A standard pre-hospital 15 lead ECG consists of taking a second "12 lead ECG" but moving 3 electrodes to V4R, V8 and V9. V4 moves to V4R; V5 moves to V8; V6 moves to V9. The last three leads on the 12 lead printout (V4, V5, V6) are re-labeled V4R, V8, V9-- this is convienient to the provider reading the (now 15 lead) ECG as the last leads of the printout are all in a line and show a quick view of the right ventricle (V4R), and the posterior wall (V8, V9).
There are three situations where a 15 lead ECG should be performed after a 12 lead ECG:
1. You suspect that the underlying cause of a patient's presentation is cardiac (e.g. a posterior wall MI) even though the initial 12 lead ECG shows no obvious acute changes (The fact that it doesn't directly show up on a standard 12 lead ECG is the reason the posterior wall MI is the most often "missed" MI).
2. Any of the leads V1, V2, V3, V4 shows ST segment depression (again, possible posterior wall MI)--in this case the ST segment depression might be reciprocal change from a posterior wall MI. (If you don't take a direct look with the 15 lead ECG who will?--that ST segment depression could just be due to medications or another benign cause, but it just might be "the BIG one!"--thats a game changer!)
3. An inferior wall MI is present--in this case you know that the complaint is due to an MI but RVI must be identified because it generally contraindicates the use of NTG and morphine administration, and may require fluid boluses (2 litres normal saline is a relatively standard intervention for inferior MI with RVI and associated symptoms). There is something else about this scenario--if there is only ST segment elevation in II III AVF and no reciprocal changes, then it raises your confidence (in your assessment that it is an MI) to see ST segment elevation extending to the posterior wall and/or right ventricle leads.