Once a target vessel is selected, cannulation can be executed using an static technique (US-guided vascular location and skin marking but without using real-time US guidance) (Video 1) or using a dynamic or real-time technique, consisting in observing the screen for direct or indirect signs of the needle entering into the vessel.2,7 (Videos 2, 3 and 4). Both US techniques are more successful for cannulation in comparison with the landmark technique.2,7 When comparing the static and dynamic techniques, the latter has demonstrated a better performance for vascular cannulation compared to the former.2,7 Advantages of the static technique is that it does not require sterile covers for the probe or a needle-screen coordination by the operators. On the other hand, in the real-time technique, cable and probe protection (as well as using sterile US gel) is needed to maintain a sterile technique, the latter usually resulting in some loss of resolution. Additionally, dynamic techniques require a perfect coordination between needle insertion, screen observation and the evaluation of blood returning from the needle, skills that necessarily require proper training and learning curve.

For US-guided real-time vascular cannulation, there are two main ways to reach the vessel: short axis and long axis1,2,7 (Video 2 and 3). There is a hybrid approach between the two techniques mentioned above called the “oblique” technique1,10 (Video 4).

Dynamic techniques can be performed by one operator (freehand technique, one operator simultaneously manipulates the probe and the needle) or two operators (one person manipulates the probe and the other person performs the procedure) and there is no formal recommendation regarding the selection of one approach over the other; however, although it requires more expertise, the freehand technique is preferred by most advanced practitioners, as it allows for real-time hand-eye coordination and it does not require additional staffing.7 While commercial needle guides are available for vascular guidance, most experienced users find that they are cumbersome and do not add value to a freehand technique. 7

In short axis cannulation, the vessel is centered on the screen and the needle is inserted in the middle, as close to the transducer as possible, and is advanced intersecting the ultrasound beam as shallow as possible to enhance needle visualization (see below). Since the tip of the needle commonly exceeds the body of the transducer as is inserted, observing the needle tip is difficult; in fact, the body of the needle (or its derived reverberation artifacts) is the portion seen7,11 (Fig. 5A). This explains why this technique is usually considered an “out of plane” technique. When using this approach, indirect signs of cannulation are important, such as the movement of the superficial tissues and the flattening of the anterior vessel wall as the needle is advanced into the vessel.

Figure 5.

Real-time US-guided vascular cannulation. (A) Short axis (out of plane) technique. Arrows: needle and its reverberation artifact; (B) long axis (in plane) technique. Arrows: needle body, Arrowhead: needle bevel; (C) oblique (in plane) technique. Arrows: needle body; arrowhead: needle bevel.

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In the long axis technique, the needle follows the path of the transducer and the ultrasound beam; therefore, it is completely visualized, including the bevel, advancing from the superficial tissues (using the ski lift technique and a shallow insertion angle, see below) up to its definite position into the vessel7,8,11,12 (Fig. 5B). This is why this technique is commonly considered an “in plane” technique. Using the probe orientation mentioned above, the needle is observed as is advanced from the right side of the screen; however, individual practitioners may prefer to see the needle from the other side (this can be achieved by changing the probe orientation or simply inverting the indicator probe positioning in the screen without changing the probe orientation).

Using the oblique technique, the needle is usually advanced using an “in plane” approach10 (Fig. 5C).

While “out of plane” and “in plane” are usually known as synonyms of short and long axis techniques, respectively, in fact they strictly refer to needle insertion in relation to the probe and ultrasound bundle, not in relation to the short or long axis of the vessels. Thus, “out of plane” and “in plane” approaches can be obtained in short and long axis approaches.

Needle visualization is sometimes cumbersome and practitioners must be aware of several useful ways to readily improve needle recognition (Table 2).

As previously mentioned, proper adjustments of gain depth and, especially, focus zone positioning parameters are basic and essential US machine manipulations intended to enhance resolution of superficial tissues aiding in needle recognition. Fundamental imaging is preferred over tissue harmonic imaging (THI), since a better needle visualization is observed when THI function is switched off.8

Either facing up or facing down bevel positioning are best to visualize the needle tip, in contrast with mid bevel positions.8

The first step to troubleshooting a “disappearing” needle is to visually inspect needle and transducer positions and exclude gross misalignment. The transducer should then be moved in a slow and controlled manner using sliding, tilting, and rotating maneuvers until the needle shaft and tip have been brought back into view.8 It is best to avoid moving the transducer and needle at the same time when trying to align them, as this makes the task more difficult and increases the risk of unintentional needle trauma.8

A common practical problem is the needle tip recognition in the superficial tissues as is inserted in the middle of the side of the transducer using in-plane techniques (long axis or oblique approaches). This situation may preclude the optimal visualization of the entire needle as is advanced and is seen when the needle is inserted too far away from the transducer. One way to best recognize the needle in these cases is to use the “ski lift” technique12 (Video 5), which consists in slightly elevating an extreme of the transducer, then inserting the needle below it and finally allowing the transducer to lean over the needle; in this way, the needle is always insonated and thus best recognized.

The most important tip for needle recognition using “in plane” techniques is the angle of insertion with respect to the ultrasound beam (needle-beam angle).8 Angle insertion close to 90 degrees is the best angle for needle visualization, according to ultrasound physics (i.e. specular reflection) (Fig. 6A). When using sloped angles of insertion (<55 degrees), the needle is not well visualized (Fig. 6B) thus, a shallow needle-beam angle must be kept to improve needle recognition at all times. In cases of unavoidable sloped angles of needle insertion (e.g. vessels that are too deep), using the electronic beam steering function of the ultrasound machine can enhance the needle visualization secondary to increasing the needle-beam angle toward 90 degrees. Using the same principle of needle-beam angle “correction”, instead of electronic beam steering, the “heel-toe” technique is also a valid alternative for this purpose (Fig. 6C).7

Figure 6.

Optimal needle-beam angle for best needle recognition. Dotted yellow line represents the 90 degrees in relation with the beam, illustrated by the solid yellow line. Solid white line represents the needle and its angle in relation with the beam. (A) Shallow needle-beam angle of insertion, allowing for perfect observation of the needle body and the bevel (facing up). Needle reverberation artifacts are also evident. (B) A slopped angle of insertion is shown, and needle visualization is blurred between the tissues. (C) With the needle in the angulation of B, inclining the transducer (“heel-toe” maneuver) greatly enhances needle visualization through approaching the need-beam angle toward 90 degrees.

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In short-axis cannulations, one way to improve the needle tip visualization is to “fan” the transducer proximally as the needle is advanced7 (walkdown technique) (Video 2).

Another way to improve needle visualization is the use of echogenic needle designs (“echo-enhanced needles”, obtained by needle dimpling, etching, and polymer coating). Although using these needles did not appear to improve important metrics such as first-pass success or number of attempts, they proved to enhance needle tip recognition and decrease the number of posterior wall perforations.13 Its main disadvantage is its high cost.

Using needle-enhancing software allows the improvement of needle visualization; however, there is not enough evidence to suggest using it.

An interesting approach to enhance needle recognition is the “WIN” technique, an acronym that stands for “wire-in needle “modified Seldinger technique14 (Video 6). It consists in the needle insertion with the wire mounted inside it (needle filled by the wire up to the bevel), with the purpose of enhancing the needle echogenicity as its hollow part (normally occupied by air or eventually primed by fluids) is occupied with metallic material. In addition to enhancing the needle, this technique provides a “one-in-all” approach, since the operator does not need to disconnect the syringe and then pass the wire; on the contrary, they insert the wire immediately after needle insertion into the vessel. This is especially important when cannulating depleted vessels since the needle tip is commonly dislodged from the target vessel when disconnecting the syringe from the needle in traditional Seldinger technique.14

Color Doppler does not aid in needle recognition and thus its use is neither useful nor recommended.

As previously mentioned, three-dimensional US, observing the needle and vessels in the three planes of the space, is an interesting technique that can improve precision when locating the needle; however, volumetric transducers are not widely available and the use of the technique is neither systemized nor widespread. Thus, although promising, three-dimensional US for vascular cannulation is not yet recommended in practice.15

Along with detecting direct and indirect signs of vessel cannulation and observing blood returning, it is recommended to have the guidewires shown into the target vessel when used2,7 (Video 7). This practice allows to finally advance the catheter into the target vessel and not in adjacent tissues if it is unintentionally placed in a wrong position. The final position of the catheter should also be directly and indirectly displayed (Video 8).2,7 In peripheral venous cannulation, an agitated saline flush (10ml) passed through the catheter allows to display microbubbles running into the vessel and thus guaranteeing its adequate position and function7 (Video 8). For central venous cannulation, an agitated saline flush is injected through the catheter, normally observing the microbubbles flowing in the right atrium7 within 1–2s16 in simultaneous echocardiographic assessment in subcostal or apical 4-chamber views, performed with a phased-array or a convex probe.

Both dynamic techniques have advantages and disadvantages that must be considered to aid in decision making regarding the selection of the most adequate technique based on the individual characteristics of the vessel and surrounding structures (Table 3). Therefore, practitioners need to have appropriate skills to perform any of them.1

Deep veins providing access to the superior vena cava are the internal jugular vein (IJV, accompanied by the common carotid artery (CCA) in the neck), and the subclavian and axillary veins (accompanied by the corresponding subclavian and axillary arteries). Common femoral vein (CFV) (accompanied by the common femoral artery (CFA) in the groin) provide access to the inferior vena cava.

For IJV and SCV cannulation, dynamic US long axis (in plane) technique has been shown to improve successful cannulation rates, lower the cannulation time, reduce needle redirections and finally decrease the number of posterior wall perforations. All this advantages are associated with a lower number of mechanical complications and thus this approach is more convenient when cannulating these vessels.17

IJV is anatomically located in the anterior neck, found anterior to the sternocleidomastoid muscle (ECM) and lateral to the airway in the higher neck and between the clavicular and sternal heads of the SCM (i.e. Sedillot's triangle) in the lower neck. IJV is accompanied by the common carotid artery (CCA) and vagus nerve in the neurovascular bundle. Key US structures when locating the IJV, easily recognized in cross sections, are the infrahyoid muscles, the airway (trachea), the thyroid gland lobes and the CCA (Supplementary material, Fig. 7A). It is worth noting that anatomic variations are reported in the IJV position in relation to the CCA. The ideal position when cannulating the IJV is when this vein is located slightly anterior and lateral to the CCA, found only in 50% of the patients2 (Supplementary material, Fig. 7A) and thus justifying the US-guided cannulation in all patients. US-guidance has demonstrated an 86% reduction in the rate of catheterization failures, an 80% reduction in arterial punctures, a 78% reduction in the number of hematomas, a 90% reduction in the number of pneumothorax cases and a 94% reduction in the rate of hematomas; thus, the risk–benefit ratio and the cost–benefit ratio clearly point toward a strong recommendation to use this technique.18

Both the anterior and posterior approaches (in relation to the ECM muscle) are well performed. The Trendelemburg position improves the venous distention and reduces the risks of air embolism; therefore, is recommended in all patients.2,7 If patient can collaborate, Valsalva maneuver can also enlarge the vein. Mechanical ventilation, rising right atrial pressure, can also distend the IJV. Contralateral head rotation can result in IJV-CCA overlapping, which may result in accidental CCA laceration if the posterior wall of the IJV is inadvertently perforated. Thus, the head must be maintained in a relatively neutral position.2

From an anatomical point of view, the subclavian vein is located by definition under the clavicle (i.e. infraclavicular region); the axillary vein is in continuum with the subclavian vein, lateral to the outer border of the first rib at the teres major muscle.7,19

The subclavian vein is accessed from a supraclavicular or an infraclavicular approach (the latter is the most common technique).2,7,20 The main problem when cannulating this vein is the presence of the clavicle bone, which interferes with the probe positioning and fundamentally with the optimal view of this vessel.7,21 In that respect, many practitioners who gain confidence with subclavian vein cannulation move more laterally and puncture the skin at the midclavicular point. This probably means performing skin puncture over the axillary vein but either the subclavian or axillary vein may be punctured depending on the distance between the skin and the vein puncture sites (Supplementary material, Fig. 7B and C).19

Uncertainties related to the utility of the subclavian vein cannulation are likely to be derived from the ambiguity implied by whether studies of US-guided access in this area are referring to direct subclavian vein access or to subclavian vein access via the axillary vein.7,22,23 Despite this, pooled data analysis of US-guided subclavian (or axillary) cannulations showed a 94% reduction in the rate of catheterization failure, an 85% reduction in arterial puncture rates, 77% reduction in the number of hematomas, 78% reduction in the pneumothorax cases and 95% reduction in the number of hemothorax cases.18

As previously mentioned, the long axis (in plane) technique is preferred when cannulating these veins since it is associated with high success rates and fewer complications. The Trendelemburg position does not seem to improve venous distention of this vessel because it is in a relatively fixed position within the surrounding tissues. However, it reduces the risks of air embolism and thus it is always recommended.2

Evaluating the lung sliding before and after cannulating these vessels is recommended, since it allows to quickly detect the presence of pneumothorax.7,16

CFV is anatomically found in the groin (femoral triangle) below the inguinal ligament and medial to the common femoral artery (CFA) and femoral nerve in the neurovascular bundle. CFV is formed by the junction of the deep femoral vein and femoral vein (previously called “superficial femoral vein”) and receives the great saphenous vein (i.e. saphenofemoral junction). While CFV is usually cannulated using the anatomical landmark technique because of the relatively constant location of the vein related to the artery at a safe entry point closer to the inguinal ligament (2–4cm) 24; however, US guidance has demonstrated an 85% reduction in the rate of catheterization failure and an 86% reduction in arterial puncture rates.18 Thus, US-guided femoral cannulation is strongly recommended over the landmark technique18, and can be especially helpful in some circumstances, such as in obese patients, when arterial pulses are weak (e.g. patient is in shock) or when there are anatomical alterations.7 Ultrasonographically, this vein is easily recognized medial to the CFA and receiving the great saphenous vein. In the short axis, these structures are called the “Mickey Mouse” sign (Supplementary material, Fig. 7D). CFV is more distended when the hip has a slightly external rotation as well as in the reverse Trendelemburg position.2 In the long axis, the vein receives the great saphenous vein (Supplementary material, Fig. 7E) and its bifurcation is distal to the CFA bifurcation (Supplementary material, Fig. 7F).

Radial artery emerges from bifurcation of the brachial artery at the elbow crease and is found in the lateral side of the distal forearm near the wrist, where it becomes more superficial and easier to palpate. Accordingly, this artery is usually well recognized by US and accessed from its distal ventral and more superficial segment. This vessel is accompanied by the paired radial veins, located at both sides of the artery (Supplementary material, Fig. 9A and B). Applying a tourniquet distal to the site of cannulation has been shown to improve the arterial dimensions and thus this maneuver can be considered in practice when the artery is too small.26

Additionally, the radial artery can be recognized and cannulated using US in its dorsal segment, found in the anatomical snuffbox (Supplementary material, Fig. 9C). This can be considered as a valid alternative when the more proximal radial artery cannot be accessed.27

Pooled data analysis showed a 36% reduction in the rate of catheterization failure at the first attempt and an 83% reduction in the number of hematomas; however, overall success rate is not reported. Despite this, benefits outweigh risks and thus US-guided radial insertion is probably recommended over the traditional technique.18

The permeability of the palmar arch must be assessed prior to cannulating this artery. This can be performed using the modified Allen test, plethysmographic Allen test28 or modified duplex Allen test.29 The last one is done placing the transducer in the anatomical snuffbox and identifying the radial artery flow by color and spectral Doppler. After spectral waveforms are obtained, the radial artery is compressed proximal to the site of insonation. Normally, reversal of flow confirms patency of the deep palmar arch (Supplementary material, Fig. 9D). Otherwise, an inadequate deep palmar arch is suspected and thus this artery is best avoided for cannulation.

As previously described, CFA is located in the femoral triangle, lateral to the CFV and medial to the femoral nerve in the neurovascular bundle. 20% of the patients have a CFA bifurcation above the inferior border of the femoral head. Too high punctures may lead to retroperitoneal hemorrage while punctures below the femoral head may lead to pseudoaneurysm formation. Also, too low punctures may lead to damage to the superficial as well as the deep femoral arteries, an important complication that may lead to significant hemorrhage since these arteries are difficult to compress in thigh soft tissues.30 There is some overlapping between the CFA and CFV in 65% of the patients; therefore, an arteriovenous fistula formation can occur if an inadvertent posterior wall perforation of the CFA occurs.31 In a recent meta-analysis including 719 patients that had US-guided femoral arterial cannulation performed, a 44% reduced overall complication rate was noted in comparison with the landmark technique. Furthermore, a 42% improvement in first-pass success was demonstrated.31 Thus, US-guided cannulation can be recommended as a first line tool to safely place a femoral artery catheter.

The authors declare that no experiments were performed on humans or animals for this study.

The authors declare that no patient data appear in this article.

The authors declare that no patient data appear in this article.