electrocardiography dr. shafali singh electrocardiography objective: to define ecg genesis of ecg...

Electrocardiography

Dr. Shafali Singh

Electrocardiography

Objective:

• To define ECG • Genesis of ECG• Identify different waves of ECG with their

causes

Introduction

Electrocardiography is the method of recording an

Electrocardiogram (ECG) with the help of the machine Electrocardiograph.

Definition

Electrocardiogram (ECG) is a graphic recording of the electrical changes that occur within the heart during the cardiac cycle.

Characteristics of SA Nodal Cells

SA Nodal (Pacemaker) Action Potential

conduction of cardiac impulse

Conduction system of the heart: SA node is the pacemaker.

Conduction Pathways and Velocity of Conduction

Pathway• SA node → atrial muscle → AV node (delay) →

Purkinje fibers → ventricular muscle

Velocity• Fastest conducting fiber = Purkinje cell• Slowest conducting fiber = AV node

Which of the following has the fastest conducting fibers in the heart?

a. SA nodeb. Atrial musclec. AV noded. Purkinje fiberse. Ventricular muscle

ORIGIN & SPREAD OF CARDIAC IMPULSE

• Correlation of ECG with phases of action potential

Principle

Electrical activities generated with each heart beat are conducted from the heart to the body surface, which are picked up and recorded by the electrocardiograph.

• The muscle mass of the atria is small compared with that of the ventricles, and the electrical change accompanying the contraction of the atria is therefore small. Contraction of atria is associated with the ECG wave called ‘P’.

• The ventricular mass is large and so there is a large deflection of the ECG wave when ventricle is depolarised. This is called the ‘QRS’ complex.

• The ‘T’ wave of ECG is associated with the return of ventricular mass to its resting electrical state – ‘repolarization’.

Normal EKG

P wave• Cause: atrial depolarization• Configuration: upright and round• Abnormalities: Absent- SA nodal block, A.Fibrillation and hyperkalemiaWide and notched- Left atrial enlargementTall and peaked- Right atrial enlargement

(inTricuspid Stenosis)

QRS complex

• Q – Septal depolarization• R – Depolarisation of ant part of ventricle• S – Depolarisation of base of ventricleAbnormalities:Deep Q wave – sign of Myocardial ischemiaTall R wave – ventricular hypertrophyLow voltage QRS complex – hypothyroidism, pericardial

effusionWide bizarre QRS complex – ventricular fibrillation

T wave• Cause- repolarization of ventricles• Configuration- upright and rounded• Abnormalities;Inverted T wave- Myocardial ischemia young childrenTall and peaked- Myocardial infarction, hyperkalemia

U Wave

• Small rounded, upright wave following T wave; • Due to slow repolarisation of papillary muscles• Prominent in hypokalemia

Intervals

• PR Interval - Distance between beginning of P wave and beginning of QRS complex;

• Measures time during which a depolarization wave travels from the atria to the ventricles;

• It measures the AV conduction time including AV conduction delay

• Normal: 0.12–0.20 sec• Abnormalities;Incr PR – AV conduction block, ischemic heart

disease, rheumatic feverDec PR – AV nodal rhythm, WPW syndrome

Intervals

• QT Interval - Measured from beginning of QRS to end of T wave;

• Represents total ventricular activity.(entire duration of depolarization and repolarization of the ventricular muscles)

• Normal: less than or equal to 0.4 sec

• ST Segment – It is an isoelectric period between the end of QRS complex and beginning of T wave.

• Measures time between complete ventricular depolarization and beginning of repolarization

• Abnormalities –• Depressed in acute myocardial ischemia• Elevated in myocardial infarction

ST segment elevation and depression

WAVE/INTERVAL/SEGMENT DURATION (sec) EVENT

P WAVE 0.08 - 0.10 Atrial depolarisation

P-R INTERVAL (onset of P to onset of QRS)

0.12 - 0.20 Conduction of impulse from the atria to the ventricles

QRS COMPLEX 0.06 - 0.10 Ventricular depolarization

T WAVE VARIABLE Ventricular repolarization

Q-T INTERVAL (from onset of QRS to end of T wave)

0.30 - 0.40* Ventricular depolarization and repolarization

S-T SEGMENT Ventricular depolarization

Recording an ECG

• The electrical signal from the heart is detected at the surface of the body through electrodes attached to the surface.

• As a result of the sequence and the timing of the spread of depolarization and repolarization in the myocardium, potential differences are established between different portions of the heart, which can be detected by electrodes placed on the body surface

• The ECG recorder compares the electrical activity detected in different electrodes and the electrical picture so obtained is called a ‘lead’.

Requirements

• ECG machine

• Cardiac jelly – to establish proper contact of the electrode plates to the body

• ECG paper

• ECG leads

ECG PAPER

• Heat sensitive plastic coated paper• Has vertical (amplitude) & horizontal (time) lines

1mm apart• Heavy line every 5mm in both planes• Conventional speed – 25mm/sec (5 big squares)• Small square = 0.04sec• Big square = 0.20sec• Sensitivity – Voltage is measured along vertical

axis• 1mV = 10mm (2 big squares)

Standard ECG Paper Grid

ECG leads

• Direct leads• Indirect leads– Limb leads – Unipolar & Bipolar (record electrical

potentials transmitted onto the frontal plane)

– Chest leads – Unipolar (record electrical potentials transmitted onto the transverse plane)

– Esophageal leads

The 12 lead ECG• Has 6 limb leads and 6 chest leads

Limb Leads• Electrodes are placed on the right arm (RA), left

arm (LA), right leg (RL), and left leg (LL). With only four electrodes, six leads are viewed.

■ Standard leads: I, II, III ■ Augmented leads: aVR, aVL, aVF

BIPOLAR LIMB LEADS

• Einthoven’s leads to record electrical potential on the frontal plane

• Two similar electrodes are placed on the body surface & the potential difference between them are recorded

BIPOLAR LIMB LEADS• Lead I – Between the right

arm (-) & the left arm (+)

• Lead II - Between the right arm (-) & the left leg (+)

• Lead III - Between the left arm (-) & the left leg (+)

• Right leg acts as ground wire to prevent external disturbance

AUGMENTED UNIPOLAR LIMB LEADS

• One limb is the active electrode & the other 2 electrodes are connected to zero potential through 5000 ohms resistance(indifferent electrode)

• aVR- RIGHT ARM ACTIVE

• aVL- LEFT ARM ACTIVE

• aVF- LEFT FOOT ACTIVE

UNIPOLAR CHEST LEADS

• Exploring electrode on the chest V1 – V6

• Reference electrode is connected to the right arm, left arm & the left leg through a high resistance – Wilson’s terminal

• Right leg – grounding electrode

UNIPOLAR CHEST LEADS

PROCEDURE

• Make the subject to lie down on a couch comfortably

• Clean the skin over both wrists & both legs above the ankle joint & apply jelly

• Connect the leads in these positions

• Switch on the machine & place the stylus at the centre of the paper

• Adjust the sensitivity to get a standard calibration

• Adjust the lead selector in order – I, II, III, aVR, aVL, aVF & record

• Place the chest electrodes V1 – V6 & record

ANATOMY OF THE WAVES

• In the ECG, each wave has either a positive or a negative deflection

• When the current flows towards the electrode, a positive deflection is recorded

• When the current flows away from the electrode, a negative deflection is recorded

HEART RATE At a speed of 25mm/sec (1500mm/min),

Ventricular rate = 1500/ RR interval in mm

1500/no of small squares (or)

300/no of big squares

Uses

A very useful noninvasive, inexpensive and versatile diagnostic tool but must be interpreted with the clinical features of the patient and other investigations

–Extent, location and progress of ischaemic damage to the myocardium–Relative sizes of the chambers

–Disturbances of rhythm and conduction

–Effects of altered electrolyte concentration

–Anatomical orientation of the heart

– Influence of drugs

Thank you