ELECTROCARDIOGRAM (ECG)ELECTROCARDIOGRAM (ECG)

Prof. Sultan Ayoub MeoMBBS, 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 CONDUCTIONNORMAL 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 ECGThe 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

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

QTPR

0.12-0.2 s approx. 0.44 s

Atrial muscledepolarization

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)

QTPR

0.12-0.2 s approx. 0.44 s

Atrial muscledepolarization

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

QTPR

0.12-0.2 s approx. 0.44 s

Atrial muscledepolarization

Ventricular muscledepolarization

P

Q

QRS

S

T

QT interval: Ventricular depolarization & re-polarization (equal . 0.4 seconds

QTPR

0.12-0.2 s approx. 0.44 s

Atrial muscledepolarization

Ventricular muscledepolarization

Ventricular muscle repolarization

P

Q

R

S

T

ST segment:(QT – QRS) Ventricular depolarization (equal . 0.32 seconds )

QTPR

0.12-0.2 s approx. 0.44 s

Atrial muscledepolarization

Ventricular muscledepolarization

Ventricular musclerepolarization

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)1. Two different points on the body (bipolar leads)

2. One point on the body and a virtual reference point 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of with zero electrical potential, located in the center of the heart (unipolar leads)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 The axis of a particular lead represents the viewpoint from

which it looks at the heart.which it looks at the heart.

STANDARD LIMB LEADS

STANDARD LIMB LEADS AUGMENTED LIMB LEADS

ALL LIMB LEADS

PRECORDIAL LEADS

SUMMARY OF LEADS

Limb LeadsLimb Leads Precordial LeadsPrecordial Leads

BipolarBipolar I, II, IIII, II, III(standard limb leads)(standard limb leads)

--

UnipolarUnipolar aVR, aVL, aVF aVR, aVL, aVF (augmented limb leads)(augmented limb leads)

VV11-V-V66

ARRANGEMENT OF LEADS ON THE ECG

ANATOMICAL REPRESENTATION(SEPTUM)

ANATOMIC PRESENTATION(ANTERIOR WALL)

ANATOMICAL PRESENTATION(LATERAL WALL)

ANATOMICAL PRESENTATION (INFERIOR WALL)

ANATOMICAL PRESENTATION(SUMMARY)

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.

DETERMINING THE HEART RATE

1. Rule of 3002. 10 Second Rule

WHAT IS THE HEART RATE?

(300 / 6) = 50 bpm

(300 / 4) = 75 bpm

(300 / 1.5) = 200 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/

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

ST Elevation - Acute MI or AnginaST depression >1 mm - Ischemia/Angina (flat), digoxin (sloping)Q waves in 2 or more leads - Previous MI (Transmural)Diffuse ST elevation with PR depression – PericarditisT 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 hypertrophyBroad (and often bifid) P waves - Left atrial hypertrophyPeaked T waves or loss of P wave – HyperkalemiaU waves - Hypokalemia ('Hump' at the end of T wave)Prolonged QT interval – HypocalcemiaShortened QT interval - Hypercalcemi

Clinical Significance of different waves and segments of ECG