From a doctors point of view...
If you need an ECG to determine prehospitally whether a patient has cardiac arrest or not.......you failed BLS, ALS, every bloody LS course there is.
Patients, that are unconscious and dont breathe have cardiac arrest and BLS procedures should be started immideately... See European Resus Council website.
In-hospital the anaesthetist uses a 5-lead ecg to monitor patient during surgery. In case of symptoms as stated in the previous post we run 2 12-leads, a short and a long one.
LP-12 and 15 or Zolls cct (that are somewhat cheaper than LPs) are great tools for the skilled user in the prehospital environment, but they can never be better than your clinical observations. They are tools, that helps YOU (or the person you transfer the ECG to) decide what to do. I totally agree with bigjl on the indications for doing a 12-lead, but dont rely on the machine, use it as a decision aid, together with SAMPLE and clinical observations.
best regs and be safe
Mike