Adequate blood flow is the ultimate value of arteriovenous fistula.
Therefore, in the ultrasound assessment of arteriovenous fistula, in addition to morphological parameters such as anastomosis and lumen size, blood flow measurement is also very important.
Although ultrasonic measurement of flow rate is not the gold standard, it is the most commonly used method because it is noninvasive and convenient. If it is measured properly, it has a good correlation with the actual flow rate.
What we want to clarify is that the blood flow of the internal fistula refers to the blood flow through the anastomosis into the vein of the internal fistula every minute, not the blood flow rate of blood removed by the needle during hemodialysis.
Generally speaking, a good autologous arteriovenous fistula (AVF) blood flow should be between 700-1300ml/min.
When the flow rate is lower than 500ml/min, insufficient blood flow may occur during dialysis, which is what the patient often says “no bleeding”.
When the flow rate is lower than 300ml/min, the probability of fistula thrombosis will increase significantly, and intervention is needed as soon as possible.
For artificial vascular arteriovenous fistulas (AVG), due to the common outflow tract stenosis, poor bleeding occurs rarely, so blood flow is mainly used to predict thrombosis.
When the flow rate is lower than 600ml/min, the probability of thrombosis increases, and a deadline is required for treatment;
When the flow rate is lower than 400ml/min, thrombosis may form at any time, requiring emergency treatment.
If the blood flow of an internal fistula is indeed very low and the stenosis is not obvious on the scan, the suspicion is that the stenosis still exists, but it has not yet surfaced, reminding us to continue to look for it patiently.
So how to measure it? The first part first talks about the location of the measurement, that is to say, which section of the blood vessel should we choose to measure, which is the vein? Is it an anastomosis? Or arteries?
Let’s talk about the anastomosis first
First, the anastomosis is usually a corner part, and the angle is complicated, which makes the Doppler sampling direction at a loss. Secondly, the shape of the anastomosis is usually more elliptical, and it is difficult to cut a good plane, and the area calculation error is large. The blood flow turbulence is also obvious here. The anastomosis seems inappropriate.
Then let’s talk about the outflow tract
Here, AVF and AVG are not the same.
For AVF, the outflow tract is an autologous vein, and under normal circumstances, this place is not suitable as a measurement site.
The reasons are as follows:
1. The blood flow in the internal fistula vein is more turbulent. Turbulence means that the direction and flow velocity of each particle in the blood flow are inconsistent. The spectrum obtained by pulse Doppler is very wide, and the wide spectrum means that the speed is very different.
For example, in a class of running, everyone’s speed is different, the fast is 20km/h and the slow is 5km/h. If you want to calculate the average speed, you need to know the speed of each person, which is difficult for fluids. Yes, let alone an indirect method such as the ultrasonic Doppler principle, so when the turbulence is large, the calculated average velocity error is large.
2. The vein is easily compressed, the diameter measurement may be unreliable. And the calculation of the area of the blood vessel may have large errors.
3. Vein distortion and large changes in diameter.
4. There may be side branches
For AVG, the outflow tract after the arterial anastomosis is an artificial blood vessel. The artificial blood vessel generally has a relatively uniform inner diameter, is not easy to collapse, has no branches, and can be used as a flow measurement site.
It should be noted that the measurement should be avoided in the uneven lumen.
In addition, the venous side of the artificial blood vessel is less turbulent than the arterial side, which is more suitable for measurement.
Artery should be the most suitable measurement site
The blood flow in the artery is closer to laminar flow, the velocity of each particle is relatively close, and the lumen is not easy to collapse.
There are also several situations below.
1. For the AVF established by the radial artery or the ulnar artery, the measurement site is in the brachial artery, so that even the internal fistula of end-to-side anastomosis, the blood supply at the proximal and distal ends of the artery has been included.
Although, in addition to supplying internal fistulas, arteries also supply limb tissues themselves, but in terms of upper limbs, these blood supply is generally very limited, in most cases only tens of milliliters per minute, which is comparable to several hundreds of internal fistulas. ignore.
What needs to pay attention to is the variation of the brachial artery. Some brachial arteries are divided into radial and ulnar arteries at a very high position, in the armpit or upper arm (as shown in the figure below, the two red Doppler blood vessels are the radial and ulnar arteries separated from the high position). At this time, the flow of the two arteries needs to be measured and then added.
2. For internal fistulas with brachial artery as the inflow tract, including high AVF and AVG, the measurement method is slightly different.
Generally speaking, the brachial artery flow upstream and downstream of the anastomosis should be measured separately. The blood flow of the internal fistula is the blood flow of the internal fistula after the blood flow of the upstream minus the blood flow of the downstream.
In this way, for the AVG with the brachial artery as the inflow tract, there are two measurement locations. One is the artificial blood vessel and the other is the brachial artery.
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