T. Rajarajan,

Senior Consultant Transplant Nephrologist, Kauvery Hospital, Tennur, Trichy

Background

Doppler is the property of a sound wave to change its frequency when a source and the observer are in relative motion. First identified by an Austrian physicist Christian Doppler in 19th century, Doppler has a tremendous impact on medical imaging sciences for imaging blood vessels, flow characteristics and their velocities.

AVF is not only the lifeline for End Stage Renal Disease (ESRD) patients but also an Achilles heel for a nephrologist. For a successful AVF creation, we need a good artery and a vein. Doppler evaluation of the vessels has superseded clinical examination in predicting the outcome [1,2]. In this section, we discuss pre-AVF vascular mapping by using Doppler ultrasound and also flow volume assessment once AVF is created.

Basics of Doppler imaging principles

Doppler imaging works on the principle of Doppler shift which is nothing but the shift of frequency of sound waves produced from the transducer and the reflected waves from the moving blood cells inside a blood vessel. This shift in frequency is calibrated and converted to electrical energy and amplified. This produces colour flow mapping images, and spectral patterns by the machine using some complex software.

Basics-of-Doppler-imaging-principles

Christian Doppler became famous after he discovered that shift in frequency depends on the velocity of the moving object and this imparts an important application of vascular imaging. The angle of insonation, the angle between the transmitted beam and the moving objects (blood cells), an important variable in determining the Doppler frequency shift and the image quality, should be parallel to the blood vessels.

Prerequisites for Doppler imaging

Doppler-machine
  1. Ultrasound Doppler machine
  2. High-frequency probe 12 MHz and above
  3. Tourniquet
  4. Comfortable room, with a comfortable temperature

Relevant Controls in Doppler machine one needs to know

  1. B mode: Grey scale imaging of vessels-vessel anatomy, intimal calcification, thrombosis.
  2. CFM (Colour Flow Mapping) images of the blood flow inside the vessel: Blue flow away from the probe, Red flow towards the probe.
  3. PW (Pulse Wave Doppler): gives spectral pattern- determines velocity and flow volume.
  4. Measure (Calliper): Measures vessel wall diameter.
  5. Doppler gate or sample volume area which the pulse wave Doppler machine interrogates. Should cover at least 2/3 of the vessel diameter.
  6. Insonation: angle formed between the transmitted wave and the moving objects (blood cells).
  7. Baseline/PRF (Pulse Repetitive Frequency): adjustment of signal transmitted frequency from the probe to prevent aliasing.
  8. VW filter-adjust to reduce the Doppler frequencies from the surrounding structures.
  9. Angle steer: adjust the interrogated direction of the transmitted wave.

Components of vascular mapping [3]

Components-of-vascular-mapping

Tips for Venous mapping

    1. Measure cephalic vein diameter at different levels
    2. Apply a tourniquet and measure the venous diameter to assess distensibility. Adequate Venous distensibility predicts good AVF maturation.
    3. Trace cephalic vein till the axilla to look for stenosis, and thrombosis.
cephalic-vein
    1. Not to forget the central veins to look for its phasic variation (variation of the spectral wave pattern of the vein with respiration; absence of phasic variation indicates proximal thrombosis.
Longitudinal-image-of-the-vein
Longitudinal-image-of-the-vein-1
  1. Longitudinal image of the vein to look for vessel wall thickening.

Tips for Arterial mapping

    1. No need to apply the tourniquet.
    2. Room temperature, and even the gel applied over the skin, should not be cold so as to avoid vasospasm and erroneous measurement of diameter.
    3. Arterial diameter is to be measured at various levels. Immediate and early failure rates of 55 and 64 % when arterial diameters of <1.5 mm(2).
    4. Look for vessel wall calcification- If present should be marked to avoid anastomosis in that part for the reason vessel wall won’t contract adequately leading to unstoppable bleeding following surgery and also it may result in failure in maturation.
Arterial-mapping-of-upper-limb
Arterial-mapping-of-upper-limb-1

Fig. 1. Arterial mapping of upper limb.

    1. Always test for hyperaemic response of the radial artery to identify its elasticity to prevent distal vascular compromise post AVF creation.
Triphasic-arterial-waveform
    1. Triphasic arterial waveform should change into the monophasic wave with prominent diastole of RI <0.7.
    2. Allen test: To assess the patency of the palmar arch can be done by Doppler imaging of the palmer arch before and after the compression of the radial artery to look for retrograde filling from the ulnar artery.
dopler-of-palmer-march
  1. It’s important to measure the artery and venous distance from the skin to alert the surgeon in order to avoid inadvertent damage to the vessel after skin incision and also inter vessel distance to plan for the length of incision.
Doppler-characteristics

Ideal Doppler characteristics of successful AVF outcome [5]

pheripheral-artheris

Normal Doppler characteristics of functioning AVF [5]

Duplex-exam-for-sonographic

Duplex exam for sonographic maturity [4]

  1. Diameter ≥ 0.40 cm
  2. Depth of the vein ≤ 0.50 cm
  3. Volume flow ≥ 500 ml/min

Flow volume assessment of AVF

Modern-Doppler-machines

Flow volume is assessed by the above formula. Modern Doppler machines have preinstalled software to calculate flow volume.

Importance of flow volume assessment

  1. Identify early access dysfunction.
  2. Diagnosis of high-flow AVF.

Ideal site to measure the flow volume

There exist two schools of thought regarding the ideal site for measuring the flow volume.

Measuring at AVF draining vein

  1. Direct representation of the flow.
  2. Should be measured 10 cm away from anastomosis.
  3. Segment should be straight with a uniform diameter.
  4. No branching veins.
  5. More amenable for compression leading to false low volume.

Measuring at Inflow artery (Brachial artery):

  1. Indirect representation.
  2. Measured 5cm from anastomosis. Not very deep.
  3. Brachial artery preferable.
  4. Advantage of non-compressibility and absence of significant branches.

Radial artery is avoided as the retrograde flow from ulnar artery will miscalculate the actual flow volume.

Valid tips for accessing flow volume:

Interpretation-of-fistula-flow-volume
  1. Brachial artery
  2. Ambient room temperature.
  3. Straight segment at least 5cm from anastomosis with uniform diameter.
  4. Angle of insonation <60 and as parallel to flow.
  5. Area of interest should not be broad.
  6. Adjust PRF/baseline to avoid aliasing.
  7. Gate should cover≥2/3rd diameter of the artery.
  8. Diameter – between the inner margins of artery.
  9. TAMEAN instead of TAMAX.
  10. Assess for 3-4 cardiac cycles.
  11. Average of 2-3 readings for final flow volume.

TAMEAN stands for timed average mean velocity whereas TAMAX denote timed average maximum velocity. Various studies utilised TAMEAN for calculating flow volume and it is found to be ideal than TAMAX [6].

Interpretation of fistula flow volume:

Conclusion

  1. Doppler evaluation is an important tool for the evaluation of vessels for the successful creation of fistula.
  2. Periodic Doppler evaluation of AV fistula can identify early access dysfunction and early intervention for fistula salvage.

References

  1. Malovrh M. Native arteriovenous fistula: preoperative evaluation. Am J Kidney Dis. 2002;39(6):1218-25.
  2. Allon M, et al. Effect of preoperative sonographic mapping on vascular access outcomes in hemodialysis patients. Kidney Int. 2001;60(5):2013-20.
  3. Simon van Hooland, et al. Duplex Ultrasonography and Haemodialysis Vascular Access: A Practical Review. Int J Nephrol Urol. 2010;2(2):283-93.
  4. Robbin ML, et al. Hemodialysis arteriovenous fistula maturity: US evaluation. Radiology. 2002;225(1):59-64.
  5. Schí­Â¤berle W. Ultrasonography in Vascular Diagnosis, Springer-Verlag Berlin Heidelberg, 2005
  6. Blanco P. Volumetric blood flow measurement using Doppler ultrasound: concerns about the technique. J Ultrasound. 2015;18:201-204.
Dr.-T.-Rajarajan

Dr. T. Rajarajan

Senior Consultant Transplant Nephrologist

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