Arterial Access at the Wrist
Ultrasound vs. Palpation
A recent meta-analysis looking at radial artery cannulation in kids by Zhang et al found the use of ultrasound significantly improved first-pass success and total success when compared to palpation. They also found fewer attempts and quicker time to success with the use of ultrasound. But as anyone that has tried cannulating an arterial line in a NICU baby can attest, arterial cannulation in young children can be challenging, and success is oftentimes influenced by patient characteristics and operator experience.
There is limited data about the exact learning curve or number of required arterial cannulation attempts to constitute a reasonably experienced operator in infants, though Tang et al found experience with ultrasound-guided vascular access plays an important role in successful cannulation. A number of smaller (mostly) randomized studies have examined US-guided radial access across a range of practitioner skill levels and patient ages. An early study from Ganesh et al looked at anesthesiology trainees with minimal experience (< 10 US-guided arterial cannulations prior to the study). It should come as no surprise that they found no benefit using ultrasound in such inexperienced hands. A later study by Ishii et al—also in anesthesiology trainees, though with much more experience placing central lines—found that the use of ultrasound more than doubled the first pass success (35.6% vs 76.3%), despite their patients being much younger than the Ganesh study (mean age of 1.5 years vs 8.3 years).
Hard to compare apples to oranges, but...
This "experience” effect is not limited to just anesthesiology trainees. Anantasit et al saw the same thing looking at PICU fellows, all of which had performed at least 10 radial artery cannulations with both techniques prior to the study. The use of ultrasound more than doubled the first-pass and total success, as well as reduced the incidence of hematomas by almost 80%!
But what about the most seasoned hands? Does the use of ultrasound help those with years, perhaps even decades of experience? Resoundingly, I would say the answer is yes! Looking at the graph above, it may seem like there is a plateau effect in the first-pass success rate beyond which additional experience offers no further benefit. However, when you also see that decreasing patient age (orange line) is mirrored by increasing practitioner experience (green line), it is hard to deny that further experience offsets the increasing challenge of placing arterial lines in our smallest patients. As alluded to above, although Jung Oh et al and Min et al found comparable first-pass success rates, the “operators” in these studies were attending anesthesiologists, each with at least 5x the experience of the previously mentioned studies. Interestingly, Min et al also found that “failed patients were younger, with higher neonatal proportion and smaller body weight.” They also noted a body weight threshold (<3.5kg), below which success was significantly impacted—even with ultrasound guidance! And while Jung Oh et al did not find the same effect, their patient cohort was typically months older, and no patient weighed ≤ 3.7kg. After performing multivariate analysis to remove confounding, they found that only radial artery cross-sectional area ≤ 1mm² and the presence of an anomalous radial artery branch were predictive of failure.
While we may never achieve 100% success on our first attempt, using the ultrasound helps us inch our way towards that ideal.
Tricks & Tips
Scanning Planes
I assume many of you have a personal preference when it comes to the short-axis vs- long-axis debate. I had long heard whispers of the the benefits of performing arterial cannulation in the long axis, and those voices would always come to me in my moments of doubt—usually as a struggled to place a right radial arterial line in a neonate for CoA repair.
The few times that I’d switch to the long-axis, I would excitedly visualize the needle advancing intraluminally, but then almost never visualize a flash. When I would then rotate the probe 90° back to the short axis, I would see the needle coursing tangential to the artery, but definitely not inside it—as they say, close, but no cigar. Only recently did I learn this phenomenon was due to an artifact, specifically the Section- or Slice-Thickness Artifact (see graphic to the right). In my eternal struggle to show Meg my needle, alas this one doesn’t count.
There are a few randomized studies in adults comparing Long Axis In-Plane (LA-IP) vs Short-Axis Out-of-Plane (SA-OOP) techniques for radial arterial line placement. While Sethi et al found comparable rates for both first-attempt success using either technique, Quan et al found SA-OOP superior, while Arora et al found LA-IP superior. A recent metaanalysis from Cao et al found the SA-OOP was associated with a higher incidence of posterior wall puncture and hematoma than the LA-IP technique, whereas success rates were similar for the two ultrasound-guided arterial cannulation techniques.
Song et al compared these two techniques in both infants and children. They found no difference in first-attempt success or total success using either technique. Furthermore, they found using a SA-OOP technique was also faster (but perhaps not that clinically relevant), but it was associated with a much higher rate of posterior wall puncture. Thankfully, while the incidence of hematoma was not documented, there were no other recorded complications using either technique.
It would seem this evidence mirrors what we see in the adult literature, that it doesn’t matter which technique you choose, you’re bound to have similar rates of success with either. With that in mind, I have one word for you: Clemmeson.
Back in 2012, Clemmeson et al looked at whether teaching ultrasound novices a novel technique, Dynamic Needle Tip Positioning (DNTP), improved success at cannulating a peripheral vessel. In DNTP, one first identifies the target, in this case a vessel in the short axis. After vessel identification, one advances an angiocatheter midline until the needle tip (a hyperechoic dot) appears in the ultrasound screen. One then advances the ultrasound (and only the ultrasound) proximally until the needle tip is almost out of view. Next, one advances the needle (and only the needle) until the needle tip reappears as a hyperechoic dot. You continue in this alternating stop-start pattern until the needle tip has been identified intraluminally AND has been advanced a few additional times using the DNTP technique.
After a brief introductory lecture on ultrasound-guided vascular access, coupled with live demonstrations, the participants attempted vascular access in both the short and long axis using a gelatine phantom “peripheral vessel”. The only modification being, they used DNTP when performing the SA-OOP technique.
They found that the DNTP SA-OOP technique had a significantly higher success rate of the needle tip inside the vessel compared to the LA-IP. 97% of novices positioned the needle tip inside the vessel using the DNTP SA-OOP technique, while only 81% were successful with the LA-IP. They also found that the needle tip was more likely to lie closer to the center of the vessel (the origin in a standard x-y coordinate system) using the DNTP SA-OOP technique.
Gelatine phantoms are one thing, but what about real patients, especially infants and small children? Look no further than Takeshita et al. And just to make things even more interesting, they only looked at kids with radial arteries with a depth ≥ 4mm, a risk factor previously associated with difficult ultrasound guidance. Takeshita found that using DNTP SA-OOP (compared to standard SA-OOP) led to much higher rates of first-pass success (85% vs 50%) and total success (95% vs 60%) even in these more challenging patients. Interestingly, they also found that DNTP was actually faster.
While comparing it to palpation, Lui et al found that DNTP had a first-pass success rate of 40% and an overall success rate of 96.7%—in neonates! It took considerably less time (1.5 vs 4.7 min), and had an 8x reduction in the incidence of hematoma. Most of the above mentioned studies were performed by skilled, experienced practitioners. Lest you think that this technique is just for attendings, Ye at al evaluated the use of DNTP for radial access by anesthesiology trainees in neonates. With one small modification. Adapting a technique described by Nakayama et al, one of the randomized cohorts first had 0.2mL of sterile saline injected subcutaneously under direct US-guidance directly above the radial artery. This accomplished two things: it increased the radial artery depth and it created an anechoic fluid collection. Not only did this collection clear the visual space between the skin and the artery, but it also made visualization of the hyperechoic needle tip that much easier. Kind of like trying to find a specific star at daytime vs nighttime. Not only was the modified technique almost 3x faster, it had an overall higher total success rate and a 64% reduction in hematomas. Gilding the lily, the modified DNTP is not!
Safe to use the Ulnar?
We both knew what would happen next. In a minute or two, once my central line was secure, I would swing around to the untouched arm and take my chances at getting the arterial line. The only thing sweeter as a trainee than nailing your procedure, is then finishing the procedure the attending is struggling with. Knowing I was seconds away, rather than let me “steal the line,” my attending did something that left me speechless: they stuck the ulnar. On the same side. As the failed radial.
I was utterly dumbfounded.
Sticking the ulnar artery was about as taboo as one could get when I was a trainee. And to do it on the same side as a failed radial arterial attempt seemed like heresy. Years later, while still emblazoned in my mind, my opinions about cannulating the ulnar artery have softened. Considerably.
And I am not alone. In a very recent retrospective analysis over a 10 year period, Gleich et al at the Mayo Clinic evaluated major complications of arterial cannulation in all pediatric patients less than 18 years old. In their analysis of over 5,000 arterial lines in over 4,000 patients, they found only “11 short-term major complications attributed to arterial cannulation,” all of which occurred in kids < 5yrs, and all were femoral arterial lines. The authors later commented that as a result of their findings they have changed their practice to “preferentially use the radial or ulnar arteries in children less than 5y of age whenever possible.” While there are some limitations to the study, and only a relatively small number of ulnar cannulations were performed (95 total, 1.8%), for an institution as prestigious as the Mayo Clinic to wholesale change their practice and integrate ulnar cannulation as a front line technique, one cannot completely discredit it either.
So, why isn’t the ulnar used more frequently? Is it because it may be deeper, and thus harder to cannulate?
In a cadaveric study by Rodríguez-Niedenführ et al, they found that the ulnar artery is actually more likely to be superficial than the radial artery. (3.75% vs. 0.2%). In an RCT in 100 adults randomized to either radial or ulnar catheterization, Karaclar et al looked at the strength of the pulse, the ease of cannulation, as well as success, failure, and complications rates. While there was a significantly lower percentage of strong ulnar pulses (73% vs 83% for radial), interestingly, cannulating a strong ulnar pulse had a much highest success rate success rate compared to for a strong radial pulse (100% vs 91.3%). In fact, in addition to frequently being more superficial, a recent study out of UCSF by Roux et al found that in 60% of pediatric patients, the ulnar artery is also was larger than the radial artery.
How about the risk of injuring ulnar nerve?
Though largely studied in the adult cath literature, this also hasn’t been found in the data.
Perhaps the most concerning is the potential for ischemia.
In a prospective study over a 12-month period, Brotschi et al evaluated over 600 intra-arterial catheters (IAC) in children. They found that arterial thrombosis occurred most frequently in the femoral artery (relative incidence of 13%), similar to Gleich et al. They also found that younger age was independently associated with an increased risk of thrombosis. In an earlier study, Kahler and Mirza also retrospectively evaluated the usage and complications of arterial cannulation in critically ill pediatric patients. In their 4-year retrospective, they had a total of 18 ulnar catheters (out of 190 total). They found that the prevalence of an ischemic event was similar for both ulnar and radial catheters (5.6 vs. 5.8%), yet lower than the average reported for femoral catheters (9.3%). In their series, only one patient with ulnar cannulation showed signs of ischemia, and this patient previously had ipsilateral radial artery cannulation for 4 days. Obviously, the danger of vascular compromise due to thrombosis from prior cannulation of the ipsilateral radial artery is a major risk factor to consider before catheterizing the ulnar artery.
While I may not be ready to stick the ipsilateral ulnar artery after a failed radial attempt, I do feel very comfortable cannulating an ulnar artery on an untouched limb. Factor in the improved success rate with ultrasound, and I think ulnar arterial cannulation may be due for a serious clinical re-evaluation and renaissance.
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