Sinoatrial block

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Sinoatrial block
Type II S-A block.png
Sinus rhythm (rate = 65/min) with Type II S-A (exit) block; (see laddergram). Note that the long cycles (191) are nearly identical to twice the short cycles (190).
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A sinoatrial block (also spelled sinuatrial block) [1] is a disorder in the normal rhythm of the heart, known as a heart block, that is initiated in the sinoatrial node. The initial action impulse in a heart is usually formed in the sinoatrial node (SA node) and carried through the atria, down the internodal atrial pathways to the atrioventricular node (AV) node. [2] In normal conduction, the impulse would travel across the bundle of His (AV bundle), down the bundle branches, and into the Purkinje fibers. This would depolarize the ventricles and cause them to contract.

Contents

In an SA block, the electrical impulse is delayed or blocked on the way to the atria, thus delaying the atrial beat. [3] (An AV block, occurs in the AV node and delays ventricular depolarisation). SA blocks are categorized into three classes based on the length of the delay.

Symptoms

Sinoatrial blocks are typically well tolerated. They are not as serious as an AV block and most often do not require treatment. In some people, they can cause fainting, altered mental status, chest pain, hypoperfusion, and signs of shock. They can also lead to cessation of the SA node and more serious dysrhythmias.

Types

In a first degree sinoatrial block, there is a lag between the time that the SA node fires and actual depolarization of the atria. This rhythm is not easily detectable using state of the art (the highest level of scientific development at the current state in time) diagnostic equipment, thus is currently not recognizable on an ECG strip because an ECG strip does not denote when the SA node fires. It can be detected only during an electrophysiology study when a small wire is placed against the SA node from within the heart and the electrical impulses can be recorded as they leave the p-cells in the centre of the node [ see pacemaker potential ], followed by observing a delay in the onset of the p wave on the ECG.

Second degree SA blocks are broken down into two subcategories just like AV blocks are:

The first is a second degree type I, or Wenckebach block.[ citation needed ] This rhythm is irregular, and the R-R interval gets progressively smaller, while the P-R interval remains constant, until a QRS segment is dropped. Note that this is quite different from the Wenckebach AV block, in which the PR interval gets progressively longer, before the dropped QRS segment. The pause of a second degree type I is less than twice the shortest R-R interval and is not a multiple of the P-R interval. The cause is a gradual lengthening of conduction time from the SA node to the atria. The p-cells in the centre of the node produce the rhythm at a regular rate, but their conduction across the node to where it meets atrial tissue is where the slowing occurs.

A second degree type II, or sinus exit block, is a regular rhythm that may be normal or slow. It is followed by a pause that is a multiple of the P-P interval usually (2-4). Conduction across the SA node is normal until the time of the pause when it is blocked.

A third degree sinoatrial block looks very similar to a sinus arrest. However, a sinus arrest is caused by a failure to form impulses. A third degree block is caused by failure to conduct them. The rhythm is irregular and either normal or slow. It is followed by a long pause that is not a multiple of the P-R interval. The pause ends with a P wave, instead of a junctional escape beat the way a sinus arrest would.

Treatment

Emergency treatment consists of administration of atropine sulfate or transcutaneous pacing.

Related Research Articles

Bradycardia Heart rate that is below the normal range

Bradycardia is a condition typically defined wherein an individual has a resting heart rate of under 60 beats per minute (BPM) in adults, although some studies use a heart rate of less than 50 BPM. Bradycardia typically does not cause symptoms until the rate drops below 50 BPM. When symptomatic, it may cause fatigue, weakness, dizziness, sweating, and at very low rates, fainting.

Electrocardiography Method to record the electrical activity of the heart through passive electrodes placed over the skin.

Electrocardiography is the process of producing an electrocardiogram. It is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances, inadequate coronary artery blood flow, and electrolyte disturbances.

Wolff–Parkinson–White syndrome Medical condition

Wolff–Parkinson–White syndrome (WPWS) is a disorder due to a specific type of problem with the electrical system of the heart which has resulted in symptoms. About 40% of people with the electrical problem never develop symptoms. Symptoms can include an abnormally fast heartbeat, palpitations, shortness of breath, lightheadedness, or syncope. Rarely, cardiac arrest may occur. The most common type of irregular heartbeat that occurs is known as paroxysmal supraventricular tachycardia.

Atrioventricular node

The atrioventricular node or AV node is a part of the electrical conduction system of the heart that coordinates the top of the heart. It electrically connects the atria and ventricles. The AV node lies at the lower back section of the interatrial septum near the opening of the coronary sinus, and conducts the normal electrical impulse from the atria to the ventricles. The AV node is quite compact.

Third-degree atrioventricular block Medical condition

Third-degree atrioventricular block is a medical condition in which the nerve impulse generated in the sinoatrial node in the atrium of the heart can not propagate to the ventricles.

Electrical conduction system of the heart Transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle

The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. The pacemaking signal generated in the sinoatrial node travels through the right atrium to the atrioventricular node, along the Bundle of His and through bundle branches to cause contraction of the heart muscle. This signal stimulates contraction first of the right and left atrium, and then the right and left ventricles. This process allows blood to be pumped throughout the body.

Heart block Medical condition

Heart block (HB) is a disorder in the heart's rhythm due to a fault in the natural pacemaker. This is caused by an obstruction – a block – in the electrical conduction system of the heart. Sometimes a disorder can be inherited. Despite the severe-sounding name, heart block may cause no symptoms at all in some cases, or occasional missed heartbeats in other cases, or may require the implantation of an artificial pacemaker, depending upon exactly where in the heart conduction is being impaired and how significantly it is affected.

Second-degree atrioventricular block Medical condition

Second-degree atrioventricular block is a disease of the electrical conduction system of the heart. It is a conduction block between the atria and ventricles. The presence of second-degree AV block is diagnosed when one or more of the atrial impulses fail to conduct to the ventricles due to impaired conduction. It is classified as a block of the AV node and is categorized in between first-degree and third degree blocks.

Supraventricular tachycardia Abnormally fast heart rhythm arising from improper electrical activity in the upper part of the heart

Supraventricular tachycardia (SVT) is a fast heart rhythm arising from abnormal electrical activity in the upper part of the heart. There are four main types: atrial fibrillation, paroxysmal supraventricular tachycardia (PSVT), atrial flutter, and Wolff–Parkinson–White syndrome. There are three subdivisions of PSVT. Symptoms common to them all may include palpitations, feeling of faintness, sweating, shortness of breath, and/or chest pain.

Bundle branch block Medical condition

A bundle branch block is a defect of the bundle branches or fascicles in the electrical conduction system of the heart.

Right bundle branch block Medical condition

A right bundle branch block (RBBB) is a heart block in the right bundle branch of the electrical conduction system.

Atrioventricular block is a type of heart block that occurs when the electrical signal traveling from the atria, or the upper chambers of the heart, to ventricles, or the lower chambers of the heart, is impaired. Normally, the sinoatrial node produces an electrical signal to control the heart rate. The signal travels from the SA node to the ventricles through the atrioventricular node. In an AV block, this electrical signal is either delayed or completely blocked. When the signal is completely blocked, the ventricles produce their own electrical signal to control the heart rate. The heart rate produced by the ventricles is much slower than that produced by the SA node.

Premature atrial contractions (PACs), also known as atrial premature complexes (APC) or atrial premature beats (APB), are a common cardiac dysrhythmia characterized by premature heartbeats originating in the atria. While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat. The exact cause of PACs is unclear; while several predisposing conditions exist, PACs commonly occur in healthy young and elderly people. Elderly people that get PACs usually don't need any further attention besides follow ups due to unclear evidence. PACs are often completely asymptomatic and may be noted only with Holter monitoring, but occasionally they can be perceived as a skipped beat or a jolt in the chest. In most cases, no treatment other than reassurance is needed for PACs, although medications such as beta blockers can reduce the frequency of symptomatic PACs.

Sinoatrial arrest is a medical condition wherein the sinoatrial node of the heart transiently ceases to generate the electrical impulses that normally stimulate the myocardial tissues to contract and thus the heart to beat. It is defined as lasting from 2.0 seconds to several minutes. Since the heart contains multiple pacemakers, this interruption of the cardiac cycle generally lasts only a few seconds before another part of the heart, such as the atrio-ventricular junction or the ventricles, begins pacing and restores the heart action. This condition can be detected on an electrocardiogram (ECG) as a brief period of irregular length with no electrical activity before either the sinoatrial node resumes normal pacing, or another pacemaker begins pacing. If a pacemaker other than the sinoatrial node is pacing the heart, this condition is known as an escape rhythm. If no other pacemaker begins pacing during an episode of sinus arrest it becomes a cardiac arrest. This condition is sometimes confused with sinoatrial block, a condition in which the pacing impulse is generated, but fails to conduct through the myocardium. Differential diagnosis of the two conditions is possible by examining the exact length of the interruption of cardiac activity. If the next available pacemaker takes over, it is in the following order:

Junctional rhythm describes an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the atrioventricular node, the "junction" between atria and ventricles.

An ectopic pacemaker is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the heart. It is thus a cardiac pacemaker that is ectopic, producing an ectopic beat. Acute occurrence is usually non-life-threatening, but chronic occurrence can progress into tachycardia, bradycardia or ventricular fibrillation. In a normal heart beat rhythm, the SA node usually suppresses the ectopic pacemaker activity due to the higher impulse rate of the SA node. However, in the instance of either a malfunctioning SA node or an ectopic focus bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may take over the natural heart rhythm. This phenomenon is called an escape rhythm, the lower rhythm having escaped from the dominance of the upper rhythm. As a rule, premature ectopic beats indicate increased myocyte or conducting tissue excitability, whereas late ectopic beats indicate proximal pacemaker or conduction failure with an escape 'ectopic' beat.

Junctional ectopic tachycardia Medical condition

Junctional ectopic tachycardia (JET) is a rare syndrome of the heart that manifests in patients recovering from heart surgery. It is characterized by cardiac arrhythmia, or irregular beating of the heart, caused by abnormal conduction from or through the atrioventricular node. In newborns and infants up to 6 weeks old, the disease may also be referred to as His bundle tachycardia or congenital JET.

P wave (electrocardiography) Represents atrial depolarization, which results in atrial contraction

The P wave on the ECG represents atrial depolarization, which results in atrial contraction, or atrial systole.

Arrhythmia Group of conditions in which the heartbeat is irregular, too fast, or too slow

Arrhythmia, also known as cardiac arrhythmia or heart arrhythmia, is a group of conditions in which the heartbeat is irregular, too fast, or too slow. The heart rate that is too fast – above 100 beats per minute in adults – is called tachycardia, and a heart rate that is too slow – below 60 beats per minute – is called bradycardia. Some types of arrhythmias have no symptoms. Symptoms, when present, may include palpitations or feeling a pause between heartbeats. In more serious cases, there may be lightheadedness, passing out, shortness of breath or chest pain. While most cases of arrhythmia are not serious, some predispose a person to complications such as stroke or heart failure. Others may result in sudden death.

Cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another.

References

  1. ↑ "sinuatrial block". TheFreeDictionary.com.
  2. ↑ Mesirca, Pietro; Torrente, Angelo G.; Mangoni, Matteo E. (2015). "Functional role of voltage gated Ca2+ channels in heart automaticity". Frontiers in Physiology. 6: 19. doi: 10.3389/fphys.2015.00019 . ISSN   1664-042X. PMC   4313592 . PMID   25698974.
  3. ↑ "sinoatrial heart block". TheFreeDictionary.com.
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