12 leads are typically done on scene if the patient presents with chest pain or an anginal equivalent. Basically, if I determine there is a reasonable possibility that these symptoms could be caused by something cardiac or there is significant history, the 12 lead is done early after making patient contact. If it's more of a "nice to know" additional assessment, I'll do it in the truck and acquire it at a stoplight or smooth stretch of road.
IVs are in the truck unless I am A. Using it to treat something immediately life threatening or treatable IE cardiac arrhythmias, hypoglycemia, pain management, allergic reactions, and cardiac or respiratory arrests or B. Waiting a while for an ambulance to respond.
Nebs are almost always started on scene shortly after patient contact. Same goes for CPAP when I use it, unless it's something I progress to after a neb treatment isn't sufficient alone. Most other meds are started in the truck apart from those I mentioned earlier or ODT Zofran, ASA, or SL and transdermal NTG. Oh, and fentanyl. If I'm administering opioids, 9/10 times I try to get some on board before we start moving the patient around and packaging them.
Physical assessment truly does depend, even though that's a bit of a cop out. All patients get a brief, general survey and enough of a symptom based physical exam to determine "sick/not sick" on scene. The closer they get to either extremely "not sick" or "extremely sick" and not treatable by me, the more of the detailed exam ends up getting deferred to being performed en route. Patients on the moderate side of the spectrum may get slightly more of an exam on scene, but I still prefer to get moving somewhat quickly rather than listening to bowel sounds or anything crazy on scene.
