ELECTROCARDIOGRAM (ECG) Prof. Sultan Ayoub Meo MBBS, M.Phil, Ph.D (Pak), PG Dip Med Ed, M Med Ed (Scotland) FRCP (London), FRCP (Dublin), FRCP (Glasgow), FRCP (Edinburgh) Professor and Consultant, Department of Physiology, College of Medicine, King Saud University, Riyadh, Saudi Arabia

LECTURE OBJECTIVES Define ECG Identify waves of ECG and the physiological cause of each Define the normal intervals and segments Discuss the bipolar and un ipolar leads and their locations Discuss the bipolar limb lead and the cardiac axis

INTRODUCTION Cardiac impulse passes through the heart, electrical current spreads from heart into adjacent tissues surrounding the heart. Current spreads all the way to the surface of the body. If electrodes are placed on the chest, opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram A record of the waves (impulses) of electrical excitation in the heart is called ECG. It helps in the diagnosis of muscle damage or electrical problems in the heart.

NORMAL IMPULSE CONDUCTION Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers

TYPES OF PATHOLOGY IDENTIFY AND STUDY FROM ECGS? Arrhythmias Myocardial ischemia and infarction Pericarditis Chamber hypertrophy Electrolyte disturbances (i.e. hyperkalemia, hypokalemia) Drug toxicity (i.e. digoxin and drugs which prolong the QT interval)

NORMAL ECG The normal ECG is composed of: P wave QRS complex T wave The separate waves: Q wave, the R wave, and the S wave. P wave: Atria depolarize before atrial contraction begins QRS complex: Ventricles depolarize before contraction. P wave and the components of the QRS complex are depolarization waves. The T wave: Ventricles recover from the state of depolarization This process normally occurs in ventricular muscle 0.25 to 0.35 second after depolarization and the T wave is known as a repolarization wave

The ECG Paper

Atrial depolarization (completed in 0.1 seconds ) P wave: Atrial depolarization (completed in 0.1 seconds ) Irregular or absent P waves may indicate arrhythmia. The shape of the P waves may indicate atrial problems QT PR 0.12-0.2 s approx. 0.44 s Atrial muscle depolarization P Q R S T

PR interval: Atrial depolarization & conductive time Measured from the P wave to the beginning of the QRS complex . Through the AV node /0.18 seconds (N. R. 0.12 to 0.2 s) QT PR 0.12-0.2 s approx. 0.44 s Atrial muscle depolarization P Q R S T

QRS complex: Ventrical depolarization Equal 0.08-0.12 s (max time 0.1seconds) Very wide and deep Q waves indicate myocardial infarction QT PR 0.12-0.2 s approx. 0.44 s Atrial muscle depolarization Ventricular muscle P Q QRS S T

Ventricular depolarization & re-polarization (equal . 0.4 seconds QT interval: Ventricular depolarization & re-polarization (equal . 0.4 seconds QT PR 0.12-0.2 s approx. 0.44 s Atrial muscle depolarization Ventricular muscle Ventricular muscle repolarization P Q R S T

(QT – QRS) Ventricular depolarization (equal . 0.32 seconds ) ST segment: (QT – QRS) Ventricular depolarization (equal . 0.32 seconds ) QT PR 0.12-0.2 s approx. 0.44 s Atrial muscle depolarization Ventricular muscle Ventricular muscle repolarization P Q R S T

ECG LEADS Leads are electrodes which measure the difference in electrical potential between either: 1. Two different points on the body (bipolar leads) 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads)

ECG LEADS The standard ECG has 12 leads: 3 Standard Limb Leads 3 Augmented Limb Leads 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart.

STANDARD LIMB LEADS

STANDARD LIMB LEADS AUGMENTED LIMB LEADS

ALL LIMB LEADS

PRECORDIAL LEADS

aVR, aVL, aVF (augmented limb leads) SUMMARY OF LEADS Limb Leads Precordial Leads Bipolar I, II, III (standard limb leads) - Unipolar aVR, aVL, aVF (augmented limb leads) V1-V6

ARRANGEMENT OF LEADS ON THE ECG

ANATOMICAL REPRESENTATION (SEPTUM)

ANATOMIC PRESENTATION (ANTERIOR WALL)

ANATOMICAL PRESENTATION (LATERAL WALL)

ANATOMICAL PRESENTATION (INFERIOR WALL)

ANATOMICAL PRESENTATION (SUMMARY)

DETERMINING THE HEART RATE 1. Rule of 300 2. 10 Second Rule RULE OF 300 Take the number of “big boxes” between neighboring QRS complexes, and divide this into 300. The result will be approximately equal to the heart rate Although fast, this method only works for regular rhythms.

WHAT IS THE HEART RATE? (300 / 6) = 50 bpm (300 / 4) = 75 bpm

10 Second Rule As most ECGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the ECG and multiply by 6 to get the number of beats per 60 seconds. This method works well for irregular rhythms.

WHAT IS THE HEART RATE? 33 x 6 = 198 bpm The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/ 33 x 6 = 198 bpm

The QRS Axis The QRS axis represents the net overall direction of the heart’s electrical activity. Abnormalities of axis can hint at: Ventricular enlargement Conduction blocks (i.e. hemiblocks)

The QRS Axis By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°. -30° to -90° is referred to as a left axis deviation (LAD) +90° to +180° is referred to as a right axis deviation (RAD)

The QRS Axis

Clinical Significance of different waves and segments of ECG ST Elevation - Acute MI or Angina ST depression >1 mm - Ischemia/Angina (flat), digoxin (sloping) Q waves in 2 or more leads - Previous MI (Transmural) Diffuse ST elevation with PR depression – Pericarditis T wave inversions and non-specific ST changes - Can be seen both in normal cases and in many diseases, therefore not useful for diagnosis. Tall P waves - Right atrial hypertrophy Broad (and often bifid) P waves - Left atrial hypertrophy Peaked T waves or loss of P wave – Hyperkalemia U waves - Hypokalemia ('Hump' at the end of T wave) Prolonged QT interval – Hypocalcemia Shortened QT interval - Hypercalcemi