Electrocardiography
Dr. Shafali Singh
Electrocardiography
Objective:
• To define ECG
• Genesis of ECG
• Identify different waves of ECG with their
causes
Introduction
Electrocardiography is the method of
recording an Electrocardiogram (ECG)
with the help of the machine
Electrocardiograph.
Definition
Electrocardiogram (ECG) is a graphic
recording of the electrical changes that
occur within the heart during the
cardiac cycle.
Characteristics of SA Nodal Cells
SA Nodal (Pacemaker) Action Potential
conduction of cardiac
impulse
Conduction system of the heart: SA
node is the pacemaker.
Conduction Pathways and Velocity of
Conduction
Pathway
• SA node → atrial muscle → AV node (delay) →
Purkinje fibers → ventricular muscle
Velocity
• Fastest conducting fiber = Purkinje cell
• Slowest conducting fiber = AV node
Which of the following has the fastest
conducting fibers in the heart?
a. SA node
b. Atrial muscle
c. AV node
d. Purkinje fibers
e. Ventricular muscle
ORIGIN & SPREAD OF CARDIAC
IMPULSE
• Correlatio
n of ECG
with
phases of
action
potential
Principle
Electrical activities generated with
each heart beat are conducted from
the heart to the body surface, which
are picked up and recorded by the
electrocardiograph.
• The muscle mass of the atria is small compared
with that of the ventricles, and the electrical
change accompanying the contraction of the
atria is therefore small. Contraction of atria is
associated with the ECG wave called ‘P’.
• The ventricular mass is large and so there is a
large deflection of the ECG wave when ventricle
is depolarised. This is called the ‘QRS’ complex.
• The ‘T’ wave of ECG is associated with the return
of ventricular mass to its resting electrical state –
‘repolarization’.
Normal EKG
P wave
• Cause: atrial depolarization
• Configuration: upright and round
• Abnormalities:
Absent- SA nodal block, A.Fibrillation and
hyperkalemia
Wide and notched- Left atrial enlargement
Tall and peaked- Right atrial enlargement
(inTricuspid Stenosis)
QRS complex
• Q – Septal depolarization
• R – Depolarisation of ant part of ventricle
• S – Depolarisation of base of ventricle
Abnormalities:
Deep Q wave – sign of Myocardial ischemia
Tall R wave – ventricular hypertrophy
Low voltage QRS complex – hypothyroidism, pericardial
effusion
Wide bizarre QRS complex – ventricular fibrillation
T wave
• Cause- repolarization of ventricles
• Configuration- upright and rounded
• Abnormalities;
Inverted T wave- Myocardial ischemia
young children
Tall and peaked- Myocardial infarction,
hyperkalemia
U Wave
• Small rounded, upright wave following T wave;
• Due to slow repolarisation of papillary
muscles
• Prominent in hypokalemia
Intervals
• PR Interval - Distance between beginning of P wave
and beginning of QRS complex;
• Measures time during which a depolarization wave
travels from the atria to the ventricles;
• It measures the AV conduction time including AV
conduction delay
• Normal: 0.12–0.20 sec
• Abnormalities;
Incr PR – AV conduction block, ischemic heart disease,
rheumatic fever
Dec PR – AV nodal rhythm, WPW syndrome
Intervals
• QT Interval - Measured from beginning of QRS to
end of T wave;
• Represents total ventricular activity.(entire
duration of depolarization and repolarization of
the ventricular muscles)
• Normal: less than or equal to 0.4 sec
• ST Segment – It is an isoelectric period
between the end of QRS complex and
beginning of T wave.
• Measures time between complete ventricular
depolarization and beginning of repolarization
• Abnormalities –
• Depressed in acute myocardial ischemia
• Elevated in myocardial infarction
ST segment elevation and depression
WAVE/INTERVAL/SEGMENT DURATION (sec)
P WAVE
0.08 - 0.10
P-R INTERVAL (onset of P to 0.12 - 0.20
onset of QRS)
QRS COMPLEX
0.06 - 0.10
T WAVE
VARIABLE
Q-T INTERVAL (from onset
of QRS to end of T wave)
0.30 - 0.40*
S-T SEGMENT
EVENT
Atrial depolarisation
Conduction of
impulse from the
atria to the ventricles
Ventricular
depolarization
Ventricular
repolarization
Ventricular
depolarization and
repolarization
Ventricular
depolarization
Recording an ECG
• The electrical signal from the heart is detected at the
surface of the body through electrodes attached to
the surface.
• As a result of the sequence and the timing of the
spread of depolarization and repolarization in the
myocardium, potential differences are established
between different portions of the heart, which can
be detected by electrodes placed on the body
surface
• The ECG recorder compares the electrical activity
detected in different electrodes and the electrical
picture so obtained is called a ‘lead’.
Requirements
• ECG machine
• Cardiac jelly – to establish proper contact of the
electrode plates to the body
• ECG paper
• ECG leads
ECG PAPER
• Heat sensitive plastic coated paper
• Has vertical (amplitude) & horizontal (time) lines 1mm
apart
• Heavy line every 5mm in both planes
• Conventional speed – 25mm/sec (5 big squares)
• Small square = 0.04sec
• Big square = 0.20sec
• Sensitivity – Voltage is measured along vertical axis
• 1mV = 10mm (2 big squares)
Standard ECG Paper Grid
ECG leads
• Direct leads
• Indirect leads
– Limb leads – Unipolar & Bipolar (record electrical
potentials transmitted onto the frontal plane)
– Chest leads – Unipolar (record electrical potentials
transmitted onto the transverse plane)
– Esophageal leads
The 12 lead ECG
• Has 6 limb leads and 6 chest leads
Limb Leads
• Electrodes are placed on the right arm (RA), left
arm (LA), right leg (RL), and left leg (LL). With
only four electrodes, six leads are viewed.
■ Standard leads: I, II, III
■ Augmented leads: aVR, aVL, aVF
BIPOLAR LIMB LEADS
• Einthoven’s leads to record electrical potential on the
frontal plane
• Two similar electrodes are placed on the body surface &
the potential difference between them are recorded
BIPOLAR LIMB LEADS
• Lead I – Between the right
arm (-) & the left arm (+)
• Lead II - Between the right
arm (-) & the left leg (+)
• Lead III - Between the left arm
(-) & the left leg (+)
• Right leg acts as ground wire
to prevent external disturbance
AUGMENTED UNIPOLAR LIMB LEADS
• One limb is the active
electrode & the other 2
electrodes are connected
to zero potential through
5000 ohms resistance
(indifferent electrode)
• aVR- RIGHT ARM ACTIVE
• aVL- LEFT ARM ACTIVE
• aVF- LEFT FOOT ACTIVE
UNIPOLAR CHEST LEADS
• Exploring electrode on the chest V1 – V6
• Reference electrode is connected to the right arm, left
arm & the left leg through a high resistance – Wilson’s
terminal
• Right leg – grounding electrode
UNIPOLAR CHEST LEADS
PROCEDURE
• Make the subject to lie down on a couch comfortably
• Clean the skin over both wrists & both legs above the ankle joint &
apply jelly
• Connect the leads in these positions
• Switch on the machine & place the stylus at the centre of the paper
• Adjust the sensitivity to get a standard calibration
• Adjust the lead selector in order – I, II, III, aVR, aVL, aVF & record
• Place the chest electrodes V1 – V6 & record
ANATOMY OF THE WAVES
• In the ECG, each wave has
either a positive or a negative
deflection
• When the current flows
towards the electrode, a
positive deflection is recorded
• When the current flows away
from the electrode, a negative
deflection is recorded
HEART RATE
At a speed of 25mm/sec (1500mm/min),
Ventricular rate = 1500/ RR interval in mm
1500/no of small squares
(or)
300/no of big squares
Uses
A very useful noninvasive, inexpensive and versatile
diagnostic tool but must be interpreted with the
clinical features of the patient and other
investigations
–Extent, location and progress of ischaemic
damage to the myocardium
–Relative sizes of the chambers
–Disturbances of rhythm and conduction
–Effects of altered electrolyte concentration
–Anatomical orientation of the heart
Thank you