So I’ve got down (kind of) the P Q R S & T, but I don’t understand some of the abnormalities.
Most books seem to just want you to memorize it. Can someone tell me why there can be an ST depression in ischemia. All I’ve foiund is that it’s endocardial, but why?
PC
“The onset of ischemia is associated with a rapid loss of intracellular potassium resulting in a diastolic current of injury, outward towards the epicardium, which alters the T-Q segment. The P and QRS are then inscribed on this elevated baseline. When ventricular depolarization occurs, inscribing the QRS, all the mycardial cells including those which are injured, are depolarized. There is no current flow. At this point, which is the time of the onset of the S-T segment, the galvanometer deflection relates to the original 0 or null point, which is located below the previously elevated diastolic baseline. The result is S-T segment depression; which, when systole is completed, is followed by the elevated diastolic baseline due to the current of injury.”
Stress Testing principles and practice edition 3
Myrvin H. Ellestad
P. 95
Now I did stress testing and EKG interpretation for a living for a decade, and I had to look this up in order to explain it. Unless you’re an electrophysiologist, it’s not something you’ll need to know on a regular basis.
QtM, MD
BTW, the above cited book has some nifty diagrams which help to further clarify the point, along with background explanations which make the above explanation more lucid. If you really, really want to grok this stuff, buy the book.
PC,
ST segment depresion?!? The only interest I’ve ever seen a dermatologist show towards EKG was when patients are allergic to the electrode adhesive.
QTM,
Thanks. Nice response. I’m starting internship this week. I’ll be sure to pull this information out of my distal alimentary orifice sometime during the first month.
Go for it, choosy. It’s bound to earn you scutpuppy points.
I could not survive another internship. Good luck!
Thanks QTM. It took a little pondering, but I think I’ve got it. I recently started my internal medicine clerkship and EKG changes are just difficult to remember so I was hoping that a better understanding of the underlying pathophysiology would help. I still think it might. 
You never know Choosy, you never know.
PC