Lone Atrial Fibrillation (LAF)

Introduction

Atrial Fibrillation (AF) is an abnormal irregular and fast heart rate where electrical signals are generated chaotically throughout the upper chambers (atria) of the heart. 

AF is common in the elderly. Its prevalence, estimated to be 2.3 percent in those over 40, increases to 5.9 percent in people 65 years and older. By age 70, 10 percent have this heart rhythm disorder. Seventy percent of the people who have AF are between 65 and 85 years old. It affects over 2 million Americans and is the cause of about one-third of all strokes in people over the age of 65. It is the most frequently diagnosed arrhythmia in the clinical setting. 

Most of the older patients with AF have associated hypertension, ischemic heart disease, congestive heart failure, and underlying conditions such as mitral valve disease, hyperthyroidism, and pulmonary disease. In some families, AF appears to have an inherited basis (although most people with AF do not have an inherited form).

In those under 40, the prevalence of AF is estimated to be less than 0.004%. AF in younger and active patients is more likely to be a manifestation of congenital heart disease, hyperthyroidism, excess alcohol or other drug use, including caffeine, and exercise-induced catecholamine release. In this group, the exact cause often cannot be found. Such cases are labeled Lone Atrial Fibrillation.

Let us first understand the basics of how electrical conduction works in the heart.

Normal Conduction Versus Atrial Fibrillation

The heart is a pump that needs an electrical system to trigger to keep the chambers contracting in a coordinated fashion. To do that, the heart has its own built-in pacemaker called the SA node (Sino-atrial node) located on the wall of the right atrium. It sends out impulses at about 60 to 100 beats per minute, to both the right and left upper chambers (atria) of the heart, telling them the exact moment to beat. The SA node sets the rate the heart beats to meet the body's demands: slow at rest, and fast during excitement. 

From the SA node, the impulse travels through the AV node (atrial-ventricular node), the "gatekeeper" of impulses, to the lower chambers of the heart (ventricles) via the bundle of His, telling the ventricles it's their turn to beat. The sequence of this electrical event causes the atria to beat first and a split second later, the ventricles to beat. This is the normal sinus rhythm characteristic of a normal functioning heart.

The Heart With AF

During AF, the normal sinus rhythm is disrupted by a very rapid, uncontrolled, and disorganized rhythm. The normal flow of electricity activity is upset by what can be best described as numerous little tornadoes wandering about in the atria. Instead of initiation of conduction from the SA node to the AV node in the normal sinus rhythm, the atrium's activity proceeds instead as multiple wavelets. Consequently, the atria beat so rapidly (as fast as 300 to 350 beats per minute) that they quiver (fibrillate). Such fibrillation does not result in effective contraction of the atrial muscles and therefore the pumping action at the atria is not strong. In addition, the rapid atrial rate effectively overrides any activity from the SA node. In other words, these rapid wavelets take control of the atrium. Most of these rapid wavelets are non-productive. Some of them reach the AV node, and then onto the ventricles. This causes the ventricles to pump in a rapid and irregular pattern in response as well. The heart with AF beats irregularly and there is no consistent pattern to such irregularity. 

Autonomic Nervous System (ANS) 

The Autonomic Nervous system controls the internal workings of the body and maintains its homeostasis. It is responsible for many bodily functions, including regulating heart rate, blood pressure, and digestive system. It originates in the hypothalamus region of the brain from where it divides into two branches - the sympathetic (adrenergic) branch and the parasympathetic (vagal) branch. The neurotransmitter used in the adrenergic branch is norepinephrine (noradrenaline) and in the parasympathetic system is acetylcholine.

Norepinephrine increases muscle contractions and heart rate, while acetylcholine does the reverse. By continuously self-adjusting the secretion of these two neurotransmitters, the body is kept in perfect balance and the heart produces normal sinus rhythm. In times of stress or emergency, the body has a backup system of releasing more adrenergic neurotransmitters (epinephrine and norepinephrine), allowing the execution of the "fight or flight" response necessary for survival. Such extraordinary release and/or excessive stimulation of adrenergic neurotransmitter often act as a trigger factor for AF. 

When the ANS is imbalanced, AF can be the result. AF can be vagally mediated or sympathetically mediated. Vagally mediated atrial fibrillation typically is preceded by slow heart rate (bradycardia), is not triggered by stress, occurs more often at night, is more common in men, and occurs at a younger age. Sympathetically mediated atrial fibrillation is less frequent, typically occurs during the day, and can be triggered by stress. The heart rate in sympathetically mediated atrial fibrillation is higher before and during the episode in comparison with the vagal type.