The final step in our list is
to evaluate the QT interval.
Now, as you can see, the QT interval extends
from the beginning of the QRS complex
through the ST segment and
to the end of the T wave.
In our cardiac cycle, that represents the
entire process of ventricular depolarization,
Now how to calculate the QT
interval is not typically a part
of your regular shift looking at an ECG.
I'm going to walk you through how we do it
but I don't expect you to memorize this math
or to know how to do this.
But I just want you to know that because
the QT interval is due to heart rate,
we use the QTc to normalize a person's heart rate.
So let's take a look at some corrected
QT intervals just for you to get a feel.
Now hang on because here comes the math.
First you measure the QT in seconds, then
you measure the R to R interval in seconds,
then you get the square root of the RR interval,
and then you divide the QT seconds
by the square root of the RR.
So if you went through that, it's
gonna be not that difficult to do
but it's not something that's easy to
just remember off the top of your head.
So, when a QTc is calculated, then
we can use it for some comparison.
A normal QTc is less than 0.46 seconds in
women and less than 0.45 seconds in men.
When we get really excited is when
a QTc is greater than .5 seconds.
This is dangerously prolonged, and
it really puts your patient at risk.
So we're not gonna walk through
how to calculate this today.
I just wanted you to be aware that this exists.
And to know that greater than .5 seconds is
dangerously prolonged for either men or women.
Now, what we call this abnormal QT
interval is a prolonged QT interval.
So that makes sense, right?
A prolonged QT interval is
also called a long QT syndrome.
We use a lot of alphabet
soup when it comes to ECGs.
And you'll see it abbreviated LQTS.
Now, Long QT Syndrome can cause
rapid and chaotic heartbeats.
This is not an effective heartbeat
to pump blood through your body.
Mild cases can lead to dizziness or seizures,
but severe cases can lead to sudden death.
So this is a significant clinical problem.
Let's take a look at the causes of LQTS.
Well, they're grouped into two categories:
non-modifiable risk factors
and modifiable risk factors.
Well, the non-modifiable risk factor
for LQTS are genetic mutation.
So there's nothing a patient can do
about that, which is why it's called
a non-modifiable risk factor.
Now the other category is modifiable risk
factors, we can intervene with those.
So that can include certain medications,
significant mineral imbalances,
or some other underlying medical condition.
So when you're thinking about
who might end up in this trouble,
LQTS can be caused by a genetic
mutation, we can't do much to fix that,
or we have medications, mineral imbalances
or an underlying medical condition.
So what do we do to treat this?
Well, one part of the treatment
could be to avoid those medications
that gives the patient LQTS
- that would be a good move.
But if that's not enough, and
we're still having trouble,
the patient can have an
implantable defibrillator device.
Now my mom got one of these and
it has been life changing for her.
It's given her so much energy,
because she has a defibrillator,
which has never gone off, thank you.
But she has a pacemaker, and that has kept
her heart beating at a consistent rate
and she has much more energy
for activities of daily life.
Now LQTS, modifiable what was the risk factor?
Ah, see, I went with the second piece first.
What I did, there was what I
want you to do when you study.
Don't keep going in the same exact order.
When you're studying with a
group, ask questions like that.
See if you can pause and recall.
Quiz each other to see if that is
making it into your long term memory.
Now, I have a list here of things that can
cause LQTS and when you go through pharmacology,
you'll probably learn about this
but take a look at this list.
There's some really common antibiotics like
erythromycin, azithromycin and there's even others,
some antifungal medications that are taken
by mouth and used to treat yeast infections.
Diuretics that causes a significant electrolyte
imbalance, like low potassium can cause LQTS.
Heart rhythm drugs, especially anti-arrhythmic
medications can lengthen the QT interval
to a point that it becomes a problem.
There's also certain antidepressants
and antipsychotic medications.
The list is too long for you to memorize but you'll
know when you have patients on these medications
when you review that possible adverse
effects that this is an option.
Now we talked about the mineral
imbalances you'll see here,
this could be low potassium, could
be low calcium, low magnesium,
and even COVID-19 (SARS-CoV2)
infection can cause LQTS.
So let's wrap this up.
We learned how to assess and measure
each of the waveforms on an ECG.
And we know it's the first step in
accurately identifying cardiac dysrhythmias.
Now, we encourage you to watch
the other videos in this series
to learn how to apply these new skills
and identifying normal, abnormal
and life threatening cardiac rhythms.
Thank you for watching our video today.