ECG INTRODUCTION(Lecture 1)

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Associate Professor Dr. Alexey Podcheko

Spring 2015

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Intended Learning Objectives:BRIEF REVIEW IN ANATOMYANATOMY & PHYSIOLOGYPHYSIOLOGY OF THE HEARTCARDIAC ELECTRICAL ACTIVITYCARDIAC ELECTRICAL ACTIVITYECG READOUT GENERATIONGENERATIONECG NOMENCLATURENOMENCLATUREPOSITIONPOSITION OF THE 12 ECG LEADS RELATIVE TO THE HEARTTIME & THE ECGTIME & THE ECGTHE PREDICTED NORMALNORMAL ECG & SOME REFINEMENTS& SOME REFINEMENTS

“CARDIAC ANATOMY &PHYSIOLOGY”

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Conductive system of the heart

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“CARDIAC ELECTRICAL ACTIVITY”“During each cardiac cycle” the atria contract in

diastole to fill the ventricles which then contract during systole to supply blood to the lungs and the systemic circulation.

Contraction of the atria and ventricles is tightly coordinated by a wave of Depolarization spreading through the muscular walls of these chambers.

The Depolarization wave reflects movement of charge across myocyte membranes and is in effect an electrical current spreading through the heart.

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………..d.d…..

- Direction of electromagnetic wave

Following contraction, cardiac muscle returns to a resting resting state state and this is associated with reversal of the movement of charge across the myocyte membranes, this second wave of electrical activity is termed cardiac repolarizationrepolarization.

Cardiac depolarization is triggered by an electrical pulse generated in the sinoatrial node.

The atria and ventricles, however, are separated by a Non-conducting fibrous septum.

The depolarization wave cannot penetrate this barrier and in order to activate ventricular contraction the wave must in order to activate ventricular contraction the wave must be transmitted into the ventricles by the specialized cardiac be transmitted into the ventricles by the specialized cardiac conducting systemconducting system. In a normal heart, the only route the only route by which the depolarizing wave can enter the ventricular conducting system is through the AV nodethrough the AV node. 12

In order to allow the ventricles to fill with blood following atrial contraction, the AV node initially delays the spread of the depolarization wave. After this short delay, the depolarizing signal is transmitted into the ventricles via the bundle of His.

The bundle of His lies in the interatrial septum and divides into right and left bundle branches. The right and left bundle branches transmit the depolarizing signal into the muscle mass of the right and left ventricles respectively.

The interventricular septum is the first part of the ventricular muscle mass to depolarize and it does so by movement of depolarization across the septum from the left towards the right bundle branch……….

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In the walls of the ventricles, Depolarization spreads from the terminal fibers of the conducting system Outwards from the Endocardium towards the Epicardial surface of the heart and also back along the ventricular wall to the atrioventricular groove.

Repolarization spreads through the ventricles in the “opposite” direction to the depolarization wave moving from the Epicardial to the Endocardial surface of the chambers.

Within the ventricular wall there is a gradient in the rate of cellular repolarization, cells in the epicardial region have the fastest rate of repolarization and repolarize first following ventricular contraction. The rate of cellular repolarization is then progressively slower as we move from the epicardium towards the endocardium.

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“ECG READOUT GENERATION”The leads of the ECG machine detect the movement of the

cardiac depolarization and repolarization waves as they spread through the atria and ventricles.

In any ECG lead, the flat line recorded on the readout when No Net current is flowing is termed the Isoelectric line.

It is very important to realize that all of the leads on the ECG machine are set up in such a way that Depolarizing Depolarizing current moving TowardsTowards a lead produces a deflection on the ECG paper above the isoelectric line, a PositivePositive deflection

while DepolarizingDepolarizing current moving AwayAway from the lead produces a deflection below the isoelectric line, a NegativeNegative deflection.

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No Net current (E.g. No signal or wave of depolarization moves at right angle

Depolarizing Depolarizing current moving TowardsTowards a lead

DepolarizingDepolarizing current moving AwayAway from the lead

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•A wave of depolarization moving toward an electrode will record a positive deflection on an ECG•A wave of depolarization traveling away from an electrode will inscribe a negative deflection on an ECG.•A wave of depolarization moving at right angles to an electrode will cause either no deflection or a very small deflection on an ECG.

Very important to remember

Therefore, RepolarizingRepolarizing current moving TowardsTowards a lead produces a NegativeNegative deflection on the paper

while RepolarizingRepolarizing current moving AwayAway from the lead produces a PositivePositive deflection.

the SA node is situated towards the back of the right atrium so the atrial depolarization wave not only spreads downwards and to the left but also outwards towards the front of chest towards the chest leads.

As this depolarizing current is moving towards the leads it produces a positive deflection on the ECG paper, this is the P wave of atrial depolarization.

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The MagnitudeMagnitude of the electrical signal generated by depolarizing muscle is directly proportional to the Mass of MuscleMass of Muscle generating it, what this means is, that the more muscle present the more electrical signal generated and the more signal the ECG machine detects.

The left ventricle has a much greater muscle mass than the right and so dominates the electrical signal of ventricular depolarization in all leads.

Therefore, as the wave of electrical activity reaches the main muscle mass of the ventricles, the left ventricular signal overwhelms all other signals and as it is moving away from V1, the deflection produced on the ECG recording from this lead becomes negative. (Overall……Negative deflection)

In contrast, however, this signal is moving towards lead V6 producing a strong positive deflection. 21

When ventricular depolarization is complete there is a brief period when No current is flowing and the recording returns to the Isoelectric line.

The deflection produced by Ventricular Repolarization is termed a T wave.

Cardiac RepolarizationRepolarization spreads relatively Slowly relatively Slowly through the muscle mass, Outside the conducting system. Hence, the T the T wavewave is considerably Longer in duration and, therefore, Longer in duration and, therefore, BroaderBroader on the ECG paper than the QRS complex.

In leads with an “overall” positive QRS complex that is the positive deflection is larger than the negative deflection, the T wave also tends to be positive above the isoelectric line, while in leads with an overall negative QRS complex the T waves tend also to be negative, inverted below the isoelectric line.

To use the jargon, in Non-diseased hearts the QRS complexes Non-diseased hearts the QRS complexes and T waves tend to be concordantand T waves tend to be concordant. 23

o atrial repolarization produces a relatively weak electrical signal which is buried in the QRS complex and is generally Not detectable on a standard 12 lead ECG.

Early left to right septal depolarization may produce small physiological q waves in left sided leads…

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“ECG NOMENCLATURE”The deflection produced by atrial

depolarization is termed a P wave

ventricular depolarization produces the QRS complex.

The diffuse deflection produced by ventricular repolarization is termed a T wave.

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Within the QRS complex: any positive deflection, that is a deflection above the isoelectric line, is termed an R wave.Any negative deflection which follows an R wave is termed an S wave.However, if the first deflection of the QRS complex is negative this deflection is termed a q wave. Remember, a q wave can only exist if, and only if, the first deflection of the QRS complex is negative.

A negative deflection following a positive deflection (no matter how small that positive deflection may be) is an S wave.29

The section of the ECG recording connecting the End of the QRS complex and the Beginning of the T wave is termed the ST segment.

the junction between the ST segment and the end of the QRS complex is termed the ‘J point’. As all the ventricular muscle mass is depolarized and there is no flow of depolarization through the heart at this time, the J point and ST segment should lie on the isoelectric line.

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“POSITION OF THE 12 ECG LEADS RELATIVE TO THE HEART”

The different leads of the ECG examine cardiac electrical activity from different perspectives. Learning the position of the leads of a standard ECG relative to the heart is not as difficult as it seems and as we’ll see later pays dividends in clinical practice.

The Chest leads Chest leads examine the heart in the

Horizontal planeHorizontal plane.

The Frontal leads (Limb leads = Standard leads + Frontal leads (Limb leads = Standard leads + Augmented leads) Augmented leads) examine the heart in the Vertical planeVertical plane.

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Augmented Limb LeadsIn addition to the three bipolar

limb leads described above, there are three augmented unipolar limb leads. These are termed unipolar leads because there is a single positive electrode that is referenced against a combination of the other limb electrodes. The positive electrodes for these augmented leads are located on the left arm (aVL), the right arm (aVR), and the left leg (aVF).

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“Chest Leads / Precordial Leads (Unipolar)”the organ is rotated toward the left, so that the right

ventricle lies anterior to the left immediately behind the sternum. Therefore, V1 and V2 face the anterior surface of the right ventricle,

V3 and V4 look at the anterior surface of the left ventricle V5 and V6 look at the lateral surface of the left ventricle. V1 is applied to the chest in the 4th right intercostal space to

the right of the sternum, lead V2 in the fourth left intercostal space to the left of the

sternumlead V4 is placed on the chest over the apex beat. Lead V3 is placed down from V2 midway between V2 and V4.V6 is placed horizontally and laterally from V4 in the midaxillary

lineV5 is placed at the midpoint between V4 and V6.40

“The Frontal (Limb) Leads: Standard Leads + Augmented Leads” (Frontal or Vertical Plane) :

the Standard leads (Bipolar leads): leads I, II and III

the Augmented leads (Unipolar leads): aVR, aVL and aVF.

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“Lead & Blood Supply”For the experienced practitioner, looking at different areas on

the ECG readout is like looking at different anatomical regions of the heart.

Leads II, III and aVF II, III and aVF examine the region of the heart supplied by the right coronary artery (RCARCA) – Inferior leadsInferior leads

Leads V1-V4V1-V4 examine the region of the heart supplied by the left anterior descending (LAD) branch of the left coronary LAD) branch of the left coronary arteryartery – Anterior leadsAnterior leads

Leads I, aVL, V5 and V6 I, aVL, V5 and V6 examine the region of the heart supplied by the left circumflex branch (LCA) of the left LCA) of the left coronary arterycoronary artery – Lateral leadsLateral leads 42

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“TIME & THE ECG”If you look at the bottom of the ECG readout you will see a

long run of recording from lead II, this is called the rhythm strip.

We use the rhythm strip to calculate the heart rate and to diagnose abnormal cardiac rhythms (arrhythmias).

Lead II is used as the rhythm strip as it is usually the easiest lead in which to see P waves

For our present purposes we can consider that in all ECG machines the recording needles run at a constant speed over the ECG paper of 25 mm/sec.

At a recording rate of 25mm/sec, 5 large squares are covered in one second. So, 300 large squares represents one minute. Therefore, the number of R waves in 300 large the number of R waves in 300 large squares is the heart rate in beats per minutesquares is the heart rate in beats per minute………….. 44

one large square corresponds to 0.2 seconds and one small square to 0.04 seconds.

In a normal heartnormal heart, the time between the onset of atrial depolarization (the beginning of the P wave) and the onset of ventricular depolarization, (the beginning of the QRS complex) varies between 0.12 and 0.2 seconds between 0.12 and 0.2 seconds that is that is between 3 and 5 small squaresbetween 3 and 5 small squares. This is the PR intervalPR interval.

The duration of the QRS complex represents the time taken for ventricular depolarisation to be completed

A normal QRS complex is less than three small squares in normal QRS complex is less than three small squares in width (<0.12s in durationwidth (<0.12s in duration).

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The time between the onset of ventricular depolarization and the end of ventricular repolarization (that is the beginning of the QRS complex and the end of the T wave on the ECG) is termed the QT interval.

“When the heart rate is 60 beats per minute”, the upper limit of normal for the QT interval is 0.44 seconds or 11 small squares.

It is important to realize, however, that the measured QT interval varies with heart rate

becoming shorter as the heart speeds up shorter as the heart speeds up longer when the heart slows down. Therefore, particularly at higher heart rates it is at higher heart rates it is

possible to miss an underlying prolonged QT intervalpossible to miss an underlying prolonged QT interval. 47

When faced with an ECG with a “heart rate other than” 60 beats per minute”, to calculate the true underlying QT interval referred to as the corrected QT interval we use the following formula: QTc = QT/√RR

QTc is the corrected QT value, QT is the actual QT interval measured on the ECG and the RR interval is the distance between consecutive R waves on the ECG measured in seconds.

To give you a rule of thumb, when looking at an ECG when looking at an ECG readout with a “heart rate other” than 60 beats per readout with a “heart rate other” than 60 beats per minute”, if the QT interval is more than half the minute”, if the QT interval is more than half the distance between consecutive R waves, at least distance between consecutive R waves, at least consider the possibility of prolonged QTconsider the possibility of prolonged QT.

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SUMMARYThe normal PR interval is 0.12 to 0.20 seconds (3 to 5 small squares). The normal QRS duration is less than 0.12 seconds (<3 small squares).

The QT interval; (approximately) The QT interval; (approximately) less than half the RR intervalless than half the RR interval.

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“To Calculate the Heart Rate – with Regular Rhythm”Rule of 300To calculate the heart rate in beats per minute from

an ECG with a regular rhythm, count the number of Large squares between two consecutive R waves and divide this number into 300.

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“To Calculate the Heart Rate”Rule of 30:To calculate the heart rate in beats per minute from an

EKG with an irregular rhythm (also regularirregular rhythm (also regular…) …) count the count the number of R waves in 30 Large squares and multiply this number of R waves in 30 Large squares and multiply this number by 10number by 10

Use “6-second EKG StripUse “6-second EKG Strip” for doing this

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“PREDICTED NORMAL ECG & SOME REFINEMENTS”

The transition from dominant S waves to dominant R waves in the chest leads occurs at or around lead V3 or V4.

The transition at this point from an overall negative QRS to at this point from an overall negative QRS to a positive QRS complexa positive QRS complex in the chest leads is termed normal normal R wave progressionR wave progression and reflects a normal healthy left normal healthy left ventricle dominating QRS morphologyventricle dominating QRS morphology with a normal pattern of flow of depolarization around this chamber.

It is quite normal for P waves to differ in morphology between the 12 leads as each lead has a different perspective on atrial depolarization. For example, as atrial depolarization spreads away from lead aVR on the right, the P wave in this lead is negative.

P waves are usually most prominent in lead II……………….(Overall Magnitude of the Depolarization Vector…………)

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in a “healthy heart in leads with dominant R waves the T wave tends to be less than half the amplitude of the preceding R wave in height.

Small physiological q waves in the left lateral leads produced by septal depolarization are a normal finding on the ECG.

We’ve seen that the interventricular septum is the first region of ventricular myocardium to depolarize and that it does so by movement of depolarization from left to right.

This early left to right depolarization wave is the key to understanding the potential existence of small physiological q waves in any left lateral lead.

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Depolarization of the septum does indeed spread from left to right but this left to right movement also travels upwards and somewhat backwards from the lower part of the left side of the septum towards the upper part of the right.

For this reason it is also perfectly acceptable to see physiological q waves in the inferior leads.

Physiological q waves may be seen in the Left Lateral and Physiological q waves may be seen in the Left Lateral and Inferior leadsInferior leads.

The square deflection at the end (or beginning) of each recording strip on the ECG readout is the calibration box, an internal standard in the ECG machine. It should be two large squares in height indicating that 1mV of electricity yields a 10 mm vertical deflection on the ECG paper. This is the standardized sensitivity of ECG machines.

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The calibration box should also be one large square in width this indicates that the machine is recording at recording at a needle speed of 25 mm/sa needle speed of 25 mm/s.

A standard calibration box of two squares A standard calibration box of two squares vertically and one box horizontally indicates that vertically and one box horizontally indicates that the ECG machine is correctly calibratedthe ECG machine is correctly calibrated.

When you start to review patients ECGs in review patients ECGs in clinical contextclinical context it will rapidly become obvious to you that there is tremendous variation in tremendous variation in normal ECG morphology in perfectly healthy normal ECG morphology in perfectly healthy individualsindividuals.

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