S/Q saline injection for pediatric art lines placed with POCUS guidance?

I was catching up on back issues of Anesthesia & Analgesia when I came across this article from the May 2014 issue: Nakayama et al. A novel method for ultrasound-guided radial arterial catheterization in pediatric patient.

This was actually 2 studies in 1, or 2 phases as the authors call it.

Common features of both phases

• Enrolled kids < 3 y.o. undergoing elective surgery who needed an art line pre-op
• Performed in Kyoto, Japan
• All lines placed by 2 anesthesiologists with lots of experience placing U/S-guided art lines
• All lines placed using out-of-plane/short axis approach and real-time/dynamic technique without a guidewire

Assessment phase
102 patients enrolled

Results: a radial artery that was more shallow than 2 mm or deeper than 4 mm were independent predictors of both decreased first-attempt and overall success.
A depth 2-4 mm had the best success rate.
Overall success rates:

• < 2 mm: 62.5%
• 2-4 mm: 89.7%
• >4 mm: 51.6%

Validation phase

Methods
With the results of the assessment phase known, the authors tested the hypothesis that a S/Q injection of saline to increase the arterial depth from < 2 mm to 2-4 mm would improve the success rate.
60 patients were enrolled and had an art line placed. These were broken down into the following groups:

• 20 had an arterial depth of 2-4 mm
• 20 had an arterial depth of < 2 mm
• 20 had an arterial depth of < 2 mm and had a S/Q injection of saline (< 2 mL)

The last 2 groups were randomized. Measurement of depth, injection of saline, and marking of the appropriate insertion point was performed by an anesthesiologist other than the two who placed the art lines. These two anesthesiologists were also blinded to the results of the assessment phase.

Results
Success rate and time to catheterization improved significantly with the saline injection and no longer differed significantly from patients who initially presented with arteries located at a 2-4 mm depth
Overall success rates:

• 2-4 mm: 85.0%
• < 2 mm: 55.0%
• < 2 mm PLUS saline: 90.0%

The authors hypothesized that an artery more shallow than 2 mm did not allow one to adjust the aim of the needle towards to the centre of the artery, simply because there is not enough trajectory to evaluate. They also suggested that a depth greater than 4 mm necessitated a needle angle that was too steep to allow easy cannulation. The injection of saline may also provide a better window to his very superficial structure, similar to an external step-off.

One last tidbit from the authors explaining why technique is so important: a 24-gauge catheter has an outside diameter of 7 mm while the mean diameter of radial arteries in this study was 10 mm.

Another question generated by this study: can a S/Q injection of saline improve the view of other superficial structures?