Arrhythmia Definition.
Arrhythmia refers to irregular heartbeat or disturbance in the rhythm of heart. In arrhythmia, heartbeat may be fast or slow or there may be an extra beat or a missed beat. It occurs in physiological and pathological conditions.
Classification of Arrhythmia.
In arrhythmia, SA node may or may not be the pacemaker. If SA node is not the pacemaker, any other part of the heart such as atrial muscle, AV node and ventricular muscle becomes the pacemaker. Accordingly, arrhythmia is classified into two types:
A. Normotopic arrhythmia
B. Ectopic arrhythmia.
Keyword : Arrhythmia and Classification of Arrhythmia, Normotopic Arrhythmia and sinus arrhythmia , Causes of Sinus Arrhythmia and Sinus tachycardia , Ectopic Arrhythmia and different Ectopic Arrhythmia, Atrioventricular block and Abnormal pacemaker .
A. Normotopic arrhythmia.
Normotopic arrhythmia is the irregular heartbeat, in which SA node is the pacemaker. Normotopic arrhythmia is of three types:
1. Sinus arrhythmia.
2. Sinus tachycardia.
3. Sinus bradycardia.
1. Sinus arrhythmia.
Sinus arrhythmia is a normal rhythmical increase and decrease in heart rate, in relation to respiration. It is also called respiratory sinus arrhythmia (RSA). Normal sinus rhythm means the normal heartbeat with SA node as the pacemaker.
Normal heart rate is 72 per minute. However, under physiological conditions, in a normal healthy person, heart rate varies according to the phases of respiratory cycle. Heart rate increases during inspiration and decreases during expiration.
ECG Changes in Sinus arrhythmia.
ECG is normal during sinus arrhythmia. Only the duration of R-R interval varies rhythmically according to phases of respiration . It is shortened during inspiration and prolonged during expiration .
Cause of Sinus arrhythmia.
Sinus arrhythmia is due to fluctuation in the discharge of impulses from SA node . During inspiration, the lungs are inflated and the intrathoracic pressure decreases. This increases the venous return. Inflation of lungs stimulates the stretch receptors of lungs, which send impulses to vasodilator area (cardioinhibitory center) through afferent fibers of vagus. It leads to reflex inhibition of vasodilator area and reduction in vagal tone. Because of these two factors, heart rate increases.
Simultaneously, increased venous return initiates Bainbridge reflex that causes increase in heart rate . During expiration, the lungs are deflated and intrathoracic pressure increases. This decreases the venous return. During deflation of lungs, the stretch receptors are not stimulated and vasodilator area is not inhibited. So, vagal tone increases, resulting in decreased heart rate. Simultaneously, decreased venous return abolishes Bainbridge reflex. It also decreases the heart rate.
2. Sinus tachycardia.
Sinus tachycardia is the increase in discharge of impulses from SA node, resulting in increase in heart rate. Discharge of impulses from SA node is very rapid and the heart rate increases up to 100/minute and sometimes up to 150/minute.
ECG Changes in Sinus tachycardia.
ECG is normal in sinus tachycardia, except for short R-R intervals because of increased heart rate .
Conditions when Sinus Tachycardia Occurs.
Sinus tachycardia occurs in physiological as well as pathological conditions.
Physiological conditions when tachycardia occurs .
1. Exercise.
2. Emotion.
3. High altitude.
4. Pregnancy.
Pathological conditions when tachycardia occurs.
1. Fever.
2. Anemia.
3. Hyperthyroidism.
4. Hypersecretion of catecholamines.
5. Cardiomyopathy.
6. Valvular heart disease.
7. Hemorrhagic shock.
Features of Sinus Tachycardia .
1. Palpitations (sensation of feeling the heartbeat).
2. Dizziness.
3. Fainting.
4. Shortness of breath.
5. Chest discomfort (angina).
3. Sinus bradycardia.
Sinus bradycardia is the reduction in discharge of impulses from SA node resulting in decrease in heart rate. Heart rate is less than 60/minute.
ECG Changes in Sinus bradycardia.
ECG shows prolonged waves and prolonged R-R interval.
Conditions when Sinus Bradycardia Occurs .
Sinus bradycardia occurs in both physiological and pathological conditions. It occurs during sleep. It is common in athletes due to the cardiovascular reflexes, in response to increased force of contraction of heart.
Physiological conditions when sinus bradycardia occurs .
1. Sleep.
2. Athletic heart.
Pathological conditions when sinus bradycardia occurs.
1. Disease of SA node.
2. Hypothermia.
3. Hypothyroidism.
4. Heart attack.
5. Congenital heart disease.
6. Degenerative process of aging.
7. Obstructive jaundice.
8. Increased intracranial pressure.
9. Use of certain drugs like beta blockers, channel blockers, digitalis and other antiarrhythmic drugs .
10. Atherosclerosis. Bradycardia due to atherosclerosis of carotid artery, at the region of carotid sinus is called carotid sinus syndrome.
Features of Sinus Bradycardia .
1. Sick sinus syndrome.
2. Fatigue.
3. Weakness.
4. Shortness of breath.
5. Lack of concentration.
6. Difficulty in exercising.
Sick sinus syndrome.
Sick sinus syndrome is the common feature of sinus bradycardia. It is the condition characterized by dizziness and unconsciousness.
Ectopic Arrhythmia .
Ectopic arrhythmia is the abnormal heartbeat, in which one of the structures of heart other than SA node becomes the pacemaker. Impulses produced by these structures are called ectopic foci.
Subtypes of Ectopic Arrhythmia .
Ectopic arrhythmia is further divided into two subtypes:
1. Homotopic arrhythmia, in which the impulses for heartbeat arise from any part of conductive system.
2. Heterotopic arrhythmia, in which the impulses arise from the musculature of heart other than conductive system.
Different Ectopic Arrhythmia.
1. Heart block.
2. Extrasystole.
3. Paroxysmal tachycardia.
4. Atrial flutter.
5. Atrial fibrillation.
6. Ventricular fibrillation.
1. Heart block.
Heart block is the blockage of impulses generated by SA node in the conductive system. Because of the blockage, the impulses cannot reach the cardiac musculature, resulting in ectopic arrhythmia. Based on the area affected, the heart block is classified into two types :
1. Sinoatrial block.
2. Atrioventricular block.
Sinoatrial Block – AV Nodal Rhythm .
Sinoatrial block is the failure of impulse transmission from SA node to AV node. It is also called sinus block. During sinoatrial block, heart stops beating. Immediately, AV node takes over the pacemaker function and produces the impulses. This leads to AV nodal (atrioventricular) rhythm.
Sinoatrial block is due to the defect in internodal fibers and it occurs suddenly. Initially, the heart stops for a while. Then after few seconds, the AV node becomes the pacemaker and the heart starts beating with decreased rate of 40 to 60/minute.
Impulses may be discharged from any part of AV node, viz.
1. In upper nodal rhythm, the impulses are discharged from the upper part of AV node. In this rhythm, the P wave of ECG is inverted. QRS complex and T wave are normal
2. In middle nodal rhythm, the impulses are by the middle part of AV node. Here, all the chambers of the heart contract simultaneously. P wave of ECG is absent as it merges with QRS complex.
3. In lower nodal rhythm, the impulses are produced by the lower part of AV node. In this condition, ventricular contraction occurs prior to atrial contraction as the impulses reach the ventricles prior to the atria. In ECG, QRS complex appears prior to P wave and R-P interval is obtained instead of P-R interval. It is called reversed heart block.
Atrioventricular Block.
Atrioventricular block is the heart block in which the impulses are not transmitted from atria (from AV node) to ventricles because of defective conductive system. Atrioventricular block is of two categories:
1. Incomplete heart block.
2. Complete heart block.
1. Incomplete Heart Block .
Incomplete heart block is the condition in which the transmission of impulses from atria to ventricles is slowed down and not blocked completely. Impulses reach ventricles late.
Incomplete heart block is of four types :
a. First degree heart block.
b. Second degree heart block.
c. Wenckebach phenomenon.
d. Bundle branch block.
a. First degree heart block.
First degree heart block is the heart block in which the conduction of impulses through AV node is very slow, i.e. the AV nodal delay is longer. It is also called delayed conduction. In ECG, the P-R interval is very much prolonged and is more than 0.2 second. First degree heart block is common in young adults and trained athletes. It is also caused by rheumatic fever and some drugs. It does not produce any symptom.
b. Second degree heart block.
Second degree heart block is the type of heart block in which some of the impulses produced by SA node fail to reach the ventricles. It is also called the partial heart block. When some of the impulses from SA node fail to reach the ventricles, one ventricular contraction occurs for every 2, 3 or 4 atrial contractions, i.e. 2 : 1, 3 : 1 or 4 : 1. In ECG, the ventricular complex (QRST) is missing accordingly. During frequent development of second degree heart block, bradycardia occurs.
c. Wenckebach phenomenon or syndrome.
Wenckebach phenomenon is a type of heart block characterized by progressive increase in AV nodal delay, resulting in missing of one beat. Afterwards, the conduction of impulse is normal or slightly delayed. In ECG, the progressive lengthening of P-R interval is noticed till QRST complex disappears.
d. Bundle branch block.
Bundle branch block (BBB) is the heart block that occurs during dysfunction of right or left branch of bundle of His. During this type of block, the impulse from atria reaches unaffected ventricle first. Then, from here, the impulse travels to the affected side. So, ECG shows normal ventricular rate, but the QRS complex is prolonged or deformed.
2. Complete Heart Block (Third degree heart block).
Complete heart block is the condition in which the impulses produced by SA node cannot reach the ventricles. It is also called complete atrioventricular block or third degree heart block. Because of this, the ventricles beat in their own rhythm, independent of atrial beat. It is called idioventricular rhythm.
Complete heart block occurs due to any one of the following causes:
a. Disease of AV node, which leads to AV nodal block.
b. Defective conductive system below the level of AV node, causing infranodal block.
a. AV nodal block.
In this type of block, a part of AV node is defective and the unaffected part becomes the pacemaker. Rhythmicity of AV node is about 45 to 60/minute.
b. Infranodal block.
Infranodal block is the heart block in which the impulses from SA node are blocked in the branches of bundle of His (below the level of AV node). In this condition, the distal part of the conductive system (i.e. the Purkinje fibers) becomes the pacemaker. The rhythmicity of Purkinje fibers is about 35/minute.
Sometimes, a part of ventricular musculature becomes the pacemaker and the ventricular rate in such conditions is about 20/minute. Third degree heart block is the serious one since it decreases the pumping action of the heart. Very often, it results in Stokes-Adams syndrome. It may also cause heart failure.
Stokes-Adams syndrome.
Stokes-Adams syndrome is the sudden attack of dizziness and unconsciousness caused by heart block. It may be accompanied by convulsions also. In many patients suffering from heart block, the complete heart block occurs intermittently. When the block occurs, the ventricles stop beating immediately. Ectopic pacemaker (AV node, Purkinje fiber or ventricular muscle) starts functioning only after 5 to 30 seconds.
During this time, the blood circulation is affected because of lack of ventricular output. Brain cannot withstand the stoppage of blood supply and oxygen supply even for 5 seconds. Before the onset of discharge from ectopic pacemaker, dizziness and fainting occurs. If the discharge of impulses from ectopic pacemaker is delayed beyond 30 seconds, death occurs.
Extrasystole and Compensatory Pause.
Extrasystole is the premature contraction of the heart before its normal contraction. It is caused by an ectopic focus (discharge of an impulse from any part of the heart other than the SA node). The ectopic focus produces an extra beat of the heart that is always followed by a compensatory pause. Compensatory pause is the period during which the heart stops in relaxed state.
Cause for the compensatory pause.
In the cardiac muscle, absolute refractory period extends throughout contraction period. When the heart is in extrasystole (because of ectopic focus), an impulse is discharged from natural pacemaker, SA node. As this natural impulse reaches the myocardium during the contraction period of extrasystole, the myocardium does not give response, because it is refractory now.
For the next beat, the heart has to wait till the discharge of next natural impulse from SA node. During this time, the heart stops in diastole. It is the cause for compensatory pause . Parts of the heart which give origin for ectopic foci are AV node, bundle of His, atrial musculature and ventricular musculature.
Accordingly, extrasystole is divided into three types:
1. Atrial extrasystole.
2. Nodal extrasystole.
3. Ventricular extrasystole.
1. Atrial Extrasystole.
Atrial extrasystole is the premature contraction produced by a stimulus arising from atrial muscle. In this condition, an extra P wave appears immediately after the regular T wave. P wave is small and shapeless. The P-R interval of this beat is short.
2. Nodal Extrasystole.
Nodal extrasystole is caused by stimulus arising from AV node. P wave is merged with QRS complex and all the chambers of the heart contract together.
3. Ventricular Extrasystole.
Ventricular extrasystole is the extrasystole that is caused by stimulus from ventricular muscle. In this condition, an extra QRS complex follows the regular T wave. This QRS complex is prolonged as the impulse is conducted through ventricular muscle and not through the conductive system. This QRS complex also has a high voltage. T wave of this beat is inverted.
Conditions when Extrasystole Occurs.
Extrasystole is associated with organic diseases of the heart. Particularly, any ischemic area of ventricular musculature can produce an ectopic focus. Other conditions which produce extrasystole:
- Emotions.
- Severe exhaustion.
- Excessive ingestion of coffee or alcohol.
- Excessive smoking.
- Hyperthyroidism.
- Reflexes elicited from abnormal viscera.
Paroxysmal Tachycardia .
Paroxysmal tachycardia is the sudden attack of increased heart rate due to ectopic foci arising from atria, AV node or ventricle. It is also called Bouveret-Hoffmann syndrome. Increase in heart rate due to ectopic foci arising from either atria or AV node is called supraventricular tachycardia (SVT).
It differs from ventricular tachycardia, which does not depend upon atria or AV node. The attack lasts for a period of few seconds to few hours. It also stops suddenly. After the attack, heart functions normally. Symptoms include palpitations, chest pain, rapid breathing and dizziness.
Paroxysmal tachycardia is of three types:
- Atrial paroxysmal tachycardia
- AV nodal paroxysmal tachycardia.
- Ventricular paroxysmal tachycardia.
1. Atrial Paroxysmal Tachycardia.
Atrial paroxysmal tachycardia is the sudden increase in heart rate caused by ectopic impulses discharged from atrial musculature. Heart rate is 150 to 220/minute. P wave in ECG is inverted, with normal QRST.
2. AV Nodal Paroxysmal Tachycardia – Bundle of Kent.
AV nodal paroxysmal tachycardia is the sudden increase in heart rate caused by ectopic foci arising from AV node due to a temporary block in the conductive system. It also involves circus movement. This type of tachycardia is very common in some healthy persons who have got
an additional conductive system.
This system is formed by some abnormal junctional tissues constituting a structure called bundle of Kent. Bundle of Kent connects the atria and ventricles directly, so the conduction is very rapid than through the regular conductive system.
Circus movement – Re-entry and atrial echo beat.
Circus movement is defined as circuitous propagation of impulses around a structural or functional obstruction, resulting in re-entry of the impulse and re-excitation of heart. When there is a sudden and temporary block in normal conductive system, the impulses from SA node reach the ventricle through bundle of Kent. By this time, the blockage in normal conductive system disappears.
Now, the impulse, which passes through bundle of Kent, after exciting the ventricular muscle, travels in the opposite direction through the normal conductive system and finally, it re-enters the AV node. Re-entered impulse activates the AV node and depolarizes the atria, resulting in atrial contraction. It is called atrial echo beat.
Re-entered nodal impulse simultaneously spreads to ventricle through normal conductive system, completing the circus movement. This circus movement is repeated producing tachycardia called AV nodal paroxysmal tachycardia. ECG shows normal QRST complex. But P wave is mostly absent.
Wolff-Parkinson-White syndrome .
Wolff-Parkinson-White syndrome is the condition characterized by repeated attacks of AV nodal paroxysmal tachycardia in persons with bundle of Kent. ECG shows short P-R interval with normal QRS complex and T wave.
Lown-Ganong-Levin syndrome.
Lown-Ganong-Levin syndrome is another condition characterized by AV nodal paroxysmal tachycardia. This occurs in persons who have got another type of abnormal conductive fibers like bundle of Kent. These fibers also connect atria and distal part of conductive system directly bypassing the AV node. So the impulse from SA node reaches ventricle through the abnormal conductive fibers.
After exciting the ventricular muscle, the impulse travels in opposite direction through normal conductive system and finally, it re-enters the AV node. The re-entered impulse activates the AV node causing atrial contraction. ECG shows short P-R interval with normal QRS complex and T wave.
3. Ventricular Paroxysmal Tachycardia.
Ventricular paroxysmal tachycardia is the sudden increase in heart rate caused by ectopic foci arising from ventricular musculature. Sometimes, a part of ventricular muscle, particularly an ischemic area is excited abnormally, followed by a series of extrasystole. This condition is dangerous as the circus movement is developed within ventricular muscle. This circus movement leads to ventricular fibrillation, which is fatal.
Atrial Flutter.
Atrial flutter is an arrhythmia characterized by rapid ineffective atrial contractions, caused by ectopic foci originating from atrial musculature. It is often associated with atrial paroxysmal tachycardia. Both the atria beat rapidly like the wings of a bird, hence the name atrial flutter. Atrial rate is about 250 to 350/minute. Maximum number of impulses conducted by AV node is about 230 to 240 /minute. So, during atrial flutter, the second degree of heart block occurs.
The ratio between atrial beats and ventricular beats is 2 : 1 or sometimes 3 : 1. Atrial flutter is common in patients suffering from cardiovascular diseases such as hypertension and coronary artery disease. Initially, it is marked by palpitations that are unnoticed. However, prolonged atrial flutter may lead to atrial fibrillation or heart failure.
Atrial fibrillation.
Atrial fibrillation is the type of arrhythmia characterized by rapid and irregular atrial contractions at the rate of 300 to 400 beats/minute. It is mostly due to circus movement of impulses within atrial musculature. P wave is absent in ECG. Atrial fibrillation is common in old people and patients with heart diseases. Though it is not life threatening, it may cause complications. If it continues for long time, it may cause blood clot and blockage of blood flow to vital organs.
Ventricular fibrillation.
Ventricular fibrillation is the dangerous cardiac arrhythmia, characterized by rapid and irregular twitching of ventricles. Ventricles beat very rapidly and irregularly due to the circus movement of impulses within ventricular muscle. The rate reaches 400 to 500/minute. This is triggered by ventricular extrasystole.
This type of arrhythmia is serious as it leads to death, since the ventricles cannot pump blood. Ventricular fibrillation is very common during electric shock and during ischemia of conductive system. It also occurs in other conditions like coronary occlusion, chloroform anesthesia, cyclopropane anesthesia, trauma of heart and disturbances of heart (due to improper handling) during cardiac surgery.
Abnormal pacemaker.
Abnormal pacemaker is the part of the heart other than SA node that becomes the pacemaker and discharges ectopic foci. Various types of arrhythmia develop when an abnormal pacemaker is activated.
Common abnormal pacemakers:
1. Atrioventricular node.
2. Atrial musculature.
3. Ventricular musculature.
1. AV Node as Pacemaker.
When AV node becomes the pacemaker, the following arrhythmias occur:
a. AV nodal rhythm.
b. AV nodal extrasystole.
c. AV nodal paroxysmal tachycardia.
2. Atrial Musculature as Pacemaker.
Following arrhythmias occur if atrial musculature becomes the pacemaker:
a. Atrial extrasystole.
b. Atrial paroxysmal tachycardia .
c. Wolff-Parkinson-White syndrome
d. Lown-Ganong-Levine syndrome.
e. Atrial flutter.
f. Atrial fibrillation.
3. Ventricular Musculature as Pacemaker.
If ventricular muscle becomes the pacemaker, following arrhythmias are developed:
a. Ventricular extrasystole.
b. Ventricular paroxysmal tachycardia.
c. Ventricular fibrillation.
Artificial pacemaker.
Artificial pacemaker is a small electronic device that is surgically implanted to regulate abnormal heartbeat. It contains a battery powered pulse generator, that produces electrical impulses capable of stimulating the heart. This pacemaker is implanted under the skin over the chest of the patient. Pulses generated by this device are transmitted to the heart through electrodes.
Electrodes connected to the device are inserted and passed through a vein and positioned in the heart chambers. The device has a lithium battery that may last for 10 to 15 years. The outer casing of the pacemaker is usually made of titanium, which is rarely rejected by body’s immune system. Pulse generator of the pacemaker has multiple functions. It is programmed to cope up with the needs of the individual patient.
Current of injury.
Current of injury means flow of current from an injured region of heart to the unaffected part. When ischemia occurs in any part of the ventricular musculature due to coronary occlusion, that part of ventricle becomes depolarized either partially or completely and the repolarization does not occur.
It causes flow of current from affected (depolarized) part to unaffected part of the ventricular muscle. Current of injury in myocardial infarction affects the ECG pattern and cardiac vector. In ECG, the J point and ST segments are displaced . Deviation of cardiac axis is also common during the current of injury.
Cardiac Axis.
In the infarction of anterior wall of the ventricle, the cardiac axis (vector) is deviated to right up to +150° due to current of injury and in the posterior wall infarction, there is left axis deviation up to –95°.
Thanks for Visiting us.
