Now that we've learned how to measure in the previous video we need something to measure,
and that will be the P, Q, R, S ,T waves generated from the heart.
(These waves are named in the order they appear in the alphabet.)
Due to the electrical path, the thickness of the tissue, and the resistance encountered,
the current passing through the heart will create different wave patterns.
Let me explain:
If the SA node directly stimulated the AV node
it would be a short distance...
but that's not what happens.
The SA node spreads current outward and across the atrial tissue eventually getting back to the AV node,
thus taking more time do the path the current took.
Thick tissue allows more current to flow through it much like
larger pipes allow more water to flow through them compared to
smaller diameter pipes.
When resistance is encountered it slows down or stops the electrical current.
For example,
The fibrous tissue that makes up the valves between the atria and the ventricle
does not allow for conduction of electricity.
Also dead tissue from a heart attack does not conduct electricity and will
slow or change the pattern of the ECG depending on where the dead tissue is located.
Now getting back to the P,Q,R,S,T wave patterns.
The "P" wave on the ECG recording is small and curved,
the reason it's small and curved is because the
path of electrical current is not direct,
but spreads outward an across the atrial tissue
taken more time.
In addition, the tissue is thin, much like the small diameter pipes it will not allow for high amplitude.
The next event is a pause.
The reason for the pause is due to the fibrous valves and tendons that separate
the upper and lower portions of the heart.
Electrical current does not conduct through the fibrous valves and tendons.
However the AV node does pass through this fibrous tissue and acts as a
resistor delaying the current from the atria to the ventricles
The "Q" wave is a small downward deflection from the
ventricular septum.
The reason it dips down below the bassline is due to the way the ECG interprets the direction of
the current passing through the heart.
The ventricular septum gets current from the left
bundle branch and sends the current in the opposite direction.
The ECG displays this movement away from the positive electrode as a negative deflection.
Almost simultaneously with depolarization of the ventricular septum,
the apex of the heart also goes to depolarization.
But this current is again moving downward, towards the positive ECG electrode,
and the ECG reflects it as an upward spike.
This is the "R" wave.
The reason for this large spike is due to
the large amount of tissue that the current is passing through.
Much like the pipe analogy we talked about earlier.
Large tissue creates more voltage.
After electrical current has depolarized the apex of the heart
you can see the walls of the heart have not yet to depolarize.
As the walls begin depolarization, current moving away from the positive
ECG electrode, it is graphically represented as a tracing below baseline.
This creates the "S" wave.
Because the entire ventricle has depolarized and no
current is moving, the ECG returns to baseline.
When the tissues in the ventricles starts to re-polarize it will start from the outer side of the
ventricles and move towards the inner. Now because the current is moving upward
you might think you'd see a "T"-wave under the baseline as a negative deflection.
but the "T"- wave is actually above the baseline. This is confusing but let me
attempt to explain. The ECG machine graphically interprets electrical
current moving from the negative electrode to the positive electrode.
So when a positive current is moving towards a positive end of the ECG
machine a positive deflection occurs.
When a positive current moves towards the negative electrode the ECG machine shows as a negative deflection.
But when a negative current moves towards a negative electrode a negative and a
negative equals a positive.
This concept may take a few times understand but once you get it move on
to the next video will start the actual diagnosing of the rhythm
strip and looking at the abnormalities of cardiac electricity.