What is atrial fibrillation (a-fib) and what are its different types?

Definition of Atrial Fibrillation

Atrial fibrillation is a supraventricular tachyarrhythmia characterized by three key ECG features: absolutely irregular R-R intervals, absence of distinct repeating P waves (replaced by rapid fibrillatory waves that vary in size, shape, and timing), and irregular atrial activity with atrial rates ≥300 bpm. 1, 2, 3

Core Pathophysiology

• AF represents uncoordinated atrial activation with consequent deterioration of atrial mechanical function, resulting in ineffective atrial contraction and loss of the "atrial kick" contribution to cardiac output 1, 3
• The fibrillatory waves reflect chaotic electrical activity in the atria with a rapid atrial cycle length less than 200 ms 2
• Any arrhythmia demonstrating these ECG characteristics that lasts at least 30 seconds should be considered atrial fibrillation, as even brief episodes carry the same risk of AF-related complications 2

Hemodynamic Consequences

• Loss of coordinated atrial contraction decreases cardiac output by 5-15%, which is particularly significant in patients with diastolic dysfunction, mitral stenosis, hypertrophic cardiomyopathy, or left ventricular hypertrophy 3, 4
• The irregular ventricular response causes beat-to-beat variability in ventricular filling, further reducing cardiac output compared to a regular rhythm at the same mean rate 1, 3
• Inadequately controlled ventricular rates can lead to tachycardia-induced cardiomyopathy and heart failure 1, 4

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Types of Atrial Fibrillation

The classification system distinguishes five clinically relevant types based on presentation and duration: first diagnosed, paroxysmal, persistent, long-standing persistent, and permanent AF. 1

First Diagnosed AF

• Every patient who presents with AF for the first time is considered to have first diagnosed AF, regardless of the duration of the arrhythmia or the presence and severity of AF-related symptoms 1
• There may be uncertainty about the actual duration of the episode and whether previous undetected episodes occurred 1

Paroxysmal AF

• Self-terminating episodes, usually within 48 hours 1
• Although AF paroxysms may continue for up to 7 days, the 48-hour time point is clinically important because after this the likelihood of spontaneous conversion is low and anticoagulation must be considered 1
• Episodes generally last ≤7 days, with most lasting less than 24 hours 1

Persistent AF

• Episodes that either last longer than 7 days or require termination by cardioversion (either pharmacological or direct current cardioversion) 1
• Termination with pharmacological therapy or electrical cardioversion does not change the designation from persistent to paroxysmal 1

Long-Standing Persistent AF

• AF that has lasted for ≥1 year when it is decided to adopt a rhythm control strategy 1
• This category represents a more advanced stage where the arrhythmia has been sustained for an extended period 1

Permanent AF

• The presence of the arrhythmia is accepted by both the patient and physician 1
• By definition, rhythm control interventions are not pursued in patients with permanent AF 1
• This designation is often arbitrary and represents a therapeutic decision rather than a distinct pathophysiological entity 1

Important Clinical Considerations

• These categories are not mutually exclusive—a particular patient may have several episodes of paroxysmal AF and occasional persistent AF, or vice versa 1
• It is practical to categorize a given patient by the most frequent presentation 1
• The pattern of AF can change over time, with patients potentially moving from paroxysmal to persistent to permanent AF as the condition progresses 1
• Both paroxysmal and persistent AF may be recurrent 1

Special Terminology

• Lone AF: Applies to young individuals (aged <60 years) without clinical or echocardiographic evidence of cardiopulmonary disease, who have a favorable prognosis with respect to thromboembolism and mortality 1
• Nonvalvular AF: Restricted to cases where the rhythm disturbance occurs in the absence of rheumatic mitral stenosis or a prosthetic heart valve 1

Critical Diagnostic Pitfalls

• Extremely rapid ventricular rates exceeding 200 bpm strongly suggest the presence of an accessory pathway (such as Wolff-Parkinson-White syndrome) or ventricular tachycardia rather than typical AF 2, 4
• AF may be misdiagnosed as atrial flutter when atrial activity is prominent on the ECG in more than one lead 1, 4
• Regular RR intervals are possible in AF when AV block or ventricular/junctional tachycardia is present, which may confuse the diagnosis 1, 4
• Diagnosis in patients with implanted pacemakers may require temporary pacemaker inhibition to expose underlying atrial fibrillatory activity 2