ECG Simplified Full Series

ECG Simplified Full Series

ECG stands for electrocardiogram, the recording of the electrical activity of the heart from the body surface. Direct recording of the electrical activity of the heart from within its various chambers are called electrograms. ECG is inexpensive widely available, and equipment needed for recording is quite portable. ECG is a very important tool to the clinician, though it was invented more than a century back.

Certain important disorders like a heart attack are still quickly diagnosed with an ECG than any other modalities. Heart rhythm disorders are diagnosed mainly by ECG, though sometimes electrograms recorded from within the heart are needed for finer details.

This is how ECG machine looked like when Willem Einthoven invented it way back in 1903! Subject had to immerse three limbs in jars filled with salt water in order to get the ECG recorded. With miniaturization and microminiaturization in electronics, now even small wearable devices can record ECG. ECG monitoring has become commonplace in all emergency departments, intensive care units and operating rooms.

A basic knowledge of the electrical system of the heart is essential to understand ECG tracings. Here is a rough sketch of the electrical system of the heart. Electrical activity starts in the pacemaker cells of the sinus node, known in short as SA node. SA node is the natural pacemaker of the heart which gives regular electrical signals to the rest of the heart. It is situated in the upper part of right atrium the right upper chamber of the heart.

Signals from the sinus node travel through muscle cells of the right atrium to the next relay centre in the lower part of right atrium known as atrioventricular node or AV node in short. There is some evidence of specialized conduction pathways within the right atrium known as internodal pathways as well. A branch of one of these pathways take signals to the left atrium, the left upper chamber.

Conduction through the AV node is quite slow compared to other regions of the heart. This delay is needed in order to complete contraction of the upper chamber before the lower chambers start contracting. Contraction of upper chambers give a booster filling to the lower chambers in addition to the passive filling of blood which they receive while relaxing after a contraction. This atrial booster function is lost in conditions like atrial fibrillation, a common rhythm disorder of the upper chambers.

From the AV node, signals pass through the specialized conduction bundle known as bundle of His into the lower chambers known as ventricles. It may be noted that the upper and lower chambers are electrically isolated and conduction between them occurs only through the specialized conduction pathways normally.

Two bundle branches, known as right bundle branch and left bundle branch, take the signals to the respective ventricles. The conduction in the bundles are quite fast, unlike in the AV node. Finer branches arise from the bundle branches, known as Purkinje fibres, which take the signals to individual heart muscle cells. Surface ECG is a sum total of the electrical activity of the heart, recorded at a distance.

This animation video shows the electrical signals travelling from the sinus node to the AV node, bundle of His and through the bundle branches. Normal activation of the heart is known as sinus rhythm and it has a rate between 60-100/minute in an adult. If the heart rate is fast, it is known as sinus tachycardia and if it is slow it is known as sinus bradycardia.

ECG uses 4 limb electrodes on each of the four limbs, of which the electrode on the right leg is considered electrically neutral while the other three are active electrodes. In addition to this 6 electrodes are placed on specifically designated parts of the chest to get chest lead recordings.

Though there are only 10 electrodes which are used for recording a usual ECG, various electrode combinations can be recorded with these electrodes so that most common recording is a 12 lead ECG. Number of leads may be increased using additional electrodes in specific situations.

Waves noted in a normal ECG are called P wave, QRS complex, T wave and sometimes a U wave. Other waves which can be rarely seen are delta waves and epsilon waves. P wave represents the depolarization of the atria while QRS complex represents the depolarisation of the ventricles. T wave is due to the repolarisation of the ventricles.

PR segment is between the P wave and the QRS complex while the ST segment is between the QRS complex and the T wave. TP segment is between the T wave and the next P wave.  TP segment is considered as the true baseline in the ECG. Intervals contain one or more waves and a segment. PR interval contains the P wave and the PR segment and is measured from the onset of the P wave to the onset of the QRS complex.

QT interval contains the QRS complex, ST segment and the T wave. It is measured from the onset of the QRS complex to the end of T wave. PP interval is measured from the onset of one P wave to the onset of next P wave. RR interval is measured from the onset of one QRS complex to the onset of next QRS complex or as the interval between the peaks of two consecutive R waves for simplicity.

Delta waves occur at the onset of QRS complex and Epsilon waves are seen at the end of the QRS complex, in certain pathological conditions. A prominent U wave after the T wave can be seen when the blood potassium is low.

An initial negative deflection which is part of the QRS complex is called Q wave. An initial positive deflection is named R wave. A second negative deflection or a negative deflection following an R wave is called S wave.

A second positive deflection will be termed R’ wave. If there is a negative deflection after the R’ it is called S’. Waves less than 5 mm amplitude may be designated by small letters.

Recording a good ECG is the first step in getting a good diagnosis using it. If the recording is technically incorrect, interpretation can go wrong. First and foremost is to place the electrodes correctly in the recommended positions.

Avoiding interferences from nearby electrically operated devices is often a challenge while recording ECG in the intensive care setting. ECG monitoring leads attached to the patient may have to be temporarily removed to prevent artefacts due to alternating current picked up from these leads.

Power cords of electrically operated beds, pneumatic compression devices, infusion pumps and other electrically operated devices in close vicinity may also have to be removed from power sockets to reduce electromagnetic interference from AC line.

AC interference is seen as a symmetrical sinewave pattern in the baseline, at the frequency of the line voltage in the vicinity. It can be either 50 Hz or 60 Hz, depending on the line voltage frequency in the locality. In most modern ECG machines, notch filters are used to suppress AC interference. In spite of this, AC interference may still appear in the recorded tracings if the interference is strong. In this ECG, AC interference is best noted in leads I and III.

Equally important is avoiding interference from muscle activity – electromyogram or EMG artefacts. If patient is restless or anxious, a good explanation of the procedure and pacification often helps.

Warming the patient with a warmer or a blanket may be needed if shivering due to the cold atmosphere is noted. Sometimes we may have to switch off the air conditioner temporarily to make the person comfortable, especially in the intensive care unit or post-operative ward.

In this ECG, V1 shows multiple small artefacts almost totally obscuring the small QRS complexes. Close scrutiny with comparison with other leads enables recognition of QRS complexes within the artefacts by their timing with other QRS complexes simultaneously recorded in leads like V3, where the amplitude of the artefacts is much lower than that of the QRS complexes. This also illustrates the advantage of monitoring multiple leads simultaneously while observing for heart rhythm abnormalities.

This ECG shows artefacts due to tremor resembling a ventricular tachycardia, a fast rhythm from the lower chambers of the heart. The upper panel shows the artefacts resembling a wide QRS tachycardia and the lower panel shows the ECG with same leads when the tremor was not severe. The spikes of the QRS complexes marked by blue arrows can be seen at regular intervals even when the tremor artefacts are strong.

ECG machines have a high pass filter which passes frequencies above it and a low pass filter which passes frequencies below it to the ECG amplifier. Low pass filter is meant for filtering out high frequency interferences like muscle artefacts and high pass filter for fluctuation in baseline during breathing. Usually for reducing artefacts, default setting of filters is 0.08 – 40 Hz.

When a permanent pacemaker has been implanted, too low setting of low pass filter can make the pacing artefacts almost invisible. Low pass filter has to be kept above 100 Hz, typically 150 Hz, to make the pacing spikes evident, as shown here.

Pacemaker spikes which were not visible when the low pass filter was set at 40 Hz, are visible when low pass filter was set at 150 Hz. Pacemaker spikes are electrical signals picked up directly from the pacemaker on the body surface before it activates the heart muscle.

Most commonly recorded ECG is a 12 lead ECG. Twelve leads in a standard ECG are as follows:

Standard Limb Leads: I, II, III

Augmented Limb Leads: aVR, aVL, aVF

Chest Leads: V1, V2, V3, V4, V5, V6

Thus, a standard 12 lead ECG does not include right chest leads known as V3R, V4R, V5R etc. But in most cases 12 lead ECG includes a long rhythm strip – either lead II or V1 or both, for facilitating rhythm analysis.

If the ECG machine has only a single recording channel, 12 leads are recorded sequentially. More sophisticated recorders have facility to record 3 or even 12 channels simultaneously. Simultaneous multichannel recording can record all leads of a single beat and is better for analysis of complex heart rhythm abnormalities.

Electrode combination for limb leads are as follows:

Lead I: Left arm positive, Right arm negative

Lead II: Right arm negative, Left foot positive

Lead III: Left arm negative, Left foot positive

Unipolar limb leads are derived using the limb lead electrode potentials. All the three limb leads are internally connected to a central terminal – Wilson’s central terminal, using fixed value resistors. Usual limb lead recordings are called bipolar recordings with one limb serving as positive electrode and another as negative electrode as illustrated above.

Unipolar recordings are obtained by taking the limb electrode as positive and central terminal as negative. Unipolar limb leads are called VR, VL and VF depending on whether the positive electrode is at right arm, left arm or left leg.

Voltages of these recordings can be augmented by disconnecting the connection to the corresponding limb electrode from the central terminal, causing almost 50% increase in the amplitude of the recordings. The augmented limb leads thus generated are called aVR, aVL and aVF. Augmented limb leads were described by Goldberger in 1942.

While monitoring ECG in an intensive care unit, the limb electrodes are placed on the body at a point close to the origin of the limbs. This is to reduce movement artefacts in the tracing. Placing the leads on the body also prevents tethering of the person and allows free movement of the limbs. Same method is followed while recording ECG during an exercise test.

Most modern monitors have the option to use chest leads in addition to limb leads. When chest lead monitoring is needed, additional electrodes are placed on the chest. Chest lead monitoring is useful in picking up ST segment changes. ST segment changes occur when there is a decrease in blood supply to heart muscle. This may be associated with chest pain.

Location of the leads on the chest depends on the type of patient. While locations on the front of the chest are available in chest pain patients, a surgical patient may require different locations depending on the location of operation and dressings. Leads placed in the food pipe may be used in some rare situations to decipher difficult heart rhythm abnormalities. These are known as esophageal leads.

Mason-Likar modification of 12 lead ECG is most popular during treadmill exercise test. It can also be used in the electrophysiology laboratory along with electrodes within the heart. In this lead system, limb electrodes are placed on the nearest location on the body to prevent artefacts due to limb movement during exercise.

Chest electrodes are placed in the conventional positions. Difference in pattern of the modified 12 lead ECG mandates caution while trying to interpret Q waves and other abnormalities on a Mason-Likar modification. This system is mainly meant for assessment of ST segment deviations and heart rhythm abnormalities during exercise test.

Lewis lead is a modified lead to enhance the amplitude of P waves and thereby enable better arrhythmia analysis. Right arm and left arm leads are repositioned on the chest as shown in the image. This lead system is useful in detecting P waves during a wide QRS tachycardia and helps in differentiating between ventricular and supraventricular tachycardia. Supraventricular tachycardias are abnormal rhythms originating above the ventricles.

Atrial repolarization wave or Ta wave is usually not evident in the ECG as it has a low amplitude of 100 – 200 microvolts and is usually hidden in the QRS complex. It can also extend into the ST segment causing ST segment depression, especially during an exercise test and cause a false positive response. A representative image of the Ta wave is shown here.

A modified limb lead system for detection of atrial repolarization on surface ECG has been devised by Sivaraman J and colleagues. This lead system has been shown to be useful in detecting atrial repolarization on surface ECG, more so in case of complete heart block, where most of the atrial repolarization activity is not masked by ventricular activity.

Now we will see how the cardiac chamber enlargements are inferred from the ECG. Enlargement of the atria are reflected in the P wave which represents the atrial electrical activity. Normal P wave has a maximum amplitude of 2.5 mm and width of 2.5 mm on the ECG.

An increase in the amplitude of P wave is an indication of right atrial enlargement and increase in width of P wave an indication of left atrial enlargement. When both atria are enlarged, both width and amplitude of the P wave are increased.

Diagrammatic representation of left atrial enlargement with wide and notched P wave, known as P mitrale.

Diagrammatic representation of right atrial enlargement with tall peaked P waves known as P pulmonale.

Ventricular enlargement is indicated in the QRS complex, the activity of the ventricles. There may be secondary changes in ST segment and T waves. ECG changes in left ventricular hypertrophy, the technical name for thickening of left ventricle, is divided into left ventricular volume overload and pressure overload patterns. Similarly, right ventricular hypertrophy is also divided into pressure and volume overload patterns.

In the ECG, left ventricular volume overload is indicated by small initial q wave, tall R wave and upright tall T waves in V5, V6. Left ventricular volume overload can occur if there is a leak in the aortic or mitral valve or large defect in the wall between the two ventricles.

Tall R wave in V5, V6 will be there in left ventricular pressure overload as well. ST segment will show a down sloping depression and the T wave will be inverted. These changes are called strain pattern. This pattern can occur in high blood pressure and in obstruction to the aortic valve.

A simple measurement for left ventricular enlargement on ECG is Sokolow-Lyon criteria, which is often printed on automated reports. Sum of S wave in V1 and R wave in V5 or V6 more than 35 mm OR R wave in V5 or V6 more than 26 mm is considered as significant as per this criteria.

This is based on tall R waves in leads facing the left ventricle and deep S waves in leads facing the right ventricle.

Right ventricular volume overload manifests as rSR’ pattern in lead V1, which is also known as incomplete right bundle branch block pattern. It is typically seen when there is a defect in the wall between the two upper chambers of the heart.

Right ventricular pressure overload manifests with tall R wave, down sloping ST segment depression and T wave inversion in V1. The pattern is similar to that of left ventricular strain pattern but noted in leads facing the right ventricle. It occurs when the pressure in the blood vessels of the lungs are high or when there is obstruction to the pulmonary valve.

Mean electrical axis can be calculated for the P wave, QRS complex or the T wave. It can even be calculated for the segments like ST segment, though it is seldom done. Most often only the mean QRS axis is calculated and when electrical axis is mentioned, generally only mean QRS axis is meant. QRS axis can be altered in various disease states like conduction defects within the ventricle, heart attack, enlargement of heart chambers and change in position of the heart.

For calculation of mean axis of the QRS complex, the amplitude of the negative waves is subtracted from that of the positive waves while plotting on the graph using lead I as X-axis and aVF as Y-axis. The electrical axis of lead I is zero and that of aVF +90 degrees. Resultant is taken as the mean QRS vector and the deviation from lead I the mean QRS axis measurement.

Now we will move on to abnormalities in the conduction of electrical signals within the heart. Conduction between the sinus node and the atrium are known as sinoatrial conduction abnormalities. Abnormalities in the conduction between the upper and lower chambers is known as atrioventricular conduction abnormalities. Abnormalities of conduction within the ventricles are called intraventricular conduction defect. Similarly, there could be inter atrial and intra-atrial conduction abnormalities as well. The former is between the two upper chambers while the latter is within the upper chamber.

Sinoatrial conduction abnormalities could be first degree, second degree or third degree SA blocks. But it is not possible to identify first degree and third degree SA blocks on the surface ECG as the sinus node activity is not visible. First degree SA block means a delay in conduction from the sinus node to the atrium, without any loss of signals.

In second degree SA block, there is intermittent conduction across the sinoatrial junction. This is manifested by intermittent absence of P waves in the surface ECG. Second degree SA block is the only type which can be diagnosed from the surface ECG.

Third degree SA block means that there is total absence of conduction from the sinus node to the atrium. In the surface ECG this will manifest indirectly as absence of P waves originating from the sinus node. P waves originating from another part of the atrium may be there, with a different shape and electrical axis. Total absence of sinus node activity and total absence of sinoatrial conduction cannot be differentiated from surface ECG.

Sinus arrest is another condition different from sinoatrial block. In sinus arrest, sinus node fails to produce signals. It could be intermittent or prolonged sinus arrest. Intermittent sinus arrest will appear like skipping of P wave, while prolonged sinus arrest will appear as a prolonged pause. Sometimes a slower pacemaker at the atrioventricular junction or from the ventricles may take over signal generation.

Sinus arrest is one of the manifestations of a disease known as sick sinus syndrome. Symptomatic sinus pauses are treated by implantation of a permanent pacemaker. Pacemaker is an electronic device, usually implanted under the skin and connected to the heart using electrode wires introduced through blood vessels known as leads.

Interatrial block has been well recognized in ECG only recently. In partial interatrial block, the P wave is wide and notched, without left atrial enlargement. In advanced interatrial block, the P wave is wide and biphasic in leads II, III and aVF. Inversion of terminal portion of P wave is because of delayed activation of the left atrium from below upwards due to the block in the conduction bundle in the upper part. Upward activation is shown as negative in leads from the lower part of the body, II, III, aVF as the activation moves away from those leads.

Atrioventricular block is divided into first degree, second degree and third degree or complete heart block. In first degree AV block, there is only prolongation of conduction without any P wave being non-conducted. It manifests in ECG as a prolonged PR interval.

In second degree AV block, there are intermittent non-conducted P waves. There are two types of second degree AV blocks. In type I, there is progressive prolongation of PR interval followed by a non-conducted P wave. In type II there is no progressive prolongation of PR interval, but there are intermittent non-conducted P waves. Type II is more serious.

In complete heart block, also known as complete AV block, none of the P waves are conducted to the ventricles. The ventricles are activated by a subsidiary pacemaker either from the AV node or the ventricles. P waves are seen at a higher rate than the QRS complexes, without any relationship between the two. If the subsidiary rhythm is originating from the AV node, the QRS complex will be narrow. When the subsidiary rhythm originates from the ventricle, the QRS is wide and the heart rate lower.

Now we will move one to heart rhythm disorders originating from the upper part of the heart known as supraventricular arrhythmias. Supraventricular means above the ventricles. Supraventricular ectopic beats are premature beats originating above the ventricles. Supraventricular ectopics are the commonest form of supraventricular arrhythmia. They can occur in isolation or in sequences of couplets. If more than three are seen in a sequence it will constitute a short run of supraventricular tachycardia.

Supraventricular ectopics are common in the setting of atrial dilatation due to chronic obstructive lung disease or reduced pumping function of left ventricle. Supraventricular ectopic beats have a narrow QRS complex preceded by an abnormal P wave. They are also called supraventricular premature complexes as they occur before the next expected sinus beat. In this ECG shown, the abnormal P wave is superimposed on the preceding T wave. QRS complex of SVPC is almost like that of the normal sinus beat. Abnormal beat is followed by a pause.

Supraventricular tachycardia is a fast regular rhythm Supraventricular tachycardia can be recognized on the ECG as a narrow QRS tachycardia with either absent P waves or P waves which are distinctly different from those in sinus rhythm. There are several types of supraventricular tachycardias, based on the mechanism. Commonest form in adults is called AV nodal re-entrant tachycardia or AVNRT. It is due to a circus movement of signals within the AV node.

Supraventricular tachycardia is often initiated by a supraventricular ectopic beat which sets off the circus movement. This ECG shows a supraventricular ectopic with a different type of P wave, marked by the red line, initiating a run of supraventricular tachycardia. This sequence was captured from the storage memory of the intensive care central monitor. SVT is the short form for supraventricular tachycardia.

Preceding sinus rhythm shows regular sequence of P waves followed by QRS complexes. During SVT, P waves are not seen because they are occurring along with the QRS complexes in AVNRT type of supraventricular tachycardia.

In this case, the supraventricular tachycardia terminated spontaneously. This was followed by a pause and then a ventricular premature complex and two beats originating from the AV junction, marked as J in the tracing. Following beat looks like a ventricular ectopic beat super imposed on a sinus P wave. Last beat in the sequence is a normal sinus beat. Such irregular sequences can occur after termination of a tachycardia.

This ECG shows a regular junctional rhythm originating from the AV junction. Tall peaked T waves marked by pink arrows, is another associated abnormality, not due to the junction rhythm. Tall peaked T waves can occur when the blood level of potassium is high. No P waves are visible in this junctional rhythm as atrial activation coincides with ventricular activation. This is because signals pass simultaneously upwards and downwards in this junctional rhythm. Junctional rhythm occurs as a subsidiary rhythm when the sinus node is suppressed due to some reason or when there is complete sinoatrial block.

This ECG shows another variety of junctional rhythm in which P’ waves are seen after the QRS complexes. This is because the rhythm originates from the lower part of the AV junction. It takes more time to reach the atria than the ventricles. So, a retrograde P’ wave is seen after the ventricular activation, which is the QRS complex. Timing of the T waves, which is different from that of the retrograde P’ waves have been marked in lead V2.

In this ECG, P waves, marked by blue arrows, are inverted in leads II, III, and aVF. These leads are directed towards the lower part of the body. Hence inverted P waves in these leads means that the atrial activation is proceeding from below upwards. Retrograde activation from below upwards indicates that activity originates from the lower part of atrium, known as low atrial rhythm. This rhythm also occurs when the sinus node is not functioning well, as a subsidiary rhythm.

This ECG shows multiple morphologies of P waves, marked M, seen in short runs of tachycardia. This is characteristic of multifocal atrial tachycardia. Sinus P waves marked S are seen after the pauses. Multifocal atrial tachycardia is sometimes called as chaotic atrial rhythm because of the variability in atrial activity. Multifocal atrial tachycardia can occur in those with chronic obstructive lung disease.

Atrial fibrillation seen in this ECG, is the most common sustained arrhythmia, originating from the upper chambers of the heart. AF can be recognized in the ECG with absence of organized atrial activity and the presence of fibrillary waves. AF can be either fine AF or coarse AF depending on the amplitude of fibrillary waves. In this ECG it is fine atrial fibrillation because the fibrillary waves are hardly seen in most leads.

In atrial fibrillation, the atrial rate is very high, of the order of 450-600/min. But most of the signals are blocked in the AV node so that the ventricular rate does not go that high. Still, it may be around 120/min in most cases. It can be slowed down by using medications. In this ECG, the ventricular rate is controlled.

Atrial flutter is a regular rhythm originating from the upper chambers of the heart. The atrial rate may be 250 to 350/min or even more. Difference from atrial fibrillation is that organized atrial activity is present. But all the signals are not usually conducted down to the ventricles in atrial flutter. In the ECG illustrated only alternated flutter waves are conducted down. Hence the ventricular rate will be only half that of the atrial rate. If all the flutter waves are conducted down, as occurs rarely, the ventricular rate will be dangerously high.

Next comes ventricular arrhythmias. Ventricular ectopic beats are also known as ventricular premature beats or complexes (VPB or VPC, sometimes PVC – premature ventricular complexes). Ventricular premature beats are easily recognized on the ECG by their wide bizarre QRS complexes, not preceded by a P wave. Secondary ST depression and T wave inversion when the QRS is dominantly positive are also seen. Ventricular ectopic beats are followed by a compensatory pause before the next sinus beat. Coupling interval is the interval between the VPC and the preceding sinus beat.

Initial two VPCs in this ECG are isolated while the last two occur in rapid sequence as a couplet. VPC couplets may be a forerunner of ventricular tachycardia. Isolated VPCs and the couplet are followed by a compensatory pause. All the VPCs have same shape or morphology (monomorphic) indicating the same focus of origin (unifocal). Unifocal VPCs usually have the same coupling interval.

ECG showing ventricular ectopic bigeminy, with grouped beating of two beats followed by a pause. Each narrow QRS sinus beat is followed by a wide QRS ventricular ectopic beat. Narrow QRS sinus beat has a preceding P wave, while the wide QRS ventricular ectopic beat does not have a preceding P wave.

Monitor screenshot showing ventricular premature complexes occurring in a trigeminal sequence, another form of grouped beating. Two sinus beats followed by a ventricular ectopic beat – ventricular trigeminy or VPC trigeminy.

ECG showing VPC salvo. The original meaning of the word salvo is a series of shots fired in rapid sequence from a gun or simultaneously from multiple guns. Here it is a salvo of three consecutive ventricular ectopic beats. It can also be considered as a short run of non-sustained ventricular tachycardia. Isolated VPC is seen subsequently.

This ECG shows multiple short runs of ventricular tachycardia. Ventricular tachycardia is a fast rhythm originating from the lower chambers of the heart. If it lasts less than 30 seconds, it is called non-sustained ventricular tachycardia (NSVT). Ventricular tachycardia is a much more dangerous arrhythmia than supraventricular tachycardia.

Here it is a sustained ventricular tachycardia, which typically lasts more than 30 seconds. Sustained ventricular tachycardia is a medical emergency. If it is not responding to medications, a controlled direct current shock is given using a defibrillator to abolish this rhythm. After that, usually sinus rhythm is restored spontaneously. This is a monomorphic ventricular tachycardia, meaning that all beats have the same shape in a given ECG lead.

This is another arrhythmia originating from the ventricles, known as accelerated idioventricular rhythm. Rate is slower than 100 beats per minute and does not qualify for the designation of tachycardia. AIVR is a classical arrhythmia seen after a blocked blood vessel to the heart muscle is opened up either by clot dissolving medication or by mechanical removal in angioplasty. AIVR usually needs no specific medical treatment as it subsides soon. QRS is wide as in other rhythms originating from the ventricles.

Ventricular fibrillation or VF is a life-threatening arrhythmia, which leads to death unless promptly corrected by electrical defibrillation (direct current or DC shock). In VF, the electrical activity is so disorganized that no ventricular contraction is possible, and ventricle remains still in a state of cardiac arrest. Ventricular fibrillation is recognized on the ECG as a highly disorganized electrical activity. Each wave has a different shape. When multiple simultaneous leads are available as in this case, the shape is different between the leads as well. If defibrillation is not immediately available, chest compressions and artificial breaths have to be started immediately, that is cardiopulmonary resuscitation or CPR.

Now we will have a look at ECG in coronary artery disease. Coronary artery disease is disease of blood vessels of the heart. Sudden blockage of a blood vessel of the heart usually produces a heart attack due to damage to a region of the heart muscle. It has certain important ECG changes which makes diagnosis possible in the emergency room or at patient side using a portable ECG machine.

Technical term for a heart attack is myocardial infarction. There are basically two types of myocardial infarction on ECG. One shown here is ST segment elevation myocardial infarction, popularly known as STEMI. Here the ST segment is seen elevated in leads II, III and aVF, marked by light blue arrows. In addition, there is second degree AV block with a non-conducted P wave marked by a pink arrow in lead II rhythm strip at the bottom of the tracing. Conducted P wave is marked by blue arrow.

This particular variety is called inferior wall STEMI because the ST elevation is seen in leads oriented to the lower part of the body, II, III, aVF. Inferior wall means the lower wall of the left ventricle. Atrioventricular conduction disturbances are common with inferior wall myocardial infarction. If there is a complete heart block, meaning that none of the electrical signals from the sinus node are conducted to the ventricles, the heart rate will be low. Some of them will require insertion of a temporary pacemaker to support the heart rate.

Another ECG showing gross ST segment elevation in leads V1 to V6, known as anterior wall myocardial infarction. Anterior wall is the front wall of the left ventricle. Maximum ST segment elevation of 7 mm is seen in V4. Extent of ST segment elevation and the number of leads showing ST segment elevation give a suggestion on the region of heart muscle affected as well as the severity. This one is more dangerous than the previous ECG showing inferior wall myocardial infarction.

ST segment elevation is seen soon after the blood vessel becomes blocked in a heart attack. When the person recovers from the heart attack, ST segment elevation subsides and becomes isoelectric as seen in several leads in this ECG. The T waves become inverted as seen in leads V2-V4. Abnormal Q waves appear, indicating loss of electrical activity of heart muscle in that region, often called an electrical window. Q waves are the reflection of unopposed electrical activity from the opposite wall of the ventricle. Q wave is an initial negative deflection of the QRS complex.

This a magnified X-ray image of a blocked blood vessel marked by yellow arrow. The test is known as coronary angiogram and is obtained by injecting radiocontrast medication directly into the blood vessel under continuous X-ray imaging. Small tubes known as catheters are introduced through the blood vessel of the wrist or groin and guided to the blood vessel of the heart under X-ray imaging in  a special procedure room known as cardiac catheterization laboratory. This test is usually done prior to mechanical removal of the block by angioplasty.

Another ECG showing old inferior wall myocardial infarction with pathological (abnormal) Q waves with T inversion in leads III and aVF. Additional findings marked are minimal ST segment depression in V6 and tall T waves in V4. These changes can occur when the blood supply to the left side of left ventricle is compromised. ST segment is isoelectric in the region of old myocardial infarction.

After a discussion on ECG changes in blocks of blood vessels of the heart, we will see what happens when there is a block in the blood vessels of the lungs. When a blood clot is carried in blood circulation to the lungs, it blocks blood vessels there. This is another life threatening condition called pulmonary embolism. Typical ECG pattern in pulmonary embolism is called S1,Q3,T3 pattern. This means that there is an S wave in lead I, Q wave in lead III and T inversion in lead III. Though this is the classical description, several other patterns are also possible in pulmonary embolism.

ECG can be abnormal when electrolyte levels in blood are abnormal. Important electrolytes in blood which can affect the ECG are potassium, magnesium, and calcium. Potassium and calcium are involved in the ionic currents which generate the electrical signals of the heart. Magnesium is important in regulating the levels of calcium and potassium by enzymes involved in transfer of these ions across the kidney. The ECG illustrated shows prolongation of ST segment with low calcium levels as calcium ions are involved in the ion currents during the inscription of the ST segment.

Coming back to our previous discussion on conduction disturbances of the heart, we have intraventricular conduction abnormalities or conduction defects in the ventricles. There could be blocks in the right bundle branch or left bundle branch, with specific patterns in the ECG. Right bundle branch block is known in short as RBBB, while left bundle branch block is known as LBBB.

ECG showing right bundle branch block with RSR’ pattern in V1 and slurred S wave in I, aVL, V5 and V6. QRS duration is 120 ms. Shallow T inversion is noted in V1,V2. R’ wave is due to delayed electrical activation of the right ventricle. When there is right bundle branch block, activation of the right ventricle occurs by conduction from the left ventricle through the heart muscle.

This is slower than the conduction through the bundle branches and causes delayed activation of the right ventricle, at a time when it is not opposed by electrical signals from the left ventricle. Slurred S wave in lead I also represents delayed activation of right ventricle. The negative wave is because lead I is oriented towards the left ventricle. Increased QRS duration is also due to the delay in activation. If the QRS duration is less than 120 ms, with the same RSR’ pattern, it is called incomplete right bundle branch block pattern or IRBBB.

Left bundle branch block or LBBB manifests with M shaped pattern in leads I, aVL, V5 and V6. A somewhat W like pattern may be seen in lead V1. But sometimes the notch in the W pattern may not be there and the pattern in V1 may be just a wide Q wave as seen in V1-V3 in this ECG. Widening of QRS is due to delayed activation of left ventricle from the right ventricle, by slow conduction through the heart muscle. Conduction velocity is much lower in heart muscle, compared to the bundle branches which are part of the specialized conduction system of the heart. The wide Q wave in left bundle branch block may be mistaken for the Q wave in old myocardial infarction.

Another ECG with LBBB pattern with much wider QRS complex. The notched R waves resembling M pattern in leads I, aVL, and V5 are more evident in this case, marked by blue arrow. Almost W pattern is seen in V1 and V3, marked by violet arrow. The wider QRS complex in this ECG indicates associated heart muscle damage. In this case it was due to a previous heart attack.

A pattern similar to LBBB is seen when the right ventricle is paced using a pacemaker, either temporary or permanent. This is because the right ventricle is activated first and left ventricle later, by conduction through the heart muscle, just as in left bundle branch block. Pacemaker signal known as pacing stimulus artifact is seen just before the QRS complex. It is a sharp vertical deflection, which can be sometimes ironed out by a lower low pass filter setting as discussed earlier. If the pacemaker artifact is not visible due to technical problem, the ECG will be mistaken for left bundle branch block. In this ECG also, pacing artifact is hardly visible in lead I.

Another interesting conduction system abnormality is the presence of an accessory atrioventricular conduction bundle. This accessory pathway bypasses the normal AV nodal delay and activates the ventricles earlier than expected. This is known as ventricular pre-excitation. Presence of accessory conduction pathway can lead to fast heart rhythms known as atrioventricular re-entrant tachycardia (AVRT) due to circus movement of signals between the normal and abnormal pathways.

Characteristic ECG finding in an accessory pathway is a delta wave due to early activation of the ventricles. Delta wave gets its name from the resemblance to the Greek alphabet Delta. Early activation of the ventricles, bypassing the AV nodal delay also causes a decrease in the PR interval. Accessory pathway with recurrent tachycardia is known as Wolff-Parkinson-White (WPW) syndrome.

Another ECG abnormality which can cause heart rhythm abnormality is Brugada syndrome. In Brugada syndrome, there is ST segment elevation in leads V1-V3, which may mimic a STEMI. There is also an R’ wave resembling right bundle branch block pattern. Brugada syndrome can lead on to life threatening heart rhythm abnormalities.

Long QT syndromes are a group of genetically mediated diseases which are prone for life threatening heart rhythm disorders. Like Brugada syndrome, it is due to defect in ion channels of the heart. Hence, they are also called cardiac channelopathies. The QT interval is prolonged in this group of disorders, which leads to heart rhythm abnormalities. Though there are several formulae for correction of QT interval for the heart rate, a simple rule of the thumb is that if QT interval is more than half of the RR interval, it is likely to be prolonged.

This is another simple ECG finding known as electrical alternans, in which the amplitude of QRS complexes alternates between beats. This occurs in a condition known as pericardial effusion, in which large quantity of fluid collects within the layers of the covering of the heart. In one cardiac cycle the heart comes closer to the ECG lead and records a higher amplitude while in the next beat it moves away, within the fluid surrounding the heart.

In this echocardiogram, an ultrasound image of the heart, fluid surrounding the heart has been marked as PE, in short for pericardial effusion. It can be seen that between the two frames, heart is swinging to either side. This changes the conduction of signals from the heart to surface ECG electrodes, producing electrical alternans. Due to the fluid covering the heart, the ECG signals are dampened, producing low voltage complexes in pericardial effusion, in addition to electrical alternans.

A simple birth defect of the heart is having the heart on the right side of the chest instead of the normal left side. This swings the electrical axis of the heart to the right. Lead I records negative waves instead of the normal positive complexes. In a normal ECG, the amplitude of R waves increases as we proceed from V1 to V5. There is a reverse progression in dextrocardia as the heart is on the opposite side. Amplitude of QRS complexes decreases from V1 to V6. Actual amplitudes can be recorded by keeping the chest leads in reverse order on the right ride of the chest, known as right chest leads. Right chest leads are designated as V3R, V4R, V5R and V6R.

This has been a very concise description of the concepts in electrocardiography in a simplified manner. The field of electrocardiography has grown a lot over the past century. Automated algorithms for analysis have come a long way due to the enhancements in information technology. One of the latest advances is ECG imaging (ECGi) which uses a large number of chest electrodes worn as a vest. ECGi can give various types of maps of the electrical activity of the heart like a similar mapping using multiple electrodes within the heart, but non-invasively. These images are integrated with computed tomography (CT) images of the heart to give valuable information.