Background and Indications for Examination
The expeditious and safe placement of venous catheters is vital to the practice of acute care medicine. Venous access includes the cannulation of both superficial peripheral and large central veins, depending on the particular clinical scenario. Traditionally, venous access is performed blindly by palpation or with the use of anatomic landmarks. More recently, ultrasound guidance has become standard of care for most central venous access, and using ultrasound for difficult peripheral access may reduce the need for central access.
Obtaining venous access in certain situations may be difficult secondary to patient and operator characteristics. These factors include obesity, intravenous (IV) drug use, history of multiple prior attempts due to chronic illness, severe intravascular volume depletion, venous thrombosis, coagulopathy, operator inexperience, and operator anxiety due to the patient’s clinical condition. There are clinical scenarios in which a patient must receive IV access expeditiously such as for the administration of fluids or medications, hemodynamic monitoring, transvenous pacemaker placement or hemodialysis.
Complications of central venous cannulation include multiple failed attempts, arterial bleeding, pneumothorax, loss of the guide wire, and catheter-related bloodstream infections. Recent studies have demonstrated that ultrasound-guided central venous access decreases the number of unsuccessful attempts and the complications when compared to the landmark technique. In addition, ultrasound reduces the time required to obtain venous access, which is critical in ill patients. With the possible exception of subclavian access in certain situations, ultrasound guidance is recommended for placing central venous catheters.
Probe Selection and Technical Considerations
A 7.5- or 10-MHz linear array probe should be used for vascular access. High-frequency probes provide higher-resolution images of superficial structures. Linear probes generate rectangular or square images, which makes it easier to follow the tip of the needle while visualizing the vessel.
The focus should be at the level of the blood vessel in order to maximize the lateral resolution of the image.
The depth should be adjusted so that the vessel of interest takes up approximately three-fourths of the screen. Most of the target vessels will be superficial; therefore, the depth should be decreased appropriately.
The machine should be used on the “vascular” setting for this procedure. This optimizes the resolution and clarity of the image.
The total gain may be increased to brighten the signal returning from the walls of the blood vessel, making it easier to identify. The far gain or time-gain compensation (TGC) may be increased alone in order to enhance the signal returning from deeper vessels and is especially helpful in obese patients.
Color-flow Doppler identifies blood flow within a vessel. It is useful for the sonographer to use this modality to correctly identify blood vessels when they are difficult to locate. It also helps to differentiate blood vessels from other surrounding structures, which will not pick up color flow.
Although peripheral venous access is not a sterile procedure, it does involve exposure to patient body fluids. A probe cover, such as a Tegaderm, is recommended for this procedure to help keep the probe clean. Central venous access is a sterile procedure; therefore, typical precautions including a sterile probe cover and gel should be used.
The static approach involves using ultrasound to locate the target vessel and marking two places along the course of the vessel about 1 cm apart. The cannulating needle is inserted at the proximal mark and directed toward the distal mark without the use of real-time ultrasound guidance. This approach has a higher failure rate than the dynamic approach and is not recommended.
The more commonly used dynamic approach involves identifying and cannulating the vessel in real time with ultrasound guidance. The dynamic approach can be performed using one or two operators. In the case of two operators, one person can hold the probe and the other performs the procedure. It has the advantage of allowing the person performing the procedure to use both hands when inserting the needle and advancing the catheter. The procedure is more often performed with one operator who holds the probe in one hand and inserts the needle with the other. With this technique, the operator can make subtle adjustments to the orientation of the probe as needed without relying on an assistant.
Ultrasound-guided venous access can be performed in either the short (transverse or “out-of-plane”) or long (sagittal or “in-plane”) axis. In the short-axis view, the probe is held perpendicular to the direction of the vessel, generating a transverse cross-sectional image (Fig. 17-1). This is the usual technique taught to the novice, as it is easier to puncture the vessel in the transverse axis. It also allows simultaneous visualization of neighboring structures, such as arteries and nerves. The disadvantage of this technique is that it is often difficult to judge how far the needle tip has advanced into the lumen of the vessel; therefore, it can be pushed in too far and puncture the posterior wall. This can result in the vessel being “infiltrated” and the development of a soft tissue hematoma due to extravasation of blood, which renders the vessel unsuitable for IV access. It can also lead to injury of nearby vessels, such as an artery, especially in central venous access. With this approach, the cannulating needle is seen only when it crosses the ultrasound beam and appears as a hyperechoic dot on the screen. The needle punctures through the proximal wall of the vessel and produces a bright white hyperechoic artifact in the center of the anechoic lumen called the “target sign” (Fig. 17-2). Once this sign is visualized, the needle is inside the vessel, a flash of blood should be seen in the angiocatheter and the needle should not be advanced any farther.
In the long-axis approach, the probe is placed parallel to the vessel, generating a longitudinal cross-sectional image (Fig. 17-3). The advantage of this technique is that the full length of the needle may be visualized within the lumen of the vessel, making it easier to avoid puncturing the posterior wall (Fig. 17-4). A disadvantage is that the probe has a slender width in this orientation and the needle must be positioned directly underneath it in order to be visualized in its entirety. Subtle movements of the probe can cause the needle to fall outside the width of the ultrasound beam and no longer be visualized on the screen. Also, it may be difficult to simultaneously determine the location of the artery (except when the artery is directly below the vein). As a result, small movements of the needle outside the ultrasound beam may lead to undesired arterial puncture.
This image illustrates the long-axis approach to peripheral venous access. Note the blood vessel (V) in its sagittal orientation and the double bright white hyperechoic lines within the lumen representing the IV catheter (C). The full lengths of the needle and catheter are visualized in this axis, making it less likely to puncture the posterior wall and infiltrate the vessel.
Oftentimes, it is recommended that the target vein be identified and punctured in the short axis and then the probe rotated 90° into the long axis to make sure the tip does not hit the posterior wall. The IV catheter can then be advanced in the long axis and can be seen in its entirety. Whichever method is selected, it is crucial to always follow the needle tip.
Performing Ultrasound-Guided Peripheral Venous Access
- 18- or 20-gauge angiocatheter: A standard angiocatheter (1.25 in) is sufficient for superficial veins, but a longer one (1.88–2.5 in) is necessary for deeper vessels. In general, ultrasound is most helpful in identifying deeper (>5 mm) vessels and unless the vessel is very superficial, the longer angiocatheters should be used for ultrasound-guided peripheral access.
- Alcohol pads or chlorhexidine applicator
- A lidocaine syringe with a 25- or 27-gauge needle
- Dressing to secure IV and vacutainer/blood tubes/IV hookup and flush as needed
The most common peripheral veins used for IV access are the cephalic, basilic, or brachial veins of the arm (Fig. 17-5). It is important for the sonographer to be familiar with the upper extremity anatomy before beginning the procedure. The cephalic vein is most lateral, coursing along the radial aspect of the extremity and best approached from the lateral surface of the upper arm. The brachial vein (or veins) runs with the brachial artery and is often cannulated in the antecubital fossa. There are often paired brachial veins on each side of the brachial artery. The artery should be identified by compressing the vessels slightly and looking for pulsations. The brachial nerve is also in close proximity and should be avoided. The basilic vein is the most medial, running along the ulnar aspect of the extremity and is best accessed from the medial surface of the upper arm. The basilic vein is not paired with an artery and is found in the subcutaneous fat of the medial arm where it is not visible or palpable and usually has not been accessed via a landmark technique. For these reasons, the basilic vein is often the best candidate for ultrasound-guided access and is often used for peripherally inserted central catheter (PICC) lines.