Here is how I was taught to do it / how it works:
I received 1 syringe with 2ml of TPA in it. #1 port was clogged completely; I couldn't push anything through it. #2 port was VERY sluggish; I could push IV meds but could not run anything IV because the pump perceived an occlusion.
For port #1, I pushed the TPA in. It seemed like nothing went it, but (in actuality), I got about 0.2ml in the line. I pushed the rest into port #2.
When I came back in 1 hr, I pulled back on both lines. Port #1 had some blood clots slowly come out, but still no flow. Port #2 was now functional. For port #2, I pulled back approx 6ml (to pull the TPA that was in the line in addition to clotted & fresh blood), then I flushed w/ 10ml NS & 10 ml heparin.
I got another syringe (another 2ml) of TPA from pharmacy. I put as much as it as I could in port #1 -- approx 1ml. Then you leave it for 2hrs. (I can't remember the exact times, but pharmacy has a protocol.) When I came back & pulled on port #1, blood came out. Again, I pulled about 6ml which included the TPA, declotted blood & fresh blood. Again, I flushed with NS & heparin.
Basically, TPA is a VERY STRONG anticoagulant. It is designed to stay in the line ONLY. (If you FLUSH the line with TPA in it, you can actually kill someone. Side note: ALWAYS label the ports, with tape or something, to CLEARLY indicate when TPA is in the line, so anyone coming in contact with it knows NOT to flush the lines! I also put a sign on the pt's door / over their bed, just to make sure.) Considering how long a PICC line is, if you push up to 2ml, that is not enough to reach the SVC. It's only enough to stay in the line. It does it's declotting work, then you pull back, and you're pulling the TPA, declotted blood, and fresh blood. Then the line is almost as good as new.
And, yes, it can be used for port-a-caths also because they have the long line like PICCs do.
It's actually pretty cool to see the stuff in action -- to see a PICC port go from completely clotted to free flushing!