Let's combine our augmented leads –
aVR, aVL, aVF –
with our standard limb leads,
1, 2, and 3.
So, now, we have six different
pictures of the heart
around one single plane.
So, this is along the frontal plane here.
And each of the arrow tips
represent where the positive pole is.
So, the negative pole is on the
opposite side of the arrow.
A depolarization traveling
towards a positive pole
is going to give you a positive deflection.
the maximal positive deflection will be
closest to that mean vector.
you need to look at
what is the wave perpendicular,
meaning that it would be a 90° angle
from the line that you drew
from that particular axis
that had the highest deflection point.
Okay, we’ll come back to this
and revisit it a couple of times,
but just have that in your mind
as we move forward.
this plane right here;
six different views, you have them.
And we can all put them around one circle.
And algebraically, one circle is 360°.
So, we're going to give certain leads a degree.
And this is going to be a helpful way
for us to assess
if someone's heart is normal or not
based upon what degree their
mean electrical axis might be.
We need to build on some of that process
and that is we need to talk about
what really is a mean electrical axis.
A mean electrical axis should be
somewhere in between about
negative 30 to 100.
That's the normal range for mean electrical axis.
Some people will say that
normal is in between 0 and 90,
and that's okay too.
I'm just giving you a little bit wider range.
Not everybody always agrees in medicine
what is normal and what's abnormal.
So, it's either 0 to 90
or negative 30 to 100.
Two good ways to think about it.
Your mean electrical axis should lie
somewhere in between that range.
Two pathologies will affect this.
If you have hypertrophy,
the mean electrical axis will
move towards a hypertrophy.
If you have a myocardial infarction –
that is a heart attack –
the mean electrical axis will move
away from the heart attack.
So, you move towards hypertrophy
or away from heart attack.
So, knowing that information,
let’s go calculate a couple.
So, let’s take this example here
and where we are looking first
for which particular wave
is the most biphasic.
What do we mean by biphasic?
Can't we just use simple words?
I know it's frustrating,
but let's work through it.
Biphasic means that your positive deflection
equals your negative deflection.
So, whichever electrode lead shows
an equal positive deflection
versus a negative deflection, okay?
So, it has to have a blip up and a blip down
and those two amplitudes
need to be very similar.
You're looking for that lead.
Then, you want to look for
the lead that’s 90° from it.
And you want to see
which one is 90° from it,
closest to 90° from it,
has a positive electrode deflection.
Let's go through two examples.
Everybody needs examples.
When you first learn this, it’s very hard.
So, you just need an example.
You need to walk through a couple.
So, let’s take one.
If standard limb lead 1 is biphasic
and aVF is the most positive,
the mean electrical axis is 90.
So, if you traced this out,
you could see standard limb lead 1.
That’s that arrow that's going towards zero here,
That's going towards zero.
That would be your most biphasic.
aVF is located perpendicular
to standard limb lead one,
and that is a direct downward movement.
If that is a positive deflection,
that gives you your mean
electrical axis a positive 90.
If we take a second example,
let's say aVL,
so here we have aVL.
If that is the most biphasic,
you'd want to take what is 90° from that,
and so that would be
standard limb lead 2.
That’s standard limb lead 2,
therefore, you would have
the most positive deflection
that would give you
a mean electrical axis of 60.
Those two examples are
normal mean electrical axes.
Because mean electrical axes should be
somewhere between negative 30 and 100.
Both 90 and 60 fall within those means,
and therefore, that is a normal ECG.
Remember our two pathologies.
If you have hypertrophy,
it will cause a shift
towards the hypertrophy.
If you have an infarction,
it will shift it away from the infarction.