Biomedical Engineering Reference
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Fig. 8.2 Heart or natural
pacemaker
All the segments and intervals used by clinicians are represented in this ECG di-
agram. Depolarisation and repolarisation of the ventricular and atrial chambers are
presented by deflection in the ECG signal. These deflections are labelled in alpha-
betic order: P-QRS-T. Letter P indicates atrial depolarisation, and the ventricular
depolarisation is represented by the QRS complex. The ventricular repolarisation is
represented by T-wave. Atrial repolarisation appears during the QRS complex and
generates a very low amplitude signal which cannot be uncovered from the normal
ECG signal.
8.3.3 ECG Morphology
Sequential activation, depolarisation, and repolarisation are distinct deflections in
the ECG, caused by anatomical differences between the atria and the ventricles.
The sequences are even distinguishable when they are not in the correct sequence
(P-QRS-T). Each beat of the heart can be observed as a series of deflections, which
reflects the time evolution of electrical activity in the heart [ 3 , 6 , 24 ]. A single cy-
cle of the ECG is considered as one heartbeat. The ECG may be divided into the
following sections.
P-wave: A small low-voltage deflection caused by the depolarisation of the atria
prior to atrial contraction as the activation (depolarisation) wave front propagates
from the SA node through the atria.
PQ-interval: The time between the beginning of atrial depolarisation and the be-
ginning of ventricular depolarisation.
QRS-complex: The QRS-complex is easily identifiable between the P- and T-
waves because it has a characteristic waveform and dominating amplitude. The
dominating amplitude is caused by currents generated when the ventricles depo-
larise prior to their contraction. Although atrial repolarisation occurs before ven-
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