The heart is a muscular pump with its own electrical system to keep it functioning independently. Electrical impulses are generated in the sinoatrial node (SA node) in the atrial wall, passes directly to the muscles of the atria, is delayed in the atrioventricular node (AV node) and then transmitted to the muscles of the ventricles. The impulses travel in only one direction and ensures that the atria contract first and then the ventricles. In this manner blood received in the atria, pushed into the ventricles and then pumped out to the lungs and rest of the body.The heart beat has to maintain a constant rate and rhythm to ensure that blood is being pumped out in manner that is sufficient for the body’s needs.
What is an arrhythmia?
An arrhythmia, or dysrhythmia, is a disturbance in the electrical system of the heart leading to an irregular heart beat. The rhythm is usually affected and in some cases there is also a disruption in the rate meaning that the heart beats faster or slower than normal. There are several different types of arrhythmias which arise with a variety of disturbances in the natural pacemaker and conduction system of the heart.
The heart rate is determined by the natural pacemaker of the heart, SA node, which initiates impulses that travel throughout the heart. It is able to generate impulses independently of the central nervous system. However, the CNS does play a role in regulating the activity of the SA node. The conduction system consisting of the internodal pathways, AV node and AV bundle, carries these impulses throughout the heart. In an arrhythmia, there may be disturbance either within the SA node or the conduction system or in both structures. Sometimes the electrical conduction system of the heart is functioning normally but structural defects of the heart disturbs its normal activity.
It is important to understand that electrical impulses travel from the atria to the ventricles and the fibers carrying these impulses are unidirectional. This prevents an impulse that has reached the ventricle from passing back to the atria. An insulating layer of fibrous tissue further serves to prevent this transmission. Arrhythmias are usually short-lived and the heart rate and rhythm quickly returns to normal. Some, however, persist or recur frequently which can adversely affect normal circulation, cause a host of signs and symptoms or even lead to death.
Types of Arrhythmias
There are several different types and subtypes of arrhythmias. Some are seen only with heart disease and other related disorders while other types may occur occasionally in an otherwise healthy person.
Tachycardia, or rapid heart rate, is a heart rate greater than 100 beats per minute while bradycardia, or slow heart rate, is slower than the 60 beats per minute. These abnormalities in the heart rate may occur on its own with no disturbance in the normal rhythm of the heart. Therefore both tachycardia and bradycardia with a normal heart rhythm are forms of arrhythmias.
This is the most common type of arrhythmia. In atrial fibrillation, there is a rapid discharge of electrical impulses causing the atria and ventricles to contract faster than normal. Due to the speed at which these structures contract, the normal rhythm is disturbed. The atria quiver and does not push blood efficiently into the ventricles.
However, the ventricles are spared to some extent since the delay in the AV node prevents all of the electrical impulses from reaching the ventricular walls. Nevertheless, the ventricular rhythm is irregular and the ventricles cannot efficiently pump out blood to the body. The inefficient pumping can cause a drop in blood pressure and even heart failure. Atrial fibrillation tends to occur in episodes (attacks).
This may originate in the atria or ventricles and are referred to as premature atrial contractions or premature ventricular contractions respectively. The atrial type is harmless. Premature ventricular contractions are common and may occur as a once off episode even in a healthy person. These are extra abnormal contractions which temporarily disrupts the normal heart rhythm.
In ventricular fibrillation, the ventricles do not contract fully but quiver instead. The pumping of blood out of the heart is therefore severely compromised. A ventricular fibrillation arises when electrical impulses discharge spontaneously in the ventricles and in a rapid manner. This causes irregular ventricular contraction.
In ventricular tachycardia, the ventricles contract rapidly due to irregular discharges in the ventricular conduction system. The ventricles are unable to fill with sufficient blood and therefore the blood supply to the rest of the body is inadequate. Ventricular tachycardia lasts for a short period and then the heart rate returns to normal. However, if it persists then it severely compromises oxygen supply throughout the body.
There are different types of supraventricular tachycardia (SVT) but the basic mechanism involves abnormal conduction. Impulses in the AV node or ventricles may travel back into the atria, or impulses from the atria may bypass the AV node. As a consequence, the heart beats faster than normal.
Long QT Syndrome
In order for the heart muscle to contract soon after a contraction, it needs to go through a period of recovery. This has to be rapid but in long QT syndrome, the recovery period is delayed. If the heart muscle is not ready to contract again as coordinated by the impulses from the sinoatrial (SA) node, the heart rhythm may be disrupted.
This is broad term to describe a number of conditions where there is delay or block of signals in the heart’s conduction system. The disruption may lie in the AV node, nodal pathways or the heart muscle itself. The block can be partial or complete and therefore affects the coordination of ventricular contraction. It may be asymptomatic and tends to occur in older patients.
Sick Sinus Syndrome
Also known as tachybrady syndrome, it is caused by an abnormality of the sinoatrial node (SA node). Electrical impulses are not generated in a regular manner and is either too rapid, slow or irregular. This changes the heart rate and rhythm accordingly.
Causes of Arrhythmias
A number of diseases or events may cause an arrhythmia. This may be related to disorders of the heart or systemic disturbances. Sometimes an arrhythmia may arise momentarily without any underlying disease. These arrhythmias are harmless but need to be investiagted if recurrent. Some of the causes of arrhythmias include :
- Coronary artery disease
- Myocardial infarction (heart attack)
- Hypertension (high blood pressure)
- Myocarditis (inflammation of the heart muscle)
- Pericarditis (inflammation of the pericardium)
- Congenital heart disease
- Fibrotic scarring of the heart muscle
- Heart valve disease
- Wolff-Parkinson-White syndrome
- Pulmonary embolism
- Thyroid disease
- Medication like beta-blockers and digoxin
- Substance like alcohol and certain illicit drugs
Signs and Symptoms of Arrhythmias
The clinical features of an arrhythmia is largely dependent on the type and severity. Some arrhythmias may occur without any signs or symptoms and a person may even be unaware of the disturbance. In severe cases, however, an arrhythmia compromises heart function, blood cannot be circulated adequately and it can be life threatening.
Some of the common signs and symptoms of an arrhythmia includes :
- Shortness of breath
- Chest pain