Most of us were taught to place central lines using the wire through needle Seldinger technique, but using the introduction catheter instead of the needle makes successful wire insertion more likely and facilitates venous confirmation. A 7 minute screencast.
operating room video of wire through catheter technique. recommend viewing at 2x speed.
Update 1 Lee 2015 compares wire through needle and wire through catheter and shows that wire through needle is (slightly) better. The operators were anesthesiologists very experienced in central line insertion, which may account for their results. In the less controlled ED environment, with providers who don’t place a lot of lines (especially learners), the tendency of the needle to move before the wire can be transmitted may be more important. Also they did not confirm venous location in this study, which should be done whenever feasible and is greatly facilitated by the wire through catheter technique. Still, I’m aware of no data comparing the two techniques other than this (which appeared a few weeks after I posted the video), so though my success rate is definitely higher since I switched to wire through catheter, your mileage may vary.
Update 2 There are two advantages of the wire through needle (conventional approach): the needle is functionally slightly longer than the catheter, and the needle doesn’t kink. Neither of these advantages make a difference at the IJ site, and where I work almost all lines are IJ, so I didn’t mention this in the video. Based on feedback, I am surprised (and pleased) at how many non-IJ lines are being placed out there. If there is a lot of flesh in between the skin and the vein, for example at the femoral site in an obese person, the catheter can kink. This can be managed by keeping the pannus out of the way and keeping the skin taut throughout the procedure (flabby groin tissue can interfere with wire advancement as well, regardless of which technique you use to transmit the wire), but this requires an extra set of hands. At the subclavian site, the problem is not flesh but distance–sometimes you enter the vein relatively far away from where you enter the skin, and in this case the couple of millimeter difference between the wire and the catheter can be relevant. You can overcome this problem, in cases where you have to hub the catheter to get into the vein, by applying gentle continuous forward pressure on the hub until the wire is in the vein.