Lecture 2: The Electrocardiogram (ECG)
1. Uses of the Electrocardiogram
An electrocardiogram, or ECG (also, in some places, called an EKG) is a measurement of
electrical activity from the heart (that spreads to the surface of the body), allowing you to
quantify heart rate, as well as see how this activity changes under different conditions over time.
Thus, the ECG is a powerful diagnostic tool.
The electrocardiogram produces the well-know trace seen on the screen of a heart monitor. There
are several important components to the trace produced by an ECG recording: there are generally
three deflections above baseline, and there are also segments between deflections - and all of
these tell us about heart function.
One of the things we can look at with an ECG is the electrical axis of the heart, which we will
talk about more in the next lecture. However, in brief, whilst electrical activity occurs throughout
the heart in all directions, there is a mean axis in which electrical activity flows, titled at about 60
degrees through the middle of the heart, and how it shifts left or right can tell us about various
disease state in the heart.
The ECG can also be used to measure heart rate, including analysing either bradychardia (a
slow heart beat) or tachycardia (an elevated one). Sometimes these terms are rather rigidly
defined (the former being less than 60 beats/minute, the latter more than 100 beats/minute), but
they can also be used in the very general sense used here, i.e., a slowing or speeding-up of heart
rate in general. We will look also at arrhythmias, and we will see how the heart has a normal
rhythm that can be disrupted. This disruption can be either superventricular (above the
ventricles) or ventricular - it is the ventricular arrhythmias that are particularly dangerous to the
health of the heart.
We will look at sequence activation disorders. In other words, an ECG trace can reveal
abnormalities in the conduction of the waves of depolarisation through the heart or disruptions in
the normal transit of electrical activity in the heart (e.g., abnormalities in movement through the
AV node or branch bundles). An ECG can tell us whether the heart has undergone hypertrophy,
which is a particular problem because a heart that has grown too much muscle will have
difficulty pumping blood properly. There are changes in the ECG when the coronary circulation
is disrupted and the heart becomes ischemic. There are also changes in the ECG if heart tissue
dies, or if there is a heart infarction (a heart attack). Drugs such as digitalis can have effects on 2
heart rhythm and rate and this too can be seen on an ECG. Electrolyte imbalances in extracellular
fluid and the and blood can also cause changes in the ECG as can infections of the heart such as
myocarditis (infection of cardiac muscle) and peritonitis (infection in the peritoneal cavity.
2. ECG Measurements, Limb Leads and Einthoven’s Triangle and Law
The ECG is essentially measuring electrical activity on the body surface that originates in the
heart. As the heart depolarises, in its normal sequence, the electrical activity spreads throughout
the body, and we can detect this using electrodes placed upon the body surface. Many who have
gone to a physical will have experienced leads being placed upon the chest and arms, and this
gives a very accurate ECG reading from multiple angles; however, simpler bipolar limb leads are
more common. For simple bipolar ECG limb leads, electrodes are placed on the left and right
arms, and the left leg. By convention, lead I goes from the left arm to the right arm, lead II goes
from the right arm to the left leg, and lead III goes from the left leg the left arm, forming a
triangle around the heart, called Einthoven's Triangle. There are positive and negative sides to
each lead, and so the left leg is positive for both leads, the right arm is negative for both, and the
left arm has one negative lead (III) and one positive lead (I).
This bring us to Einthoven's Law, which states that in the electrocardiogram, in any given
instant, the potential in any wave in lead II is equal to the sum of the potentials in leads I and III.
We will look at measuring potential differences in the leads, when we come to measuring the
electrical axis of the heart. 3
So we have these three leads in an equilateral triangle around the heart. If we map the courses of
the leads over an image of the heart, we realise that the leads form a star pattern over the heart,
with lead I crossing through the center horizontally, and leads II and III going through the centre
at opposing 60 degree angles (i.e., lead I is at 0 degrees, lead II is at 60 degrees and lead III at
120 degrees). Movements of electrical axes around these ranges can be used as a diagnostic tool
for different diseases. We are going to be looking at bipolar limb leads primarily from the
perspective ECG activity measured via lead II.
Before we see what the various components in the ECG actually reflect, we will look at see what
are actually causing negative or positive deflections in these traces. If we look at traces from all
three leads, we see quite similar patterns, with two slightly rounded positive deflections, and a
much sharper one in the middle. These are typical ECG patterns, though there are many other
lead configurations, and physicians will often have maybe twelve to fifteen different leads,
providing many more views of the heart’s electrical activity.
If we look at lead I, with a negative side on the right arm and a positive side on the left arm, if a
wave of depolarisation heads toward the positive electrode (left arm), then we will get a positive
deflection in lead I. if a wave of depolarisation travels away from the left arm, then a negative
deflection will appear in lead I. The reverse is true for waves of repolarisation. Similarly, a wave
of depolarization travelling toward the left leg will appear as positive deflections in leads II and
III. The maximum possible deflection will occur when the waves (either depolarization or
repolarisation) occur exactly parallel to the lead.
So as we look at the stages of electrical transmission in the heart, starting from the SA node
down through the AV node, up through Purkinje fibers into the ventricles, we can tell which
direction current is flowing at any particular moment by seeing whether a positive or negative
deflection occurs on any given lead in the ECG trace. 4
3. Components of the ECG
In the standard bipolar limb lead configuration (we will focus on lead II), there are three standard
deflections. The first, small deflection is called the P-wave, and it is associated with the
depolarization of the atria. We'll see that there are three general stages of the P-wave; the first is
due to the pacemaker potential, the second is due to the spread of electrical activity through the
internodal pathways, and the final phase is due the depolarisation of the muscle tissue within the
The next component consists of three points: a small downward deflection called the Q-wave,
then a large positive deflection called the R-wave, and then another downward deflection (this
time slightly larger) called the S-wave. This combined QRS deflection is what makes up the blip
on an ECG that is most familiar to us. It reflects the depolarisation of the ventricles, however,
also hidden within this blip is the activity associated with repolarisation of the atria. Given that
the ventricular muscle is much more massive than the atrial muscle, it is hard to distinguish the
two separate events (i.e., ventricular depolarisation masks atrial repolarisation), and we simply
group the two events together as the QRS complex.
The final deflection comes a little while after the QRS complex, and is called the T-wave. It
represents the repolarisation of the ventricle, and looks like a somewhat larger version of the
These three deflections will always appear on an ECG trace recorded from a bipolar limb lead,
no matter which lead you are looking at. Soon, we're going to look at the segments between these
components, and they too will be indicative of the various phases of electrical activity in the
4. ECG Components and the Stages of Electrical Transmission in the Heart
We can relate the various components of the ECG trace, whether the positive or negative
deflections, or the segments in between these events, to the stages of depolarisation of the
cardiac muscle, and the changes in electrical conductivity in the heart. On an ECG, we see that
the P-wave is significantly smaller than the QRS complex; this is because the electrical activity
generated (and detected by the ECG electrodes) is proportional to the amount of muscle tissue
there is to depolarise and since the atria has less muscle than the ventricles, its depolarisation
produces much smaller deflections than does the depolarisation of the ventricles.
Any disease state that leads to growth of muscle mass in the ventricles will result in a larger QRS
complex, and when we look more closely at the electrical axis of the heart we'll see how it too is
affected by hypertrophy of the cardiac muscle.
The pacemaker potential in the heart is reflected in the ECG trace as the first phase of the
P-wave. So the early phase of the P-wave reflects the electrical activity that is travelling, from
the pacemaker cells in the SA node, across the body, to be picked up by t