00:00
I’m going to show a number of diagrams of
the electrocardiogram and take you through
showing you what each part consists of. You'll
see the first small wave there is called
the P wave and that is the atrial depolarization,
it’s followed by a large upward deflection
that’s the QRS, that’s ventricular depolarization
and then followed by a final rounded wave
that’s called the T wave, which is repolarization
of the ventricle. You may say, “Wait a minute,
why call them P, QRS and T? Why not call them
A, B, C, D, E and F?” In fact, in the early
history of electrocardiography, back at the
beginning of the twentieth century, a number
of waves were described A, B and C and so
forth, that all turned out to be artefacts
and the real ones actually ended up being
finally called P, QRS and T. They went through
a lot of artefacts in the alphabet before
they got the P, QRS and T and those are terms
we’ve used now for over a hundred years.
What the electrocardiogram is reflecting is
the electrical wave that is being conducted
through the heart muscle and you'll remember
from our earlier lectures, the electrical
wave triggers the mechanical activity. So,
each electrical complex is actually telling
us something about the mechanical activity
of the heart. So, where does the impulse begin?
It begins high in the right atrium, in the
sinus node. The sinus node is the pacemaker
of the heart. It’s the one that we hope
is always controlling the rhythm in the heart,
it’s the normal pacemaker of the heart.
01:36
The impulse, the electrical impulse, passes
down through some special channels in both
atria and enters the atrioventricular node
which is right where the atria and the ventricles
meet with the valves, the mitral and tricuspid
valve on either side and the beginning of
the septum that separates the two ventricles.
There's a delay there. I'll tell you about the...
02:00
why there's a delay there. The impulse then
enters the septum of the ventricles between
the right and left ventricles and passes into
some special electrical fibers - the bundle
of His and the Purkinje fibers and passes
out into the ventricle resulting in activation
of the ventricle with contraction. So, why
should there be a delay? The reason you have
to have a delay is you want the electrical
activity in the atria to cause an atrial contraction
and then you want the valves to be able to
be open and empty all of the atrial blood
into the ventricle and then you want the ventricle
to contract. If there were no delay, the atria
and ventricles would contract at the same
time and the blood wouldn’t move forward.
02:45
So, the delay in the AV node is important,
but you don’t want excessive delay and we'll
talk about conditions where there's
excessive delay later and where often patients
need pacemakers, but that’s in the final
phase of this lecture because that’s very
advanced.
So, let’s talk about each component in place.
03:04
Here's the sinoatrial node, it has a spontaneous
depolarization, so it slowly slowly depolarizes
and then fires, resets and slowly slowly depolarizes
and then fires. It is affected by the autonomic
that is the automatic nervous system. So,
when you exercise, impulses reach the sinus
node increasing the heart rate of the patient.
When you rest and you're quiet, there are
impulses from the vagus nerve that slow the
sinus node and this is, of course, in response
to your activity which is exactly what you'd
like. If you're running up a flight of stairs,
you want the heart to increase an activity
to pump more blood. If you're lying down,
sleeping, you want the heart to rest and relax
and have a nice slow heartbeat. From the sinus
node, which is high in the right atrium, the
electrical impulse passes through both atria,
both the right and the left atrium, and into
the AV node. And as I have already told you,
there is a delay here which is required so
that the mechanical activity, the systole -
the squeeze of the atria can empty all of
their blood down into the ventricles before
the ventricles start to contract. In the AV
node, there are a number of specialized fibers
which in long life they can become disrupted
and even injured and fibrotic and the patients
may develop a very slow heart rhythm because
the impulse cannot get through from the atria.
04:36
You can see from the electrocardiogram, the
P wave is… begins with the sinus node and
the rest of the P is the atrial depolarization
and then you can see a little pause before
the big deflection, which is the ventricular
deflection. That little pause shown in green
on this is the delay in the AV node. And then
we get to the ventricle, the QRS is ventricular
depolarization. When the electrical impulses
passing out through the His-Purkinje system -
the specialized fibers in the ventricular
muscle transmitting the electrical wave to
the ventricles, which then contract and you
can see again in green where the QRS is and
the impulse is in the ventricles.