Sequence of Depolarization – Electrocardiogram

by Thad Wilson, PhD

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    So, let's trace through now the standard ECG and how that links up with the spread of depolarization throughout the heart. So, where do we start? We start at the primary pacemaker – the sinoatrial node. The SA node is our starting point. Here, you have a depolarization that occurs, and that forms a wave that we call a P wave. You have a little bit of a delay that happens and that delay occurs in the AV node. The QRS complex is a little bit more complex. So, let's talk through that in step-by-step form. The first portion of the QRS complex is a small downward deflection. This small downward deflection is because the septum of the heart depolarizes from the left to the right. Now, this is kind of hard to think about. Why would a depolarization moving from the left to the right cause a small negative deflection on the ECG? Remember that it's all about the mean electrical or the mean axis of this particular depolarization. That can be seen here as a small travel towards the negative pole, so if a negative deflection is a small positive force traveling towards the negative pole. That’s hard for a second. Let’s step through the other three examples and come back to this one because we started with the hardest one. Let’s go to the next one. We have a positive deflection as you have depolarization going to the apex of the heart. This makes a lot more sense, right? You have a large vector traveling towards the positive pole. If you had traveling towards the positive pole, what do you get? A positive deflection. Okay? Now, as you continue to de-polarize the ventricle, especially the left ventricle, it starts to travel down towards baseline. And once it's...

    About the Lecture

    The lecture Sequence of Depolarization – Electrocardiogram by Thad Wilson, PhD is from the course Cardiac Physiology.

    Included Quiz Questions

    1. Septal depolarization
    2. Atrial depolarization
    3. Apex depolarization
    4. Base depolarization
    1. Negative (downward) deflection
    2. Positive (upward) deflection
    3. No deflection (flatline)
    1. … 0.1 second.
    2. … 0.03 second.
    3. … 0.01 second.
    4. … 0.2 second.
    5. … 0.3 second.
    1. Ventricular depolarization
    2. Atrial repolarization
    3. Right ventricular repolarization
    4. Left ventricular repolarization
    5. Atrial depolarization

    Author of lecture Sequence of Depolarization – Electrocardiogram

     Thad Wilson, PhD

    Thad Wilson, PhD

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