Introduction - ECG Acquisition and Automated Remote Processing - page 4

The electrical activity of heart muscles is initiated at sinoatrial (SA) node
located at the upper region of the right atrium. A specialized group of cells named
pacemaker cells at this node spontaneously depolarize and repolarize at a rate of
60-100 times/min. The electrical impulses gradually spread over the atria at a
speed of 4 m/s, causing them to contract together. This produces a noticeable
deflection on the ECG record, which is named as P wave. After flowing through
the atria, the electrical impulses reach at the atrioventricular (AV) node, located at
the lower end of the right atria. Here, the conduction is delayed, at speed of 0.1 m/
s. AV node is the only route through which these electrical impulses can reach the
ventricles, since the rest of atrial myocardium is separated from the ventricles by a
non-conducting ring of fibrous tissue. The conduction at the AV node provides a
gap between the atrial conduction and ventricular conduction and reflected as an
equipotential line, named PR segment in the ECG record. From the AV node, the
electrical impulses enter the bundle of His which is bifurcated to left bundle and
right bundle branches. As the impulses flow through the bundle branches, the
contraction (depolarization) of the ventricles starts together. This generates a
combination of sharp downward and upward deflection in the ECG record, named
as QRS complex. The right bundle branch conducts the pathway of conduction to
the right ventricle, while the left bundle branch is divided into anterior and pos-
terior fascicles that conduct the wave to the left ventricle. The conduction is
gradually distributed in Purkinje fibers, which are spread out to the left and right
atria. Since the systemic circulation dominates in overall contraction of the ven-
tricles in terms of muscle activity, the QRS complex mainly represents ventricular
contraction due to left ventricle. During ventricular contraction, the atria also
expand (repolarizes). However, the generated electrical activity due to this is very
feeble and suppressed by the ventricular activity. There is a small time gap
between the ventricular contraction and expansion, where no electrical activity is
recorded in the ECG record. This is represented by the equipotential ST segment,
after which the ventricles start expanding (repolarization) together. The ventricular
repolarization is represented by a T wave in the ECG record. In certain electrodes,
a U wave is recorded as small after-potentials after the T wave, representing slow
repolarization of inter-ventricular septum or slow repolarization of ventricles. The
cardiac events and corresponding ECG waves are summarized in Table 1.1 and
Fig. 1.3 .

Table 1.1 ECG waves and events
Wave/segment

Event name

P wave Atrial depolarization

PR interval Start of atrial depolarization to start of ventricular depolarization

QRS complex Ventricular depolarization

ST segment Pause in ventricular electrical activity before repolarization

T wave Ventricular repolarization

U wave (uncertain) Slow ventricular repolarization

Or, inter-ventricular septal repolarization