The right and left atria (singular, atrium) receive blood from the vena cava (inferior and superior) and the pulmonary veins, respectively. This dichotomy represents a division between deoxygenated blood and oxygenated blood, with the lungs between these two circuits. From the atria, which contract, blood is sent to the right and left ventricles, which then send it to the lungs or to the rest of the body, respectively. Thus, the heart is a 4-pump mechanism, i.e., the two atria and the two ventricles.
The cells of the heart have automaticity, sending electrical pulses in sequence. The sinoatrial (SA) node is the “pacemaker” that supersedes all of the other cells’ automaticity and determines the rhythm of contractions in the heart, which generally travel right to left and superior to inferior. These contractions are skewed in their timing such that there is one vector force that propels the blood onward. One-way valves prevent backflow.
Atrial Fibrillation (AF)
Atrial premature beats are an important trigger for atrial fibrillation. In on-going AF, there is no organized wave of depolarization that sets up an organized contraction. The atrium basically quivers without an organized net vector force to drive blood into the ventricles normally for effective cardiac output. The ventricles and their own pacemaker, the atrioventricular node, can “rescue” the process with their own automaticity, but this is not as effective a pump.
AF is usually due to some underlying heart disease, atrial enlargement, irritation, or inflammation:
- Coronary heart disease
- Previous myocardial infarction
- Rheumatic heart disease and other valvular abnormalities
- Heart failure
- Cardiomyopathy–disease of the heart muscle, associated with hypertrophy
- Congenital heart disease
- Chronic obstructive pulmonary disease (COPD)
- Obstructive sleep apnea
- Heart surgery
- Medications (theophylline, adenosine, and drugs for osteoporosis)
AF is classified as new-onset, paroxysmal, persistent, longstanding persistent, or permanent.
Other Atrial Arrhythmias
Supraventricular arrhythmia, as opposed to rogue automaticity from disease in the sinoatrial node or ectopic sites of automaticity, involves re-entry of electrical signals into the circuit.
Increased vagal tone, low serum magnesium (hypomagnesemia), alcohol, and caffeine have all been known to be triggers for AF.
Atrial flutter differs from atrial fibrillation by having a regular rhythm and identifiable P waves. Its rate can be 250-350 beats/minute.
Complications from AF
AF impairs cardiac output, leading to symptoms of syncope, near-syncope, and other cognitive impairment. It is also a prime cause of arterial emboli that can cause stroke and peripheral embolization, requiring anticoagulant therapy.