00:01
The way we measure this is by electrocardiography.
00:06
An electrocardiograph can detect the electrical
currents that are generated by the heart.
00:14
An electrocardiogram is the graphing
record of that electrical activity.
00:21
It's important to note or understand that the peaks
and the electrocardiogram, the peaks and valleys
do not correspond to actual membrane
potentials because it's going to be a culmination
of all of the action potentials that
are happening over the entire heart.
00:42
In this process, electrodes are placed at various points
on the body in order to measure the voltage differences.
00:51
So how does this work?
The main features of an electrocardiogram
include a P wave which is going to correspond
to the depolarization of the SA node and the atria.
01:06
From there, we have the P to R interval
which is the beginning of atrial excitation
all the way to the beginning of ventricular excitation.
01:18
We also have the QRS complex.
01:22
This is going to correspond to ventricular
depolarization as well as atrial repolarization.
01:32
Also, we have the QT interval which is
the beginning of ventricular depolarization
through the ventricular repolarization.
01:44
the ST segment of the electrocardiogram is going to be
the entire ventricular myocardium being depolarized.
01:53
And finally, we have our T waves which
corresponds to ventricular repolarization.
02:03
In a normal ECG trace, when we have sinus
rhythm, you notice that for about a second,
you have a P wave followed by your
to QRS peak followed by the T wave
and in a second you usually can
get about two of these occurring.
02:24
Sometimes when you do a trace you'll
notice that there is some abnormalities.
02:29
For example if the AV node fails
to conduct some SA node impulses,
what you'll find is that you have
multiple P waves per QRS wave.
02:43
This lets you know that there's some type
of blockage and usually in this circumstance,
you see two P waves per QRS wave
instead of one P wave per QRS wave.
02:58
Sometimes in ventricular fibrillation,
we can see just a disorganized tracing
where are the electrical activity
is just kind of all over the place.
03:09
This is because action potentials are
occurring randomly throughout the ventricle.
03:14
This is a grossly abnormal ECG deflection and this
is what you can see sometimes in acute heart attacks
or after an electrical shock.