The electrocardiogram The electrocardiogram (ECG or EKG)(ECG or EKG)

The electrocardiogram (ECG) is a record The electrocardiogram (ECG) is a record of the electrical activity of the heart from of the electrical activity of the heart from

the surface of the bodythe surface of the body..

The body tissues function as electrical The body tissues function as electrical conductors because they contain conductors because they contain

electrolyteselectrolytes..

* *If a recording electrode is applied on any point on If a recording electrode is applied on any point on the surface of the trunk, it will detect electrical the surface of the trunk, it will detect electrical waves reflecting the electrical activity in the heartwaves reflecting the electrical activity in the heart . .

* *A positive wave is recorded when depolarization is A positive wave is recorded when depolarization is propagating towards the electrode or when propagating towards the electrode or when repolarization is propagating away from the repolarization is propagating away from the

electrodeelectrode . . * *A negative wave is recorded when depolarization is A negative wave is recorded when depolarization is

propagating away from the electrode or when propagating away from the electrode or when repolarization is propagating towards the repolarization is propagating towards the

electrodeelectrode . . * *When there are no propagating potentials, no waves When there are no propagating potentials, no waves

are recorded and the recording needle will be on are recorded and the recording needle will be on the line of zero potential, which is called the the line of zero potential, which is called the isoelectric lineisoelectric line . .

The active and reference The active and reference electrodeselectrodes

For recording an ECG, two electrodes are For recording an ECG, two electrodes are requiredrequired..

One of the them is the active electrode (also One of the them is the active electrode (also called searching electrode or exploring called searching electrode or exploring electrode) which is applied to a recoding electrode) which is applied to a recoding

point on the surface of the bodypoint on the surface of the body..

The other is the reference electrode which The other is the reference electrode which serves a reference to the active electrodeserves a reference to the active electrode..

Recording points on the body Recording points on the body surfacesurface

By convention, there are nine standard By convention, there are nine standard points on the surface of the body from points on the surface of the body from

which an ECG should be recordedwhich an ECG should be recorded..

Six points are on the chest wall and the other Six points are on the chest wall and the other three points are on the limbsthree points are on the limbs . .

Chest pointsChest points

VV11: at the right fourth intercostals space near the : at the right fourth intercostals space near the sternumsternum..

VV22: at the left fourth intercostals space near the : at the left fourth intercostals space near the sternumsternum..

VV33: midway between V: midway between V22 and V and V44..

VV44: at the left fifth intercostals space at the : at the left fifth intercostals space at the midcalvicular linemidcalvicular line..

VV55: at the left fifth intercostals space at the anterior : at the left fifth intercostals space at the anterior axillary lineaxillary line..

VV66: at the left fifth intercostals space at the midaxillary : at the left fifth intercostals space at the midaxillary lineline . .

Chest pointsChest points

Chest pointsChest points

Limb pointsLimb points VL: at the junction of the left arm with the trunk. VL: at the junction of the left arm with the trunk. Any point on the left upper limb has the same Any point on the left upper limb has the same potentialpotential..

  VR: at the junction of the right arm with the VR: at the junction of the right arm with the trunk. Any point on the right upper limb has the trunk. Any point on the right upper limb has the same potentialsame potential..

  VF: at the junction of the left lower limb with the VF: at the junction of the left lower limb with the trunk. Any point on the left or right lower limbs trunk. Any point on the left or right lower limbs has the same potentialhas the same potential . .

Limb pointsLimb points

The ECG leadsThe ECG leads

A lead is the ECG record obtained A lead is the ECG record obtained when the recording electrodes are when the recording electrodes are

placed at specific points on the bodyplaced at specific points on the body..

Unipolar leadsUnipolar leads

These are the ECG records obtained when the reference These are the ECG records obtained when the reference electrode is at zero potential. The active electrode is electrode is at zero potential. The active electrode is applied to the recording points on the body surface. applied to the recording points on the body surface. So, the records are actually the vectors of these pointsSo, the records are actually the vectors of these points..

There are six standard unipolar chest leads recorded There are six standard unipolar chest leads recorded from the six standard chest points and designated as from the six standard chest points and designated as VV11, V, V

22, V, V33, V, V

44, V, V55 and V and V

66. There are another three . There are another three standard unipolar limb leads recorded from the standard unipolar limb leads recorded from the standard limb points and designated as aVL, aVR, standard limb points and designated as aVL, aVR, aVFaVF..

Unipolar Limb leadsUnipolar Limb leads

Unipolar Chest leadsUnipolar Chest leads

Bipolar leadsBipolar leads

These are the ECG records obtained when These are the ECG records obtained when the active electrode is applied to a the active electrode is applied to a recording point and the reference recording point and the reference

electrode is applied to another recording electrode is applied to another recording pointpoint..

The ECG will be a record of the changes in The ECG will be a record of the changes in electrical potential at the active electrode electrical potential at the active electrode

relative to the reference electroderelative to the reference electrode..

Lead I: records the potential between left arm Lead I: records the potential between left arm and right arm. The active electrode is at VL and right arm. The active electrode is at VL and the reference electrode is at VRand the reference electrode is at VR..

  

Lead II: records the potential between left leg Lead II: records the potential between left leg and right arm. The active electrode is at VF and right arm. The active electrode is at VF and the reference electrode is at VRand the reference electrode is at VR..

  

Lead III: records the potential between left leg Lead III: records the potential between left leg and left arm. The active electrode is at VF and left arm. The active electrode is at VF

and the reference electrode is at VLand the reference electrode is at VL . .

Bipolar Limb LeadsBipolar Limb Leads

-

+

-

+

Normal ECGNormal ECG

The normal ECG has five waves, The normal ECG has five waves, designated as P, Q, R, S, and T waves. designated as P, Q, R, S, and T waves. The shape and amplitude of the waves The shape and amplitude of the waves

differ in different leadsdiffer in different leads..

P wave:P wave: It is caused by atrial depolarization. Depolarization It is caused by atrial depolarization. Depolarization wave starts at the S-A node then moves to the atria. It is a wave starts at the S-A node then moves to the atria. It is a positive wavepositive wave..

  QRS complex:QRS complex: It is caused by ventricular depolarization It is caused by ventricular depolarization..

Q wave: Depolarization of the ventricular septum (direction of Q wave: Depolarization of the ventricular septum (direction of depolarization: left to right → -ve wave)depolarization: left to right → -ve wave)..

R wave: Depolarization of the ventricular wall. Depolarization R wave: Depolarization of the ventricular wall. Depolarization wave of the left ventricle predominates over that of the right wave of the left ventricle predominates over that of the right ventricle (direction of depolarization: right to left → +ve ventricle (direction of depolarization: right to left → +ve wave)wave)..

  T wave:T wave: It is caused by ventricular repolarization. (direction of It is caused by ventricular repolarization. (direction of

repolarization: left to right → + ve wave)repolarization: left to right → + ve wave) . .

Normal ECGNormal ECG

Electrical Activity in the HeartElectrical Activity in the Heart

The Normal EKG recorded on the Bipolar The Normal EKG recorded on the Bipolar Limb LeadsLimb Leads

Normal Normal Unipolar Unipolar and Chest and Chest

EKGEKG

Cardiac arrhythmiasCardiac arrhythmias

Arrhythmia means abnormal rate or Arrhythmia means abnormal rate or rhythm of the heat beatsrhythm of the heat beats . .

It may be caused by abnormal excitability It may be caused by abnormal excitability of some cardiac fibers or abnormal of some cardiac fibers or abnormal conduction of impulses in the heartconduction of impulses in the heart..

Arrhythmias due to abnormal Arrhythmias due to abnormal

excitabilityexcitability

Extrasystoles (premature beats = premature Extrasystoles (premature beats = premature complexes = ectopic beats)complexes = ectopic beats)

Paroxysmal atrial tachycardia (PAT)Paroxysmal atrial tachycardia (PAT)

Paroxysmal ventricular tachycardia (VT)Paroxysmal ventricular tachycardia (VT)

Atrial fibrillation (AF)Atrial fibrillation (AF)

Ventricular fibrillation (VF)Ventricular fibrillation (VF)

Arrhythmias due to abnormal Arrhythmias due to abnormal

conductivity (heart block)conductivity (heart block)

Heart block means failure of conduction of Heart block means failure of conduction of impulses from the S-A node down to the impulses from the S-A node down to the

ventriclesventricles..

A-V blockA-V block::  In this condition, the A-V node and bundle In this condition, the A-V node and bundle conduct impulses at an abnormally slow conduct impulses at an abnormally slow rate or fail to conduct some or all of the rate or fail to conduct some or all of the impulses from the atria to the ventriclesimpulses from the atria to the ventricles..

Extrasystoles (premature beats = Extrasystoles (premature beats = premature complexes = ectopic beats)premature complexes = ectopic beats)

An extrasystole is an abnormal systole that occurs An extrasystole is an abnormal systole that occurs early during diastoleearly during diastole..

Extrasystoles are caused by impulses generated in Extrasystoles are caused by impulses generated in a hyperexcitable focus, other than the S-A nodea hyperexcitable focus, other than the S-A node..

If the focus is in the ventricle, the extrasystole is If the focus is in the ventricle, the extrasystole is called ventricular extrasystole. If the focus is in called ventricular extrasystole. If the focus is in the atrial wall, the extrasystole is called atrial the atrial wall, the extrasystole is called atrial extrasystoleextrasystole..

An atrial extrasystole is followed by An atrial extrasystole is followed by the normal period of diastolethe normal period of diastole . .

This is because when the abnormal This is because when the abnormal impulse reaches the S-A node, it impulse reaches the S-A node, it

depolarizes the node. The node then depolarizes the node. The node then repolarizes and starts the repolarizes and starts the

depolarization again for the next depolarization again for the next normal impulsenormal impulse..

A ventricular extrasystole is usually followed by a A ventricular extrasystole is usually followed by a longer diastole called the compensatory pauselonger diastole called the compensatory pause . .

The long compensatory pause occurs because the The long compensatory pause occurs because the conducting system does not allow the abnormal conducting system does not allow the abnormal impulse from the ventricular ectopic focus to pass impulse from the ventricular ectopic focus to pass

up to the atriaup to the atria . .The next impulse from the S-A node reaches the The next impulse from the S-A node reaches the ventricles in its normal timing. It finds the ventricles in its normal timing. It finds the ventricles still in the absolute refractory period of ventricles still in the absolute refractory period of

the extrasystole. So, it will be ineffectivethe extrasystole. So, it will be ineffective . .The ventricles then, wait in diastole for a longer The ventricles then, wait in diastole for a longer period (compensatory pause) until the next period (compensatory pause) until the next impulse from the S-A node arrives to excite them impulse from the S-A node arrives to excite them

and start the next systoleand start the next systole . .

Paroxysmal atrial tachycardia Paroxysmal atrial tachycardia (PAT)(PAT)

This is tachycardia that occurs in attacks This is tachycardia that occurs in attacks that last from few seconds up to few days. that last from few seconds up to few days. Usually the atria beat at a regular rate of Usually the atria beat at a regular rate of

about 200 beats/minuteabout 200 beats/minute..

PAT is caused by one of three mechanismsPAT is caused by one of three mechanisms : :

* *An ectopic hyperexcitable focusAn ectopic hyperexcitable focus that discharges that discharges impulses at a high rateimpulses at a high rate..

* *Lewis’ circus movement (reentry mechanism):Lewis’ circus movement (reentry mechanism): Normally, there are circles of cardiac muscle fibers in Normally, there are circles of cardiac muscle fibers in which impulses travel. When an impulse enters the which impulses travel. When an impulse enters the circle, it splits into two, each one travels in the opposite circle, it splits into two, each one travels in the opposite direction to the other. Each impulse is followed by an direction to the other. Each impulse is followed by an absolute refractory period. When they normally meet, absolute refractory period. When they normally meet, they both die off and vanish. If a transient block occurs they both die off and vanish. If a transient block occurs at one side, one impulse is blocked and dies off, the at one side, one impulse is blocked and dies off, the other one goes on around the ring. By the time it other one goes on around the ring. By the time it reaches the block area, the block would have finished reaches the block area, the block would have finished and the impulse passes this area and continues to and the impulse passes this area and continues to circulatecirculate..

* *Wolff-Parkinson-White (WPW) syndromeWolff-Parkinson-White (WPW) syndrome..

Re-entry by Partial conduction BlockRe-entry by Partial conduction Block

The long cardiac refractory period normally prevents re-excitement and extinguishes the AP wave.

Partial conduction block can allow re-entry of the AP into previously stimulated regions. This allows the AP to continue cycling through the heart.

Wolff-Parkinson-White (WPW) syndromeWolff-Parkinson-White (WPW) syndrome:: Normally the only conducting pathway between the atria and the Normally the only conducting pathway between the atria and the ventricles is the A-V bundle. Persons with WPW have another ventricles is the A-V bundle. Persons with WPW have another abnormal conducting bundle; the bundle of Kent. This bundle abnormal conducting bundle; the bundle of Kent. This bundle conducts faster than the A-V node but has a longer absolute conducts faster than the A-V node but has a longer absolute refractory period. It can conduct impulses in both directions. Under refractory period. It can conduct impulses in both directions. Under normal conditions, impulses from the S-A node pass through the normal conditions, impulses from the S-A node pass through the bundle of Kent and the A-V bundle. Both impulses meet then die off bundle of Kent and the A-V bundle. Both impulses meet then die off in the right ventricle. If an early impulse is fired in the atrium, it in the right ventricle. If an early impulse is fired in the atrium, it passes through the A-V node, but finds the bundle of Kent still in the passes through the A-V node, but finds the bundle of Kent still in the absolute refractory period. The impulse passes through the right absolute refractory period. The impulse passes through the right ventricle then passes through the bundle of Kent up to the right ventricle then passes through the bundle of Kent up to the right atrium. The impulse reenters the A-V node, to the ventricles and atrium. The impulse reenters the A-V node, to the ventricles and

reentry circuit is establishedreentry circuit is established..

Paroxysmal ventricular Paroxysmal ventricular tachycardiatachycardia

This is tachycardia that occurs in attacks This is tachycardia that occurs in attacks that last from few seconds up to few daysthat last from few seconds up to few days..

The heart rate is about 200 beats/minuteThe heart rate is about 200 beats/minute..

The mechanism of VT is a high rate of The mechanism of VT is a high rate of discharge from an ectopic focus or circus discharge from an ectopic focus or circus

movement in the ventriclesmovement in the ventricles..

An attack of SVT can be terminated by An attack of SVT can be terminated by reflex stimulation of the vagus nerve by reflex stimulation of the vagus nerve by massaging the carotid sinus area. This massaging the carotid sinus area. This

depresses the excitability of the depresses the excitability of the myocardial cellsmyocardial cells . .

However, carotid sinus massage can not However, carotid sinus massage can not stop an attack of VT because the vagus stop an attack of VT because the vagus

nerves do not supply the ventriclesnerves do not supply the ventricles . .

Atrial fibrillation (AF)Atrial fibrillation (AF)

This is a pathological condition in which This is a pathological condition in which atria beat at an extremely high rate (350-atria beat at an extremely high rate (350-500 beats/min). AF is caused by multiple 500 beats/min). AF is caused by multiple circus movements in the atria operating circus movements in the atria operating at the same time leading to at the same time leading to desynchronization of the atrial muscle desynchronization of the atrial muscle

fibersfibers..

Ventricular fibrillation (VF)Ventricular fibrillation (VF)

This is a pathological condition in which This is a pathological condition in which the ventricles beat at an extremely high the ventricles beat at an extremely high rate (350-500 beats/min). VF is caused by rate (350-500 beats/min). VF is caused by multiple circus movements in the multiple circus movements in the ventricles operating at the same time ventricles operating at the same time leading to desynchronization of the leading to desynchronization of the ventricular muscle fibersventricular muscle fibers

Arrhythmias due to abnormal Arrhythmias due to abnormal conductivity (heart block)conductivity (heart block)

Heart block means failure of conduction of Heart block means failure of conduction of impulses from the S-A node down to the impulses from the S-A node down to the ventriclesventricles..  A-V blockA-V block::

  In this condition, the A-V node and bundle In this condition, the A-V node and bundle conduct impulses at an abnormally slow rate or conduct impulses at an abnormally slow rate or fail to conduct some or all of the impulses from fail to conduct some or all of the impulses from the atria to the ventriclesthe atria to the ventricles..

AV NODE AND AV BLOCKSAV NODE AND AV BLOCKS

FOCUS ON N REGION

NORMAL ECG

1ST DEGREE

PROLONGED AVCONDUCTION TIME

2ND DEGREE

1/2 ATRIAL IMPULSES CONDUCTED TO VENTRICLES

3RD DEGREE

VAGAL MEDIATIONIN N REGION/COMPLETEBLOCK