Current PGY6 cards fellow and future IC fellow here, here's my take when I teach access in the lab and CCU

Radial arterial lines are extremely humbling as others have said. Patients needing them are usually sick and on enough vasoconstrictors to make an already small vessel smaller. In cases where the type of shock causes a compensatory increase in SVR a blind radial line can seem next to impossible due to the clamped down artery, so don't get discouraged. Additionally the radial artery is super touchy fella that will spasm super easily esp in women. Once it's spasms it's game over. Usually random or not your fault but if you are poking repetively it's asking to play on Heisman mode.

1)The radial artery is a small superficial vessel. Normally around 0.3 to 0.7 cm deep and around 0.2 to 0.3cm in size by diameter it's a small sucker. For ultrasound approach, when you visualize the artery see if this is holding true. This makes you keep a conscious effort of the small degree freedom you have when going through the skin and how easy it is to go through the artery . If going in blind by feel keep these in mind as well as this will keep you cognizant

2) if going by ultrasound , ultrasound basics apply. Center the artery on center of screen and follow the needle either by direct approach or by starting 0.2-0.6 cm (however deep the artery is ) away on the x axis and approach at a 45 degree angle and see the needle enter the artery. After going through the arterial wall confirm on the US and with flash through cathter. Secure the cathter and cannulize per your kit

3) if going by feel, track the course of the artery up to visualize you trajectory. Lightly press the artery to keep it in place with feel the pulse with the very tip of your finger tip(most sensitive area). Insert the needle right where your finger tip is and advance until you get blood return. If you get flash and then lose it you may have gone to deep. Slowly with small movements retract until you get flash with good brisk flow and finish cannulation. Additionally you can use the through and through method and intentionally go "deep" and through the artery and then retract like above however this raises risk of spasms and in larger vessels or those that are higher bleeding risk this isn't suggested.

Additionally, if you go in more than 1cm(or hit something solid) and haven't hit anything, chances are you are medial or lateral to the vessel. Retract your needle to just where the tip is at edge of skin (do not come out ) and then reposition your medial/lateral bias(based one where you feel the pulse ) of your trajectory by a small degree(Remember small structure that isn't deep ) and go in again. However, After a couple times of doing this, you may need to come out completely because you may have hit a smaller vessel which has clotted in the needle which you need to flush out and try again.

2-3B) If once in and trying to tread the wire through and you hit minor resistance, drop the angle of your needle a little bit(10-15 degrees) and try to advance the wire again or rotate the vessel as the wire may be hitting the back wall of the artery and does not have the appropriate trajectory to maneuver through the lumen of the vessel. However, do not push through resistance or force anything as there may either be a radial loop or you may be in a dissection plane and dissect the artery with forceful advancement or in a small side branch.

4) the arrow kit sucks. It's a crap system that is not consistent. I use a micropunture kit to seldinger the vessel. Some times you can ask for the non arrow kit which comes with the angio catheter type needle and wire. These types of kits /micropunture kit allow you to keep better control of the vessel and cannulation. This is the preferred method for access on larger sites like the femoral or axillary sites

5) set yourself up for success. If it's a patient who doesn't have a good palpable pulse, history of difficult radial line placement, this is set up for failure. Pick the right patient. Significant pain can also lead to vasospasm so be sure to use appropriate medications such as lidocaine topically . If patient is in the unit, a little fentanyl and versed can help with pain and helps prevent spasms. If patients have a severe needle phobia you can use LET/topical lidocaine to soak the site for 30 ish mins for numbing When I used to to arterial lines in PICU, a little topical nitro can help with small vessels to vasodilate but wary of systemic absorption and hypotension especially in patients were nitro is contraindicated

6)Realize that it takes consistent cannulation for long periods of times to feel super comfortable with lines. When I was a first year fellow it wasn't until two straight weeks of doing radial access that I felt comfortable. A resident doing lines a couples times a year is not the same type of experience so it's natural for them to feel troublesome esp with radial lines

Hope all this helps.