BIOC34 – Lecture II – Electrocardiogram (ECG)
Figure The standard ECG patterns when referencing ECG from the leads
Bipolar Limb Leads
ECG essentially measures surface electricity originating from the heart. As the heart
depolarizes, electrical activity from the heart spreads out to the rest of the body which is then
picked up by the electrodes placed on the body.
• Bipolar ECG limb leads record the voltage between electrodes placed on the wrist and
• Equilateral triangle surrounding the heart is formed (Einthoven’s triangle)
Standard Bipolar Limb Leads
• Lead I: right arm (-) to the left arm (+)
• Lead II: right arm (-) to the left leg (+)
• Lead III: left arm (-) to the left leg (+)
In the electrocardiogram, at any given instant, the potential of any wave in lead II is equal to the
sum of the potentials in lead I and III (E = electrical potential; EI+ E III ) II
• Lead I = E LAE RA
• Lead II = E LLE RA
• Lead III = ELL E LA
Einthoven’s Triangle and the Axial Reference System
• Positive electrode for lead I: 0° relative to the heart
• Positive electrode for lead II: + 60° relative to the heart
• Positive electrode for lead III: + 120° relative to the heart
Deflections or movements of electrical axis above and beyond these different ranges can be
used as diagnostic tools for different diseases. • Lead II is the most important. ECG interpretation will predominantly be viewed through
the perspective of lead II.
Positive and Negative Deflections in the ECG Trace
• Wave of depolarisation toward the left arm: positive deflection in lead I.
• Wave of depolarisation away from the left arm: negative deflection in lead I.
• Wave of repolarisation away from the left arm: positive deflection in lead I.
• Wave of repolarisation toward the left arm: negative deflection in lead I.
• Wave of depolarisation toward the left leg: positive deflection in leads II and III.
• Wave of depolarisation away from the left leg: negative deflection in leads II and III
• Maximum positive deflection occurs when wave of depolarisation heads toward the
positive electrode in a path that is parallel to the given axis.
o Ex: if the wave of depolarization moves down at a 120° angle, then the maximum
depolarization occurs in lead III.
• An ECG recording is very similar to a compound AP Thiruvarangan
• The first component is a P-wave which is indicative of electrical activity associated with
depolarization of the atria.
• QRS Complex (combination of deflections) predominantly reflects the depolarization of
the ventricles. Also hidden within this, is the repolarization of the atria (hard to distinguish
the two events because the ventricular depolarization is so large).
• T Wave (from Lead II and III) represents the repolarisation of the ventricle.
Phases of the ECG relate to the waves of depolarization and repolarization during a heart beat
• The P Wave is significantly smaller than the QRS Complex because the amount of
electrical generated and detected by the ECG electrodes is proportional to the amount
muscle tissue that is available to depolarize (more muscle in ventricle than atria)
• Any disease state that leads to enlarged ventricles (hypertrophy) will lead to muscle
mass and can affect the magnitude of the QRS complex.
The early phases of the P-wave reflect the electrical activity traveling from the pacemaker cells
across the surface of the body and being picked up by the ECG electrodes.
Conduction through the atria
As we begin to get the waves of depolarization moving through the atria and we begin to get
phases of depolarization of the muscle, this conduction of electricity through the atria and the
time of this conduction is reflected in the second half of this initial upward swing.
P Wave: Atrial Depolarisation
The P Wave represents pacemaker potentials, transmission of electrical activity through the
atria, and full contraction of the atrial muscle. Delay at the AV node and beginning of conduction through the branch bundles & Purkinje fibres
The waves of depolarization (above) converge on the AV node which is the sole sight from
which electricity can be transmitted from the atria to the ventricles (in non-diseased states).
• There is a slight delay between the P-wave and the QRS complex and is indicative of
the time it takes electrical activity to conduct through the AV node and begin its
conduction down branch bundles and Purkinje fibers.
QRS Complex: Ventricular Depolarization (and atrial repolarization)
***Q wave also represents some of the branch bundle / PF depolarization ***
• Represents mass depolarization of the ventricles and repolarization of the atria
T Wave: Ventricular Repolarization
• In reality, in a clinical setting, there will be more leads than these standard limb leads Thiruvarangan
• There are two other sets of limb leads used (see below)
Augmented Unipolar Limb Leads – these leads use the exact electrodes as leads I, II, and III
(same positions), but the difference in this configuration, these new leads are referred to as the
positive side of the augmented leads.
• aV L Left arm positive; -30° relative to lead I
• aV R Right arm positive; - 150° relative to Lead I
• aV F Left leg positive; + 90° relative to Lead I
***All referenced against combinations of other electrodes***
If we look at aVR, you can see that it is oriented almost 180° to lead II and thus, the ECG traces
of aV Rs almost a mirror image of the ECG recorded by lead II (i.e. going in opposite direction). Unipolar Chest Leads (V – 1 ) 6
These allow a reference of electrical activity both at the frontal and perpendicular planes and in
combination can view the heart in different perspective.
By looking at all the different forms of ECG traces, you can get a complete picture of the
electrical activity of the heart and looking at the different perspectives allows for a more
R-R Distance – distance between R wave of one QRS complex and the R wave of the next.
• It is the time between two heart beats
• 60/R-R interval = heart rate (e.g. if R-R = 0.75 sec, then HR = 80 bpm) (i.e. measures
instantaneous heart rate).
In practice, bradycardia and tachycardia would be referred as a reduction or elevation of heart
rate from what is generally normal.
P-Q (or P-R) Distance
• Reflects the time of conduction through the AV node and down to the branch bundles. Thiruvarangan
• From the end of the T wave until the start of the QRS complex
• It represents the time period in which the heart is relaxing and this referred to as
• The contraction of the ventricles is the most important event in the heart as that is when
the blood is being pumped to the lungs or systemic circulation