The electrical impulses from the SA node can be detected through electrodes placed on the skin, usually on the chest, arms, and legs. The ECG provides a graphical representation of the electrical impulses generated by the heart's muscle cells.
• An inherent and rhythmical electrical activity is the
reason for the heart’s lifelong beat.
• Autorhythmic fibers repeatedly generate action
potentials that trigger heart contractions.
• SA node cells do not have a stable resting potential.
Rather, they repeatedly depolarize to threshold
• The spontaneous depolarization is a pacemaker
• Spontaneous depolarization in SA node cells is the
self-generated, gradual increase in the electrical
potential across the cell membrane without any
Why the SA node do not have stable resting
• This unique characteristic is due to the presence of specific ion channels
and the spontaneous depolarization property of the SA node cells.
• SA node cells exhibit a gradual depolarization phase known as the
pacemaker potential (also called the diastolic depolarization).
• This phase is characterized by a slow, steady increase in membrane
potential toward the threshold potential, at which point an action potential
• When the threshold potential is reached, L-type calcium channels open,
triggering an action potential. After the action potential, the membrane
potential returns to its initial level, and the pacemaker potential phase
• The lack of a stable resting potential in SA node cells enables them to
generate rhythmic, spontaneous electrical activity, which is essential for
initiating and regulating the heartbeat.
• There is a delay of action potential reaching from AV
node to Bundle of His due to differences in the cell
• This delay provides time for the atria to empty their
blood into the ventricles.
• This bundle is the only site where action potentials can
conduct from the atria to the ventricles.
• If the action potential is conducted between the atria
and ventricles at other sites, it can lead to a range of
potential issues and abnormal heart rhythms, known as
What is ECG?
• The electrical impulses from the SA
node can be detected through
electrodes placed on the skin,
usually on the chest, arms, and
• The ECG provides a graphical
representation of the electrical
impulses generated by the heart's
What is ECG?
• The 12-lead ECG comprises three standard
limb leads (I, II, and III), three augmented
limb leads (aVR, aVL, and aVF), and six
precordial leads (V1, V2, V3, V4, V5, and
• By comparing these records with one another and
with normal records, it is possible to determine
• if the conducting pathway is abnormal,
• if the heart is enlarged,
• if certain regions of the heart are damaged, and
• the cause of chest pain.
Limb Leads- I, II, and III
Lead I: Primarily provides information about the heart's electrical
activity in the left-to-right direction.
Lead II: Particularly useful for assessing the heart's electrical
activity in the superior-to-inferior direction.
Lead-III: Helps evaluate the heart's electrical activity in the
inferior and left direction.
Limb Leads- Significance
Detection of myocardial infarction: leads II, III, and aVF
provide crucial information about the inferior wall, while
leads I and aVL focus on the lateral wall.
Identification of arrhythmias: lead II, are commonly
used for rhythm analysis and detecting arrhythmias
such as atrial fibrillation, atrial flutter, and various
forms of heart block.
Augmented Limb Leads- aVR, aVL, aVF
aVR: Augmented vector right provides an "augmented" view
of the heart's electrical activity from the right side.
aVL: Augmented vector left provides an "augmented" view
of the heart's electrical activity from the left side.
aVF: Augmented vector foot provides an "augmented" view
of the heart's electrical activity from the inferior aspect
Precordial leads- V1, V2, V3, V4, V5, and V6-
Detection of myocardial infarction: help identify the
presence and location of myocardial infarction (heart
attack) in the anterior and lateral walls of the heart.
Identification of arrhythmias: Precordial leads
provide additional information about the origin and
nature of arrhythmias.
Precordial leads- V1, V2, V3, V4, V5, and V6-
Evaluation of ventricular hypertrophy: can help detect
left or right ventricular hypertrophy, which is the
enlargement and thickening of the heart's ventricular walls.
Assessment of conduction abnormalities: can help
identify various conduction abnormalities, such as bundle
branch blocks or hemiblocks, by revealing specific
patterns in the QRS complex.
• P-wave: Atrial depolarization
• PR interval: Onset of P-wave to onset of
QRS complex. Determine whether the
impulse conduction from the atria to
ventricles is normal.
• PR segment: Line between the end of P-
wave and the onset of the QRS complex.
Reflects the slow impulse conduction
through the AV node.
• Depolarization of the ventricles.
• QRS duration: Time interval between
onset and end of QRS complex.
• Short QRS complex: Rapid
depolarization of ventricles and proper
• Wide QRS complex: Slow ventricular
depolarization and due to dysfunction
in the conduction system.
• J point in an ECG is the point where the
QRS complex meets the ST segment.
• Represents the beginning of
• J point helps assess the presence and
magnitude of ST segment deviations,
which can indicate various cardiac
• ST segment elevation (when the J point
and ST segment are above the baseline)
can be indicative of an acute myocardial
• The ST segment corresponds to the
plateau shape of the action potential.
• The ST segment is of particular
interest in the setting of acute
QT interval and QTc
• Reflects total duration of ventricular
depolarization and repolarization.
• Onset of QRS complex to the end of
• QT duration is inversely related to the
• The heart-rate adjusted QT interval is
• A long QTc interval increases the risk
of ventricular arrythmias
• Reflects ventricular repolarization.
• The T wave is flatter than normal when
the heart muscle is receiving
insufficient oxygen—as, for example,
in coronary artery disease.
• The T wave may be elevated in
hyperkalemia (high blood K+ level).