As a paramedic student, I learned basic 12 Lead interpretation and what ST Depression and ST Elevation looked like on a 12 lead. However, I was a student who like to ask “why” questions. My instructors probably saw me as an anyoing toddler!
ME: Why is there ST deflection from a MI?
Instructor: Because the tissue with either ischemic or REALLY ischemic.
ME: Thats not what I meant…I mean why does a MI cause the ekg to deflect positively or negatively.
Instructor: As I said….because the tissue with either ischemic or REALLY ischemic.
This back and forth went on for a bit… but never got me anywhere! So I had to go find the answer myself!
- A EKG is the surface recording of the electrical charges of th heart.
- At the cellular level those electical charges are know as Action Potiential.
- Inpulses traveling towards the positive electrode, will result in a positive deflection (ST Elevation).
- Inpulses traveling away from the positive electrode, will result in a negative deflection (ST Depression).
- On a 12 Lead, the named Lead ie- V1 or V2, indicates the Positive Lead.
Current of Injury
In a normal myocardium, there is no current of injury. In a heart owith regions of ischemia, neighboring regions will have diffent voltages as a result of the action potential reaching diffrent values. A voltage diffrence between two regions generates a small current- this is the Current of Injury.
The Positives and Negatives
The current is made of primarly K+ ions. Therefore, the flow will occur from the more positive to the more negative region. During systole, the ischemic tissue is LESS positive. Therefore, current will travel from the normal tissue to the ischemic tissue.
Current of Injury- ST Depression
If the ischemia affects the subendocardial area. Then the current of injury will be running from the epicardium (outermost layer) towards the endocardium (innermost layer)
This results in ST Depression in the affected leads.
Current of Injury- ST Elevation
If the injuried area is the whole thickness (transmural). Then the current of injury will be running from the neighboring healthy myocardium towards the injuried. The overall current will be directing outwards.
This results in ST Elevation in the affected leads.
So you may say….Bryan, not knowing about the Current of Injury and why we see elevation or depression of the EKG- Beside knowing its could be due to a MI, won’t affect my day to day patient care. Well maybe your right, it won’t affect it. However, understanding this concept helps you to better understand how the conduction system works in the heart and more importantly how issues translate on a EKG.