Atrial fibrillation

Atrial fibrillation is characterized by an irregular atrial activation, leading to an irregular ventricular response. The atria are activated simultaneously by several ectopic foci producing an average atrial rate of 300 to 600 per minute. This irregular and rapid activation prevents the atria from pumping effectively. The atrial "kick" is absent and ventricular filling during diastole may be suboptimal.

Atrial fibrillation may be caused by cardiovascular and non-cardiovascular conditions. Cardiovascular causes of atrial fibrillation include:

  • Long-standing hypertension
  • Ischemic heart disease
  • Myocarditis
  • Cardiomyopathy
  • Congestive heart failure

Non-cardiovascular causes of atrial fibrillation include:

  • Hyperthyroidism
  • Pulmonary embolism
  • Pneumonia
  • Lung cancer
  • Electrolyte abnormalities (hypocalcemia and hypomagnesemia)
  • Medications (such as anti arrhythmics, dopamine, and sympathomimetic drugs) and alcohol

 



 

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Patients with atrial fibrillation are usually asymptomatic but they may present clinically with symptoms including palpitations, syncope, shortness of breath, and chest pain. In an ECG tracing, atrial fibrillation is characterized by:

  • The absence of recognizable P waves
  • Disorganized fibrillatory wave pattern (representing disorganized activation of multiple atrial foci)
  • Irregular R-R interval

In atrial fibrillation the AV node is not capable of conducting all atrial activation fronts to the ventricles. The frequency or rate of ventricular activation during atrial fibrillation may vary from less than 50 to more than 200 per minute. Depending on the ventricular rate, atrial fibrillation may be classified into the following:

  • Atrial fibrillation with slow ventricular response – the ventricular rate is less than 60 per minute
  • Atrial fibrillation with rapid ventricular response – the ventricular rate is more than 100 per minute
  • Atrial fibrillation with controlled ventricular response – the ventricular rate is between 60 to 100 per minute

Remember: Do not use the terms "slow" and "fast" atrial fibrillation. Atrial fibrillation is always fast (300-600 per minute).

Origin of atrial fibrillation: In the past, atrial fibrillation was believed to originate from chaotic spontaneous depolarization from different atrial ectopic foci. Recent experiments have shown a less chaotic mechanism of atrial fibrillation than what was previously assumed. As demonstrated in the animation, atrial fibrillation seems to originate from ectopic atrial foci, usually situated near the opening of the pulmonary vein in the left atrium. These foci initiate reentry in different atrial circuits and the resulting reentrant arrhythmia is atrial fibrillation.

In some cases, persistent atrial fibrillation can be prevented by cleaving the potential reentry circuits, thus eliminating the potential pathway for initiation of atrial fibrillation. This can be achieved through a procedure called radiofrequent ablation of the reentry circuit (MAZE procedure).