Difficult fistula cannulation: deep, tortuous, with collaterals or after a hematoma

Some fistulas will not be needled easily: the vein cannot be felt, it changes direction, it splits, or it is surrounded by hematomas. This guide explains what makes an AV fistula difficult, how to plan cannulation with ultrasound, what changes in each situation and when to stop trying, because sometimes difficult cannulation is a symptom of an access problem.

For professionals · Updated October 25, 2026 · 11-minute read

What makes a fistula difficult

Difficult cannulation rarely has a single cause. It helps to identify the main one, because each needs a different solution. Some units grade their accesses by color according to how hard they are to needle, and keep the hardest ones for the most experienced staff: that is how patients were selected for one of the ultrasound-guided cannulation trials [6].

Before you needle: the plan

1. Examine the whole access. Look, feel and listen from the anastomosis up the arm, and raise the arm to see whether the vein collapses (step by step in the guide to AVF cannulation technique). Ask the patient: they usually know which areas work and which do not.
2. Scan the vein in short axis. Start at the anastomosis and slide the probe up the arm. Note depth, diameter, straight segments, branches, hematomas and narrow areas. Keep probe pressure minimal: compressing the vein flattens it and makes it look shallower than it is [1].
3. Choose the segment. It should be straight and long enough for the whole needle, free of aneurysms, scabs, hematomas and red skin [4], and respect your protocol's distances (see AVF needle angle, gauge and spacing).
4. Measure depth at that exact spot. It sets the angle and the needle length. You can mark the skin before prepping (ultrasound-marked cannulation) or needle while watching the screen (real-time guidance). For the hardest accesses, use real time.
5. Decide who cannulates. First cannulations and difficult accesses go to the person with the most experience and the best success rate [2,3,4].
6. Record it. A cannulation map in the chart, with the course, the depth and the areas to avoid, means the whole team needles the same way and nobody has to rediscover the access.

Deep fistula: the patient with obesity

A fistula is considered deep when its anterior wall lies more than 6 mm below the skin, one of the criteria in the rule of 6s. It is common in people with obesity. The vein may have a good diameter and good flow and still not be palpable. The problem is geometric: the deeper the vein, the more needle is used up getting there. At 10–12 mm and 25°, a 25 mm needle barely reaches the wall, leaving nothing to advance inside the lumen.

• Measure first. The depth of the anterior wall, at the exact spot, without pressing with the probe.
• Longer needle, steeper angle. Long 32 mm needles are used for deep fistulas [1]. Enter at a steeper angle than usual and, once you see flashback, lower it to advance along the center of the lumen. The table in the guide to needle angle and length shows how much needle each depth uses up.
• Ultrasound guidance, in plane. A deep fistula is one of the clear indications for ultrasound-guided cannulation [1]. In plane you see the whole needle; in a deep vessel without ultrasound it is easy to stop short or go through the back wall.
• Adjust the image: more depth, the focus at the level of the back wall and more gain in the deep field.
• Watch the neighbors. In a brachiocephalic fistula, the brachial artery and the median nerve lie medially, toward the inner side of the arm.

If it still cannot be cannulated safely, there are surgical options: superficializing the vein, that is, bringing it closer to the skin [3,11], or removing the fat over it (lipectomy) [8]. In a Brazilian series of patients with obesity, lipectomy reduced the mean depth of the cephalic vein from 7.9 to 4.7 mm [8]. The vascular access team makes that decision.

Tortuous fistula

The vein winds and changes depth. In long axis the whole vessel cannot be kept in plane, and a straight needle placed in a curve exits through the side wall.

• Map it in short axis. Slide the probe along the arm and watch the vein shift sideways. That is how you find the straight segments.
• Choose a straight segment long enough for the whole needle [4], mark its axis on the skin and align the needle with it, not with the arm.
• In plane, rotate the probe to follow the vein's local axis. If the straight segment is very short, the out-of-plane approach with tip tracking (DNTP) lets you center the vessel, though it takes more practice.
• After flashback, advance only as far as the straight segment allows: beyond it, the tip will meet the wall of the curve.

Collaterals, duplications and accessory veins

Branches leaving the main vein are thinner and sometimes shallower, which makes them tempting. Needling them gives poor flow and hematomas, and a puncture right at a bifurcation can go through the wall where the two vessels separate.

• Follow the main channel in short axis from the anastomosis. At a bifurcation you will see two lumens drifting apart.
• Cannulate away from junctions, on a segment with a single lumen.
• Color Doppler shows which branch carries the main flow (see AV fistula Doppler ultrasound).
• If the fistula is new and not maturing, a large accessory vein may be the cause: in one study, an accessory vein over 70% of the cephalic vein's diameter predicted non-maturation [10]. Discuss it with the vascular access team.

Calcified arteries

In diabetes and advanced kidney disease, arterial walls calcify. On ultrasound they look like two very bright parallel lines, like rails, with an acoustic shadow behind them that can hide what lies beneath. If there is also more subcutaneous tissue, the vein sits deeper.

• Tilt the probe or change where you rest it to look behind the shadow.
• Identify each vessel: the vein compresses with light pressure and the artery does not. Color Doppler separates the arterial lumen from the venous one.
• Plan the depth as for a deep fistula.

After a hematoma or an infiltration

A hematoma compresses the vein, distorts the anatomy and takes cannulation sites away. On ultrasound it is a heterogeneous hypoechoic or anechoic collection without color Doppler flow [11]. Map its extent and cannulate a healthy segment away from it. If the patient needs dialysis on the same day as the infiltration, KDOQI suggests cannulating above the injury, closer to the heart [2]. Needling through the hematoma adds the risk of another extravasation. Full management is in the guide to fistula infiltration.

New or underdeveloped fistula

The first cannulations carry the highest risk. In a 5-year series, 43.5% of major infiltrations happened in fistulas less than 6 months old [9]. According to data cited by KDOQI, a single infiltration before the first two-needle cannulation is associated with 56% lower odds of the fistula maturing [2]. That is why, in a new fistula, the Spanish GEMAV guideline advises [3]:

• cannulation by experienced staff;
• a 17G needle with a blood pump speed of about 200 mL/min;
• checking the needle with a saline syringe before connecting;
• not persisting: a hematoma in a new fistula can delay its use by weeks.

If the vein does not yet have enough diameter or flow, technique is not the problem: review the criteria in the guide to the rule of 6s and fistula maturation.

When to stop and ask for help

• Two failed attempts in the same session: ask another experienced colleague to needle instead of persisting [4]. The GEMAV guideline considers repeated attempts by the same person unacceptable [3].
• Pain, swelling or saline that will not flush: stop. It may be an infiltration.
• Tingling or an electric jolt as you advance: withdraw. You may be next to a nerve.
• Record every incident so it can be investigated and corrected [3].

If cannulation cannot be done safely, the decision about the session (single needle, a catheter if the patient has one, postponing) belongs to the team and follows your unit's protocol.

When difficult cannulation is a symptom

A fistula that used to be easy and is now hard has not become "difficult" for no reason. The DICAF study looked at 43 patients with difficult cannulation [5]:

• the most common causes were stenosis in the cannulation zone (23%), an immature fistula (21%) and outflow stenosis (19%);
• immaturity dominated when the difficulty started in the first months, and cannulation-zone stenosis when it started later;
• every patient with arm edema had a central vein stenosis;
• two in three ended up needing a bridging catheter.

The authors recommend early imaging for fistulas that are hard to cannulate [5]. Tell the vascular access team if:

• needling gets harder each time;
• flashback is poor;
• bleeding after needle removal lasts more than 20 minutes without excess anticoagulation;
• the arm swells;
• the thrill or the bruit changes.

Bleeding that long can point to a stenosis [3] (see AV fistula stenosis). Doppler ultrasound is the first step; in the DICAF study, fistulography had a higher diagnostic yield than Doppler [5].

Educational content. It does not replace supervised hands-on training or your unit's protocols.

References

1. Moyano Franco MJ, Salgueira Lazo M, Roca-Tey R. Punción ecoguiada del acceso vascular para hemodiálisis [Ultrasound-guided cannulation of the hemodialysis vascular access; in Spanish]. Nefrología al día. 2023. nefrologiaaldia.org
2. Lok CE, Huber TS, Lee T, Shenoy S, et al. KDOQI Clinical Practice Guideline for Vascular Access: 2019 Update. Am J Kidney Dis. 2020;75(4 Suppl 2):S1-S164. doi.org
3. Ibeas J, Roca-Tey R, Vallespín J, Moreno T, et al. Spanish Clinical Guidelines on Vascular Access for Haemodialysis (GEMAV). Nefrología. 2017;37(Suppl 1):1-191. doi.org
4. Vascular Access Society of Britain and Ireland (VASBI) and British Renal Society Vascular Access Special Interest Group. Clinical Practice Recommendations for Needling of Arteriovenous Fistulae and Grafts for Haemodialysis. 2018. thinkkidneys.nhs.uk
5. Behera MR, John EE, Thomas A, et al. Difficult cannulation of hemodialysis arteriovenous fistula: role of imaging in access management (DICAF study). J Vasc Access. 2022;23(6):877-884. doi.org
6. Eves J, Cai P, Latham R, et al. A randomised clinical trial of ultrasound guided cannulation of difficult fistulae for dialysis access. J Vasc Access. 2021;22(4):635-641. doi.org
7. Chen S, Liu JS, Chai CC, et al. Handheld ultrasound-guided cannulation of difficult hemodialysis arteriovenous access: a randomized controlled trial. Hemodial Int. 2023;27(1):21-27. doi.org
8. Reis JMC, Ribeiro FRCM, Melo GS, Reale Neto HB, Andrade MC. Lipectomy as an alternative for superficialization of autologous AVF in obese patients: experience of a referral center in Amazon. J Vasc Bras. 2024. doi.org
9. Lee T, Barker J, Allon M. Needle infiltration of arteriovenous fistulae in hemodialysis: risk factors and consequences. Am J Kidney Dis. 2006;47(6):1020-1026. doi.org
10. Planken RN, Duijm LE, Kessels AG, et al. Accessory veins and radial-cephalic arteriovenous fistula non-maturation: a prospective analysis using contrast-enhanced magnetic resonance angiography. J Vasc Access. 2007;8(4):281-286. pubmed.ncbi.nlm.nih.gov
11. Sharbidre KG, Alexander LF, Varma RK, et al. Hemodialysis access: US for preprocedural mapping and evaluation of maturity and access dysfunction. RadioGraphics. 2024;44(1):e230053. doi.org