Echo color Doppler ultrasound: a valuable diagnostic tool in the assessment of arteriovenous fistula in hemodialysis patients

Introduction

A reliable and durable vascular access (VA) is a critical requirement to ensure adequate efficiency in dialysis and it plays an important role in long-term survival and quality of life of patients on chronic hemodialysis.

In fact, VA failure continues to be one of the most common causes of morbidity in hemodialysis patients with a consequent increase in costs.

The arteriovenous fistula (AVF) with native vessels is closest to the ideal VA, characterized by long life, low incidence of complications (stenosis, thrombosis, aneurysms, infections, limb ischemia) and adequate blood flow to dialysis prescription (1, 2).

Both for the first and the next access, the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines provide recommendations for preparing the largest number of native AVF as well as supervision and monitoring programs (clinical and instrumental) for the identification of potentially treatable causes of VA failure in order to act quickly and thus prevent thrombosis. Therefore, a careful objective examination is necessary before each dialysis session and a constant and periodic VA evaluation must be carried out through specific parameters (recirculation, arterial-venous pressure and static-dynamic flow measurement). Today, the flow calculation is mainly recognized by the guidelines as the best VA surveillance method (3).

Several studies (4-5-6) have demonstrated the utility of ultrasound assessment and its critical role:

1)

in the pre-operative assessment for the planning of VA placement in terms of evaluation of arteries and veins;

2)

as a predictor of VA maturity (since the first 24-48 hours from the placement) intended either as increased flow and adequacy to venipuncture or as part of continuous monitoring;

3)

in the early recognition and localization of abnormalities and complications that could threaten the function and the patency of a AVF in order to carry out a timely treatment.

The echo color Doppler ultrasound (CD-US) allows a detailed morphological and functional VA assessment; it is a cheap and non-invasive technique, suitable for the study of superficial structures, it does not use contrast agents and it is becoming a tool directly used by nephrologists.

Methodology

In our five-year experience, a study protocol of the AVF has been used and improved with the help of CD-US. Moreover, in our Department, a detailed preoperative ultrasound assessment of the arterial and venous drainage of the upper limb is usually carried out through the analysis of several morphological and functional parameters. The study focuses mainly on both the venous and arterial side. With regard to venous side, the study is founded on the evaluation of cephalic and basilic veins in the arm, with the analysis of the walls, path, patency, internal diameter, distensibility and the presence of side circuits. A small diameter (<1.6 mm) is associated with a high risk of early AVF failure. As regards the arterial side, the assessment focuses not only on the diameter and walls but especially on the functional ability of the artery to dilate using the active hyperemia test.

The preoperative assessment for AVF placement, in accordance with the literature, has led to an increasing number of VA interventions, especially as first access. The CD-US is an advantage for all patients, particularly for those who require complicated procedures in which central venous cannulas are used, for patients with pacemakers, and risk factors due to central vein stenosis or to previous failures (7).

By studying the exact vascular anatomy, the preoperative ultrasound assessment has certainly addressed the right site of AVF placement and caused a reduction in complications but not in the percentage of early failures.

In addition to the preoperative study, ultrasonographic monitoring was carried out following the AVF maturation with flow calculation based on the first 24-48 hours until its use as suggested by the NKFK/DOQI guidelines. The AVF was used according to ultrasound parameters (RULE “6”): a blood flow of 600 mL/min, vein diameter greater than 0.6 mm and vein depth of about 0.6 cm (Tab. I) (Guidelines 3. 2 Maturation and cannulation) (8).

TABLE I -

The ultrasound parameters of an AVF for the first cannulation as suggest by guidelines

RULE “6”

K-DOQI Guidelines

Blood flow 600 mL/min

Vein diameter greater than 0.6 mm

Vein depth of about 0.6 cm

The ultrasound mapping of the efferent veins has facilitated the first punctures, especially in obese subjects, with vessels not perfectly visible and not easily palpable to an objective examination. Hemodynamic assessments were later useful in judging the suitability of a fistula at the time of its use.

The CD-US of the AVF was performed by studying the anastomosis, afferent arterial side, efferent venous side (distal/proximal) in hemodialysis patients with distal AVF and middle arm (Fig. 1).

Fig. 1 -

The CD-US is a valuable diagnostic tool for documenting the low flow of an AVF and leading to research of causes (9-10-11-12). Therefore, the flow (QB) in AVF on the brachial artery was monitored after its placement by at least three measurements in order to minimize the error caused by fluctuations of the diameter of the vessel during the cardiac cycle (Fig. 2).

Fig. 2 -

The routine examination was carried out during the interdialytic period with the patient in the supine position with the arm slightly abducted, after an appropriate period of stability of at least 10 minutes. The CD-US has never been executed during the dialysis session or at the end because of the phenomena of hypotension that can affect the operation of a fistula and/or to the presence of the needles-fistula and hemostatic swabs which constitute an obstacle to the execution of the examination.

A linear probe with medium-high frequency (5-7 MHz) was used.

Insonation was oblique to the ultrasound beam, through the use of the “steering”, in order to obtain an insonation angle between the ultrasound beam and vessel ≤60°. During the recording of the Doppler tracking, the volume sample was successfully inserted at the center of the vessel, taking into account that the velocity of red blood cells in the vessels is recorded during a Doppler sampling. So, if the sample volume is too small, it only tracks the higher speeds (with overestimation of the flow) but if it is too large, it may be affected by the vibrations of the walls.

As a result, the volume sample is between 50%-70% of the vessel lumen, insertion of a filter that absorbs the interferences due to the vibrations of the vessel walls. Moreover, it is important to consider that velocity of red blood cells is maximum at the center of the vessel and decreases gradually approaching the walls.

Ultrasound setting: it is the maximum enlargement of the section of insonated vessel (in order to limit the measurement error) through a large adjustment of the sample volume.

Measurement of average speed time is carried out through the evaluation of the spectral curve.

As suggested by the K/DOQI guidelines, the brachial artery was chosen as the site of sampling in a straight section, 2 cm above the bend of the elbow in order to obtain a right measurement of the vessel diameter as well as optimal paths for the calculation of the hemodynamic parameters (Fig. 3) (10).

Fig. 3 -

This site can be easily reproduced and divided into samples, it is not compressible (low percentage of error in the measurement of its area as small changes in the diameter of the vessel may lead to large variations of flow), it has a laminar flow (unlike the turbulent flow in the veins with arterial flow or near the anastomosis) and it facilitates a precise positioning of the sample volume at the center of the vessel in a direction parallel to the flow and with an insonated angle between 30° and 60°; all these characteristics allow to easily measure the section area and the diameter as well as the recording of tracks suitable for the calculation of hemodynamic parameters.

Conditions that may be sources of error deserve particular attention:

1)

measurement of the diameter of the vessel;

2)

the angle of insonation;

3)

the amplitude of the sample volume;

4)

the patient’s hemodynamic stability.

Most of the devices with color Doppler and echo color Doppler have algorithms able to measure AVF flow taking into account the following elements:

AVF flow (mL/min) = area of the vessel (cm) × medium velocity (m/sec) × 60 (sec).

A particular attention for the steal phenomenon, frequent in patients with forearm, upper arm AVF and graft, because it is usually clinically silent and it evolves into a steal syndrome when compensatory mechanisms to maintain peripheral arterial perfusion fail (13).

Timing used for the execution of the examination is described in Table II.

TABLE II -

(A) and (B) B-mode and color, transverse plane image of anastomosis and site post-anastomotic. (C) Longitudinal plane of anastomosis: aliasing phenomenon, it is an artifact. (D) B-mode and color transverse plane: image of a thrombus in the anastomosis, showing both low and high echogenicity thrombotic material. The absence of color flow throughout the access.

(A) B-mode and color sonography: progressively thinner lumen, presence of aliasing, colour Doppler mode as a sign of turbulent blood flow. (B) A high-degree stenosis is noted: flow acceleration with peak flow velocity of over 600 cm/sec systolic and of about 400 cm/sec diastolic in the stenosis. (C) Access dysfunction with recirculation and volume flow about 300 mL/min. (D) Different size of the vein with intimal hyperplasia phenomenon.

(A) and (C) B-mode and color in the longitudinal plane of the brachial artery as site of sampling in a straight section, 2 cm above the bend of the elbow in order to obtain a right measurement of the vessel diameter as well as optimal paths for the calculation of the hemodynamic parameters. (B) The arterialized draining vein and feeding radial artery. In this picture the cephalic vein is superficial, sitting ≤0.5 cm below the surface of the skin. This is superficial enough for easy punctures. (D) Color Doppler ultrasound measurement of flow volume in the access feeding brachial artery and Doppler waveform analysis: good quality of the access. Doppler sample volume placement: great care must be taken to avoid that an improper angle of insonation may thus result a false impression of stenosis where it is not present.

Timing used for ECD examination

CD-US AVF assessment
Flow values mL/min	Interval of execution
>1200 in absence of simptoms	4 months observation of developing symptom
>1500 with distal hypoperfusion ischemic	The treatment strategy depends on severity of symptoms and increase of patient co-morbidity
800-1200	12 months
600-800	6 months
<600 (continuous)	Monthly CD-US, further instrumental assessment and a potential correction

Ultrasound assessment has always been carried out in the presence of continuous abnormalities of the AVF function (difficult insertion of needles, increased venous pressures, frequent and prolonged bleeding after dialysis, high circulation, KT/V inadequate), and of signs and symptoms for AVF insufficiency (inadequate arterial inflow, lack of maturation, weak trill, signs of infection).

In native AVF, more than 85% of cases of thrombosis are due to the presence of stenosis, the most frequent cases, about 50% are related to juxta-anastomotic venous stenosis and one of the rare cases is instead the artery feeding the fistula. The stenosis is the most common complication of AVF and is responsible for the reduction of the AVF QB, for this reason its early recognition is necessary in order to prolong the life of VA.

Therefore, we have always observed not only the presence of aliasing, which identifies an acceleration of flow sign of a significant reduction in size and/or in stenosis, but also the high-speed of systolic-diastolic flow in the spectral analysis. We have also evaluated the high velocity of the peak systolic velocity (PSV), the relationship between the PSV at the site of stenosis and the speed detected in an adjacent segment of regular or non-stenotic caliber, and also indirect signs of stenosis in terms of VA flow reduction and of variations of the afferent artery Doppler spectrum typical of a circle of high downstream resistance. The qualitative aspect of Doppler examination deserves particular attention. It is about the “harsh and sibilant sound” detectable on vascular constrictions in the case of stenotic fistulas. This element acquires importance and significance with the increased experience of the operator who can easily distinguish the different sound qualities of stenosis from simple, involuntary, and excessive compressions of the probe on the vessel under consideration.

The wall vessel in question has always been assessed by researching and defining any potential irregularities (calcifications, interruption and/or hematomas) (Fig. 4).

Fig. 4 -

Moreover, our team have also selected a group of patients at increased risk of health problems such as vascular calcification and atherosclerosis, age, female gender, obesity, diabetes, hypertension and prior thrombotic events. In such cases CD-US monitoring was more precise and helped us to recognize and correct VA dysfunctions earlier. In fact, once the type and origin of the problem are identified at the level of VA, the ultrasound study prepares the patient for angiography by affecting and limiting the segments of punctures, thus reducing not only such possible complications as hematoma and thrombosis, but also the examination time.

Discussion

The rationale for monitoring and surveillance of VA is to extend and improve the life of the AVF, reduce stenosis and thrombosis, and decrease the use of catheters in hemodialysis patients. Therefore, nearly all thrombosed mature fistulas have an underlying stenotic lesion.

In addition to physical examination, routine laboratory, dialysis adequacy, documented recirculation, difficulty in venopuncture and/or achieving hemostasis after needle withdrawal, many methods have been proposed and implemented for evaluation of VA.

The usefulness of access blood flow measurement is an ongoing controversy, but all VA clinical guidelines recommend monitoring and surveillance protocols to screen for subclinical dysfunctions and prevent VA thrombosis. Although randomized clinical trials have failed to consistently show the benefits of based surveillance protocols.

The three major types of access surveillance are:

1)

intra-access blood flow monitoring;

2)

static dialysis venous pressure;

3)

duplex ultrasound.

There are several different methods for monitoring intra-access blood flow (14), but all the methods yield similar values, as suggested in the literature. Ultrasound dilution is the most commonly used, which quantifies access blood flow by injection ice-cold saline via the dialysis needle after reversing the lines of the dialysis. There is a sensor which measures the rate of increase in temperature following the injection and used for the calculation of the access flow rate.

In a recent study by Aragoncillo et al (15), the measurement of access blood flow combining Doppler ultrasound and ultrasound dilution method, after one-year follow-up, shows a reduction in thrombosis rate and an increased assisted primary patency rate in AVF.

Other methods include dilution based on urea, or thermal techniques; glucose pump infusion techniques and differential conductivity.

Static venous pressure is another technique for detecting significant stenosis, but it is unknown if this measure is predicting failure of AVF, because it has a lower positive predictive value for stenosis in fistula as compared with grafts (16). Kumbar et al (17) described this technique, then the same group evolved a computerized method and measurement of the static intra-access pressure has evolved over time. The authors conducted a prospective observational study to test the utility of static venous pressure (832 patients with a long observation period of 7.75 years; 65% to 80% of the accesses were prosthetic grafts). The result of this study was very promising; static venous pressure/systolic blood pressure was found to provide excellent criteria for angiographic referral and intervention of >50% stenosis using angioplasty or surgical revision.

However, its usefulness in predicting thrombosis or access failure in AVF is currently unknown.

Tessitore et al (18) indicated that the best test to detect a given stenosis depends on its location. Flow measurement is useful for identifying inflow stenosis, whereas derived static venous pressure is a better tool for outflow lesions. Also, an access can have multiple lesions involving both inflow and outflow and it is necessary to implement a process rather than a single method in detecting stenosis.

There are very limited studies about the use of CD-US in patients with native arteriovenous fistulas, but data also suggest that duplex flow measurements may be useful. CD-US is an additional diagnostic method to predict the ultimate maturation of newly created AVF and is also very useful in further defining problems that have been detected by physical examination.

CD-US allows a detailed morphological and functional assessment of the VA, it measures the PSV on side of stenosis and the ratio of PSV measured across the stenotic lesion (PSV ratio >2 is suspicious for significant stenosis).

CD-US, also provides additional information that is of the utmost importance for surgical or interventional treatment.

In a recent study, Lomonte et al (19) underlined the key role of CD-US in the work-up to hemodialysis VA. In addition, the authors reaffirmed that clinical monitoring remains the backbone of any VA program, but very important is the adoption of a VA surveillance program based on CD-US.

The study conducted by Kudlicka et al (20) emphasized that the patency of VA is mostly limited by the growing stenosis and the therapy of choice is often percutaneous angioplasty, but it can injure the vessel wall with subsequent faster development of re-stenosis. The authors compared ultrasonographic and angiographic measuring of residual diameter (2 mm) as the additional criterion of significant stenosis and they affirmed ultrasonographic measurement of the residual diameter is stable in experienced hands and compares well to angiography results.

Angiography is the most sensitive and specific imaging modality in order to identify and characterize stenotic lesions. In addition, it can examine the anastomosis, the arterial inflow and outflow veins, define the anatomy of AVF, immediate percutaneous transluminal coronary angioplasty with or without stenting can be performed, if indicated, or the angiogram can serve as a guide for surgical revision, but it is a very expensive and invasive tool. Angiography can be reserved for patients with physical findings or in presence of abnormalities as an unexplained decrease in the delivered dialysis dose.

Conclusions

In conclusion, our experience underlines the essential role and the direct involvement of the nephrologist not only in the VA planning, that begins with a preservation of vessels for placement, but also in the precise choice of the VA type, and it continues with careful management of its maturity and functionality by the use of different processes such as physical examination, clinical laboratory data and skill in the knowledge and use of sonography with hemodynamic assessments.

In our opinion the CD-US should have a crucial place in the interdisciplinary cooperation in AVF monitoring because it provides a detailed picture of the vascular anatomy, it has a high sensitivity for stenosis, and it provides indications and quantitative measurements of blood flow through the flow measuring, which can be considered an important prognostic value for the dialysis adequacy and, consequently, an effective predictor of early thrombotic risk. The instrumental assessment, however, cannot and should not replace a careful clinical examination, but our experience confirms that a meticulous ultrasound examination can be a valued instrument for the definition of anomalies and for the diagnosis and follow-up of AVF complications; thus allowing the best problem management related to VA. Finally, CD-US should be included as a part of an integrated access management program and this thesis is supported by different clinical outcome data.

(A) and (C) Vessel with intraluminal atherosclerotic plaques, hyperechogenic. (B) and (D) Large hematoma after puncture both low and high echogenicity.

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