The Electrocardiogram - Part III


Making sense of the waves:

Remember how I said that a lead has a positive electrode? Well this comes into play when looking at a specific lead. The isoelectric line is your baseline so-to-speak. The isoelectric line has no amplitude. Anything above the isoelectric line is considered a positive deflection and anything below it is considered a negative deflection.


- When deflections are positive, the mean electrical current is heading in the general direction of the positive electrode.


- When deflections are negative, the mean electrical current is heading away from the positive electrode.


In the above description, you can replace the word 'deflection' with the word 'wave'.


- Deflections or Waves on an ECG strip are representations of heart chamber depolarization and/or replorization (for definitions see basic cardio 2).


Below is an image from Wikipedia which illustrates what I just said:

Take a look at the ECG strip below. Lets say that this is lead II. The yellow circles and lines point out all the positive deflections. The white circle depicts the single negative deflection and the blue lines in the magnified portion are pointing to the isoelectric line.


The isoelectric line is normally at the beginning and end of all waveforms.




Remember where the positive electrode is in lead II? Lets take a look at it again:



Lead II shows the negative electrode on the right arm and the positive on the left leg. This may be modified to the torso, with placement near the lower left quadrant of the abdomen. The axis should be the same.

Since we know that the positive electrode is on the left leg, we know which way the impulse is traveling based on the deflections. Take a look at this animated graphic I have stolen from Google:


As with our ECG, the mean electrical impulse is traveling towards the positive electrode. This is where we get the waves that show up on our ECGs. Now the same wave will look completely different based on which lead you are looking at. This is due to the placement of the positive electrode. Take a look below:


All three complexes inside the blue circle are of the same electrical impulse. Remember, the strip is measured horizontally by time. So every wave or complex is the same as the ones above and below it on a multi-lead ECG strip. They are just different angles of view. In an upcoming section I am going to explain these different angles.


1 comment:

Stepan&Anna said...

Yeh, just a recall from physics class: electrical current is movement of electrons, and that is what is inside of repolarized cell and come outside during depolarization, total mass of electrons rushing outside create electrical current and has a direction. total vector of which we see on google and wiki pics. So for example lead I runs from left arm(-) to right arm(+) so direction of current(anions rushing outside) to cause positive deflection should be in the same direction, and in reverse.
Ta-Da.