DIALYSIS & TRANSPLANT

Ultrasound of hemodialysis access
Ultrasound has an important role in planning temporary and permanent hemodialysis access, for examining permanent access fistulas and grafts prior to first use, and in the investigation of complications in these fistulas and grafts. Ultrasound may be used for routine monitoring of access flow, although other methods are also available.
Portable ultrasound scanners may be used in dialysis units to aid needling where access is difficult. For a more detailed examination of the access, radiology departments and vascular laboratories are equipped to undertake ultrasound investigation of the whole access circuit. The high flows, unusual hemodynamics, and anatomy of fistulas and grafts can make this a challenging investigation but it is potentially a rewarding one. Ultrasound is a quick, safe, and effective means to identify existing and impending problems, enabling early radiological or surgical intervention to prolong the use of the existing access and to plan effective alternatives.


HEMODIALYSIS ACCESS

Hemodialysis requires high blood flow, from 250 to 400 ml/min, to the extracorporeal dialyzer. Hemodialysis is used in both acute and chronic renal failure.

Acute renal failure

Following a sudden loss of renal function,1/ dialysis is usually via a central venous catheter. Ultrasound is used to 2/ examine central, neck, and arm veins and to 3/ guide catheter placement. In patients with reversible failure, dialysis is discontinued as the kidneys recover. In kidneys with 4/ irreversible failure, permanent access is planned, since central venous catheters have a limited life.

Chronic renal failure

Once chronic end-stage renal failure is identified, 1/ measurement of the patient’s creatinine levels and estimated glomerular filtration rate (eGFR) are used to determine the impending need for dialysis. Ideally, as the prospect of dialysis approaches, permanent access is planned and surgery undertaken so that the access is ready in time for first use.

Permanent 2/ access, through a fistula or graft, requires high flows through a superficial vein or graft that can be repeatedly needled (Fig. 16.1), is easy to keep clean, and is comfortable for the patient during periods of dialysis – typically 4 hours three times a week. American and European reviews and guidelines have been produced and are regularly updated to foster good hemodialysis practice (National Kidney Foundation 2006; Tordoir et al. 2007). Both contain comprehensive bibliographies. Amongst challenges for the clinical teams that may involve ultrasound investigation are the need to increase the number of patients with a permanent access in place when they first present for dialysis, identifying the most appropriate access site for patients, and maintaining effective surveillance to reduce the incidence of complications and access failure.

Figure 16.1 Needles inserted into the basilic vein of a brachial artery/transposed basilic vein fistula prior to dialysis. The distal, upstream, needle is referred to as the arterial needle (A) from which blood is drawn for dialysis. Blood is returned through the venous needle (V).

Access sites

Permanent hemodialysis accesses can be either an arteriovenous fistula where the vein is used for needle access or a prosthetic graft between an artery and vein. Fistulas generally have a lower incidence of complications than grafts (0.2 events/patient per year versus 0.8–1.0 events/patient per year for European Best Practice Guidelines) and superior long-term patency. Grafts can be used earlier following surgery whereas fistulas need more time for the vein to develop and diameter to increase, typically 6 weeks postoperatively. Recommendations are to use fistulas where possible initially.
An AV fistula is a connection that's made between an artery and a vein for dialysis access. A surgical procedure, done in the operating room, is required to stitch together two vessels to create an AV fistula

There are several possible sites for fistulas and grafts in the arms. Once arm accesses are exhausted, femoral artery to femoral vein grafts may be used. Several unusual variations of grafts and veins are possible where access is difficult, depending on the preference of the surgeon and the availability and access of suitable arteries and veins.
These fistulae are typically fashioned to connect the radial artery to the cephalic vein, the brachial artery to the cephalic vein, or the brachial artery to a basilic vein.
A surgeon usually performs the procedure in the operating room. You receive a local anesthetic (numbing medicine) at the proposed site along with IV sedation to relax you. Discomfort is minimal and you may even fall asleep during the 1 to 2 hour-long procedure.
An AV fistula is the best means of access to the blood stream for long-term haemodialysis. It means that there is no plastic dialysis line to become infected. This is particularly important as infections damage the lining of the blood vessel and cause it to become narrow.
hat are the rules of 6 AVF?
The Rules of 6 (flow volume >600 mL/min, vein diameter >6 mm, vein depth <6 mm) are widely used to determine when an arteriovenous fistula (AVF) will support dialysis.Feb .
disadvantages of AV fistula
AV fistulas may not be suitable for people with small or weak veins. – The most common problem with the AV fistula is stenosis, the narrowing of the blood vessel, which may lead to a decrease in blood flow or 
formation of blood clots. – Strengthening the AV fistula requires daily exercises

• Do not let anyone draw blood or put an IV in your access arm. ...
  
• Do not have your blood pressure taken in your access arm.
  
• Do not wear a tight sleeve, a watch, or other constricting jewelry on your access arm.
  
• An AVF can occur anywhere in the body, though we mostly find them in the head, neck, spine, and liver. The connection between a high-pressure artery and a low-pressure vein can increase the blood flow through the area, which often expands both the artery and the vein.
  
• Rule of 7 in dialysis
  
• An informal algorithm that has been commonly advocated for decades is the “rule of seven”, in which the pre-dialysis serum potassium level is subtracted from seven to determine the dialysate 
  
• potassium that should be assigned
  
• Data in the literature on DUS flow volume assessment indicate that a well-functioning AVF will be characterized by a flow rate of 700–1,300 ml/min
  

The main sites for fistulas and grafts are listed in Box 16.1 and shown diagrammatically in Figure 16.2. The nondominant arm is generally used initially. Box 16.1 shows an approximate order of preference; the choice for an individual patient depends on many factors. For example, a young patient with good peripheral arm vessels may be a better candidate for a radiocephalic fistula whereas an elderly patient with diabetes and known peripheral vascular disease might start dialysis with a more proximal access. A major advantage of a peripheral fistula is that it may leave more proximal vessels available for future access

Figure 16.2 Diagram of common arm fistula and graft sites. (A) Main arm vessels. In the other images, the arterial supply and venous drainage for the high-flow dialysis access circulation are shown in bold. (B) Radiocephalic. (C) Brachiocephalic. (D) Brachial transposed basilic. (E) Forearm loop graft. (F) Upper-arm graft.

Temporary access – role of ultrasound

Ultrasound is useful to assess central veins to ensure patency for emergency central dialysis catheter placement in patients with acute renal failure or failed permanent access. The patient should be lying flat so that the central veins are not collapsed. The internal and external jugular veins are examined using a low-frequency (4–8 MHz) linear array in B-mode and with color and pulsed Doppler to ensure patency and normal venous flow patterns. The subclavian veins may be imaged in long section either supraclavicularly or infraclavicularly (Fig. 16.3). The subclavian vein often collapses during the cardiac cycle or in response to breathing. The right and left proximal internal jugular vein and subclavian veins may be difficult to image with a linear array due to clavicle, sternum, and ribs which restrict ultrasound access. A small footprint phased or tightly curved array (Fig. 16.4) aids in color flow imaging of these veins (Fig. 16.5) and the brachiocephalic veins, although B-mode images are poor and compression of all these veins, to rule out deep-vein thrombosis, is impossible.
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Figure 16.3 Infraclavicular approach for imaging the subclavian vein with a linear array.


Figure 16.4 High-frequency curvilinear array used to image the brachiocephalic vein.


Figure 16.5 Color flow and spectral Doppler image of the right brachiocephalic vein. The B-mode image is poor but contiguous color flow and venous waveform fluctuations indicate normal venous return.

PERMANENT ACCESS

Preassessment

A successful dialysis access requires good inflow and outflow. Poor selection of vessels for permanent access is associated with high failure rates. As dialysis is increasingly offered to older patients and those with diabetes and arterial disease, ultrasound has an important role to complement physical examination in identifying the most suitable site for access surgery. Trials have shown that ultrasound measurement of arterial and venous diameters can be predictive of likely success of a radiocephalic fistula. In reviewing the results of published investigations, European recommendations are that the minimum diameter of radial artery and cephalic vein at the wrist level should be 2 mm.

Technique for assessing arm veins and arteries

The arm to be imaged is supported comfortably with the palm of the hand uppermost (Fig. 16.6). For the superficial veins, a high-frequency (for example, 8–14 MHz) linear array is used. A tourniquet is applied proximal to the measurement sites to occlude venous return and the veins are allowed to expand (Fig. 16.7).

Figure 16.6 Imaging the cephalic vein. A proximal tourniquet causes the vein to enlarge.


Figure 16.7 Transverse image of cephalic vein with (A) and without (B) a proximal tourniquet.

It is important to use very light pressure; even with a tourniquet, veins are readily compressed and the diameter may be underestimated. The veins are measured in transverse section and are scanned through their length to check for patency, narrowing, particularly near confluences/bifurcations, anatomical variations (for example, large veins communicating with the deep veins), and continuity of flow. The proximal cephalic vein may be compressed extrinsically by surrounding tissue just before its insertion to the subclavian vein. This is sometimes relieved by relaxation of the arm.

The deep veins are imaged to ensure patency for the outflow of the fistula. For the proximal arm veins, axillary and subclavian veins, a lower-frequency transducer may be required. Color filling of the deep veins is helpful where compression is impossible (Fig. 16.5). Flow waveforms in the proximal veins should show phasicity with respiration (Fig. 16.8). Lack of or reduced phasicity is an indication of possible proximal occlusion or stenosis; compare the waveforms with those on the contralateral side. A phased or curvilinear array may be helpful to determine central vein patency.

Figure 16.8 Subclavian vein imaged from an infraclavicular approach. The flow waveform is phasic with changes in right atrial pressure, indicative of unimpeded venous return.

The arteries should be examined for normal pulsatile flow to wrist level. Radial artery internal diameter should be measured and note is taken of arterial disease, including general calcification in the arm arteries. Even moderate stenoses should be noted as they can become more hemodynamically significant with the larger flow volumes after fistula creation. It is important to identify the brachial bifurcation level if surgery in the cubital fossa is contemplated. For patients with evidence of arterial disease, measurement of right and left brachial artery blood pressure may reveal proximal disease.

For radiocephalic fistulas, it has been suggested that measurement of flow waveform changes in an induced hyperemic response in the radial artery prior to surgery is a good measure of the likelihood of success of the fistula. A low resistance index (< 0.7) in the initial period of hyperemia was associated with a markedly improved outcome when compared with fistulas formed from arteries with an index ≥ 0.7 (Malovrh 2002) (Fig. 16.9), although others have been unable to show the same discrimination between fistula success and failure (Lockhart et al. 2004).

Figure 16.9 Radial artery flow waveform changes following a 2-minute hand clench. The hyperemic response is evidence of higher velocities throughout the cardiac cycle, with a marked change in diastolic flow which can be measured by a resistance index of 0.64.
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