The way we measure this is by electrocardiography.
An electrocardiograph can detect the electrical
currents that are generated by the heart.
An electrocardiogram is the graphing
record of that electrical activity.
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.
In this process, electrodes are placed at various points
on the body in order to measure the voltage differences.
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.
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.
We also have the QRS complex.
This is going to correspond to ventricular
depolarization as well as atrial repolarization.
Also, we have the QT interval which is
the beginning of ventricular depolarization
through the ventricular repolarization.
the ST segment of the electrocardiogram is going to be
the entire ventricular myocardium being depolarized.
And finally, we have our T waves which
corresponds to ventricular repolarization.
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.
Sometimes when you do a trace you'll
notice that there is some abnormalities.
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.
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.
Sometimes in ventricular fibrillation,
we can see just a disorganized tracing
where are the electrical activity
is just kind of all over the place.
This is because action potentials are
occurring randomly throughout the ventricle.
This is a grossly abnormal ECG deflection and this
is what you can see sometimes in acute heart attacks
or after an electrical shock.