Finding a vein in a dehydrated arm: why cannulation sometimes takes two attempts
The cannula itself is smaller than most first-timers expect: typically 22 or 24 gauge, a plastic tube around the diameter of a pen refill's tip, threaded over a needle that is withdrawn immediately once the vein is entered. Nothing metal stays in your arm. The insertion stings for a second or two; a competent nurse gets it on the first attempt most of the time, and professional convention is to stop and reassess after two failed attempts rather than keep fishing. If someone is on attempt four, you are entitled to end the session.
Once the line is in, flow is set to run the bag over roughly 30 to 60 minutes. Faster is not better: infusion rate is a safety parameter, especially for additives like magnesium or high-dose vitamin C, which is one more reason these belong under physician-approved protocols rather than improvisation.