MEDICAL DOCTOR (MD) PROGRAMME PHASE I

CARDIOVASCULAR SYSTEM COURSE (GMT 107)

PHYSIOLOGY PRACTICAL
ELECTROCARDIOGRAPHY (ECG)

INTRODUCTION
The electrical potential generated by the cardiac muscle can be detected at the surface of
the body and a record of such potential is called electrocardiogram (ECG or EKG). The
machine used for recording this potential is called an electrocardiograph (abbreviated also
ECG or EKG).
The electrocardiograph is essentially an electronic device that amplifies the very small
potentials present at the surface of the body. These potentials can be displayed on a video
screen or recorded permanently on a paper. The potential is picked up by electrodes placed
at the specified anatomical sites on the body surface. The electrical activity of the heart
causes potential difference between two electrodes. Potential difference is very minute
(about 1 mV) and hardly causes a deflection of a tracer. Therefore, the potential differences
are amplified before they are recorded on a moving strip of paper.
By convention when current flows toward positive electrode, an upward deflection known as
"positive wave" is produced. When current flows away from positive electrode, a downward
deflection known as "negative wave" is produced. A wave with both upward and downward
deflections (in any sequence, without an interval between the two deflections) is known as
“biphasic wave”. A baseline segment (no deflection) is known as "isoelectric segment”.

The Standard ECG Leads.

A. Bipolar Leads – 3 leads.
These are so called because they actually compare the electrical activity between two
parts of the body (compare with other leads below). They are all limb leads because the
electrodes are placed on the limbs.
The three leads are
1. Limb lead I (+ ve electrode on left forearm)
(- ve electrode on right forearm)
2. Limb lead II (+ ve electrode on left leg)
(- ve electrode on right forearm)
3. Limb lead III (+ ve electrode on left leg)
(- ve electrode on left forearm)

B. Unipolar Leads (3 unipolar limb leads + 6 unipolar chest leads)

These leads measure the potential difference between a given point in the body and a
zero point (Indifferent electrode) artificially created by connecting three electrodes placed
at the three corners of an equilateral triangle with the heart in the middle. In practice the
three electrodes are placed on the right forearm, left forearm and the left leg. The
imaginary triangle formed by joining these three points to each other is an equilateral
triangle. This concept was first introduced by Einthoven and this triangle is referred to as
the Einthoven's triangle.
 a. Unipolar Limb Leads – 3 leads.
These are recorded with positive electrodes on the right arm, left arm or left leg. The
negative electrode is connected to the other two limbs. This increases the size of the
potential by 50% without any changes in the configuration from the non-augmented
leads. These are called augmented limb leads and the small letter 'a' denotes this
(aVR, aVL, aVF).
The augmented limb leads are:
1. aVR (+ ve electrode on right forearm)
(- ve electrode on left arm + left leg)
2. aVL (+ ve electrode on left forearm)
(- ve electrode on right arm + left leg)
3. aVF (+ ve electrode on left leg)
(- ve electrode on right arm + left arm)

b. Unipolar Chest Leads – 6 leads

Here the positive electrode is placed on a given point on the anterior chest wall. The
negative electrode is the original indifferent point obtained by joining three limb
electrodes together.

LEARNING OUTCOMES
At the end of this practical, students should be able to

1. know the method of recording a standard electrocardiogram with 12 leads.

2. identify and label the various waves and intervals.

3. calculate the duration of the cardiac cycle, heart rate, duration of all the intervals and
waves as well as voltages of the waves.
4. relate the various waves and segments of the ECG to the electrical and mechanical
activities of the heart.
EQUIPMENT
1. A standard 12-lead
electrocardiograph
2. Electrodes
3. Electrode jelly
4. ECG paper
5. Couch and screen

Figure 7.1: Electrocardiograph

ECG Paper
A standard strip of paper specially made for recording ECG is used in electrocardiography
and is marked with thick and thin lines horizontally and vertically. The thick lines are drawn at
5mm intervals. The distance between two adjacent thick lines is divided into five spaces of
1mm. each by thin lines (placed at 1mm intervals).
The speed of the movement of paper in the machine is also standardized. For normal
recording a paper speed of 25mm per second is used. At this speed the distance between
two thick lines (5mm) represents 0.2 seconds and the distance between two thin lines (1mm)
represents 0.04 sec.
Most machines are equipped with a speed selection switch and faster speed of 50mm/sec. is
available for use if necessary.

Figure 7.2: ECG recording paper.

 Figure 7.3: Calibration and ECG recording.

METHODS
A. Subject Preparation
The subject should ideally be lying down comfortably in the supine position. Muscle electrical
activity can interfere with the recording of the ECG and this is minimized if the subject is lying
down. However, if necessary the recording may be made with the subject in the sitting or
standing position. The top half of the body and the distal parts of the limbs must be bare.
Metal chains, bracelets and wrist watches are best removed.

B. Leads Placement
Limb leads - place the electrodes on the right forearm, left forearm, right leg and left leg.

Chest leads - place the electrodes in the appropriate places for V1 to V6 and connect them
to the six wires.

Figure 7.4: Placement of electrode for chest leads.

The chest leads are:-

1. V1 (+ ve electrode on 4th. RICS, sternal margin)
2. V2 (+ ve electrode on 4th. LICS, sternal margin)
3. V3 (+ ve electrode midway between V2 and V4)
 4. V4 (+ ve electrode on 5th. LICS - mid clavicular line)
5. V5 (+ ve electrode on 5th. LICS, anterior axillary line)
6. V6 (+ ve electrode on 5th. LICS - mid axillary line)

(R = right, L = left and ICS = Intercostal space).

C. Recording ECG
When all the electrodes are properly positioned and connected to the electrocardiograph,
recording can be started. Machines will record all 12 leads automatically when the start
button is pressed. ECG machines will also generate a calibration mark while recording.
Now you have recorded a full 12-lead ECG. Record a rhythm strip from lead II for 30
seconds. Record the name and age of the subject as well as date and time of recording on
the ECG strip.

D. Calculation of Heart Rate

Paper speed = 25mm/sec

1. If the QRS complexes are at regular interval;

Heart rate = 300
Total num. of large squares between RR

OR = 1500
Total num. of small squares between RR

2. If the QRS complexes are at irregular interval;

Calculate how many QRS complexes in 6 seconds (30 big boxes) = A
Heart rate = A x 10
QUESTIONS
1. Please attach ECG sample used for analysis.

2. Calculate the heart rate and identify P wave, QRS complex and T wave on the ECG
obtained.
3. Comment on the rhythm of the heart.
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4. What is sinus arrhythmia?

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5. Relate the waves and complexes to the electrical and mechanical events of the heart.

Electrical Mechanical
events events
P wave
QRS complex
T wave

6. Calculate the duration of each wave or wave complex and segment. Compare with the
normal ranges.

Duration Normal Range

P wave
QRS complex
PR interval
QT interval
7. Describe the normal shape of the different waves and complexes (in lead II).

Normal Shape
P wave
QRS complex
PR segment
ST segment