Practical Step 2: Recognize a "Healthy" Arteriovenous Fistula
Two-Dimensional Grayscale (B-mode Ultrasound):
Evaluate the structure and morphology
The two-dimensional (2D) mode is our “eyes” for visualizing the anatomical structure of blood vessels.
Arterial End (Feeding Artery)
Vessel Wall: The distinct “hyperechoic-hypoechoic-hyperechoic” three-layer structure (intima-media layer, media layer, adventitia layer) is clearly visible, with obvious pulsation.
Lumen: Appears as anechoic (black), with a clear interior and no abnormal echoes (e.g., thrombus).
Lumen Diameter: Compensatory dilatation occurs after surgery. The diameter is generally required to be ≥ 2.0 mm (for the radial artery) or ≥ 3.0 mm (for the brachial artery).
Venous End (Draining Vein)
Vessel Wall: The venous wall is thin, appearing as a slender hyperechoic line, with weaker pulsatility than that of the artery.
Lumen: Also anechoic (black) and unobstructed.
Lumen Diameter: The venous segment of a mature fistula should be uniformly dilated. It is generally required to be ≥ 4.0 mm (for the forearm) or ≥ 5.0 mm (for the upper arm), with a depth of less than 6 mm from the skin to facilitate puncture.
Anastomosis: This is the key site connecting the artery and vein, and a common location for stenosis. Special attention should be paid to observing whether its structure is clear and whether there is hyperplasia or abnormal echoes.
Key Point: A sufficient lumen diameter is the primary condition for the fistula to be puncturable and to maintain the blood flow rate required for dialysis.
Two-Dimensional Grayscale (B-mode Ultrasound):
Evaluate the structure and morphology
1. Arterial End Spectrum:
Waveform: Presents a low-resistance blood flow spectrum. This is the most characteristic change after fistula creation! It is manifested as continuous, high forward blood flow throughout the diastole, with a “spike-like” spectral envelope.
Velocity: The peak systolic velocity (PSV) is usually between 150–400 cm/s or even higher. What matters is the serial comparison of the same vessel rather than a single absolute value.
2. Venous End Spectrum:
Waveform: Presents an arterialized spectrum. This is the result of arterial blood impacting the venous wall, manifested as a broad-spectrum waveform that pulsates with the cardiac cycle.
Velocity: Usually high and stable. For a well-functioning fistula, the access flow (Qa) should be > 500 mL/min (Qa needs to be calculated by the device software).
Key Point: The spectrum shows “arterial supply and venous arterialization”, which is a typical feature of a normal arteriovenous fistula.
Four Key Points of a Normal Arteriovenous Fistula
1. Sufficient lumen diameter: ≥ 4 mm
2. Adequate blood flow: ≥ 500 mL/min
3. Turbulence visible at the anastomosis: without significant stenosis
4. Arterialized venous spectrum: persistent low-resistance and high-velocity flow
A healthy arteriovenous fistula should present the following features on ultrasound: qualified lumen diameter, adequate blood flow filling, normal turbulence, and arterialized venous spectrum. Mastering these characteristics can help us quickly determine whether the fistula is mature, thereby ensuring the safety and effectiveness of hemodialysis. The SonoMaxx handheld ultrasound is easy to operate and quick to learn, with images comparable to those of large ultrasound machines. It supports one-click measurement and has been widely used in clinical hemodialysis practice.
Next Episode Preview: Now that we’ve learned to recognize a “healthy” fistula, the next step is to identify a “diseased” one. In Episode 4, we will delve into Ultrasound Diagnosis of Arteriovenous Fistula Stenosis: Teach You to Detect the Clues of the “Flow Killer”. This is the most common and crucial complication in clinical practice—stay tuned!