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The Electrocardiogram (ECG)
 Notes electrical activity of
the heart.
 Reports on cardiac electrical
 Provides useful information
about the heart's function
and structures.
The Myocardium
 Myocardium
 Myo = Muscle
 Cardium = Heart
 ECG represents the
electrical activity of
the myocardium.
Depolarization / Contraction
 The inside of the heart's muscle
cells are negatively charged at rest.
 During depolarisation the inside
becomes positive.
 The myocytes contract.
 A wave of depolarization moves
through the heart.
 This causes the myocardium to
Depolarisation / Repolarisation
 Depolarisation
 The inside of the cell becomes positive.
 Repolarisation
 The inside of the cell becomes negative.
 Abovementioned is noted on the ECG.
 Both depolarization and repolarisation of
the myocardium is because of a
movement of ions (electric).
ECG electrodes
 The heart's electrical activity
can be observed from the
skin surface by electrodes.
 When a wave of positive
charge (Na⁺ ions) moves to a
positive electrode there is a
equal upward deviation on
the ECG.
 In general an upward wave
on an ECG = depolarisation.
Electrical conduction of the heart
Electrical conduction of the heart
SA-node / P-wave
 Heart's dominant pacemaker.
 The SA-node initiates a wave of
depolarisation that spreads
 The atriums are stimulated to
 Automaticity = The ability of the
SA-node to generate a stimuli.
 Atrial depolarisation / Atrial
contraction is observed as the Pwave on the ECG.
AV-node / Pause
 Depolarization becomes
slower within the AV-node.
 Therefore there is a rapid
delay or pause before the
ventricles are depolarised.
 The necessary delay allows
blood to move from the atria
through the AV-valves into
the ventricles.
 AV-node makes use of slow
Ca⁺ ions.
Ventricular Conduction System / QRS-complex
 Starts at the Bundle of Hiss.
 Conduction is slow within the AV-node,
but faster in the right and left bundle
 The terminal filaments of the Purkinje
fibers depolarise the ventricular
 Depolarization of the ventricular
myocardium registers as the QRScomplex.
 Contraction of the ventricles occurs.
 The ventricular conduction system makes
use of the fast Na⁺ ions.
The QRS-Complex
 The Q wave is always present at the beginning of the QRS-complex.
 This is the first downward deflection of the complex.
 The Q-wave is followed by an upward R wave.
 A downward S wave follows the R wave.
 The total QRS-complex represents ventricular contraction.
Name the following waves
Name the following waves
Name the following waves
The ST-Segment
 Horizontal segment
following the QRS-complex.
 It is very important to
observe that the segment is
on the same level as other
areas of the baseline.
 Any elevation / depression
is an indication of serious
The T wave
 Ventricular repolarisation
occurs when the inside of
ventricular myocytes is
negatively charged, so it
can undergo depolarization
 The T-wave represents
ventricular repolarisation.
The QT-interval
 Ventricular systole
 In other words ventricular
 QT-interval
 Represents the duration of
ventricular systole from the
beginning of the QRS-complex
to the end of the T-wave.
 The QT-interval varies with
different heart rates.
The cardiac cycle
 Represents atrial
contraction, followed by
ventricular contraction –
as well as the rest stage
before a new cycle
Ion tranport during the Cardiac cycle
 Sodium ions cause rapid myocyte
contraction (Na⁺)
 Like in the SA-node, Bundle of Hiss
and Purkinje fibers.
 Calcium ions cause slow myocyte
contraction (Ca⁺)
 Like in the AV-node.
 Outflow of Potassium ions causes
repolarisation / relaxation of the
 Like during repolarisation of all the
electrical parts in the heart.
ECG paper and Graphs
Measurement of voltage
 The height and depth of a
wave is measured vertically
from the baseline in
 The vertical amplitude is a
measurement of voltage.
 Positive deviations are upward
on the ECG (depolarization).
 Negative deflections are
downward on the ECG.
Time on the ECG paper
 The duration of any wave can be determined by
measuring the horizontal axis.
Limb conductors (limb leads)
 Electrodes are placed on
Right arm
Left arm
Left leg
 Each limb conductor has two electrodes,
one positive and the other negative
(bipolar conductors).
 The bipolar limb conductor set up is
sometimes called “Einthoven's triangle”.
Bipolar conductors
 Conductor I
 Horizontal
 From negative right arm to positive
left arm.
 Conductor II
 From negative right arm to positive
left foot.
 Conductor III
 From negative left arm and positive
left foot.
Unipolar limb conductors
 AVF (Augmented Voltage left Foot)
 Left foot electrode is positive.
 The negative electrode is formed by
both the right and left arm electrodes.
 AVR (Augmented Voltage Right arm)
 Right arm electrode is positive.
 The negative electrode is formed by
the left arm and left foot.
 AVL (Augmented Voltage Left arm)
 Left arm electrode is positive.
 The negative electrode is formed by
the right arm and left foot.
Limb conductors (limb leads)
 The six limb conductors consist of:
 The abovementioned form 6 intersecting lines on
the patient's chest on a FRONTAL LEVEL.
 Each limb conductor records from another angle,
so that cardiac activity on another level can be
Limb conductors (limb leads)
 Remember = as a depolarisation
wave moves to a positive
electrode there is a positive
(upward deflection) on the ECG
 Conductor I and AVL
 Lateral conductors
 Conductors II , III and AVF
 Inferior conductors
Cardiac / Chest conductors
 Positive electrodes are placed on
different levels on the chest.
 Chest conductors are numbered
V1 - V6.
 Runs from right to left.
 Each of the chest conductors pass
through the AV-node and project
through the patient's back, which
is negative.
Limb and chest conductors
Modified areas for limb electrodes
 Instead of putting
limb electrodes on
the extremities, it can
be placed on the
body of the patient.
 Wilders incorporated
records ECG’s in this
Autonomic Nervous System
 Regulates vital functions of all organs through
reflex and CNS control.
 But not conscious control.
 Controls the heart and systemic arteries - as
they are related to blood pressure.
 The Autonomic nervous system has two
divisions viz:
One stimulates the heart and systemic
The other inhibits the heart and systemic
The Autonomic
nervous system
nervous system
nervous system
Secretes acetylcholine that
Secretes Norepinephrine as
activates cholinergic
a neurotransmitter that
activates adrenergic
Sympathetic Nervous System
Thus the sympathetic system’s
cardiac excitatory effects are the
Norepinephrine stimulates the heart’s ß1 receptors (adrenergic)
Stimulates SA-node to provide faster pace.
1. ↑ rate of SA-node
2. ↑ rate of conduction
Improved AV-node conduction accelerates conduction
through the atrial- and ventricular myocardium
3. ↑ force of contraction
This increases the force of myocardial contraction
Increases the irritability of foci
4. ↑ irritability of foci
Epinephrine (adrenaline) is secreted by the
adrenal gland during the fight- or - flight
response and also has a stimulating effect on
ß1 receptors (adrenergic).
Parasympathetic Nervous System
Thus the parasympathetic system 's
cardiac inhibitory effects are the
Acetylcholine activates cholinergic receptors
Inhibits the SA-node, that leads to reduced heart rate
1. ↓ rate of SA-node
2. ↓ rate of conduction
Reduces the speed of myocardial conduction and inhibits
the AV-node
3. ↓ force of contraction
Reduces the force of myocardial contraction
Reduces the irritability of foci
4. ↓ irritability of foci
Autonomic Control of Blood Flow and Blood Pressure
 The autonomic nervous system controls blood
flow and blood pressure by regulating
constriction and dilation of arteries throughout
the body.
 Sympathetic stimulates α1 (adrenergic)
receptors leading to:
Constriction of arteries throughout the body
Resulting in increased blood pressure and blood
 Parasympathetic activation of cholinergic
receptors leads to:
Dilates arteries
Resulting in reduced blood pressure and blood
Merciful Syncope
 Syncope (unconsciousness / fainting)
 Severe pain / seeing your own blood
sometimes results in a parasympathetic
reflex that inhibits the SA-node and leads
to a reduced heart rate.
 Bradycardia = Low Heart Rate
 Dilatation of systemic arteries leading to
reduced blood pressure = hypotension
 Reduced blood flow to the brain leads to
Vagal Maneuver
 Vagal = parasympathetic reflex maneuvers
such as:
 Gastrointenstinal stimulation
Gag reflex
 Carotid sinus massage
Can be used therapeutically to suppress irritable
Sympathetic response during the standing action
 It is logical to think that when people are standing
blood will accumulate in the lower extremities due
to gravity.
 However, this is not the case.
 Standing causes a compensatory sympathetic
response that constricts peripheral arteries and
increases heart rate to prevent distal blood
 If the normal sympathetic response is ineffective it
may lead to syncope due to reduced blood flow to
the brain.
 Abovementioned is called Orthostatic hypotension.