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.
