The electrocardiogram (ECG or EKG) records the electrical activity of the heart. The ECG machine detects electrical changes in the heart with each heartbeat that are conducted throughout the body and can be measured on the skin surface. The ECG provides important information about heart rate, rhythm, and the size and function of the heart's chambers that can help diagnose cardiovascular conditions like ischemia, myocardial infarction, and arrhythmias. ECG leads are attached at various locations on the limbs and chest to detect the heart's electrical signals from different perspectives.

1. RK Goit, Lecturer
Department of Physiology
Electrocardiogram

2. • Electrocardiography is the method of recording of an
electrocardiogram (ECG)
• ECG or EKG is the graphic recording of the electrical
activities of the heart
• Electrocardiograph is the machine that records the ECG
• It is an important diagnostic & prognostic tool for
assessment of cardiovascular function

3. • body fluids are good conductors of electricity
• electrical changes occurring in the heart with each heart
beat, are conducted all over the body & can be picked up
form the body surface
• ECG recorded at the body surface represents the
algebraic sum of the action potential of the individual
cardiac muscle fibers

5. The ECG is recorded to study the following parameters:
– Anatomical orientation of the heart
– Relative size of the chambers of the heart
– A variety of disturbances of the rhythm & conduction
– To detect ischemia of the myocardium, if present
– The location, extent & progress of myocardial infarction
– The effects of altered electrolyte concentration
– The influence of certain drugs like digitalis
– Evaluation of electrical pacemaker function

8. Requirements:
– ECG machine
– Cardiac jelly
– ECG paper
– ECG leads

9. ECG leads
• Direct leads
– Leads applied directly to the surface of the heart
– These leads are used to record cardiac activities during cardiac
surgery or during an experiment
• Indirect leads
– Leads applied away form the heart to record the cardiac
activities
– Different indirect leads are limb leads, chest leads

10. Limb leads (bipolar & unipolar)
• Bipolar limb leads
• Bipolar standard limb leads are original leads selected by
Einthoven to record electrical potential on frontal plane
• electrodes are attached to right arm, left arm, & left foot
– another electrode is applied to the right leg, which acts as a
ground wire to prevent external disturbance during recording
• Lead I: between right arm (negative electrode) & left arm (positive
electrode)
Lead II: between right arm (negative electrode) & left leg (positive
electrode)
Lead III: between left arm (negative electrode) & left leg (positive
electrode)

13. Augmented limb leads (unipolar limb leads)
• positive electrode is connected to one limb, & negative
electrode to the other two through high resistances
• There are three augmented limb leads
– aVR: between right arm (positive electrode) & left arm + left leg
(negative electrode)
– aVL: between left arm (positive electrode) & right arm + left leg
(negative electrode)
– aVF: between left leg (positive electrode) & right arm + left arm
(negative electrode)

15. • Unipolar chest leads
• six chest leads are used routinely; V1 to V6 (V7-V9)
• The reference electrode is connected to the right arm, left
arm & left leg through a high resistance.
• V1: in the right fourth intercostal space at the right border of the
sternum
• V2: in the left fourth intercostal space at the left border of the
sternum
• V3: at the midpoint between V2 & V4
• V4: in the left fifth intercostal space on the midclavicular line
• V5: in the left fifth intercostal space on the anterior axillary line
• V6: in the left fifth intercostal space on the mid-axillary line
• V7: in the left fifth intercostal space on the posterior axillary line
• V8: in the left fifth intercostal space on the posterior scapular line
• V9: in the left fifth intercostal space on the back just left to the
spine

18. Einthoven’s Triangle
• is drawn around the area of the heart
– this illustrates that the two arms & the left leg form apices of a
triangle surrounding the heart
Einthoven’s Law
• if the electrical potentials of any two of the three bipolar
limb electrocardiographic leads are known at any given
instant, the third one can be determined mathematically
by simply summing the first two

28. Abnormalities of rhythm
• Tachycardia
– fast heart rate (>100 bpm)
• Bradycardia
– slow heart rate (<60 bpm)
• Sinus arrhythmia
– ↑ heart rate during inspiration & ↓ during expiration

29. • Arterial extrasystole
– an abnormal focus of impulse generation in atria may discharge
sporadically giving atrial extrasystoles & corresponding P waves
in the ECG

30. • First degree block
– when the P-R interval ↑ 0.20s
• Second degree block
– when the P-R interval ↑ 0.35 to 0.45s
• Third degree block
– complete block of the impulse from the atria into the ventricles
occurs

31. • Atrial tachycardia
– This is similar to atrial extrasystoles except that the abnormal
focus generates impulses at a regular rate
• Atrial flutter
– causes a rapid rate of contraction of the atria, usually between
200 & 350 beats per minute
• Atrial fibrillation
– If the abnormal focus of impulse generation discharge at a rate
exceeding 300 per min

32. • Ventricular extrasystole
– It may occurs because a papillary muscle may fire an impulse
before normal impulse reaches the ventricles, leading to
premature contraction (extrasystole) of ventricles.

33. • Ventricular fibrillation
– due to extremely high frequency discharge of an abnormal
focus of impulse generation in the ventricles

34. References
• Ganong Review of Medical Physiology, 23/E
• Textbook of Medical Physiology, 12/E Guyton & Hall
• Understanding Medical Physiology, 4/E Bijlani &
Manjunatha
• http://www.n3wt.nildram.co.uk/ECG/

35. Thank you