An electrocardiogram - or ECG - or the dutch and german version of the word - EKG, is a
tool used to visualize the electricity that flows through the heart.
An ECG tracing specifically shows how the depolarization wave during each heartbeat
- which is a wave of positive charge.
And the way it looks depends on the set of electrodes you’re using.
This particular set of electrodes is called lead II, for example, with one electrode on
the right arm and the other on the left leg, so essentially when the wave’s moving toward
the left leg electrode, you get a positive deflection, like this big positive deflection
corresponding to the wave moving down into the left and right ventricles.
To understand the basics, let’s start with an example of how we can look at the heart
with only one pair of electrodes - a positive one and a negative one.
Remember that at rest, cells are negatively charged relative to the slightly positive
outside environment and when they depolarize, the cells become positively charged, leaving
a slightly negative charge in the outside environment.
So let’s say that when this set of cells is at rest they’re red and then they turn
green as the wave of depolarization moves through them.
Now if we freeze this “wave of depolarization” as it’s moving through the cells, half of
the cells are positive or depolarized and half are negative or resting, and so there’s
a difference of charge across this set of cells.
You can think of the charge difference as being a dipole because there are two electrical
poles, and we can draw this dipole out as an arrow or vector pointing towards the positive
charge, and remember the electrodes detect charge on the outside of the cell, so this
points toward where the positive charge is outside.
Now, if there’s a dipole vector pointing toward the positive electrode, then the ECG
tracing shows it as a positive deflection - the bigger the dipole, the bigger the deflection.
If we unpause this, then everything becomes depolarized, and since there’s no difference
in charge, now, there’s no dipole, and so no deflection.
Moments later, a wave of repolarization goes through.
Pausing halfway through again, now the vector dipole goes in the opposite direction, and
faces the negative electrode, which means there will be a negative deflection on the
ECG tracing.
Again, the bigger the dipole, the bigger the negative deflection.
Now even though it’d be nice if the depolarization wave lined up perfectly with the electrodes,
usually that’s not the case, so what we end up looking at is the vector component
that is parallel to the electrode.
For example let’s say that the depolarization happened this way - at an angle, then we’d
break the vector into two parts.
One going parallel with the electrodes and one going perpendicular.
The one we care about is the one that’s going towards the positive electrode which
causes the deflection, though since this arrow is shorter is going to cause a slightly smaller
deflection than previously, in other words, the size of the deflection on the ECG tracing
always corresponds to the magnitude or size of the dipole in the direction of the electrode.
The perpendicular component isn’t pointing at the electrodes, so it doesn’t cause any
deflection.
In fact, if there’s a depolarization wave that goes straight up, which is perpendicular
to the positive and negative electrodes, then there’d be no deflection at all!
In a standard ECG there are 10 electrodes - four limb electrodes, one on the left arm,
right arm, left leg, and the right leg, and six precordial electrodes - V1 through V6
that wrap around the chest.
The right leg electrode is usually used as a neutral lead.
Now, the heart is a 3-dimensional organ, right, so V1 through V6 line up in the transverse
or horizontal plane of the heart, and each electrode is set up to detect any wave of
positive charge coming towards them, which based on what we know already means they’re
positive.
These are collectively called the chest leads.
Meanwhile, in the coronal plane, the non-neutral leads are called augmented vector right, or
aVR, on the right arm, and augmented vector left, or aVL, on the left arm- both of which
are represented as vectors that are 30 degrees up from the horizontal line.
Finally there’s the augmented vector foot, or aVF, on the left foot which anatomically
isn’t straight down, but it’s close enough that it ends up representing the vector facing
straight down on the diagram.
Just like the precordial electrodes, aVR, aVL, and aVF - each detect any positive deflection
coming towards them.
Now, in addition to these three limb leads, there are also bipolar limb leads called lead
1, 2, and 3, which are recorded using two electrodes instead of just one.
Lead 1 uses the Right Arm as the negative pole and Left Arm as the positive pole, forming
a vector that goes to the right.
Lead 2 uses the right arm as the negative pole and the left leg as the positive pole,
forming a vector that goes to the +60 degree mark.
And lead 3 uses the left arm as the negative pole and the left leg as the positive pole,
forming a vector that goes to the +120 degree mark.
So, in total you’ve got 6 leads from the limb leads, 6 from the chest leads, leading
to a grand total of 12, which gives you your 12-lead ecg.
Now the point of having all these leads is to get different views of the heart.
Making it easier to see exactly how the wave of depolarization moves through the heart.
As an example, consider how the 6 chest leads - V1 through V6 - register this depolarization
wave form called the QRS complex.
The exact same depolarization wave might appear mostly negative in V1 and V2, isoelectric
in V3, and mostly positive in V4, V5, and V6, all because of the exact direction and
magnitude of the vectors at different points in time.
Similarly, each of the limb leads produces its own viewpoint of the depolarization wave
as well.
Now, all the limb leads and chest leads can be grouped based on the regions of the heart
that they are nearest.
Problems in specific leads or groups of leads suggests that there may be a specific region
of the heart that may be affected by a disease.
Leads II, III, and aVF are “inferior” leads because they’re near the inferior
wall of the heart which receives blood from the right coronary artery.
Leads I and aVL, along with two of the chest leads, V5 and V6, are all considered “lateral”
leads and they’re near the lateral wall of the heart which receives blood from the
left circumflex artery.
Finally V1 and V2 are considered “septal” leads because they’re nearest to the interventricular
septum, and V3, and V4 are “anterior” leads because they’re nearest the anterior
wall of the heart.
Both of the septal and anterior regions are served by the left anterior descending artery.
All right as a quick recap - In a standard ECG there are 10 electrodes - four limb electrodes
and six precordial electrodes that wrap around the chest.
These electrodes are used to make 12 leads, each of which illustrates the movement of
positive charge on the outside of heart cells.
The ECG tracing shows a depolarization wave moving towards an electrode as a positive
deflection, and one moving away as a negative deflection, each of which is proportional
to the size of the dipole.
The point of being able to get different views of the heart is that it makes it easier to
see how the wave of depolarization moves, which provides valuable information about
the heart's structure and function.