and you can see that.
Usually, what happens is
there's a lot of small jittery –
what looks like noise
in front of the QRS complex.
That is denoted as atrial fibrillation.
A first-degree AV block is
a block and where you have
a normal P wave,
but it takes too long to capture a QRS complex.
And how do you know it's too long?
It’s greater than 0.2 seconds.
That is a first-degree AV block.
A second-degree AV block is more serious than a first.
It means that there are certain times
when you have a P wave
and you don't capture a QRS complex.
Therefore, there is some
block in the AV system
that's preventing it from getting through.
A first-degree AV block
is just slowing down.
The second-degree AV block involves
some QRS complexes
that are not captured from that P wave.
The most serious is a third-degree AV block
and here you can see
a number of P waves with
no normal QRS complexes.
If there are QRS complexes that are generated,
they are generated ectopically
and therefore, have a wide QRS complex.
How do I know it's wide?
It’s greater than 0.1 seconds.
That’s where all those
intervals become important.
You have to be able to
identify what the interval is
and then know the normal numbers
that are associated with it.
PVCs can be seen quite often.
These can happen if you have,
again, a type III AV block,
but you can also get PVCs
to occur on a regular basis.
And here, you can see some PVCs
that are occurring in a sequence.
And if you have them occur fast,
that can be a ventricular tachycardia.
You notice that there is no isoelectric line.
You don't see any P waves.
All you have are these PVCs.
Even more serious than ventricular tachycardia
is ventricular fibrillation.
So, at least in ventricular tachycardia,
you are getting contractions of the ventricles
that was pushing out blood.
In ventricular tachycardia,
you have so many contractions happening,
it's not pushing out enough blood per beat.
And if you don't push out enough blood,
it doesn't circulate to the tissues.
And you end up having hypoxic tissues
and actually could die in this condition,
unless you were cardio-averted
which means that you got put on a defibrillator
and you received shocks,
and that is the best way to get out
a ventricular fibrillation.
There are number of other ways to look at the ECG.
And as you move through medicine,
you will just deepen your
understanding of what an ECG is.
I'll go through one other pathology with you
and this involves the
STEMI myocardial infarction,
which is a heart attack.
And I go through it just because it is
something that you all have heard about
are heart attacks
and be able to understand
or to recognize someone who had a
heart attack with an ECG is helpful.
So, you can notice the normal QRS complex
where you have a P wave,
a Q, R and S,
and then you have a T wave.
This can change
if you're exposed to severe ischemia
induced by lack of blood flow
to certain areas of the heart.
What happens in ischemia is
you have a decrease in membrane potential.
What do I mean by a
decrease in membrane potential?
It’s lower than normal.
So, if you have a lower-than-normal
you start off on a lower level.
You have a normal P wave.
You might have a normal QRS complex.
But then you don't
return back to this low level,
which you had,
because of this ischemic event.
And so, you end up
being at a higher value for your ST segment.
And then you have a normal T wave.
This is called ST segment elevation.
And one of the key reasons why ST elevation happens
is because of a little bit of a trick on your eye,
which looks like the ST segment is elevated,
when actually it's the normal isoelectric line
that is before the P wave
or after the T wave,
which is suppressed, because of ischemia
and lack of oxygen to a certain portion of the heart.
And so, this is just a very
classic way to look at
or to find out
that someone has a heart attack
if they show up with an ST segment elevation.