Why does atrial depol cause upward deflection on EKG?

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Discussion Overview

The discussion revolves around the mechanics of atrial depolarization as it relates to the upward deflection observed on an EKG, specifically when analyzed from Lead II. Participants explore the relationship between current flow, voltage measurement, and the conventions of EKG readings.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that if the current flows from the negative terminal at the right arm to the positive terminal at the left leg, the atrial depolarization would lead to an upward deflection due to positive summation of currents.
  • Another participant questions the notion of a positive current flowing from a negative terminal to a positive terminal, raising the possibility that the leads measure voltage changes instead.
  • A participant acknowledges the confusion about current and voltage, seeking clarification on whether a positive current would indeed cause an upward deflection if the recording terminal is positive.
  • There is a reiteration that during heart cell depolarization, positive ions are released, which may contribute to the observed positive current, but the EKG measures changes in voltage over time across a lead.
  • One participant cites a Wikipedia article to support the idea that EKG electrodes measure voltage rather than current.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the definitions and roles of current and voltage in EKG readings. There is no consensus on the interpretation of how atrial depolarization translates to the observed deflection on the EKG.

Contextual Notes

There are unresolved assumptions about the definitions of current and voltage in the context of EKG measurements, as well as the implications of these definitions on the interpretation of the EKG deflection.

drana91
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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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drana91 said:
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,

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?
 
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?
 
Errr, I thought you just said that an EKG doesn't measure current...
 
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!
 
drana91 said:
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!

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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