Imaging of blood vessels is performed primarily for 2 reasons in the ICU. One is to detect deep vein thrombosis (DVT), commonly in the veins of the lower extremities. The other is to guide central venous and arterial cannulation.
Deep Vein Thrombosis
DVT is not uncommon in the ICU and it may often not be clinically obvious. It is important to rapidly diagnose DVT, allowing for immediate treatment with the reduced chances of pulmonary embolism Sometimes detecting a DVT strengthens a difficult diagnosis of pulmonary embolism. While venography and CT venography are more sensitive, Doppler ultrasound has been the most common modality of DVT diagnosis at the bedside.
Venous thrombosis can occur in the upper limbs and jugular veins too, but is most common in the lower limbs. Hence the focus.
The traditional radiological approach of systematically scanning the entire venous tree of both lower limbs is meticulous, but, it needs a lot of time and skill to perform. This examination has been adapted for use by the bedside in the ICU.
1.Common femoral vein, just below the inguinal ligament
The artery is located lateral to the vein here. Sometimes, the junction of the superficial with the deep femoral vein may be visible at this level.
2.Deep femoral vein, at the mid-thigh
The artery and vein are located much deeper at this level and an increase in the depth settings of the image may have to be made.
3.Popliteal vein, in the popliteal fossa
The popliteal artery and vein lie next to each other in the upper part of the popliteal fossa. If the trifurcation of the politeal vein is seen or if any smaller vessels are sen around the two main ones, then the probe is probably too distal and needs to be moved up.
The high frequency linear array vascular probe with a frequency range of 7 to 12MHz is used for DVT studies. A lower frequency may be required on overtly obese or edematous lower limbs. CFI and PWD capabilities should be available with the probe.
Patient and Probe positioning
The supine position with neutral positioning of the lower limb is adequate for imaging of the common femoral vein at the inguinal ligament, while the limb must be externally rotated and mildly flexed at the knee to allow imaging of the other 2 locations.
The probe is placed tranversely, i.e. the long axis of the probe foot is perpendicular to the long axis of the vessel being studied.
While no angulation of the probe is necessary for 2D imaging, CFI and PWD provide accurate information when the probe is slightly angulated to face toward or away from the direction of blood flow.
Identifying the vein and artery
On the 2D image the vein and artery appear as a hypo or anechoic lumen surrounded by an echogenic wall. The artery and the vein can be differentiated based on
1.Location - for example, just below the inguinal ligament, the artery is normally placed lateral to the vein. Anatomical aberrations are possible, however and this is not to be depended on.
2.Shape and pulsatility - the artery is round while the vein is ovoid. The artery is pulsatile whereas the vein is usually not.
3.Compressibility - On applying downward pressure vertically over the vessels, the muscular wall of the artery resists deformation and stays open, while a non-thrombosed vein gets compressed and the walls meet each other.
4.Color flow imaging - Angulating the transducer slightly downwards and selecting the CFI brings up the color box, which should be placed to cover both visualized vessels. The artery shows an intermittent pulsatile flow with color alaising, whereas the vein shows a gradually undulating continuous flow. Do not go by the color of the flow (eg. Blue for vein, red for artery), as this can be changed by changing the direction of angulation of the probe.
5.Pulsed wave Doppler - Selecting the PWD and placing the cursor at the center of the vessel and obtaining a trace will show a steady gradually changing flow in the vein and sharply accelerating, pulsatile flow in the artery.
Looking for thrombosis
Once the vein is identified, downward pressure is applied to the ultrasoud transducer until the vein collapses completely on the ultrasound screen. If the vein is not collapsing and the artery is starting to deform, consideration must be given to the posibility of thrombus within the veins lumen. The amount of pressure required to collapse the vein will differ from patient to patient, and with experience, the sonologist will be able to ascertain if enough pressure has been applied. Care must be taken because downward pressure at the wrong angle or down the wrong vector can greatly decrease the actual pressure felt by the vein and make it appear uncollapsible. As the vein lumen disappears and the walls meet each other, pressure on the transducer is relaxed and the vein allowed to take its normal shape.
Compressibility indicates a lack of thrombus in the vein at the location being compressed, and hence for a complete study, the entire vein will have to be sequentially compressed along its entire length. However, it is enough to limit oneself to performing this test at the three designated locations described above and use other techniques to assess the rest of the segments.
The probe is slightly angulated downwards and a PWD cursor is placed in the center of the lumen of the vein. A PWD tracing is then obtained and observed for any significant variation in the flow velocity (Fig.1). The tracing shows flows below the baseline, but this may change if the probe is angulated upwards.
Two variations are normally noticed, cardiac and respiratory. Variations with respiration are the more marked of the two. In spontaneously breathing patients, there is an increase in flow with inspiration, while in mechanical ventilation, the reverse is true.
The absence of such variation suggests a thrombosis above the point in the vein being tested. For example, if an absence of phasicity was noticed in the common femoral vein, then an iliac or IVC thrombus should be suspected.
With the PWD still in place in the vein lumen, a PWD trace is obtained and the calf muscles are gently squeezed. This normally results in a surge of blood flowing through the vein and a peaking of the plow velocity trace (Fig.2).
If this response is absent, it suggests thrombosis between the point being studied and the calf veins. It is important that the sqeezing of the calf is not done too close to the probe, as this may result in spiky noise artefacts on the PWD trace
4.Direct visualization of thrombus
Sometimes the thrombus may be visible as a slightly echogenic mass seen inside the vein lumen. It may be sessile, fixed to the wall or be floating in the flow tethered at one point to the wall. As a clot becomes chronic and gets organized, it becomes more firmly attached to the wall and appears more echogenic. The degree to which the lumen is obstructed is variable.
It is important not to do the compressibility and augmentation tests when a thrombus is visualized as it may result in its embolization.
Upper limb and Jugular thrombosis
While less common that the femoral veins, central line associated thrombosis of the brachial, subclavian and jugular veins are sometimes encountered. The principles of identifying the vein, compressibility, phasicity and direct visualization of thrombus can be applied to these veins as well.
Ultrasound guided Vascular Access
Ultrasound can be used to guide both arterial and venous cannulation.
Traditional approaches depend on anatomical landmarks to guide safe and successful cannulation. Therefore, in this approach successful cannulation depends on a knowledge of surface anatomy, an expectation that the anatomy will be normal and personal experience. However, the fact that patient anatomy does not always follow the rules and even if it does, the possibility of a thrombosis in the vein make needle malposition with its attendant complications a real possibility. The incidence of complications depends on patient factors such as short neck, obesity, scarring, cachexia and coagulopathy, operator experience, number of attempts and urgency of insertion.
The fundamental concept behind ultrasound guidance is that if the anatomy and patency of the vein can be visualized, the success of cannulation increases and complications related to multiple needle passes or aberrant anatomy can be avoided.
There is mounting evidence that using ultrasound guidance for venous cannulation reduces arterial puncture rates, counter-intuitively reduces time to cannulation and bridges the complication rate gap between inexperienced and experienced operators.
While ultrasound guidance is used most commonly for Internal Jugular and Femoral venous cannulation, it can be used for Subclavian veins as well. Most evidence exists only for Internal Jugular venous cannulation.
Techniques of ultrasound
There are different ways in which ultrasound can be used to guide vascular access
2D or B-mode
X marks the spot
2 hand technique
3 hand technique
This is similar to unguided access, except that an ultrasound is done prior to the procedure to ensure that the vein is patent and the anatomy normal. No marking of the vein is done
In this method, a Doppler probe (not a visual ultrasound) is placed over the area of interest and the artery and vein are identified by the pattern of audio output from the Doppler machine. There is no visual correlation. This is less effective than 2D in preventing arterial punctures and takes longer to localize the vein.
2D or B-mode
X marks the spot