00:01
So let's go through the electrical activity in the heart.
00:04
So, when a myocardial cell depolarizes; that is it has a resting potential electric current across its membrane,
and then suddenly that goes to zero.
00:15
When that happens in a process called the action potential
it results in a trigger of the myocardial contractile machinery.
00:24
So you have the electrical depolarization, contraction, electrical depolarization, contraction.
00:30
So, there is a pacemaker which spontaneously depolarizes high in the right atrium.
00:37
That's the little round green dot high in the right atrium in the diagram on the left here.
00:44
And that area has very specialized myocardial cells. They depolarized spontaneously.
00:52
Sodium spontaneously leaks into the cell through some channels
that are called the funny channels cuz no other sodium channel leaks like this.
01:02
And when they get to a certain point, it triggers the action potential.
01:06
And then what happens is the wave of electrical depolarization
passes from the sinus node through the atria down into the septum of the -
between the two ventricles.
01:19
There's a pause at the AV node and that's the second little round green circle. Why a pause?
Because if it went immediately into the ventricle,
both the atria and the ventricle will be contracting at the same time.
01:32
Not a good idea because then nothing would go off the aorta,
there would be no effective cardiac output.
01:39
So there's a pause while the atria finishes its contraction, filling the ventricles.
01:46
And then the way the depolarization passes down,
past the AV node into the His-Purkinje system and throughout the ventricle,
passing down first the septum to the apex and then backwards to the base of the heart.
02:01
And when that happens, it triggers contraction and the ventricles squeezes.
02:06
And here you see - here's a whole series of action potentials for the different myocardial cells
as the wave of depolarization passes down.
02:15
This is the - you'll see the action potential for the sinus node
which is as you can see slowly rising.
02:25
There's a spontaneous depolarization.
02:27
But once you get into the atria and to the ventricle, the depolarization is not spontaneous,
it's immediate as the wave of depolarization arrives.
02:36
And so - then here we see again going down through the atrium and into the - past the mitral valve,
past the tricuspid valve into the ventricle, you're in the His-Purkinje system now.
02:49
And then eventually, you finish and you get to the T wave as everything resets itself,
getting ready for the next beat.
02:58
It's interesting that the wave of depolarization doesn't go through the ventricle all at the same place.
03:07
It comes down the septum, arrives at the apex and then turns back and does the base.
03:13
So what that means is that if you're an observer - you're an electrode watching this,
when the electrical current and the wave of depolarization is coming towards you,
the EKG records an upstroke.
03:27
When the electrical depolarization passes the apex and starts to go back towards the base,
in other words going away from an electrode, you see a negative deflection.
03:37
And that's why for example, you saw on the QRS,
there's some positive and there's some negative depending on where the electrode is looking.
03:45
If an electrode's looking from here, it's going to see different than if the electrode's looking from here
because the way the depolarization from here will be coming towards this one at a certain point
when it's going away from the one over here and vice versa.
03:59
And we'll see some examples of that.
04:01
And here in the -- we continue -- we see the wave of depolarization
passing from the apex back up to the base until the entire -
both right and left ventricle have depolarized and have contracted.