Arrhythmia Physiology Notes

Arrhythmia Definition And Classification Of Arrhythmia

Arrhythmia refers to irregular heartbeat or disturbance in the rhythm of the heart. In arrhythmia, the heartbeat may be fast or slow or there may be an extra beat or missed beat. It occurs in physiological and pathological conditions.

Pacemaker in Arrhythmia

• In arrhythmia, the SA node may or may not be the pacemaker.
• If the SA node is not the pacemaker, any of the other parts of the heart such as the atrial muscle, AV node, and the ventricular muscle becomes the pacemaker.

Accordingly, arrhythmia is classified into two types:

1. Normotopic arrhythmia
2. Ectopic arrhythmia.

Read And Learn More: Medical Physiology Notes

Normotopic Arrhythmia

Normotopic arrhythmia is an irregular heartbeat in which the SA node is the pacemaker.

Normotopic arrhythmia is of three types:

1. Sinus arrhythmia
2. Sinus tachycardia
3. Sinus bradycardia

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 the SA node as the pacemaker.
• The heart rate is 72/min. However, under normal physiological conditions in a normal healthy person, the heart rate varies according to the phases of the respiratory cycle.
• Heart rate increases during inspiration and decreases during expiration.

ECG Changes

• ECG is normal during sinus arrhythmia. Only the duration of the R-R interval varies rhythmically according to the phases of respiration.
• It is shortened during inspiration and prolonged during expiration.

Sinus Arrhythmia Cause

• Sinus arrhythmia is due to the fluctuation in the discharge of impulses from the SA node.
• During inspiration, the lungs are inflated and the intrathoracic pressure decreases.
• This increases the venous return. Inflation of the lungs stimulates the stretch receptors of the lungs which send impulses to the vasodilator area (cardioinhibitory center) through afferent fibers of the vagus.
• It leads to refit-A inhibition of vasodilator area and reduction in vagal tone Because of these two factors heart rate increases.
• Simultaneously, increased venous return produces the Bain-Bridge reflex that causes an increase in heart rate.
• During expiration, the lungs are deflated, and the intrathoracic pressure increases.
• This decreases the venous return. During the deflation of the lungs, the stretch receptors are not stimulated and the vasodilator area is not inhibited.
• So, vagal tone increases resulting in decreased heart rate.
• Simultaneously, decreased venous return abolishes the Bain-Bridge reflex. It also decreases the heart rate.

Sinus Tachycardia: Sinus tachycardia is the increase in the discharge of impulses from the SA node resulting in an increase in the heart rate.

ECG Changes

ECG is normal in sinus tachycardia except for short because of increased heart rate. The discharge of impulses from the SA node is very rapid and the heart rate increases up to 100/minute and sometimes up to 150/minute.

Conditions when Sinus Tachycardia Occurs: Sinus tachycardia occurs in physiological as well as pathological conditions.

Physiological conditions when tachycardia occurs are:

1. Exercise
2. Emotion
3. High altitude
4. Pregnancy.

Pathological conditions when tachycardia occurs are:

1. Fever
2. Anemia
3. Hyperthyroidism
4. Hypersecretion of catecholamines
5. Cardiomyopathy
6. Valvular heart disease
7. Hemorrhagic shock.

Features

Common features of tachycardia are:

1. Palpitations (sensation of feeling the heartbeat)
2. Dizziness.
3. Fainting
4. Shortness of breath
5. Chest discomfort (angina).

Sinus Bradycardia

• Sinus bradycardia is the reduction in the discharge of impulses from the SA node resulting in a decrease in heart rate.
• The heart rate is less than 60/minute.

ECG Changes: 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 the heart.

Physiological conditions when sinus bradycardia occurs are:

1. Sleep
2. Athletic heart.

Pathological conditions when sinus bradycardia occurs are:

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 the carotid artery at the region of the carotid sinus is called carotid sinus syndrome.

Features

1. Sick sinus syndrome
2. Fatigue
3. Weakness
4. Shortness of breathing
5. Lack of concentration
6. Difficulty in exercising
7. Sick sinus syndrome

Sick sinus syndrome is a common feature of sinus bradycardia. It is a condition characterized by dizziness and unconsciousness.

Ectopic Arrhythmia

• Ectopic arrhythmia is an abnormal heartbeat in which one of the structures of the heart other than the SA node becomes the pacemaker.
• The impulses produced by these structures are called ectopic foci

Ectopic arrhythmia is further divided into two subtypes:

1. Homotopic arrhythmia is when the impulses for a heartbeat arise from any part of the conductive system and
2. Heterotopic arrhythmia is when impulses arise from the musculature of the heart other than the conductive system.

Different types of ectopic arrhythmia are:

1. Heart block
2. Extrasystole
3. Paroxysmal tachycardia
4. Atrial flutter
5. Atrial fibrillation
6. Ventricular fibrillation.

Heart Block

• Heart block is the blockage of impulses generated by the 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:

Sinoatrial block-Atrioventricular block.

• Sinoatrial Block – AV Nodal Rhythm
• The sinoatrial block is the failure of impulse transmission from the SA node to the AV node. It is also called sinus block.
• AV nodal (atrioventricular) rhythm is the cardiac rhythm in which the AV node takes over the pacemaker function during the sinoatrial block.

• The sinoatrial block is due to a defect in internodal fibers and it occurs suddenly. Initially, the heart stops for a while.
• Then after a few seconds, the AV node becomes the pacemaker, and the heart starts beating with a decreased rate of 40-60/minute.
• The impulses may be discharged from any part of the AV node, viz.

1. In the upper nodal rhythm, the impulses are discharged from the upper part of the AV node. In this rhythm, the P wave of ECG is inverted. The QRS complex and T wave are normal
2. In the middle nodal rhythm, the impulses are by the middle part of the AV node. Here, all the chambers of the heart contract simultaneously. The P wave of ECG is absent as it merges with the QRS complex
3. In a lower nodal rhythm, the impulses are produced by the lower part of the AV node. In this condition, the ventricular contraction occurs prior to atrial contraction as the impulses reach the ventricles prior to the atria. In ECG, the QRS complex appears prior to the P wave, and R- P interval is obtained instead of the P-R interval. It is called reversed heart block.

Atrioventricular Block

The atrioventricular block is the heart block in which the impulses are not transmitted from the atria (from the AV node) to the ventricles because of a defective conductive system.

The atrioventricular block is of two categories:

1. Incomplete heart block
2. Complete heart block.

Incomplete Heart Block

It is the condition in which the transmission of impulses from the atria to the ventricles is slowed down and not blocked completely.

The impulses reach the ventricles late. Incomplete heart block is of four types:

1. First-degree heart block
2. Second-degree heart block
3. Wenckebach phenomenon
4. Bundle branch block.

First-degree heart block

• It is the heart block in which the conduction of impulses through the AV node is very slow, 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 seconds.
• 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 symptoms.

Second-degree heart block

• It is the type of heart block in which, some of the impulses produced by the SA node fail to reach the ventricles.
• It is also called the partial heart block. When some of the impulses from the SA node fail to reach the ventricles one ventricular contraction occurs for every 2, 3, or 4 atrial contractions, e. 2:1, 3:1 or 4:1.
• In ECG, the ventricular complex (QRST) is missing intermittently.
• During the very frequent development of second-degree heart block, bradycardia occurs.
• Wenckebach phenomenon or syndrome
• It is a type of heart block characterized by a progressive increase in AV nodal delay resulting in missing one beat.
• Afterward, the conduction of the impulse is normal or slightly delayed.
• In ECG, the progressive lengthening of the P-R interval is noticed till a QRST complex disappears.

Bundle branch block (BBB)

• The bundle branch block is the heart block that occurs during the dysfunction of the right or left branch of the bundle of His.
• During this type of block, the impulse from the atria reaches the 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.

Complete Heart Block (Third-degree heart block)

• Complete heart block is the condition in which the impulses produced by the 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 the atrial beat.

It is called idioventricular rhythm. Complete heart block occurs due to any one of the following causes:

• The disease of the AV node, which leads to AV nodal block
• The defective conductive system below the level of the AV node causes an intranodal block.

AV nodal block: In this type of block, a part of the AV node is defective and, the unaffected part becomes the pacemaker. The rhythmicity of the AV node is about 45-60/minute.

Infranodal block

• Infranodal block is the heart block in which the impulses from the SA node are blocked in the branches of the bundle of His (below the level of the AV node).
• In this condition, the distal part of the conductive system (e. the Purkinje fibers) becomes the pacemaker.
• The rhythmicity of Purkinje fibers is about 35/minute.
• Sometimes, a part of the ventricular musculature becomes the pacemaker, and the ventricular rate in such conditions is about 20/per minute.
• Third-degree heart block is a 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 a 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. The ectopic pacemaker (AV node, Purkinje fiber, or ventricular muscle) starts functioning only after 5-30 seconds.
• During this time, blood circulation is affected because of a lack of ventricular output.
• The brain cannot withstand the stoppage of blood supply and oxygen supply even for 5 seconds.
• Before the onset of discharge from an ectopic pacemaker, dizziness and fainting occur.
• If the discharge of impulses from the ectopic pacemaker is delayed beyond 30 seconds, death occurs.

Extrasystole

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 a relaxed state.

Cause for the compensatory pause

• In the cardiac muscle, the absolute refractory period extends throughout the contraction period.
• When the heart is in extrasystole (because of ectopic focus), an impulse is discharged from the natural pacemaker — SA node.
• As this natural impulse reaches the myocardium during the contraction period of extrasystole, the myocardium does not give a response, because it is refractory now.
• For the next beat, the heart has to wait till the discharge of the next natural impulse from the pacemaker — the SA node. During this time the heart stops in diastole.
• It is the cause for compensatory pause.
• The parts of the heart that give the origin for the ectopic foci are the AV node, the bundle of His, atrial musculature, and ventricular musculature.

Accordingly, extrasystole is divided into three types:

1. Atrial extrasystole
2. Nodal extrasystole
3. Ventricular extrasystole.

Atrial Extra systole

• Atrial extrasystole is the premature contraction produced by a stimulus arising from the atrial muscle. In this condition, an extra P wave appears immediately after the regular T wave. The p wave is small and shapeless.
• The P-R interval of this beat is short.

Nodal Extrasystole: It is the extrasystole caused by stimulus arising from the AV node. The p wave is merged with the QRS complex and, all the chambers of the heart contract together.

Ventricular Extrasystole

• Ventricular extrasystole is the extrasystole that is caused by the stimulus from the 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. The wave of this beat is inverted.
• Conditions when Extrasystole Occurs
• Extrasystole associated with organic diseases of the heart in any ischemic area of ventricular Musculature can produce an ectopic focus.

Other conditions, which produce extrasystole:

1. Emotions
2. Severe exhaustion
3. Excessive ingestion of coffee or alcohol
4. Excessive smoking
5. Hyperthyroidism
6. Reflexes elicited from abnormal viscera.

Paroxysmal Tachycardia

• Paroxysmal tachycardia is the sudden attack of increased heart rate due to ectopic foci arising from the atria, AV node, or ventricle.
• It is also called Bouveret-Hoffmann syndrome.
• An 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 a few seconds to a few hours. It also stops suddenly.
• After the attack, the heart functions normally. The symptoms include palpitations, chest pain, rapid breathing, and dizziness.

Paroxysmal tachycardia is of three types:

1. Atrial paroxysmal tachycardia
2. AV nodal paroxysmal tachycardia
3. Ventricular paroxysmal tachycardia.

Atrial Paroxysmal Tachycardia

• It is the sudden increase in heart rate caused by the ectopic impulses discharged from atrial musculature. The heart rate is 150-220/minute.
• The P wave in ECG is inverted, with normal QRST.

AV Nodal Paroxysmal Tachycardia – Bundle of Kent

• It is the sudden increase in heart rate caused by ectopic foci arising from the 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 an additional conductive system.
• This system is formed by some abnormal junctional tissues constituting a structure called a bundle of Kent.
• The bundle of Kent connects the atria and ventricles directly, so the conduction is more rapid than through the regular conductive system.

Circus movement – Re-entry and atrial echo beat

• Circus movement is defined as the circuitous propagation of impulses around a structural or functional obstruction resulting in the re-entry of the impulse and re-excitation of the heart.
• When there is a sudden and temporary block in the normal conductive system, the impulses from the SA node reach the ventricle through a bundle of Kent.
• By this time, the blockage in the normal conductive system disappears. Now, the impulse, which passes through the bundle of Kent, after exiting the ventricular muscle, travels in the opposite direction through the normal conductive system, and, finally, it re-enters the AV node.
• The re-entered impulse activates the AV node and depolarizes the atria resulting in atrial contraction. It is called an atrial echo beat.
• The re-entered nodal impulse simultaneously spreads to the ventricle through the 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 the P wave is mostly absent.

Wolff-Parkinson-White syndrome

• The Wolff-Parkinson-White syndrome is a condition characterized by repeated attacks of AV nodal paroxysmal tachycardia in persons with a bundle of Kent.
• ECG shows a short P-R interval with normal QRS complex and T wave.

Lawn-Ganong-Levin syndrome

• Lawn-Ganong-Levin syndrome is another condition characterized by AV nodal paroxysmal tachycardia.
• This occurs in persons who have another type of abnormal conductive fibers like a bundle of Kent.
• These fibers also connect the atria and the distal part of the conductive system directly bypassing the AV node.
• So the impulse from the SA node reaches the ventricle through the abnormal conductive fibers.
• After exiting the ventricular muscle, the impulse travels in the opposite direction through the normal conductive system, and, finally, it re-enters the AV node.
• The re-entered impulse activates the AV node causing atrial contraction. ECG shows a short P-R interval with normal QRS complex and T wave.

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 the 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 a ventricular muscle.
• This circus movement leads to ventricular fibrillation, which is fatal.

Atrial Flutter

• Atrial Flutter is an arrhythmia characterized by rapid affective striae contractions caused by ectopic foci originating from atrial musculature.
• It is often associated with atrial paroxysmal tachycardia. Both atria beat rapidly like the wings of a bird hence the name atrial flutter.
• The atrial rate is about 250-350/minute. The maximum number of impulses conducted by the AV node is about 230240 /minute.
• So, during atrial flutter, a 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

• It is the type of arrhythmia characterized by rapid and irregular atrial contractions at the rate of 300-400 beats/ minute.
• It is mostly due to the circus movement of impulses within atrial musculature. The 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 a long time it may cause blood clots and blockage of blood flow to vital organs.

Ventricular Fibrillation

• Ventricular fibrillation is a dangerous cardiac arrhythmia characterized by rapid and irregular twitching of ventricles.
• The ventricles beat very rapidly and irregularly due to the circus movement of impulses within a ventricular muscle.
• The rate reaches 400-500/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 the conductive system.
• It also occurs in other conditions like coronary occlusion, chloroform anesthesia, cyclopropane anesthesia, trauma of the heart, and disturbances of the heart (due to improper handling) during cardiac surgery.

Abnormal Pacemaker

• An abnormal pacemaker is part of the heart other than the SA node that becomes the pacemaker and discharges ectopic foc
• Various types of arrhythmia develop when an abnormal pacemaker is activated.

The common abnormal pacemakers are:

1. AV node
2. Atrial musculature
3. Ventricular musculature.

AV Node as Pacemaker

When the AV node becomes the pacemaker, the following arrhythmias occur:

1. AV nodal rhythm
2. AV nodal extrasystole
3. AV nodal paroxysmal tachycardia.
4. Atrial Musculature as Pacemaker

The following arrhythmias occur if atrial musculature becomes pacemaker:

1. Atrial extrasystole
2. Atrial paroxysmal tachycardia
3. Wolff-Parkinson-White syndrome
4. Lawn-Ganong-Levine syndrome
5. Atrial flutter
6. Atrial fibrillation.

Ventricular Musculature as Pacemake

If the ventricular muscle becomes pacemaker following arrhythmias are developed:

1. Ventricular extrasystole
2. Ventricular paroxysmal tachycardia
3. Ventricular fibrillation.

Artificial Pacemaker

• An 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.
• The pacemaker is implanted under the skin over the chest of the patient.
• The pulses generated by the device are transmitted to the heart through electrodes.
• The 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 10-15 years. The outer casing of the pacemaker is usually made of titanium which is rarely rejected by the body’s immune system.
• The pulse generator of the pacemaker has multiple functions, it is programmed to cope with the needs or the individual patient.

Current Of Injury

• A current of injury means the flow of current from an Injured region of the heart to the unaffected part.
• When ischemia occurs in any part of the ventricular musculature due to coronary occlusion, that part of the ventricle becomes depolarized either partially or completely and the repo-variation does not occur.
• It causes the flow of current from the affected (depolarized) part to the unaffected part of the ventricular muscle.
• The current of injury in myocardial infarction affects the ECG pattern and cardiac vector.
• In ECG, the J point and ST segments are displaced. The deviation of the cardiac axis is also common during the current of injury.

Cardiac Axis

In the infarction of the anterior wall of the ventricle, the cardiac axis (vector) is deviated to right up to + 150° due to the current of injury and in the posterior wall infarction, there is left axis deviation up to -95°.