I'm going to assume you will have ruled /in/out arrhythmias, bundle branch blocks, ventricular hypertrophies, Printzmetal's, or J point elevations before applying these steps. Also, all bets are off if WPW or LBBB is present
Reciprocal change refers to deviation of the ST segment, T wave, or Q waves in the opposite direction from that of their MAXIMAL deviation.
Easiest way to not get confused anymore:
1. Check V1-V2 for abnormal R wave (V1: R amp > S amp, R dur >= 0.04s, R amp >= 0.60 mV; V2: R amp >= 1.5 x S amp, R dur >= 0.05s, R amp >= 1.50 mV) and ST segment depressions (Just eyeball it, if their is a R wave present that's bigger than the S wave, you're probably dealing with posterior)
2. If you don't see those findings on the ECG then you can rule out a transmural posterior MI
3. If you ruled out a transmural posterior MI, then you don't need to worry about reciprocals anymore, as it's the only time "reciprocals" are used for a dx on ECG (and, even then, you need to get posterior leads before moving to additional testing). In other words, scan the ECG for ST elevations first. Then, if you find any, you can see if there are any reciprocal changes, though just b/c they aren't there doesn't mean it's not still a STEMI
4. If you don't see any STEMIs, and you ruled out the transmural posterior MI, then you are left w/ either subendocardial infarctions, subendocardial ischemia, and/or transmural ischemia. Remember, subendocardial infarctions by definition can't cause ST elevations and the damage they cause aren't typically extensive enough to cause any reciprocal changes either way (basically, don't start w/ a ST depression and look for its reciprocal ST elevation, do it they other way around)
5. Now, without cardiac enzymes or a timeline, it's difficult to separate these out. With an ischemic attack, the ST segment should return to baseline after the angina. While with non-Q-wave subendo infarction, the ST segment can stay depressed for up to 48 hrs (and you need positive cardiac enzymes). But, there are some tricks to try an localize the ischemia and differentiate transmural vs subendocardial:
LAD localized ischemia --> T wave inversions in V2, V3, V4
RCA localized ischemia --> T wave inversions in III & aVF (inferior) or V2 & V3 (anterior)
LCX localized ischemia --> T wave inversions in I, aVL, V4, V5, V6
For subendocardial ischemia, the MEAN direction of the T wave shifts AWAY from the involved ventricle
For transmural ischemia, the MEAN direction of the T wave shift TOWARDS the involved ventricle (may see hyperacute T waves with increased amplitude)
