explain st elevation/depression and pr depression
Started by fiasco19
Alright so first lets remind ourselves that A depolarising current towards the electrode is viewed as a positive deflection, and away from it is viewed as negative.
So lets consider the situation of a transmural infarct. When the ventricle is at rest and depolarised, the depolarised ischemic region generates a current that travels away from the electrodes. Therefore baseline voltage prior to the QRS complex will be depressed (negative deflection). When the entire ventricle is depolarised, zero voltage is recorded, as an isoelectric ST segment. After repolarisation, the voltage following the T-wave is once again depressed, as in the resting state mentioned earlier.
This gives the ST segment an appearance of being falsely elevated. (Portions before and after it are depressed).
Hope this helped.
So lets consider the situation of a transmural infarct. When the ventricle is at rest and depolarised, the depolarised ischemic region generates a current that travels away from the electrodes. Therefore baseline voltage prior to the QRS complex will be depressed (negative deflection). When the entire ventricle is depolarised, zero voltage is recorded, as an isoelectric ST segment. After repolarisation, the voltage following the T-wave is once again depressed, as in the resting state mentioned earlier.
This gives the ST segment an appearance of being falsely elevated. (Portions before and after it are depressed).
Hope this helped.
It doesn't. That's the problem. Instead of all the muscle being depolarized, only some of it is, thus generating a different electrical potential floor.
according to wise wikipedia
"During transmural ischemia, the Na+/K+ATPase which is responsible for the final stages of myocyte repolarization (-80mV to -90mV) is impaired, thus the ischemic cells will only be partially depolarized compared to surrounding healthy (non-ischemic) myocytes. The difference in membrane potential between healthy and ischemic cells causes negative charges to accumulate on their surfaces, generating a vector that points towards the normal cardiac cells (which have positive charges on their surface). This vector points away from the chest EKG leads, causing a downward deflection in the TP segment. However, since the TP segment is the baseline of the EKG, the machine corrects for this by raising TP to baseline which results in ST elevation. Also see ST depression."
so b/c the TP segment is deflected downward, the ST elevation is actually just a fault on the machine?? can you explain that last part to me?
why would it bring the ST segment up that high?
also why is it the TP segment that is affected specifically?
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It's saying that it's not really the ST segment that gets elevated, it's that the TP segment gets depressed. EKG machines set the TP segment as the baseline. So if you lower the TP segment, the ST segment looks elevated in comparison.
I suppose that makes some sense. I didn't know that, though. Bottom line from my perspective is that we get ST elevation because of incomplete depolarization and resultant potential difference across the myocardium.
do you know why the TP segment is affected specifically?
CV Physiology | Electrophysiological Changes During Cardiac Ischemia
Here's a page with a nice little diagram that might help. You can sort of see that it's actually the TP segment that's really changing due to the current induced by the infarcted myocardium.
I suppose we all learned something today.
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