Electrocardiography

RK Goit, LecturerDepartment of Physiology

Electrocardiogram

• 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

• 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

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

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

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

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)

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)

• 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

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

Abnormalities of rhythm• Tachycardia– fast heart rate (>100 bpm)

• Bradycardia– slow heart rate (<60 bpm)

• Sinus arrhythmia– ↑ heart rate during inspiration & ↓ during expiration

• Arterial extrasystole– an abnormal focus of impulse generation in atria may discharge

sporadically giving atrial extrasystoles & corresponding P waves in the ECG

• 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

• 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

• 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.

• Ventricular fibrillation– due to extremely high frequency discharge of an abnormal

focus of impulse generation in the ventricles

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/

Thank you