Download Dubin Ch 1 - 3

The Electrocardiogram (ECG)
 ECG
 Notes electrical activity of
the heart.
 Reports on cardiac electrical
activity.
 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
contract.
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
outward.
 The atriums are stimulated to
contract.
 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
branches.
 The terminal filaments of the Purkinje
fibers depolarise the ventricular
myocytes.
 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
pathology.
The T wave
 Ventricular repolarisation
occurs when the inside of
ventricular myocytes is
negatively charged, so it
can undergo depolarization
again.
 The T-wave represents
ventricular repolarisation.
The QT-interval
 Ventricular systole
 In other words ventricular
contraction.
 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
begins.
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
myocytes
 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
millimeters.
 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
1.
Right arm
2.
Left arm
3.
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:
1.
I
2.
II
3.
III
4.
AVR
5.
AVL
6.
AVF
 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
observed.
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
way.
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:
1.
One stimulates the heart and systemic
arteries
2.
The other inhibits the heart and systemic
arteries
The Autonomic
nervous system
Parasympathetic
Sympathetic
nervous system
nervous system
Secretes acetylcholine that
Secretes Norepinephrine as
activates cholinergic
a neurotransmitter that
receptors
activates adrenergic
receptors
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.
following
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
following
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
flow
 Parasympathetic activation of cholinergic
receptors leads to:

Dilates arteries

Resulting in reduced blood pressure and blood
flow
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
syncope.
Vagal Maneuver
 Vagal = parasympathetic reflex maneuvers
such as:
 Gastrointenstinal stimulation

Gag reflex
 Carotid sinus massage

Can be used therapeutically to suppress irritable
foci.
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
accumulation.
 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.