Depolarization, Waves, and Deflection?

[AnthonyM83]

Just reviewing my cardiology and want to clarify some points. Looking for pretty in-depth explanations, so more technical, exact, and detailed the better...I think I get stuff moderately well, but want to clarify (cough:: Fiz ::cough)

1) I need a better precise explanation of depolarization and repolarization. I was initially taught K+ is inside during resting polarized state. But just re-read Bledsoe's cardiology section that's making it seem like Na+ is mainly inside right before depolarization. When does stuff rush in/out, relative rates, effects, etc.

2) What counts as the exact borders of waves? When does a wave end? (When it passes the isoelectric line?) For example, in QRS. Is the S wave the entire downward slope from the very tip of the R? Or does it start when the downward slope from the R passes isoelectric line and end when it comes back up to isoelectric line (think I know, but gotten different stuff from some people)

3) And one I still haven't been able to get answered, though I think I know just based on working it out in my head, but want to be sure:

If the QRS complex is mainly positive, then why is the T-wave ALSO positive? Is repolarization also positive ions going into the cells even though overall negative wins out inside? Is it because of physical direction in which myocytes are repolarized? It's not intuitive (you'd think it would be opposite deflection than QRS...)

Edited March 17, 2009 by AnthonyM83

[chbare]

Just reviewing my cardiology and want to clarify some points. Looking for pretty in-depth explanations, so more technical, exact, and detailed the better...I think I get stuff moderately well, but want to clarify (cough:: Fiz ::cough)

1) I need a better precise explanation of depolarization and repolarization. I was initially taught K+ is inside during resting polarized state. But just re-read Bledsoe's cardiology section that's making it seem like Na+ is mainly inside right before depolarization. When does stuff rush in/out, relative rates, effects, etc.

2) What counts as the exact borders of waves? When does a wave end? (When it passes the isoelectric line?) For example, in QRS. Is the S wave the entire downward slope from the very tip of the R? Or does it start when the downward slope from the R passes isoelectric line and end when it comes back up to isoelectric line (think I know, but gotten different stuff from some people)

3) And one I still haven't been able to get answered, though I think I know just based on working it out in my head, but want to be sure:

If the QRS complex is mainly positive, then why is the T-wave ALSO positive? Is repolarization also positive ions going into the cells even though overall negative wins out inside? Is it because of physical direction in which myocytes are repolarized? It's not intuitive (you'd think it would be opposite deflection than QRS...)

During depolarization, sodium enters the cell rather quickly. During repolarization, sodium is actively pumped out of the cell, while potassium enters. Obviously, many other things are occurring during depolarization and repolarization as well.

Remember, positive deflection of waves has nothing to do with the positive or negative ion flow through cells. A complex (QRS, T, or other wave) is positive on the ECG when the electrical activity is moving toward the positive electrode. If it moves toward the negative electrode, the waves will have negative deflection.

Hope this clears things up.

Take care,

chbare.

[FL_Medic]

The influx of Sodium or Calcium may cause cellular depolarization. The outflux of Potassium may also cause depolarization.

Positive and negative deflections as chbare, "the dude" stated has to do with the position of the positive and negative electrode in the lead you are monitoring.

Your most readable lead will change depending on the R-axis. Lead 2 will usually be most inline with the axis and provide the clearest picture. Of coarse, looking at other views of the heart may provide some valuable information. That's why we do 12-lead ECGs.

The U wave (if present) is thought to be the repolarization of perkinje fibers.

Different QRS morphologies

A wave ends at the isoelectric line. As you thought.

During repolarization the electricity may be flowing in either direction but is usually concordant with the QRS. If it is discordant, this may indicate cellular damage. Some leads may present you with a biphasic p-wave, and inverted T-wave (usually v1) without any cardiac damage.

Edited March 18, 2009 by FL_Medic

[fiznat]

Since the other two questions have been mostly answered, I'll post the more detailed info you wanted about depolarization and repolarization you wanted.

At rest, it is important to remember that sodium (Na+) is the most prominent extracellular ion, while potassium (K+) is the most prominent intracellular ion. Its silly, but I remembered this when I learned it by thinking that the neuron says OK to potassium (K+) and Nah to sodium (Na+).

This balance is maintained by the sodium/potassium pump, which uses ATP (energy) to actively pump 3 sodium ions out and 2 potassium ions in. Remember that even though we are dealing with all cations here (positively charged ions), the relative amounts of those ions is what causes the differential in charge across the membrane. Because three positively charged ions are being pumped out for every two that are pumped in, the outside of the cell becomes more positive than the inside of the cell. Therefore, at rest, the inside of the neuron is negative and the outside is positive.

Depolarization is the process by which those charges (negative in, positive out) momentarily switch places in response to an electrical stimuli.

When the cell is stimulated:

1. the "channel" that normally allows free passage of potassium closes

2. the "channel" that normally allows free passage of sodium remains open.

Because there is less sodium inside the cell than outside the cell, sodium rushes in. This causes a rapid change in the relative charge. Inside the cell becomes more positive, outside the cell becomes more negative.

That is depolarization.

After a certain period of time (note, this period, phase 2 of the action potential, is longer in cardiac cells due to inflow of Calcium (Ca++) ions), the cell will repolarize:

1. Sodium channel closes

2. Potassium channel opens.

This causes potassium to rush out of the cell, helping to restore a balance of charge. The sodium/potassium pump then does the rest of the work to re-establish the resting potential (negative in, positive out).

Hope that helps. Its a difficult thing to explain. Try checking out this flash file, it explains it really well:

http://outreach.mcb.harvard.edu/animations...onpotential.swf

Edited March 18, 2009 by fiznat

[logos]

2) What counts as the exact borders of waves? When does a wave end? (When it passes the isoelectric line?) For example, in QRS. Is the S wave the entire downward slope from the very tip of the R? Or does it start when the downward slope from the R passes isoelectric line and end when it comes back up to isoelectric line (think I know, but gotten different stuff from some people)

3) And one I still haven't been able to get answered, though I think I know just based on working it out in my head, but want to be sure:

If the QRS complex is mainly positive, then why is the T-wave ALSO positive? Is repolarization also positive ions going into the cells even though overall negative wins out inside? Is it because of physical direction in which myocytes are repolarized? It's not intuitive (you'd think it would be opposite deflection than QRS...)

2. The letters represent deflections, technically not waves. Thus the p is the first upward deflection, the Q is the next deflection (also downward in the stereotypical EKG...but often you don't see this), the R is the upward deflection which creates most of the "QRS complex", the S is the following downward deflection (again all of these directions only apply to a sterotyped EKG) and the T is the upward deflection. That's not much help but it explains why I wouldn't worry about the question too much (unless I was in the business of computer EKG interpretation).

3.) No one understands this very well. You are correct in expecting the T-wave direction to be opposite the QRS since it represent repolarization. The typical explanation for why the T wave happen in the direction it does is that the myocardium REpolarizes from epicardium endocardium to (outside to inside) whereas it DEpolarizes in the opposite fashion (endocardium to epicardium, or inside to outside). There are several possible explanations for why this might happen, none of which are particularly convincing (to me anyway).

This supposedly (can you tell I'm a little skeptical of this as a complete explanation...but its well known often used explanation) reverses the direction of travel of the sum of the electrical potential. Thus its sort of a 3-D explanation where two negatives cancel out (ie you can think of it as -1*-1 = 1.

Does that make sense?

The truth is that there are several observed EKG phenomena which are not well explained by known physiology. The same is even more true of heart sounds.

Remember, positive deflection of waves has nothing to do with the positive or negative ion flow through cells. A complex (QRS, T, or other wave) is positive on the ECG when the electrical activity is moving toward the positive electrode. If it moves toward the negative electrode, the waves will have negative deflection.

Here is where the confusion typically comes from in the physics. There are two ways to think of an electrical signal. That is, a positive charge flowing in one direction is the same as a negative charge moving in the opposite direction.

Here is a quick summary for EKG:

+ charge wave moving towards electrode: positive deflection

- charge wave moving towards electrode: negative deflection

+ charge wave moving away from electrode: negative deflection

- charge wave moving away from electrode: positive deflection