How to read ECG Part 1

The reading an electrocardiograph fascinates every medical student and practitioner. But what looks so simple and clear cut in the books does not appear so when it comes to actual ECG reading in wards and clinics. This cassette aims to solve this problem by developing an eye to pinpoint the abnormalities and analyze them systematically. The cassette teaches simple things first and more complicated problems later towards the end of the cassette. So until you master the first five sections do not view the last two. Study each section individually through rewind and replay and master it before proceeding to the next.

Now let us begin with an introduction to the PQRST terminology. This is the terminology used to describe the various wave forms of an electrocardiographic tracing. The first wave is the P wave which represents depolarization and contraction of the atria. The next is the QRS complex which represents the depolarization and contraction of the ventricles. It is followed by the T wave representing repolarization of the ventricles. The repolarization of the atria is not recorded on the routine electrocardiograph. Sometimes another small upright wave follows the T wave and is termed as the U wave.

Now coming to the QRS complex. The initial downward deflection of the QRS complex is termed as the Q wave. It may or may not be present. The first upward deflection of the QRS complex is termed as the R wave. And the downward deflection following the R wave is termed as the S wave. Sometimes there may be a second upward deflection following an R wave. It is termed as R-wave. And if a downward deflection occurs after R-wave, it is termed as S-wave. Now we shall see a few QRS complexes to understand the terminology better.

Here we see a small R wave and a deep S wave, an RS pattern. This is a tall R wave and a deep S wave, another RS pattern. And this is a tall R wave and a small S wave, again an RS pattern. You must have noted by now that prominent waves are written in capital letters and small waves in small letters. This is a R pattern with no Q wave, no S wave. In this complex we see a small Q wave, a tall R wave and a small S wave, a QRS complex.

And in this complex there is only a downward deflection, no R wave. We do not know whether it is a Q wave or a S wave. So it is termed as QS complex. This is a RS R- with a deep S wave. This is a RS R- with a tall R- wave. This is a R R- pattern and this is R R- S- pattern. Now we shall study the various intervals. The time from the onset of the P wave to the onset of the QRS complex is termed as the PR interval.

It represents the conduction time from the atrium to the ventricles, from the onset of P wave to the onset of QRS complex. The time from the beginning of the Q wave to the end of the S wave is called the QRS interval. It indicates the time taken by the impulse to spread through the two ventricles. The time from the beginning of the Q wave to the end of the T wave is called the QT interval. It represents the total electrical activity of the ventricles. The line between the QRS complex and the T wave is termed as the ST segment.

And the point of junction between the QRS complex and the ST segment is called the J point. The time interval from the apex of one R wave to the apex of next R wave is termed as the R R interval. R R interval is related to the heart rate or rate of ventricular contractions. The time interval from the beginning of one P wave to the beginning of next P wave is termed as the PP interval. The PP interval indicates the rate of atrial contractions. Under normal circumstances, the R R interval and the PP interval are both equal.

In a normal ECG, the PR, ST and TP segments are at the same horizontal level and form the baseline or the iso-electric line. The amplitudes of all the waves are measured from this line. The standard electrocardiograph is a recording made from 12 leads. Each lead records the electrical potential difference between a positive and a negative body surface electrode. And each lead gives us a view of the electrical activity of heart from a different direction. Six of these leads, that is 1, 2, 3, AVR, AVL and AVF, give different views in the frontal plane.

And six chest leads, V1 to V6, give the views in the horizontal plane. The ECG machine has four limb leads and one chest lead. The white lead marked V is for the chest and the other color-coded leads are for the right foot, left arm, left foot and right arm respectively. The leads are placed in contact with the body surface at different fixed points. An electrode jelly is applied to have proper skin contact with minimal resistance. The four limb leads are tied to the four limbs over the forearms and the shins, where there are minimal muscles because muscle twitchings can cause electric interference.

The electrode on the right leg is the ground electrode or the earthing electrode. So in effect we have three active electrodes whose combinations will give us six leads. Three bipolar leads, 1, 2 and 3 and three unipolar leads, AVR, AVL and AVF. Leads 1, 2 and 3 are bipolar leads and they record the electric potential difference between two extremities at a time. Lead 1 as you can see records the potential difference between left arm and right arm. Lead 2 records the potential difference between left leg and right arm.

And Lead 3 records the potential difference between left leg and left arm. Now note the direction of each lead. This is the Eindhoven's triangle showing positive terminal of each lead. So lead 1 is directed from the left arm to the right arm. Lead 2 is directed from the left leg to the right arm and lead 3 is directed from left leg to left arm. Now the directions of each lead are depicted. Red arrow for lead 1, blue arrow for lead 2 and orange for lead 3. Let us now overlap the directions over the heart.

And the next diagram will show the leads as to how they look at the heart or the directions from which they look at the heart. Lead 1 from the left side and leads 2 and 3 from the inferior side. All other leads are unipolar leads. The letter V stands for a unipolar lead. So AVR, AVL and AVF are unipolar limb leads. And V1, V2, V3, V4, V5 and V6 are unipolar chest leads. The unipolar limb leads however have a very low electric potential. So they are instrumentally augmented by connecting the other two limbs to form the neutral electrode.

Hence the letter A is used to designate an augmented lead. Thus AVR means augmented unipolar lead of the right arm. The exploring electrode is on the right arm while the left arm and leg are connected to the neutral electrode to augment its potential. AVL means augmented unipolar lead of the left arm with exploring electrode on the left arm. And AVF means augmented unipolar lead of the left leg with exploring electrode over the left leg. This diagram now shows how the six different limb leads look at the heart from six different directions.

Leads 1 and AVL from the left side leads 2, 3 AVF from the inferior side and lead AVR from the right side. The chest leads are recorded with an exploring electrode placed at various points over the chest while all the three limb electrodes are connected together to form the neutral electrode. The points are marked as follows. Pulpates the sternal angle. The space just below the sternal angle is the second intercostal space. Mark the fourth intercostal space at right sternal border as V1. Then mark at the same level that is in the fourth intercostal space at the left sternal border as V2.

Mark V4 in the mid-clavicular line in fifth intercostal space. The point midway between V2 and V4 is V3. Now mark V5 and V6 in the same horizontal level as V4 in the anterior axillary line and mid-axillary line in the sixth and seventh intercostal spaces respectively. Thus we have six standard chest leads and this is a diagrammatic horizontal cross section of the chest. Observe that the leads V1 and V2 face the right ventricle. V3 faces the interventricular septum and V4, V5 and V6 face the free wall of the left ventricle.

Thus limb leads give different views of the electrical activity of the heart in the frontal plane and chest leads give different views in horizontal plane. What we must remember always is that lead I and AVL are anterior leads directed to the anterior and left wall of the left ventricle while leads II, III and AVF are inferior wall leads directed to the inferior wall of the left ventricle. Leads I and AVL are leads to the anterior wall and leads II, III, AVF are leads to the inferior wall.

Now let us overlap the two diagrams and we find that leads I, AVL, V5 and V6 are left sided leads that look at the left side of the heart while V1 and V2 are right sided leads that look at the right side of the heart. To revise leads I, AVL, V5 and V6 are left sided leads and leads V1 and V2 are right sided leads. Now we come to the cardiac cycle or the chain of electrical events that occur during a heartbeat. The process of depolarization begins at I that is at SA node, passes to the atria that is number 2

and then at 3 it passes across the AV node to the ventricles. In the ventricles it travels along the right and left bundle branches marked 4 and then through the purkinje fibers to the ventricles. Now correlating the events to the ECG recording, the process of activation that is depolarization begins at the sinoatrial node or the SA node situated in the right atrium. Within 0.06 to 0.08 seconds the atria get depolarized which is recorded on the ECG as the P wave. The wave of depolarization has to reach the ventricles through the AV node which delays the impulse by 0.1 second.

During this time there is no significant electrical activity and ECG records it as the isoelectric PR segment. Now as the impulse spreads throughout the ventricles within about 0.1 second the septum gets depolarized first then left and right ventricular walls almost simultaneously but a little earlier on the left side. This activity is recorded as the QRS complex. Once the ventricles are totally depolarized there is no electrical activity for a brief period so the ECG returns to the isoelectric line and records the ST segment. Then the repolarization of the ventricles begins and is recorded as the T wave.

The repolarization is much slower than the depolarization and so the T wave is broader than the QRS complex but it is in the same direction as the main wave of the QRS complex. Now we come to the second section on interpretation of PQRST patterns. Calculation of the heart rate. The heart rate is calculated by measuring the number of small squares between two successive R waves. 1500 divided by this number gives us the heart rate per minute. The standard ECG paper runs at a rate of 25 millimeters per second.

So each small square represents 0.04 seconds and each large square represents 0.2 seconds and 1500 small squares represent 1 minute. In this ECG the RR interval measures 10 small squares so the rate is 1500 divided by 10 equal to 150 per minute. A rate above 100 per minute is termed as tachycardia. In the second ECG the RR interval is much prolonged. Let us count it taking help of the large squares in thick lines after every 5 small squares. The RR interval is 28 small squares so the heart rate is 1500 divided by 28 equal to 54 per minute.

A rate below 60 per minute is termed as bradycardia. In this ECG count the RR interval. It is 18 small squares so the rate is 84 per minute that is within normal range. So normally the RR interval should be between 15 to 25 small squares or 60 to 100 per minute. Now in this ECG the rhythm is irregular. Each RR interval is different. So in this case count the number of R waves in 15 large squares which represent 3 seconds and multiply the number by 20 to get an approximate heart rate per minute.

This tracing shows 5 R waves in 15 large squares so the rate is about 100 per minute. So before heart rate is calculated see whether the rhythm is regular. We must also remember that RR interval actually measures the ventricular rate. In total heart block where P waves do not correspond with the QRS complexes the atrial rate should be calculated separately by measuring the number of small squares between two P waves and dividing 1500 by this number. In this tracing the P P interval is 15 small squares and the RR interval is 50 small squares.

So the atrial rate is 100 per minute while ventricular rate is 30 per minute. Clinically what we count by palpating the pulse is ventricular rate. Atrial rate can be seen only on ECG. Access deviation. To look for deviation of electrical axis of heart we must study leads 1 and 3. Normally QRS complexes are predominantly upwards in both these leads. In left axis deviation there is a prominent R wave in lead 1 and a prominent negative deflection either Q or S in lead 3. In right axis deviation there is a prominent negative deflection in lead 1 and a prominent R wave in lead 3.

To avoid confusion remember a simple mnemonic. Left leaves and right reaches. Imagine lead 1 above and lead 3 below. The QRS complexes seem to leave one another in left axis deviation that is left leaves. And QRS complexes seem to reach each other in right axis deviation that is right reaches. This ECG shows left axis deviation. Left axis deviation is seen in left ventricular hypertrophy, left bundle branch block and inferior wall in fact. This ECG shows right axis deviation. Right axis deviation is seen in right ventricular hypertrophy, right bundle branch block and anterior wall in fact.

Atrial hypertrophy. The atrial hypertrophy can be diagnosed from the contour of the P waves seen best in lead 2. The P wave represents depolarization of both right and left atria. The SA node starts the depolarization in right atrium. So the initial part is contributed by the right atrium and the later part of the P wave shaded in the diagram is contributed by the left atrium. In left atrial hypertrophy the second component is delayed and prominent. So we get a wide and notched P wave which is wider than 2.5 small squares.

Since this is common in mitral valve diseases it is termed as P mitralae. Note the P mitralae notched and 3.5 small squares wide. Width more than 2.5 squares is the main criterion. This is another P mitralae not notched but 3 small squares wide. In right atrial hypertrophy the initial component is prominent. So the P wave is tall and peaked in lead 2 which is taller than 2.5 small squares. Since this is common with pulmonary hypertension it is termed as P pulmonale. This is a very prominent P pulmonale.

5 small squares tall it suggests right atrial hypertrophy. In this ECG the lead 2 shows tall and peaked P wave which is 3 small squares tall. This is P pulmonale suggesting right atrial hypertrophy. This ECG shows tall and peaked P waves in 2, 3 and AVF typical P pulmonale. This is P mitralae no notched but 2.5 small squares wide suggesting left atrial hypertrophy. If both the atria are hypertrophied the P wave in lead 2 will be taller than 2.5 small squares and broader than 2.5 small squares.

This is an ECG of biatrial hypertrophy. Ventricular hypertrophy. Ventricular enlargement is diagnosed from the pattern and amplitude of the QRS complexes in the chest leads V1 to V6. This is the normal pattern of QRS complexes in chest leads. The lead V1 shows a small r wave and a deep S wave. As we proceed towards V6 the height of r wave progressively increases and depth of S wave progressively decreases. Somewhere in V3 and V4 the r and S waves are equal and in V6 there is a tall r wave and very small or absent S wave.

This ECG shows a normal QRS pattern in the chest leads. Note the S wave in right sided chest leads which disappears as we reach V5 or V6. R and S are equal in V4 and the r wave progressively increases from V1 to V6. In left ventricular enlargement this pattern remains the same but the amplitude of the waves increases. This is the normal pattern. Now in left ventricular hypertrophy note that there are deep S waves in V1 and V2. R and S waves are equal in V3 and there are very tall r waves seen in V5 and V6.

That is in left ventricular enlargement there is normal progression from negative deflection in lead V1 to positive deflection in lead V6. But the amplitude of the waves increases. So we have to measure the amplitudes of S wave in leads V1 or V2 whichever is larger and amplitude of r wave in V5 or V6 whichever is larger to diagnose left ventricular hypertrophy. If any of these is more than 25mm S V1 or R V6 or if the radiation is more than 35mm then a diagnosis of left ventricular hypertrophy is made.

It will usually be associated with left axis deviation and a p-mitralae of left atrial hypertrophy may be seen in lead 2.

In this ECG there is normal pattern of QRS complexes from negative in V1 to positive in V6. So let us measure S V1 and R V5 from the isoelectric line. S V1 is 19mm and R V5 is 18mm. S V1 plus R V5 is 37mm that is more than 35mm. So left ventricular hypertrophy is present. This is another ECG where gross inspection of the chest leads immediately reveals that the amplitudes of QRS complexes is very high. But there is normal progression from negative deflection in V1 to positive deflection in V6.

Now measure the amplitudes. S V1 is 21mm.

R V6 is 25mm. So S V1 plus R V6 is 46mm which is more than 35mm. So left ventricular hypertrophy is present. In this ECG again note the normal progression from deep S waves in V1 to tall R waves in V6. S V2 is 28mm more than 25mm. So left ventricular hypertrophy is diagnosed. R V6 is 25mm. So S V2 plus R V6 is 53mm. All in support of left ventricular hypertrophy. Now observe leads 1 and 3. Left axis deviation is seen with positive QRS in lead 1 and negative QRS in lead 3.

Now study the P wave in lead 2 shown enlarged in the inset. It is 2.5 small squares wide suggesting that there is left atrial hypertrophy also. Right ventricular hypertrophy. In right ventricular hypertrophy the pattern of the QRS complexes in the chest leads will be changed. This is the normal pattern from S V1 to R V6. Let us now compare and study the pattern of right ventricular hypertrophy. In right ventricular hypertrophy lead V1 shows a prominent R wave. Lead V2 also shows a prominent R wave. Leads V3 and V4 are similar with equal R and S.

Lead V5 shows a deep S wave. And lead V6 again shows a prominent S wave. So whenever there is a prominent R wave in lead V1 or deep S wave in V6, right ventricular hypertrophy must be suspected. The criteria for diagnosis of right ventricular hypertrophy are R wave in V1 more than 7 mm or S wave in V5 or V6 more than 7 mm or addition of both more than 10 mm. Right ventricular hypertrophy may or may not be associated with right axis deviation. And often there will be P pulmonale of right atrial hypertrophy.

In this ECG, V1 shows a tall R wave taller than S wave. So immediately think of right ventricular hypertrophy. There is a corresponding deep S wave in V6. So the normal QRS pattern in the chest leads has altered. Now measure R V1. It is 11 mm. R V1 more than 7 mm is diagnostic of right ventricular hypertrophy. S V6 is 5 mm. Their addition is 16 mm. More than 10 mm is the criterion for right ventricular hypertrophy. Now come to limb leads. There is no gross deviation of axis, but there is a negative wave in 1 and AVL

and positive wave in lead 3, suggesting tendency to right axis deviation. Now note the P wave in lead 2. A very typical tall and peaked P pulmonale is seen in leads 2, 3 and AVF, diagnostic of right atrial hypertrophy. So this ECG shows right ventricular hypertrophy with right atrial hypertrophy and right axis deviation. The next ECG shows a prominent R wave in V1 and a deep S wave in V6. So right ventricular hypertrophy is suspected. Let us measure the amplitudes of R V1 and SV6. R V1 is 6 mm.

SV6 is 16 mm. R V1 plus SV6 is 22 mm. So the criteria for diagnosis of right ventricular hypertrophy are met with. Coming to limb leads, negative QRS in lead 1 and positive QRS in lead 3 denotes right axis deviation. And P wave in lead 2 is broad, suggesting left atrial hypertrophy. This ECG shows a prominent R wave in V1 and deep S wave in V6. That is a change in the QRS pattern. R V1 is 9 mm. More than 7 mm. And SV6 is 7 mm.

So R V1 plus SV6 is 16 mm. That is more than 10 mm. So right ventricular hypertrophy is present. Now look at the limb leads. Imagine lead 1 above and lead 3 below. Right reaches. So right axis deviation is present. And the P wave in lead 2 is wide as well as tall. Suggesting by atrial hypertrophy. Strain pattern in ventricular hypertrophy. Ventricular enlargement is of two types. Dilatation and hypertrophy. When the left ventricle has to work against resistance as in hypertension or aortic stenosis. There will be hypertrophy of its muscle.

Which will be seen on the ECG as strain pattern in leads V5 and V6. In addition to the very tall R waves. The ST segment is slightly depressed. And the T wave is inverted. This is the strain pattern that indicates hypertrophy of left ventricle. In this ECG the strain pattern is seen. Note the very tall R waves. Slightly depressed ST segment and inverted T wave. This ECG shows left ventricular hypertrophy without strain. As in aortic regurgitation or mitral regurgitation. Note the tall R wave of left ventricular hypertrophy.

But normal ST segment and upright T wave. This ECG with left ventricular hypertrophy in leads V5 and V6. Shows mild strain. ST segment is depressed slightly. And T wave is reduced in amplitude but not inverted. This is another case of left ventricular hypertrophy. And note the strain pattern. Depressed ST segment and inverted T wave. A similar strain pattern may be seen in right ventricular hypertrophy. Due to pulmonary stenosis. Leads V1 and V2 will show depression of the ST segment and inversion of the T waves. In this ECG leads V1 and V2 show prominent R wave of right ventricular hypertrophy.

And strain pattern with ST segment depression and T wave inversion. Now this ECG shows biventricular hypertrophy. Note the tall R waves in V1 of right ventricular hypertrophy. And also note the deep S wave in V1 and V2. With tall R waves in V5 and V6 of left ventricular hypertrophy. And the strain pattern in V5 and V6. RV1 is 10 mm. So right ventricular hypertrophy is present. SV2 is 24 mm. RV5 is 22 mm. Their total is 36 mm. So left ventricular hypertrophy is also present with strain.

Once again note the picture of biventricular hypertrophy. RV1 of right ventricular SV1 and V2. And RV5 and V6 of left ventricular with strain pattern. Note again in close up. RV1 SV2. Tall R waves in V5 and V6 with strain pattern. Bundle branch blocks. A bundle branch block is suspected when the width of the QRS complex is more than 3 small squares. That is more than 0.12 seconds. This is because when one bundle branch is blocked. Only one ventricle will be activated first. And then the impulse will pass to the other ventricle across the septum.

Resulting in delayed depolarization in abnormal direction. In this diagram the left bundle branch is interrupted at 4. So the right ventricle gets depolarized in the normal sequence at 4 and 5. And then the impulse passes across the septum to the left ventricle at 6. The width of the QRS complex or the QRS interval is counted from the beginning of the Q or R wave. Whichever is first to the end of the QRS complex. And whenever the QRS complex in any of the leads is wider than 3 small squares.

A bundle branch block is diagnosed. Now let us learn to differentiate between right and left bundle branch blocks. When the QRS complexes are noted to be wide. First look for a positive wide and slurred M shaped pattern. That is RSR dash or RR dash pattern in the chest leads. If it is seen over the right chest leads. That is in V1 or V2. It is a right bundle branch block. And if it is seen over the left chest leads. That is V5 or V6. It is a left bundle branch block.

At the same time the opposite chest leads will display a wide and slurred S wave. So the features of right bundle branch block are. Wide QRS complexes 3 millimeters or more wide. An RSR dash pattern also called as M pattern or rabbit ears in lead V1. And broad slurred S wave in leads V5 and V6. It may also be associated with right axis deviation. To revise right bundle branch block shows. Wide QRS complexes 3 millimeters or more wide. RSR dash pattern in V1. Broad S wave in V6.

And right axis deviation may be present. In this ECG. Major the width of the QRS complex in lead V1. It is 3 small squares. Now note the RSR dash pattern in V1 and V2. Note that the T wave is opposite to the direction of the wide terminal QRS deflection. In V5 and V6. A wide S wave is seen. So the diagnosis is right bundle branch block. Note again the rabbit ears in V1 and wide S wave in V6. This is another ECG of right bundle branch block.

Note the RSR dash pattern in V1. Typical rabbit ears in V2. And shallow but wide S wave in V5 and V6. That is right bundle branch block. Note in close up view RSR dash in V1 and wide S wave in V6. Left bundle branch block. The features of left bundle branch block are number one. The width of the QRS complex is 3 millimeters or more. Number two. Very deep and broad S wave in lead V1 with no R wave. Number three. Broad slurred R wave or R dash pattern without a Q wave in leads V5 and V6.

And number four. Left bundle branch block is always associated with left axis deviation. To revise left bundle branch block shows wide QRS complexes more than 3 millimeters wide. Deep and broad S wave in V1. Broad R wave or RR dash wave in V6. And left axis deviation. This ECG shows wide QRS complexes. The lead V1 shows broad and deep S wave without an initial R wave. The leads V5 and V6 show broad and slurred R waves. So this is left bundle branch block. Note in close up view the broad and deep S wave in V1 and broad and slurred R wave in V6.

In particular note the absence of Q wave in V6. And note that the T waves are opposite in direction to the QRS complexes. Now let us see the limb leads. There is left axis deviation or left leaves pattern. And the changes of left sided leads V5 and V6 are also seen in 1 and AVL which are basically left sided leads. This is another ECG of left bundle branch block. See the broad and deep S wave in V1. The inside shows RR dash pattern in V5. This is broad slurred R wave in V6.

And also note the inverted T wave. Left anterior hemiblock. The left bundle branch has 2 phasicals anterior and posterior. Sometimes only the anterior phasical of the left bundle branch is affected causing pure left axis deviation. This is called as left anterior hemiblock. It is suspected when there is left axis deviation with no other causative factor with QR pattern in lead 1 and RS pattern in lead 3. The QRS is slightly widened but not to 3 small squares. Sometimes right bundle branch block may be associated with left anterior hemiblock because both bundles have common blood supply.

This ECG shows RS R dash pattern in V1 and broad shallow S wave in V5 and V6. So this is right bundle branch block but in limb leads there is left axis deviation. So this is a right bundle branch block with left axis deviation or right bundle branch block with left anterior hemiblock. Myocardial infarction. Myocardial infarction produces 3 basic changes in the leads facing the infarcted wall. Number 1. Elevation of the ST segment indicating zone of injury. Number 2. Inversion of the T wave indicating the effect of surrounding ischemic zone.

And number 3. A deep and wide Q wave indicating the zone of infarct or dead muscle. These are the 3 cardinal signs on ECG in acute myocardial infarction. The diagnosis is supported by reciprocal changes in the leads facing the opposite wall. These leads will show a depression of the ST segment and tall upright T waves. Now in clinical practice these changes do not appear simultaneously. When an infarct occurs the first change to appear in ECG is an elevation of ST segment with tall peaked T wave. This appears within a few minutes or may take a few hours to be seen.

Which means that an ECG taken immediately after the onset of chest pain may be normal. And it must be repeated after a few hours if clinically myocardial infarction is suspected. Note the ST segment elevation of about 3 millimeters in this ECG. This ST segment elevation will stay for 2 to 3 days and gradually return to base level over 1 to 2 weeks. The T wave which becomes tall immediately on blocking of coronary artery becomes inverted after 2 days and remains inverted for 3 weeks. Then gradually it becomes flat and then upright over 2 to 3 months.

The Q wave which is an indicator of muscle death appears after about a week and then gradually deepens and remains permanent. Its size being proportional to the size of the infarct. Now let us study the events in chronological order. On the first day the ST segment will be elevated with upright tall T wave but no Q wave. Over the next 2 days the T wave will slowly become inverted. ST segment is still raised. Towards the end of the first week the ST segment will be returning to the base level.

The T wave will be deeply inverted and Q wave starts appearing. The T wave is typically pointed and inverted with symmetrical limbs. In the third week the Q wave will become fully developed and the T wave will gradually become flat and start returning to normal. By the end of the 3 months the ST segment and T wave will return to normal and only the Q wave will remain permanent. Of course if the size of the infarct is very small the Q wave may completely disappear. Now let us see the same changes in a lead with negative QRS complex.

This is ST segment elevation with upright T wave on the first day. This is ST segment elevation with inverted T wave during the first week. Now a QS complex is formed with deeply inverted T wave during second and third week. And this is the picture after 3 months where the T wave has become upright and only a deep QS complex persists. Now that we have learned to recognize the typical changes in the leads facing the infarct. Let us learn how to localize the infarct. Leads 1 AVL and chest leads face the anterior wall and show the classical changes in anterior wall infarction.

Leads 2-3 AVF face the inferior wall and hence show the classical changes in inferior wall infarction. This also means that in anterior wall infarction leads 2-3 and AVF will show the reciprocal changes of ST segment depression and tall T waves. And in inferior wall infarction leads 1 AVL and the chest leads will show these reciprocal changes. To understand this concept better let us view the heart from the lateral side. These are the anterior, inferior and true posterior walls. The anterior wall is supplied by anterior descending branch of the left coronary artery.

Posterior wall is supplied by the left circumplex branch of the left coronary artery. And the inferior wall is supplied by the posterior descending branch of the right coronary artery. Now when we overlap the leads the anterior wall is picked by 1 AVL V2 to V6. V5 and V6 picking the lateral most part. Inferior wall is picked by 2-3 and AVF. And the posterior wall is picked by V1 and V2 in the form of reciprocal changes as no lead actually faces the posterior wall.

The extent of anterior wall infarction can be further studied in the chest leads. If the ST segment elevation is seen in the leads V2 to V4 it is an anterior septal infarction. In anterior septal infarction the changes may not appear in leads 1 and AVL. If the ST segment elevation is seen in leads V4 to V6 that is lateral chest leads it is an anterior lateral infarction. And if it is seen in all the chest leads it is an extensive anterior wall infarction. In this ECG leads 2, 3 and AVF on the right show marked ST segment elevation while leads 1 and AVL on the left show reciprocal ST depression.

So this is an inferior wall infarction a fresh inferior wall infarction. Now see the chest leads reciprocal ST depression is seen in V1, V2, V3 and V4 because chest leads face the anterior wall. This ECG shows ST elevation in leads 1 and AVL and reciprocal ST depression in leads 2, 3 and AVF. This is an anterior wall infarction. The chest leads show ST elevation from V1 to V5. So this is an extensive anterior wall infarction. Note in V3 ST elevation of more than 5 millimeters. In this ECG leads 1 and AVL show ST segment elevation.

The T waves are slightly inverted. So this is a 2 to 3 day old anterior wall infarction. The leads 2, 3 and AVF show marked ST segment depression suggesting an extensive infarction. More the depressions and elevations more extensive is the infarction. Note in close of view ST elevation with Q wave in anterior leads and reciprocal ST depression in inferior wall leads. Note the very tall ST segment elevations from V1 to V5. This is a fresh and extensive anterior wall infarction. This ECG shows leads 2, 3 and AVF with slight ST elevation and deeply inverted symmetrical T waves.

This indicates an inferior wall infarction which is 2 to 3 weeks old. Note the reciprocal tall T waves in 1 and AVL. This T wave inversion persists for 2 to 3 months. In this ECG the chest leads show ST elevation in V2 and V3 only. This indicates a small anterior septal infarction. It is about 2 to 3 days old as T wave has started inverting. The limb leads may not show any changes in a small anterior septal infarction. Here there is minimal ST elevation only in AVL.

In this ECG note the marked ST segment depression in inferior wall leads 2, 3 and AVF. The ST segment is depressed by up to 10 mm. There is ST elevation in AVL with tall T waves in 1 and AVL. This is a very fresh anterior wall infarction. Note again in close of view the marked ST changes. More the amplitude of ST elevations and depression more is the severity of infarct. In this ECG note the marked ST elevation in leads 2, 3 and AVF. There is reciprocal ST depression in leads 1 and AVL.

This is an inferior wall infarct. The ST elevation with upright tall T waves indicates that it is fresh less than 1 day old. In the chest leads there is reciprocal and marked ST depression from V2 to V6. More the number of leads showing STT changes more is the severity of the infarct. So this is fresh and extensive inferior wall infarct. In this ECG strip note the deep Q waves in leads 2, 3 and AVF. The ST segment is isoelectric and there is no T wave inversion. So this is an old inferior wall infarct at least 3 months old.

The same ECG note in close of view that the Q wave is deep as well as wide. A pathological Q wave. At this stage we will learn an important point about Q wave. This ECG shows a Q wave in lead 3 but there is no Q wave in 2 and AVF. When a Q wave is present only in lead 3 it may be due to a high diaphragm. Take the ECG again in deep inspiration and if the Q wave is non pathological it will reduce in size or disappear.

See this continuous tracing. In deep inspiration there is no Q wave. But on expiration the Q wave reappears. This is a normal non pathological Q wave. So if Q wave is seen only in lead 3 take a tracing in deep inspiration. Now see another tracing. No Q wave in inspiration marked I. And note the appearance of Q wave during expiration. Lead 3 of this ECG shows deep Q wave, isoelectric ST segment and deeply inverted symmetrical arrowhead like T wave of inferior wall myocardial infarction. This picture is seen between 3 weeks to 3 months.

If ST segment elevation persists after 6 months it suggests a development of ventricular aneurysm. In leads 1 and AVL of this ECG note the ST segment elevation with inverting T wave indicating a 2 to 3 day old myocardial infarction. The chest leads show ST elevation and T wave inversions in V2 to V6 and extends you anterior wall infarct. This is another inferior wall infarct with ST elevation in 2, 3 and AVF and ST depression in 1 and AVL. The T waves are just inverting so the infarct is 2 to 3 days old.

In close up view note the deep Q wave ST elevation in 3 and AVF and the reciprocal ST depression in AVL and V3.