The Michigan Vascular Access/MiDRE primer in Vascular Access

Chapter 5 - Ultrasound evaluation of the dialysis access candidate

The creation and maturation of fistulas for a greater percentage of dialysis patients has come to be understood as an important goal and a major quality advance. One contributing factor in allowing more fistulas to be created these days is ultrasound mapping - identifying veins that are not always readily apparent on clinical exam, and hence overlooked. Many patients who are referred for dialysis access evaluation have been advised to undergo "venous mapping"

Many experts are recommending that ultrasound mapping be done prior to surgical consultation: that the nephrologist examine the patient, make a decision about the most appropriate vascular access, and then communicate that expectation to the surgeon. Certainly, there may be surgeons in most communities who can benefit from guidance regarding a procedure they do less frequently, and other surgeons whose care of the dialysis patient is improved by having their feet held in the fire by the nephrologist. Each nephrologist must make an assessment of the service their patients get from the local surgeons, and adjust their practices accordingly. For many, more nephrological involvement in the access process may mean better results.

As the importance of vascular access for dialysis is increasingly recognized, however, more surgeons are becoming sophisticated in the assessment and treatment of the dialysis patient. In larger metropolitan areas there are surgeons like myself focusing on this area as a large or exclusive part of our practices. For many of us, the office ultrasound is an essential extension of the physical examination, used in every new patient assessment and in many of the follow-up visits.

A Sonosite 1800 office ultrasound
The veins
In my "new patient" assessment, the office ultrasound is used to map the veins - measure the size, identify branches, identify or exclude clot from intravenous needle injury or other factors, follow the course of the vein, note variations on the norm, and identify the dominant outflow. An attractive large vein at the wrist may lead to a long stenotic stretch in the mid-forearm, dooming a wrist fistula, and setting up the patient for an early failure that will color his or her perception of access surgery. Futile operations can be predicted with better screening of the anatomy, and can thence be avoided. The high-yield procedure can be correctly identified in the initial office evaluation with the use of ultrasound in most instances.

The veins of the forearm and arm
Overuse or injudicious use of IVs and PICC lines have created havoc with the venous anatomy, and close evaluation of the target veins and their outflow is essential to avoid surprises in the operating room. In many instances, veins identified as usable will be mangled between the office visit and the procedure if the patient has intercurrent problems. Re-evaluation with ultrasound in the operating room is highly recommended before committing yourself to a given procedure (see image one below). In this era of ever-expanding waistlines, obese patients are more and more common, and the veins are deep (see image two below). Rather than tell these patients that they cannot have a fistula, we have developed more complex fistulas using transposition and superficialization. Depth pf the veins is something to recognize and address with the patient at the first visit if possible to avoid the sense of betrayal that may occur at the third post-operative visit when you tell the pa4eitn that they will need a second operation to "raise the vein". Discussing this reality and the plan for a two stage procedure at the outset is critical.

Thrombosed vein

Deep vein
Knowledge of the runoff allows long-term planning. A wrist fistula that does not mature sufficiently to be usable may still develop the antecubital veins. A forearm graft may then succeed where it would not have prior to vein growth. On the other hand, a graft in the forearm correctly placed can grow the cephalic vein in the upper arm, allowing an upper arm fistula at a later date. Or, an antecubital fistula useless for cannulation may grow a basilic vein to sufficient size and toughness to allow for transposition. All these stepwise approaches to providing vascular access are only made possible by an expanded knowledge of individual venous anatomy beyond what is easily visible.

The arteries

We tend to focus on the veins, but arterial status can also be important. A huge vein at the wrist coupled with a two millimeter calcified radial artery in an older diabetic woman may be eagerly jumped on by the unwary surgeon who can be fooled by a "good pulse" in a superficial incompressible artery. This can be a recipe for intraoperative agony for the surgeon, early fistula failure, insufficient flows in any fistula that does develop, or digital ischemia due to steal. As a result, we are very cautious about advising wrist fistulas in the very elderly, in small-stature women, and in long-term diabetics.

A small radial artery

A fistula built on a thready artery and which will never perform as desired

High bifurcation of the brachial artery

As another example, up to 15 or 20% of patients have a brachial artery that bifurcates in the upper arm rather than below the elbow, and a higher rate of access failures is noted in these patients. Forewarned is forearmed, as they say, and pre-operative knowledge of the anatomy derived from ultrasound examination may help identify the high-yield option for the patient without frustrating and time wasting exploratory surgery.

The ultrasound appearance of hign bifurcation, with the generally smaller radial-equivalent more superficial


The angiographic appearance of high bifurcation, with division of the artery in the upper arm

A diagram showing high-bifurcation


Increased recognition that the arterial side of the equation is as important as the veins is reflected in changes in terminology many patients are now advised to go for "vascular mapping" rather than venous mapping.

Discussion

The precision of measurement achieved by a trained vascular lab technician will most often eclipse that of the surgeon, but in those practices where surgeons are adept enough to do a basic vascular ultrasound exam, a step can be eliminated, and time and money saved as the patient comes directly to the surgeon without stopping at the vascular lab. In my practice, I refer the patient for a formal vascular lab exam only if my exam precludes a fistula, for an ultrasound "second opinion".

One important consideration is also the change in vascular tone that can occur due to room temperature, hydration status and other factors. A spastic vein invisible on one day may become a soda-straw on another. I routinely request an interscalene block for my vascular access operations where possible not because it gives the best pain control (it doesn't always), but because frequently the vasodilatory effects of the sympathetic block dilates the veins sufficiently to make the operation technically easier, to allow a fistula to be created at a lower level than previously thought possible, or even unveil a fistula option where a graft was planned. For this reason, the vascular access patient is almost invariably re-examined with a portable ultrasound in the operating room after the block is placed, and immediately before prepping the skin.

All too often I will examine a patients PTFE graft with ultrasound for some problem and find a large cephalic vein running underneath the graft. "Why didn't they do a fistula?" I ask myself. Many times it is obvious that because of thick skin or chubby forearms, the vein was not palpable or visible, and so the opportunity was missed. Other times it is possible that the vein grew in size over time under the influence of a graft and its extra blood flow in that arm.

Michigan Dialysis Research and Education (MiDRE)
Saint Mary's Mercy Hospital