Ultrasound Guidance for Radial Access: Getting in the First Time

Arnold Seto, MD, MPA, University of California-Irvine and Long Beach VA Medical Center, Orange, California

Arnold Seto, MD, MPA, University of California-Irvine and Long Beach VA Medical Center, Orange, California

Many cardiologists trained in the femoral catheterization approach find transitioning to radial arterial access difficult, if not outright intimidating. The radial artery is much smaller, averaging 2.6 mm in diameter, compared with 7 mm for the femoral artery. Neurology textbooks suggest that the two-point discrimination limit of our fingertips is somewhere between 2-4 mm, making accurate palpation and subsequent cannulation of the radial artery at times a frustrating exercise. In addition, calcification of the artery may cause the artery to move away from the needle tip. Although it improves with experience, the rate of access site crossover from transradial to femoral catheterization is 5-10%, with much of that due to failure of vascular access.

We have found that the use of real-time vascular ultrasound facilitates and speeds our radial artery access. Speaking to my colleagues around the country, a number of operators have also found ultrasound to be useful, especially for cases where initial palpation-guided attempts have failed. The following is a brief guide to the technique.

Step 1: Prepare the ultrasound machine (first turn it on). Cover the probe with a sterile sheath cover. Put ultrasound transducer gel in contact with the probe tip inside and outside of the plastic sheath cover. Securing the probe cover with a rubber band supplied with the probe cover is useful, but optional. The image display settings should be set at a minimum of depth penetration (i.e., 2 cm) and high gain. Turn on the centerline guide (green dots) on the display.

Step 2: Image the radial artery in the axial plane by holding the ultrasound probe perpendicular to the course of the artery. The artery will be an echolucent circle that pulsates on gentle compression. There may or may not be veins adjacent to the artery, which will completely disappear on compression (Figure 1).

Step 3: Align the artery with the centerline guide on the display by moving the probe. This ensures that the artery is directly beneath the center of the probe.

Step 4: Inject lidocaine above the radial artery (if not done prior to imaging). You may see the location of the lidocaine swelling the skin above the radial artery under ultrasound.

Step 5: Insert the radial micropuncture needle into the skin directly underneath the center marking of the probe, as close to the probe as possible without puncturing the sterile drape at about a 45-degree angle.

Step 6: Use short jabs (short in/out movements of the needle) to see the approximate course of the needle. Readjust the angulation if needed to have the needle move towards the artery.  Eventually, you should see the needle compress the artery wall, and subsequently see a successful puncture. Blood will be in the needle at this point (Figure 2).

Step 7 (Optional): If the entire forearm is exposed and sterilized, the ultrasound can be used to follow the guidewire up the arm to confirm appropriate placement without fluoroscopy.

Ultrasound imaging machines

In choosing an ultrasound machine for the cath lab, it is critical that the image be high resolution, that the depth of the display can be reduced to show the shallow radial artery easily, and that the display can be easily viewed from a distance.

The Sonosite M-Turbo provides a low profile, 6-13 MHz transducer (L25x) that is easy to use, boots up quickly, and provides a high quality image for the shallow radial artery. The Bard Access Site-Rite 6 and Site-Rite Vision machines have a 5-10 MHz transducer. The Bard machines have a proprietary needle guide system which is widely available and useful for femoral access, but which is not feasible to use in radial access. Other machines such as the Escalon Medical Vascuview and the Arrow InView also provide high frequency vascular images with a depth of 0.5-6 cm. Older machines such as the Acuson Cypress or Site-Rite 1-4, tend to have smaller screens or insufficient near-field displays which limits their usefulness in radial access.

Insights from radial artery ultrasound access experience

We have learned a number of insights on the radial access procedure from performing routine ultrasounds. 

  1. Artery sizing

The size of the artery can be assessed prior to the procedure (Figure 3), which may affect the likelihood of radial artery occlusion following the procedure. It has been shown that when the sheath outer diameter exceeds the radial artery diameter, radial artery occlusion may be more likely.1 In patients with peripheral vascular disease or diabetes, despite a normal Allen’s test, the radial artery might be particularly small. The modified Allen’s test is a marker of the size and dominance of the ulnar artery, and not a marker of radial artery size.

  1. Compression

Particularly in heart failure patients, the veins adjacent to the radial artery can become engorged  (Figure 4). Accidental cannulation of these veins can be difficult to detect using micropuncture needles, and can lead to a hematoma or worse. One of our fellows accidentally but successfully placed a 5 French sheath and catheter through a small radial vein. We have found that a small amount of manual compression is often helpful in closing these veins and making it less likely they are accidentally cannulated.

  1. Needle angulation

In most patients, the radial artery is shallow enough that inserting the needle from an angle, i.e. lateral to medial, is feasible. Some of my colleagues prefer this technique, thinking that it helps trap the artery from moving. However, in patients with thick arms, the radial artery may be up to 1.5 cm below the skin surface, and the needle angulation introduces an extra degree of freedom that may result in the needle inserting superficial or deep to the artery. For ultrasound-guided techniques, we find keeping the needle in line with the course of the radial artery maximizes our chance of successful cannulation.

  1. Blood pressure

Patients with marginal blood pressure often have very collapsible radial arteries on ultrasound.  As a result, the micropuncture needle may more easily puncture through and through the back wall, requiring manual pullback in order to insert the micropuncture wire. When a collapsible radial artery is seen, or blood pressure is <110 systolic, the double wall or Angiocath cannulation technique may be preferable.

  1. Radial artery anomalies

We have found dual radial artery systems (Figure 5) and radial artery loops in patients using ultrasound. Following the radial artery up to the elbow helps to screen patients for these anomalies, which can make the transradial procedure difficult or impossible. 

  1. Patient preparation

Prepping and exposing the entire forearm with a sterile gloved hand2 allows for maximal use of ultrasound, including pre-puncture screening for anomalies, following the guidewire in the artery up into the antecubital fossa, and more proximal radial puncture in the event of access failure, spasm, or hematoma.


Ultrasound guidance has been shown to reduce the number of attempts required to access the radial artery.3 It takes only a moment to set up, and only about 15 procedures before you become proficient. Practice on the easy ones, and you will find that you will be able to cannulate even the most difficult of radial arteries. Rapid radial artery access allows reluctant operators to eliminate one hurdle to moving fully into radial artery catheterization, with all its attendant benefits.

Dr. Seto can be contacted at: arnoldseto@yahoo.com.


  1. Saito S, Ikei H, Hosokawa G, Tanaka S. Influence of the ratio between radial artery inner diameter and sheath outer diameter on radial artery flow after transradial coronary intervention. Catheter Cardiovasc Interv 1999 Feb; 46(2): 173-178.
  2. Kern M. The Armen glove for radial access prep – a better way. Cath Lab Digest 2010; 18(4): 4-6.
  3. Shiloh A. Ultrasound-guided catheterization of the radial artery: a systematic review and meta-analysis of randomized controlled trials. Chest 2011 Mar;139(3):524-529.