDU in preoperative assessment of the shunt