Thread by @AvrahamCooperMD, 1/Ever wonder why a "STEMI" causes ST elevation on EKG?The answer is [...]

Avraham Z. Cooper, MD

AvrahamCooperMD

1/
Ever wonder why a "STEMI" causes ST elevation on EKG?

The answer is mind-bending.

#medthread #tweetorial #medtwitter

2/
We need to review a few basic EKG facts.

First:

If electrical signals travel away from an EKG lead they appear as negative deflections

If electrical signals travel toward an EKG lead they appear as positive deflections

https://bit.ly/2PIRGWw

2/We need to review a few basic EKG facts.First: If electrical signals travel away from an EKG lead they appear as negative deflections If electrical signals travel toward an EKG lead they appear as positive deflections https://bit.ly/2PIRGWw

3/

Second:
Recall that the ST segment represents electroneutrality in the cardiac cycle, between depolarization and repolarization, w/ no voltage gradient.

That's why the ST and TP baseline segments are normally flat on EKG (aka isoelectric).

https://bit.ly/2PGiLJZ

4/
There are many channels that contribute to ventricular repolarization.

An important one is K-ATP, a potassium exporter channel that needs adequate ATP to remain closed.

Less ATP available = more potassium export from the myocardial cell.

https://www.ncbi.nlm.nih.gov/pubmed/9349389?dopt=Abstract

5/
Not surprisingly, during ischemia mitochondrial ATP production goes down.

Https://physiology.org/doi/full/10.1152/physrev.1999.79.3.917

6/
This decrease in ATP production leads to more potassium export through K-ATP

faster ventricular depolarization of ischemic cells relative to healthy ones

a shorter cardiac cycle.

Https://physiology.org/doi/full/10.1152/physrev.1999.79.3.917

6/This decrease in ATP production leads to more potassium export through K-ATP faster ventricular depolarization of ischemic cells relative to healthy onesa shorter cardiac cycle. Https://physiology.org/doi/full/10.1152/physrev.1999.79.3.917

7/
Here's another way to view the shorter ischemic electrical cycle.

https://www.physiology.org/doi/full/10.1152/advan.00105.2016

8/
This difference in cycle speeds between ischemic and non-ischemic cells causes a relative current during repolarization:

Ischemic

non-ischemic, away from overlying EKG leads

This shifts the EKG baseline down, for reasons we discussed above.

https://bit.ly/2s99b9l

8/This difference in cycle speeds between ischemic and non-ischemic cells causes a relative current during repolarization: Ischemicnon-ischemic, away from overlying EKG leadsThis shifts the EKG baseline down, for reasons we discussed above. https://bit.ly/2s99b9l

9/
Conversely, there is little difference between ischemic/non-ischemic depolarization speeds.

Thus the voltage gradient disappears during the QRS complex and the EKG returns to isoelectric baseline in the ST segment (aka zero voltage).

https://bit.ly/2s99b9l