ECG - PrinciplesAbbas A. A. Shawka

Medical student ( 1st stage )

VOLTAGE AND TIME CALIBRATION OF THE ELECTROCARDIOGRAM

In normal ECG we notice: - PQ interval- QRS complex - ST Segment - QT interval

Current flow around the heartTwo facts : 1. the average current flow occurs with negativity toward the

base of the heart and with positivity toward the apex. 2. Thus, in normal heart ventricles, current flows from negative

to positive primarily in the direction from the base of the heart toward the apex

THREE BIPOLAR LIMB LEADS • Lead I : In recording limb lead I, the negative terminal of the

electrocardiograph is connected to the right arm and the positive terminal is connected to the left arm. When right arm is positively respect to the left one the ECG record positively.

• Lead II. To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal is connected to the left leg. Therefore, when the right arm is negative with respect to the left leg, the electrocardiograph records positively.

• Lead III. To record limb lead III, the negative terminal of the electrocardiograph is connected to the left arm and the positive terminal is connected to the left leg. This configuration means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.

Einthoven’s LawEinthoven’s law states that if the ECGs are recorded simultaneously with the three limb leads, the sum of the potentials recorded in leads I and III will equal the potential in lead II.

Lead I potential + Lead III potential = Lead II potential

Note• Because the recordings from all the bipolar limb leads are

similar to one another, it does not matter greatly which lead is recorded when one wants to diagnose different cardiac arrhythmias, because diagnosis of arrhythmias depends mainly on the time relations between the different waves of the cardiac cycle. However, when one wants to diagnose damage in the ventricular or atrial muscle or in the Purkinje conducting system, it matters greatly which leads are recorded because abnormalities of cardiac muscle contraction or cardiac impulse conduction change the patterns of the ECGs markedly in some leads yet may not affect other leads

CHEST LEADS (PRECORDIAL LEADS)

Normal ECGs of chest leads

• Why QRS complex negative in V1,V2 and positive in V3,V4,V5,V6 ?

• In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because the chest electrode in these leads is nearer to the base of the heart than to the apex, and the base of the heart is the direction of electronegativity during most of the ventricular depolarization process. Conversely, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is nearer the heart apex, which is the direction of electropositivity during most of depolarization.

AUGMENTED UNIPOLAR LIMB LEADS• In this type of recording, two of the limbs are connected

through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal

Normal ECGs of aVR, aVL and aVF

• Why QRS complex in aVR is inverted ?! when the direction of current flow is opposite to the axis of the lead, a negative deflection is produced. since the direction of current is basically down and to the patients left, this is opposite the axis of the lead (as shown by the fact that you correctly identified the positive terminal on the right arm

Vectorial Analysis of ECGs• What is a “ Vector “ ?A vector is an arrow that points in the direction of the electrical potential generated by the current flow, with the arrowhead in the positive direction. Also, by convention, the length of the arrow is drawn proportional to the voltage of the potential.

This vector is summation of all electrical impulse generated by the heart and referred to as the direction of impulse ( current ) flow.

Axes of Bipolar & unipolar lead

VECTORS THAT OCCUR AT SUCCESSIVE INTERVALS DURING DEPOLARIZATION OF THE VENTRICLES—THE QRS COMPLEX

• When the cardiac impulse enters the ventricles through the atrioventricular bundle, the first part of the ventricles to become depolarized is the left endocardial surface of the septum. Then depolarization spreads rapidly to involve both endocardial surfaces of the septum.

• Next, depolarization spreads along the endocardial surfaces of the remainder of the two ventricles

• Finally, it spreads through the ventricular muscle to the outside of the heart

A

the ventricular muscle has just begun to be depolarized, At this time, the vector is short because only a small portion of the ventricles—the septum—is depolarized.

Therefore, all electrocardiographic voltages are low, as recorded to the right of the ventricular muscle for each of the leads.

The voltage in lead II is greater than the voltages in leads I and III because the heart vector extends mainly in the same direction as the axis of lead II.

B

represents about 0.02 second after onset of depolarization. the heart vector is long because much of the ventricular

muscle mass has become depolarized. Therefore, the voltages in all electrocardiographic leads have

increased.

C

about 0.035 second after onset of depolarization, the heart vector is becoming shorter and the recorded

electrocardiographic voltages are lower because the outside of the heart apex is now electronegative, neutralizing much of the positivity on the other epicardial surfaces of the heart.

Also, the axis of the vector is beginning to shift toward the left side of the chest because the left ventricle is slightly slower to depolarize

D

about 0.05 second after onset of depolarization, the heart vector points toward the base of the left ventricle,

and it is short because only a minute portion of the ventricular muscle is still polarized positive.

Because of the direction of the vector at this time, the voltages recorded in leads II and III are both negative— that is, below the line—whereas the voltage of lead I is still positive.

E

about 0.06 second after onset of depolarization, the entire ventricular muscle mass is depolarized so

that no current flows around the heart and no electrical potential is generated.

The vector becomes zero, and the voltages in all leads become zero.

Why Q depression in the beginning of QRS complex ?

Sometimes the QRS complex has a slight negative depression at its beginning in one or more of the leads, which is not shown in; this depression is the Q wave. When it occurs, it is caused by initial depolarization of the left side of the septum before the right side, which creates a weak vector from left to right for a fraction of a second before the usual base-to-apex vector occurs

Different types of QRS complex

Rhythm1. A P wave morphology P wave (atrial contraction) precedes

every QRS complex2. The rhythm is regular, but varies slightly during respirations3. The rate ranges between 60 and 100 beats per minute4. The P waves maximum height at 2.5 mm in II and/or III5. The P wave is positive in I and II, and biphasic in V1

Heart rate ( HR ) • The square counting methodThe square counting method is ideal for regular heart rates. Use the sequence 300-150-100-75-60-50-43-37. Count from the first QRS complex, the first thick line is 300, the next thick line 150 etc. Stop the sequence at the next QRS complex. When the second QRS complex is between two lines, take the mean of the two numbers from the sequence or use the fine-tuning method listed below.

• The marker methodNon-regular rhythms are best determined with the "3 second marker method". Count the number of QRS complexes that fit into 3 seconds (some ECG writers print this period on the ECG paper). Multiply this number by 20 to find the number of beats/minute.

Conduction – PQ interval• The PQ interval starts at the beginning of the atrial

contraction and ends at the beginning of the ventricular contraction.

• sometimes referred to as the PR interval as a Q wave is not always present)

• indicates how fast the action potential is transmitted through the AV node (atrioventricular) from the atria to the ventricles. Measurement should start at the beginning of the P wave and end at the beginning of the QRS segment.

• The normal PQ interval is between 0.12 and 0.20 seconds.

Conduction- PQ interval• A prolonged PQ interval is a sign of a degradation of the

conduction system or increased vagal tone, or it can be pharmacologically induced. This is called 1st, 2nd or 3rd degree AV block.

• A short PQ interval can be seen in the WPW syndrome in which faster-than-normal conduction exists between the atria and the ventricles.

*What is the diagnosis ?!

Conduction-QRS Interval• The QRS duration indicates how fast the ventricles depolarize.

The normal QRS is < 0.10 seconds

• he ventricles depolarize normally within 0.10 seconds. When this is longer than 110 miliseconds[1], this is a conduction delay. Possible causes of a QRS duration > 110 miliseconds include:

1. Left bundle branch block2. Right bundle branch block3. Electrolyte Disorders4. Idioventricular rhythm and paced rhythm.

Conduction-QT interval• indicates how fast the ventricles are repolarized, becoming

ready for a new cycle.• The normal value for QTc is: below 450ms for men and below

460ms for women.• the QT interval gets shorter as the heart rate increases• The QT interval is prolonged in congenital long QT syndrome,

but QT prolongation can also occur be acquire as a results of:1. Medication (anti-arrhythmics, tricyclic antidepressants,

phenothiazedes2. Electrolyte imbalances3. Ischemia.• QT prolongation is often treated with beta blockers

Conduction-QT interval What do you see in this ECG ?!

Conduction-QT interval• If the QT segment is abnormal, it can be difficult to define the

end of the T wave. Below are a number of examples that suggest how QT should be measured in these patients.

Abnormal heart axis• A left heart axis is present when the QRS in lead I is positive

and negative in II and AVF. (between -30 and -90 degrees)• A right heart axis is present when lead I is negative and AVF

positive. (between +90 and +180)• An extreme heart axis is present when both I and AVF are

negative. (axis between +180 and -90 degrees). This is a rare finding.

Abnormal heart axis• The direction of the vector can changes under different

circumstances:1. When the heart itself is rotated (right ventricular overload),

obviously the axis turns with it.2. In case of ventricular hypertrophy, the axis will deviate toward the

greater electrical activity and the vector will turn toward the hypertrophied tissue.

3. Infarcted tissue is electrically dead. No electrical activity is registered and the QRS vector turns away from the infarcted tissue

4. In conduction problems, the axis deviates too. When the right ventricle depolarizes later than the left ventricle, the axis will turn to the right (RBBB). This is because the right ventricle will begin the contraction later and therefore will also finish later. In a normal situation the vector is influenced by the left ventricle, but in RBBB only the right ventricle determines it.

IS this Right OR left axis deviations ?

IS this Right OR left axis deviations ?

IS this Right OR left axis deviations ?

IS this Right OR left axis deviations ?

Left axis deviation Causes of left axis deviation include:1. Normal variation (physiologic, often with age)2. Mechanical shifts, such as expiration, high diaphragm

(pregnancy, ascites, abdominal tumor)3. Left ventricular hypertrophy4. Left bundle branch block5. left anterior fascicular block6. Congenital heart disease (e.g. atrial septal defect)7. Emphysema8. Hyperkalemia9. Ventricular ectopic rhythms10. Preexcitation syndromes11. Inferior myocardial infarction12. Pacemaker rhythm

Right axis deviation Causes of right axis deviation include:1. Normal variation (vertical heart with an axis of 90º)2. Mechanical shifts, such as inspiration and emphysema3. Right ventricular hypertrophy4. Right bundle branch block5. Left posterior fascicular block6. Dextrocardia7. Ventricular ectopic rhythms8. Preexcitation syndromes9. Lateral wall myocardial infarction10. Right ventricular load, for example Pulmonary Embolism or

Cor Pulmonale (as in COPD)

P wave morphology• The P wave morphology can reveal right or left atrial

hypertrophy or atrial arrhythmias and is best determined in leads II and V1 during sinus rhythm.

• Characteristics of a normal p wave:1. The maximal height of the P wave is 2.5 mm in leads II and /

or III2. The p wave is positive in II and AVF, and biphasic in V13. The p wave duration is shorter than 0.12 seconds

The Abnormal P wave• Elevation or depression of the PTa segment (the part between

the p wave and the beginning of the QRS complex) can result from atrial infarction or pericarditis.

• If the p-wave is enlarged, the atria are enlarged.

• If the P wave is inverted, it is most likely an ectopic atrial rhythm not originating from the sinus node

QRS morphologyThe basic questions in judging QRS morphology are:1. Are there any pathological Q waves as a sign of previous

myocardial infarction?2. Are there signs of left or right ventricular hypertrophy?3. Does the QRS complex show microvoltage (roughly QRS <

5mm)?4. Is the conduction normal or prolonged (QRS-interval > 0,12s)?5. Is the R wave propagation normal? Normally R waves become

larger from V1-V5. At V5 it should be maximal. If the R wave in V2 is larger than in V3, this could be a sign of a (previous) posterior myocardial infarction. Other causes are noted in the chapter Clockwise and Counterclockwise rotation.

If all these items are normal you can go on to the next step: ST morphology.

Normal QRS morphology

ST Morphology & T-waves• The ST segment represents ventricular repolarization.• The T wave is usually concordant with the QRS complex. Thus if

the QRS complex is positive in a certain lead (the area under the curve above the baseline is greater than the area under the curve below the baseline) than the T wave usually is positive too in that lead.

• Accordingly the T wave is normally upright or positive in leads I, II, AVL, AVF and V3-V6. The T wave is negative in V1 and AVR. The T wave flips around V2, but there is likely some genetic influence in this as in Blacks the T wave usually flips around V3.

• The T wave angle is the result of small differences in the duration of the repolarization between the endocardial and epicardial layers of the left ventricle. The endocardial myocytes need a little more time to repolarize (about 22 msec). This difference causes an electrical current from the endocardium to the epicardium, which reads as a positive signal on the ECG

ST segment elevation• The most important cause of ST segment elevation is acute Ischemia.

Other causes are 1. Early repolarization2. Acute pericarditis: ST elevation in all leads except aVR3. Pulmonary embolism: ST elevation in V1 and aVR4. Hypothermia: ST elevation in V3-V6, II, III and aVF5. Hypertrophic cardiomyopathy: V3-V5 (sometimes V6)6. High potassium (hyperkalemia): V1-V2 (V3)7. During acute neurologic events: all leads, primarily V1-V68. Acute sympathic stress: all leads, especially V1-V69. Brugada syndrome.10. Cardiac aneurysm.11. Cardiac contusion12. Left ventricular hypertrophy13. Idioventricular rhythm including paced rhythm

Pathological ST elevation

ST segment depressionThe most important cause of ST segment depression is Ischemia. Other causes of ST segment depression are:1. Reciprocal ST segment depression. If one lead shows ST

segment elevation then usually the lead 'on the other side' shows ST segment depression. (This is usually seen in ischemia as well.

2. Left ventricular hypertophy with "strain" or depolarization abnormality

3. Digoxin effect4. Low potassium / low magnesium5. Heart rate-induced changes (post tachycardia)6. During acute neurologic events.

T wave changesA concise list of possible causes of T wave changes:

1. Ischemia and myocardial infarction2. Pericarditis3. Myocarditis4. Cardiac contusion5. Acute neurologic events, such as a subarachnoid bleed.6. Mitral valve prolapse7. Digoxin effect8. Right and left ventricular hypertrophy with strain

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