Haemodialysis vascular access
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Vascular access for haemodialysis is used for end-stage renal failure that requires renal replacement therapy. Options include temporary/permanent and non-surgical vs surgical methods. This article will focus on surgical arteriovenous fistulae.
Types of vascular access
temporary vascular access dialysis catheter
permanent tunneled haemodialysis catheter
a long term alternative, usually inserted into either the internal jugular or subclavian veins
Usually created in the non-dominant arm.
native arteriovenous fistula (AVF) is the preferred option as it has better outcomes. The three main types are:
radiocephalic AV fistula
brachioephalic AV fistula
brachiobasilic transposition AV fistula
synthetic polytetrafluoroethylene (PTFE) graft: can be used in case of unsuitable vascular anatomy, or after the failure of a pre-existing AVF
endovascular arteriovenous fistula creation: an emerging endovascular alternative to surgical AVF creation
Preoperative assessment of venous anatomy is essential in the selection of the most appropriate approach. Doppler ultrasound has largely replaced venography for this, as it is a quick and ionizing radiation-free alternative (although venography remains the gold standard). Doppler ultrasound can also be used to assess fistula maturation and potential complications such as stenosis and thrombosis.
The patient is examined in the supine position with the upper limb in a neutral anatomical position. The hand is relatively dependent (hanging from the side of the bed).
The superficial veins are scanned for patency and course. Multiple measurements of the diameters of the veins and distance from the skin should be obtained. The suitable veins should be marked on the skin surface. The veins with a diameter >0.2 cm (0.25 cm if a tourniquet is applied) and distance from skin <0.6 cm have better outcomes regarding the maturation of the arteriovenous fistula and vessel cannulation respectively.
The deep veins are scanned for patency using compressibility until the peripheral part of the subclavian vein. The central veins can be indirectly assessed by Doppler wave pattern analysis (venography may be required if central venous stenosis or occlusion is suspected).
The arteries are scanned for patency, stenosis and variants. A high bifurcation of the brachial artery is a common anatomic variant.
Arterial wall compliance can be evaluated with Doppler. The triphasic wave pattern recorded in the radial artery with a clenched fist should normally become biphasic with a resistive index (RI) <0.7. RI >0.7 and arterial diameter (inner-to-inner edge) <0.2 cm are poor prognostic factors for the maturation of the arteriovenous fistula.
AVF maturation and access volume flow
Volume flow across the feeding brachial artery for arteriovenous fistula and along PTFE graft is measured by machine-based software using the formula (area x mean velocity x 60, where the area is the cross-sectional area of the vessel in cm2).
Automatic calculation of the volume flow can be obtained by equipment software after measuring the inner diameter of the brachial artery/graft, placing a sample volume covering the entire luminal cross-section, using Doppler angle ≤60° and defining the time of the cardiac cycle.
AVF volume flow <300 mL/minute is suggestive of AVF failure
PTFE graft volume flow <650 mL/minute is suggestive of graft failure
Thrombosis and aneurysm formation
Thrombosis is the most common cause of vascular access failure. Usually, it is seen along the out-flow vein or the graft itself.
Vascular access stenosis
AVF: high resistance Doppler wave pattern in the brachial artery or reduced flow volume is suggestive of hemodynamically significant stenosis
PTFE graft: luminal diameter reduction >50% or a peak systolic velocity (PSV) >400 cm/second is suggestive of hemodynamically significant stenosis
postoperative hematoma may cause external compression and lead to stenosis
Steal syndrome (access-induced ischemia)
The steal phenomenon is converted into a steal syndrome (painful limb at rest/during hemodialysis) when compensatory mechanisms to maintain peripheral arterial perfusion fail.
The access-feeding artery is evaluated by color Doppler for a change in the flow direction. The flow in the distal arterial tree usually improves with transient occlusion of the arteriovenous fistula during the examination.
high-output cardiac failure may occur with AVF flow volume >2000 mL/minute