(1) P-wave - what EXACTLY causes the down slope of the P-wave? I understand the P-wave is atrial depolarization, so UPWARD portion/slope of the P-wave makes sense, BUT if depolarization is still occurring during the P-wave, WHY would the slope then go DOWNWARD? I would think that (a voltage drop as indicated by the downward slope on the 2nd half of the P-wave) indicates REpolarization (ie voltage drop), BUT repolarization of the atrium doesn't occur until later during QRS (masked by depolarization of the ventricles). Basically, it seems there should be NO downward slope on P.
(2) QRS - same question: WHY the downward slope (on the second half) if in DEpolarization? (ie since repolarization of vetricles doesn't occur until T)
(3) why the UPward slope on the T-wave if T-wave is REpolarization? (Would think REpolarization would be DOWNward)
Thanks in advance
Sep 29, '10
The tracing goes up or down d/t an impulse traveling toward or away from the positive electrode, not d/t depolarization vs repolarization. Question #1 - We will just use lead II as our example. I would guess maybe the p wave slopes down because the atrial conduction fibers are converging and there is not as large of an area being depolarized and then of course the conduction stops and the p wave is back at baseline by this point. So the less surface area conducting, the lower the voltage. #2 - It downslopes because the impulse is traveling away from the positive electrode. That is the main concept you have wrong. This concept is well illustrated when looking at the precordial leads V1-V6. Note that in the normal ecg the R wave becomes progressively larger as the positive electrode moves accross the chest. V1 the QRS is almost completely negative as the impulse moves away from the positive electrode. V6 is the exact opposette since the impulse should be traveling directly at the positive electrode creating a completely positive QRS. And question #3 - You get the idea.
Hope I've explained that in and understandable and (more importantly) correct manner.
Sep 29, '10
good questions and good explanation.
also remember that the electricity is always trying to get back to the isoelectric line, as the na+ and k+ exchange across the cell membrane. basically what goes up must come down and vice versa, otherwise you'd have a serious problem.