00:00
And then there is resetting
of the ventricular muscle
and that’s the T wave. And here we see an
enlarged example of one heartbeat. You see
the P wave, this starts with the sinus node
beat, atrial depolarization is the P wave.
00:15
You'll see a little flat area that’s the
delay in the AV node, then there's the QRS
ventricular depolarization. There's a little
pause then and the T wave is the ventricle
resetting itself getting ready for the next
beat for the next P wave.
00:35
So, one of the first things we do when we
do an electrocardiogram is check the voltage
calibration. The machine is set to have a
certain calibration. If you set the calibration
wrong, you’re going to make for either very
large or very small complexes and you may
misread the electrocardiogram. So, the first
thing the technician does when they're doing
a cardiogram is to check and make sure that
the calibration is accurate. It’s like any
test you do, you have to make sure that the
equipment you're using to do the test is
accurate before you start the test. What you
see in this diagram is that there are 12 different
positions where the leads look at the heart
electrical activity. It’s as if you are
looking at the heart from multiple points
of reference. You look at it from one side,
you look at it from the other side, you look
at it from below, you look at it straight
through and that’s what each of these little
electrodes do, they record the electrical
activity as seen at that particular point
and when you put that all together, you actually
could create a 3-dimensional electrical activity
of the heart contraction. We have six so called
limb leads that means there's leads on the...
both arms and both legs and we have six precordial
leads that is chest leads and you can see
them in the fourth diagram from the left.
01:57
Each one of these goes around the left side
of the chest and almost out into the armpit.
02:04
What is happening here is that we are looking,
again, as I said, at the heart electrical
activity from different angles. So, here you
see six different leads, these are the leads
that are obtained from the limb leads. And
I always tell people it’s like this, one
of the leads is out here, it’s looking at
the heart coming in from the left arm. Another
lead is looking at the heart activity, electrical
activity coming from below. Another lead is
looking at the heart activity coming from
the right shoulder and we put all of these
together. We know what a normal series of
patterns would be for limb leads. If we then
look at the precordial leads, you'll see
that what you’re getting here are views
as if we were putting needles through the
heart as we go around the chest. So, we get
not only a frontal view from the limb leads,
but we also get a transverse view from the
precordial leads. And you put all of those
six different views together and there's
a normal series of patterns for these 12 leads
and there are abnormal and learning the abnormal
and recognize them is the pattern recognition
we talked about that has to be, of course,
combined with more accurate and more careful
analysis of the electrocardiogram. You've
seen this cardiogram before in two previous
lectures. I hope you recognize that this is
a normal electrocardiogram. We’ll look at
it, each component of this in a little more
detail, but if you looked up close of this,
you will discover there's a P wave in front
of each QRS that is there's an atrial depolarization
preceding each QRS depolarization and there's
a T wave repolarization after each QRS.
The axis, that is the direction of the electrical
impulse in the heart is normal here. The PR
intervals - the interval between the P wave
and the QRS, is normal. The intervals following
the T wave are normal. This is a completely
normal heart rate and a normal electrocardiogram
from a normal person.