Leads
and Axis
When you perform an ECG , you see the recording of the
bipolar limb leads (I, II, III) , the amplified unipolar limb leads ( aVR aVL aVF ) and precordial leads (V1 -6), for a total of 12 leads . But
what exactly are these leads ?
Bipolar leads :
The I
measures the difference in electrical potential between the left arm and right
arm . All (pathological signs) that we see on this derivation will cover the
left side of the heart.
The II
measure the potential difference between the foot ( either left or right) and
the right arm
The III
measure the potential difference between the foot ( either left or right) and
the left arm
Both the III and II leads concern the lower part of
the heart.
The three lines that you see in the picture above ,
form the Einthoven triangle.
The aVR (Amplificated Vector Right) is a lead exploring the right
side of the heart.
The aVL ( Amplificated Vector Left) explores the left side of the
heart while the aVF ( Amplificated Vector Foot ) the bottom / diaphragmatic part
of heart .
If we take now the three lines (axes) of the three
bipolar leads (I, II, III) and move each one toward the center
so that they intersect and put them in a circle, and we also add the amplified
unipolar leads, we form the hexaxial
reference system of the heart.
I = 0 °
II = + 60 °
III = +120 °
aVR = -150 °
aVL = -30 °
aVF = +90 °
With this system we can determine the electrical axis
of the heart of the patient and that has a great clinical importance as we
shall see later. Generally the axis of the QRS complex is evaluated because of
its great clinical interest, although you can find the axis of any wave of the
ECG.
To find the axis of the QRS complex we have to observe them in all the limb
leads (not the precordial leads ) . There is a system to calculate the axis
with accuracy but for the sake of simplicity we will not discuss it here .
You can locate the QRS axis (or P or T ) with three
empirical methods :
1 ) The axis corresponds with the R-wave (P, T) of the higher lead (
between limb leads ) .
2 ) Of all the limb leads , if there is one that is not negative nor
positive, this is the transition zone between the positive and negative half of
the circle. The axis will be found at 90 degrees angle to it , in the half positive
part.
3) You can found the electrical axis of the heart considering the
polarity of the QRS in just two leads, I and aVF :
If I (+) and aVF ( + ) = mean vector of the QRS is normal
If I ( -) , and aVF (+) = right axis deviation
If I ( +) and aVF (-) = left axis deviation
If I ( -) , and aVF (-) = extreme right axis deviation
The normal electrocardiogram of adult (healthy) people
has the mean vector of the QRS between 0 (I) and 90 (aVF). Between 0 and -90
you have the left axis deviation ,
and between 90 and 180 the right axis
deviation . In the area between -90 and 180 we have an extreme right axis deviation .
What is the
clinical utility of the hexaxial reference system of the heart?
Look at the circle with axes and imagine that you have
in front of you a person who is watching you . The I lead (and aVL ) is his
left arm , the aVR is his right arm, while the II, III and aVF are at his feet.
We said that an ST -segment elevation means acute myocardial infarction, but
which part of the heart is suffering ? If the ST segment elevation is on the II
or III , it means that regards the lower part of the heart, near the diaphragm,
which "looks" towards the feet, while if it was on I would be the
left side of the heart to suffer , and so away.
A right axis deviation could mean a right ventricular
hypertrophy , a posterior fascicular block or acute pulmonary embolism , while
a left axis deviation could mean left ventricular hypertrophy or an anterior
fascicular block .
The precordial leads are also unipolar and explore the
heart wall directly below: V1 V2 V3 and explore the front and going to the V6
the left side of the heart. So if we find an ST segment elevation on the V2
means that the front heart wall or the interventricular septum are suffering
from an acute ischemic event , with probable involvement of the anterior
descending coronary artery , while in V6 ST above means that there is a
suffering of the wall left heart with probable involvement of the circumflex
artery , and so forth.
…