The document describes how an electrocardiogram (ECG) works by placing electrodes on the skin to record the electrical activity of the heart over time. 10 electrodes are placed on the chest, arms, and legs to measure voltage fluctuations between different electrode pairs. The ECG machine records these voltages on paper or a screen to produce a tracing showing the P, QRS, and T waves that make up the cardiac cycle and provide information about heart rate and rhythm and signs of conditions like heart attacks. Key intervals measured include the P-R, Q-T, T-P, and P-P intervals which help evaluate conduction delays and other cardiac issues.

2.  It refers to extracellular recording of the summed up
electrical events of all the cardiac muscle fibres
generated with each other.

3.  10 electrodes in total are placed on the patient
 Firstly self-adhesive ‘dots’ are attached to the
patient. These have single electrical contacts on
them
 The 10 leads on the ECG machine are then clipped
onto the contacts of the ‘dots’

4.  ECG lead refers to the two electrodes which are placed
on the body surfaces and connected to ECG machine
for measuring the potential fluctuations between only
points.
 6 are chest electrodes(called V1-6)
 4 are limb electrodes
I. Right arm(red)
II. Left arm(yellow)
III. Right leg(black)
IV. Left leg (green)
THE RIGHT LEG ELECTRODE IS A NEUTRAL(DUMMY)

5.  Chest electrodes
 V1- in the right 4th intercostal space(ICS), just near
the sternum
 V2- in the left 4th ICS, just near the sternum
 V3- halfway between V2 and V4
 V4- in the left 5th ICS at mid clavicular line
 V5- in the left 5th ICS at anterior axillary line
 V6- in the left 5th ICS at mid-axillary line

6.  Chest electrodes
 These are unipolar
leads where one
electrode is an active
electrode and the
other is an
indifferent electrode
at zero potential.

7.  Limb leads
 These are bipolar leads where both the electrodes are
active and one of the active electrode is connected to
negative terminal of ECG machine and the other to the
positive terminal.

8.  Three standard limb leads used in the bipolar
recording are based on Einthoven’s assumption that
the body is like an electrically homogenous plate in
which the right shoulder, left shoulder and the pubic
region form the corners of an equilateral triangle with
heart at the centre of the triangle(Einthoven’s triangle)
and the two active electrodes need to be placed at the
two corners of this triangle.

10.  In three standard limb leads, the two active electrodes
are connected as
 Lead I is formed using the right arm electrode (red) as
the negative electrode and the left arm (yellow)
electrode as the positive .
 Lead II is formed using the right arm electrode (red) as
the negative electrode and the left leg electrode as the
positive .
 Lead III is formed using the left arm electrode as the
negative electrode and the left leg electrode as the
positive

11.  aVL, aVF, leads, aVF , and aVR are composite leads
computed using the information from the other leads.
 These are unipolar limb leads

12.  Limb leads
 Limb leads look at the heart in the coronal plane
 aVL, , I and II = lateral
 II, III and aVF = inferior
 aVR = right side of the heart

13.  Each lead can be thought of as ‘at’ looking at an area of
myocardium ..
Chest leads
 V1 to V6 ‘look’ at the heart on the transverse plain
 V1 and V2 look at the anterior of the heart and Right
ventricle
 V3 and V4 = anterior and septal
 V5 and V6 = lateral and left ventricle

14.  AKA ECG machine
 It is essentially a sophisticated string galvanometer.
 Special paper is used which turns black on exposure to
heat.
 The recording pen is made hot by the current flowing
through its tip.

15.  It refers to the record of the potential fluctuations
during the cardiac cycle
 Cardiac cycle
 Systole –muscles contract, blood exits heart
 Diastole- muscles relax, blood enters heart

16. Height
 10mm = 1mV
 Look for a reference pulse which should be the
rectangular looking wave somewhere near the left
of the paper. It should be 10mm (10 small squares)
tall
Paper speed
 25mm/s
 25 mm (25 small squares / 5 large squares) equals
one second

17. Configuration of a typical electrogram from a bipolar
limb lead II is described…
P WAVE
 Configuration- positive deflection
 Cause – depolarisation of atrial muscles
 Duration – not more than 0.1 seconds
 Amplitude- o.1 to 0.12mV
 Clinical significance- it is a guide to functional activity
of atria.
 Larger and prolonged in mitral stenosis and tall and
peaked with normal duration in tricuspid stenosis

19.  Configuration – Q wave is a small negative wave, it is
continued as a tall positive R wave which is followed by
a small negative S wave.
 Cause – ventricular depolarisation
 Duration – less than 0.08s.
 Amplitude of Q wave is 0.1-0.2mV, R wave is 1.0mV and
S wave is 0.4 mV(Total – 1.5-1.6mV)
 Deep Q wave is one of the important signs of
myocardial infarction(heart attack)
 Tall R wave is seen in ventricular hypertrophy(increase
in size)

20.  Configuration – it is the last, positive, dome shaped
deflection
 Cause – ventricular repolarisation
 Duration – 0.27 secs
 Amplitude – 0.3mV
 Inverted T wave is an important sign of myocardial
infarction
 Tall and prolonged T wave occurs in
hyperkalemia(high potassium in blood)

21. P-R interval
 Measured from the onset of P wave to the onset of QRS
complex.
 It measures the AV conduction time, including the AV
nodal delay.
 Its duration varies from 0.12 to 0.21s depending on the
heart rate.
 Significance- prolonged P-R interval indicates
Atrioventricular conduction block.

22. QT interval
 It is the time from the start of QRS complex to the end
of T wave.
 Duration- 0.4s
 Indicates total systolic time of ventricles i.e.
ventricular depolarisation and repolarisation.
 Clinical significance- ischamia(death of cells due to
lack of oxygen) and ventricular conduction defects
prolong the QT interval.
 In hypocalcaemia also the QT interval is prolonged.

23. TP interval
 Measured from end of T wave to the beginning of P
wave.
 It measures the diastolic(filling) period of the heart.
 Variable TP interval indicates AV(atrioventricular)
dissociation.

24. P-P interval
 Interval between two successive P waves.
 Equal P-P intervals indicate rhythmic depolarisation
of the atria.
ST segment
 It is an isoelectric period between the end of the QRS
complex and the beginning of T waves.
 Duration- 0.04-0.08s
 Corresponds with ventricular repolarisation.
 Significance- it is elevated in the patients with
myocardial infarction.