Under these conditions the second set of cells which is firing is called
an ectopic foci.
And they can interfere with the sequential movement of electrical activity
that we have just described.
And when that occurs, you can get arrythmias, you can get a skipped beat,
you can have improper filling of the Of the chambers either of the atria,
if it occurs in the atria or of the ventricle, if it occurs in the ventricle.
And under these conditions, if the chamber is contracting so
rapidly because of an arrhythmia, then you may not be able to fill it.
And if this occurs in the ventricle, this could be lethal.
You can have a heart attack.
And the other thing that we have to remember then are the electrical
activities of the entire heart can be observed on the clinical ECG.
That the time intervals, as well as whether a specific
wave occurs gives information to the cardiologist as to how well
the electrical conduction system of the heart is performing.
There are some diseases where for instance, it takes too long for
the ventricles to depolarize and so you will have a widening of the QRS.
Or you can have other diseases where you have problems with repolarization of
the heart and under those conditions then it affects the T wave and the ST segment.
All right, so let's look at a case.
So our case is Mrs. R, she's 80 years old,
and her normal resting heart rate is 85 beats per minute.
She usually goes to the gym every day and she works out on an electrical stepper.
And on Friday, she was unable to do her morning exercises.
She got up in the morning and didn't feel very well, and
by the time she got to the gym, she tried to get onto the electrical stepper,
but she just couldn't get enough energy and was unable to do her exercises.
She felt so badly that she decided to go and see her cardiologist.
When she came in, he took her heart rate and
he found that her heart rate was 30 beats per minute.
She has bradycardia and is this from due to athletic training?
No.
Her normal resting heart rate was 85 on Thursday and on Friday is now 30.
So, something dramatic has happened to the electrical conduction system
within her heart.
So he decided to run electrocardiogram to see what changes had occurred
within her heart, and the electrocardiogram is what showing here.
So on the Y-axis we have millivolts and on the X-axis we have time.
And as you can see, there are P waves which are present, and
that the P waves have a very set time interval.
So the P waves are occurring, so that means that the SA node is firing.
The SA node is firing and it's firing on a regular basis, so
she has a normal SA node.
But then if you look at the R in the QRS complex,
there's a P wave followed by a QRS here, but
then we have a P, which is not followed by the QRS.
And then we have another P which again, is not followed by the PRS and
that the R to R intervals are actually longer than the P to P intervals.
So, the R to R intervals are more R regular but
they are at in much longer interval.
Which means that they have a different pacemaker.
So, we've somehow have uncoupled the electrical activity that's occurring
the atria from the electrical activity that's occurring in the ventricle in this
particular heart.
And what that is, is that there is a complete block at the AV node.
So the AV node then, is not taking the information and
listening to the pace which is being set by the SA node.
But instead, the atria are then contracting at one pace,
but the ventricles at a much slower pace.
So the new pacemaker, is a pacemaker that's giver her a 30 bpm.
And that new pacemaker then would be the, HIS bundle or the Prekinjes.
Okay, so one of our
key concepts.
So the first is, each heartbeat or
one cardiac cycle involves electrical activation of the atria and
the ventricles in the right and the left chambers.
Secondly, that the action potential of the pacemaker and
the contractile cells, they're both cardiac myocytes,
but their pacement, their action potentials vary or are very different.
The pacemakers, the time of the action potential is 150 milliseconds.
And we have an unstable resting membrane potential, and
in the cardiac myocyte that’s contractile.
The time of the action potential is 200 to 220 milliseconds and
there's a stable resting membrane potential.
The pacemaker cells and have these unstable resting membrane potential.
The SA node is the fastest pacemaker in the heart and
in the normal heart it sets the beat.
So all of the other pacemakers are trained by the SA node,
are entrained by the SA node.
Fourth, our heart rate is determined by the autonomic nervous system.
The sympathetic nervous system increases heart rate, speeds up heart rate and
its acting through the beta-1 adrenergic receptors.
The parasympathetic is what slows the heart rate, is the break for the system
and it's acting through the vagus innervation or the vagal innervation.
And it is through the muscarinic receptors which are present on the heart.
Five, the electrocardiogram is the sum of the electrical activity
of the entire heart.
So the P waves depict the atrial depolarization,
the QRS complex depicts the ventricular depolarization.
And the T wave is the ventricular repolarization.
And six, disease of the electrical conduction system
can be manifested by changes in the electrocardiogram itself.
So both the timing, and whether or not a specific wave is occurring
tells the cardiologist something of what may be part of the disease
process that's occurring within the electrical conduction system.
Okay, so the next time we come in, we're going to talk about how electrical
conduction system coordinates the contractile activities of the heart.
Okay, so see you then