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Why does atrial depol cause upward deflection on EKG?

  1. Jun 2, 2015 #1
    Given a standard 3-lead ekg where the leads are read as below:
    ecg_534.gif
    Why does atrial depolarization cause an upward deflection? Assuming the EKG is being read from Lead II, I believe that if the current flowing from the negative terminal at RA (right arm) to the positive terminal at LL (left leg) is positive, then atrial depolarization (whose current moves parallel to this vector from upper right heart to lower left heart) would cause a positive summation of currents leading to the upward deflection. However, why would Lead II (which runs from negative terminal to positive) cause a positive current? Doesn't convention state that current is defined as the apparent movemnt of positive charge, in this case, in a direction opposite to what is shown in the graphic above? Am I even thinking about this correctly?? Thank you!
     
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  3. Jun 2, 2015 #2

    Drakkith

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    Staff: Mentor

    I'm not familiar with the details of an EKG setup, but a positive current cannot flow from a negative terminal to a positive terminal. Is there really a current flowing here, or are the leads measuring a voltage change?
     
  4. Jun 2, 2015 #3
    Ah, I guess I assumed current was flowing. You're right, the leads are simply measuring voltage change. Just to make sure this makes sense then... if a positive current causes an upward deflection on an EKG, the recording terminal must be the positive one, correct?
     
  5. Jun 2, 2015 #4

    Drakkith

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    Errr, I thought you just said that an EKG doesn't measure current...
     
  6. Jun 2, 2015 #5
    So when the heart cell depolarizes, positive ions are let out. Because this propogates down the heart this causes the positive current. I believe what the ekg is measuring is the change in voltage, over time, across a lead. Does that make sense? Sorry, I'm trying to make it make sense!
     
  7. Jun 2, 2015 #6

    Drakkith

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    Staff: Mentor

    Based off of what the wikipedia article is saying about EKG's, I believe that is correct. The electrodes are measuring voltage, not current.
     
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