Jones’s Rule and Subendocardial Ischemia
Jones's Rule...
Any ST depression in the 12- lead ECG of a patient with credible chest pain (symptoms suggestive of an Acute Coronary Syndrome) represents a reciprocal change of acute epicardial ischemia until proved otherwise.
This pertains even if there is no ST elevation on the ECG! Remember: reciprocal changes can appear long before the primary changes of ST elevation. And, when the ST elevation DOES eventually appear, the reciprocal changes may appear much more impressive!
Law of ST Depression
Whenever we see ST elevation in Leads II, III and aVF, we can proclaim that the patient has an acute inferior MI (more correctly, acute inferior epicardial ischemia). However, the same cannot be said of ST depression. ST depression due to acute (or chronic) ischemia is generally a phenomenon of the subendocardial layer and does not localize! Therefore, to say that a patient has "inferior wall ischemia" or "anterolateral ischemia" makes no sense. It doesn't matter that the ST depression appears only in Leads I and aVL or only in Leads II, III and aVF - you still have no idea where or how extensive the ischemia is!
Imagine you are walking down a street in your neighborhood late one evening and you see a house that you know to be vacant with light coming out of some of the windows. Some windows are shuttered closed and you can't see anything from them but a few windows have the shutters open and you see light. You approach to see if you can discern exactly where the light is coming from inside the house. You look through the open windows and - although light is illuminating those rooms that you can see - it is apparent that the light is coming from a source that is not visible to you. Perhaps the light is behind a door or in a portion of the ceiling you can't see or even in a closet. You know it's there, but you just can't locate it. That, in a nutshell, is subendocardial ischemia! When you see the ST depression of real subendocardial ischemia "peeking" through leads II, III and aVF or through leads I and aVL or through leads V4 through V6, just think of those leads as the "windows" you are peering into. You definitely see the light (ST depression) but you just can't see the source (actual area of ischemia).
Read Jones's Rule again!
Never mistake reciprocal changes of an acute epicardial ischemia for subendocardial ischemia! If you cannot recognize the difference, then you are NOT interpreting ECGs at an advanced level!