Catheter is just an IV line.
The Needle punctures the vein , just like drawing blood.
Then a little plastic over the needle sheath is slid up the needle and then the needle withdrawn.
I was thinking as in human catheter that goes up the urethra to the kidneys for urine elimination. That's uncomfortable/painful for humans. Are you sure? If so, then past nightmares would have been for naught. What's the point of the sheath? Why don't they just inject the sedative in the vein and another injection for the fatal dose? Please explain.