ECG – electrocardiography Enas Sabah MSc. Human Physiology

ECG – electrocardiography It is the recording of cardiac electrical activity through placing electrodes on the surface of the body. Waveforms are caused by change in the voltage measured between 2 points on the body.

Normal components of ECG: waveforms P wave represents atrial depolarization (normal range = 0.12 sec) QRS complex ventricular depolarization (normal range = 0.08 sec) T wave represents ventricular repolarization (normal range = 0.3 – 0.35 sec) Note : Depolarization wave = positive charge

intervals PR interval, measured from the beginning of the P wave to the first deflection of the QRS complex. (normal range = 0.12-0.2 sec) QT interval, measured from first deflection of QRS complex to end of T wave (normal range = 0.4 sec) Each small box = 0.04 sec For example the no. of small boxes in the pervious ECG = 5 The PR interval = 0.04 x 5 = 0.2 For testing rhythm: by measuring the distance between successive R waves (regular rhythm means equal distance).

Placement of the ECG leads chest leads The chest leads are attached to the skin over the intercostal spaces as follow: V1 over 2nd right intercostal space V2 over 2nd left I.C.S V3 in between V2 and V4 V4 over 5th I.C.S at midclavicular line V5 over 5th I.C.S at ant. axillary line V6 over 5th I.C.S at mid-axillary line

R L

2. The limb leads: The augmented limb leads (aVR, aVL, aVF) and the leads ( I, II, III) aVR placed on the right arm aVL placed on the left arm aVF placed on left foot The lead I is between the right arm and the left arm. The lead II is between right arm and left foot. The lead III is between left arm and left foot.

Uses of ECG: To find the heart rate (300 divided by the number of large squares in between two successive R waves when the heart rate is regular). For finding cardiac arrhythmias like (Ventricular tachycardia, Heart block) For finding cardiac ischemia ( impairment of coronary blood flow) and infarction Finding electrolyte disturbance Myocarditis and cardiomyopathy Cardiac axis (vector)

conduction system of the heart

Cardiac vector: It is the direction of current flow that represents the net electrical potential flowing through the heart mass or (its the average direction of the flow of the electricity or depolarization through the heart) Normal cardiac axis is downwards towards the left in the direction of + 59 °. The normal range is from – 30° to +90°. If the cardiac axis lies outside this range it is called axis deviation.

Common causes of axis deviation: Right axis deviation right ventricular hypertrophy anterolateral myocardial infarction Left axis deviation inferior wall myocardial infarction Ventricular tachycardia