BIOMEDICAL INSTRUMENTATION

Electrocardiogram (ECG)
The conduction system of the heart
• It consists of the sinoatrial (SA) node, the internodal tracts, the
atrioventricular (AV) node, the bundle of His, the right bundle branch
(RBB), the left bundle branch (LBB), and the Purkinje network.
• The conduction system of the heart and representative electrical activity
from various regions is shown in the figure below.
The conduction system of the heart and representative electrical activity
Note: The contribution of electrical activity of various tissues of the heart in
genesis of the ECG signal (the lowest trace)

• The rhythmic electrical activity of the heart (cardiac impulse) originates
in the SA node.
• The impulse then propagates through internodal and interatrial
(Buchmans’s bundle) tracts.
• As a consequence, the pacemaker activity reaches the AV node by cellto-
cell atrial conduction and activates the right and left atrium in a very
organised manner.
• Because atria and ventricles are separated by fibrous tissue, direct
conduction of cardiac impulse from the atria to the ventricles can not
occur and activation must follow a path that starts in the atrium at the AV
node.
• The cardiac impulse is delayed in the AV node for about 100 msec. It
then proceeds through the bundle of His, the RBB, the LBB and finally to
the terminal Purkinje fibres which arborize and invaginate the
endocardial ventricular tissue.
• The delay in the AV node allows enough time for completion of atrial
contraction and pumping of blood into the ventricles.
• Once the cardiac impulse reaches the bundle of His, conduction is very
rapid, resulting in the initiation of ventricular activation over a wide
range.
• The subsequent cell-to-cell propagation of electrical activity is highly
sequenced and coordinated resulting in a highly synchronous and
efficient pumping action by the ventricles.
• Essentially, an overall understanding of the genesis of the ECG
waveform (cardiac field potentials recorded on the body surface) can be
based on a cardiac current dipole placed in an infinite (extensive) volume
conductor.
• In the early 1900, Einthoven (the father of electrocardiography)
postulated that the cardiac excitation could be viewed as a vector.

• He drew an equilateral triangle with two vertices at two shoulders and
one at the navel.
• With the cardiac vector representing the spread of cardiac excitation
inside the triangle, the potential difference measured between two
vertices of the triangle (known as the limb leads), is proportional to the
projection of the vector on each side of the triangle (see the figure
below.)
Einthoven equilateral triangle. The vertices are: LA (left arm), RA (right arm) and LL
(left leg). RL is the right leg used as a reference for potential difference measurements
and is not shown. I, II and III represent electrocardiographic frontal limb leads. The + and
– represent connection to the terminals of a biopotential amplifier. Lead I is the potential
difference between LA and RA. Lead II is the potential difference between LL and RA.
Lead III is the potential difference between LL and LA.
• Based on the aforementioned concepts, electrocardiographers have
developed an oversimplified model to explain the electrical activity of the
heart.
• In this model, the heart is considered as an electric dipole (points of equal
positive and negative charges separated from one another by a distance),
denoted by a dipole moment vector M, that can change its magnitude and
direction.

• This dipole moment (amount of charge x distance between positive and
negative charges) is called the cardiac vector.
• As the wave of depolarisation spreads throughout the cardiac cycle, the
magnitude and orientation of the cardiac vector changes and the resulting
bioelectric potentials appear throughout the body and on its surface.
• The potential differences (ECG signals) are measured by placing
electrodes on the body surface and connected to biopotential amplifier
(described in detail later).
• In making these potential measurements, the amplifier has a very high
input impedance to minimally disturb the cardiac electric field that
produces the ECG signal.
• As it was discussed before, when the depolarisation wavefront points
towards the recording positive electrode (connected to the + input
terminal of the bioamplifier), the output ECG signal will be positivegoing,
and when it points towards the negative electrode, the ECG signal
will be negative-going.
• The time varying cardiac vector produces the surface ECG signal with its
characteristic P wave, QRS complex and T wave during the cardiac
cycle.

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