Atrial Fibrillation (AF)

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Diagnosis | Management

Definition

Atrial fibrillation (AF) is an irregular and often very rapid heart rhythm (arrhythmia) that can lead to blood clots forming in the heart. Normally, the heart rate at rest should be between 60 and 100 beats per minute. AF significantly increases the risk of strokes, heart failure, and other heart-related complications. AF episodes can be intermittent or persistent. Although AF is not usually life-threatening, it is a serious medical condition that requires proper management to prevent stroke and other complications.

Aetiology

Several underlying conditions and factors can lead to the development of AF:

• Myocardial Infarction (MI): Heart tissue damage from a heart attack can disrupt the normal electrical pathways.
• Atherosclerosis: Narrowing and hardening of the arteries can affect blood flow to the heart.
• Congenital Heart Disease: Structural heart abnormalities present from birth can predispose to AF.
• Cardiomyopathy: Disease of the heart muscle can alter the heart's electrical system.
• Hypertension: High blood pressure can strain the heart, leading to AF.
• Pulmonary Disease: Conditions like chronic obstructive pulmonary disease (COPD) can affect the heart's function.
• Previous Heart Surgery: Scarring and changes to the heart post-surgery can trigger AF.
• Sleep Apnoea: Interrupted breathing during sleep can lead to AF.
• Hyperthyroidism: Overactive thyroid gland increases the risk of AF.
• Asthma and COPD: Chronic lung diseases can also contribute to the development of AF.

Pathophysiology

The heart consists of four chambers: two upper chambers (atria) and two lower chambers (ventricles). The sinoatrial node (SAN) in the right atrium acts as the heart's natural pacemaker, controlling the rhythm of the heart (see Figure 1).

Figure 1: This image illustrates the anatomy of the heart, focusing on the electrical conduction system. The sinoatrial (SA) node, located in the right atrium, generates electrical impulses that travel through the atria to the atrioventricular (AV) node and then to the ventricles, leading to coordinated heart contractions. In AF, this orderly conduction is disrupted, leading to rapid and irregular heartbeats.

In normal heart rhythm:

• The signal originates from the sinus node (SA node).
• The signal travels to the atrioventricular (AV) node, which connects the atria and ventricles.
• The coordinated contraction of the heart muscles pumps blood efficiently to the body and heart itself.

In AF, the heart's electrical signals become disorganized, leading to:

• Tachycardia: The heart rate may rise to between 100 and 175 beats per minute, compared to the normal 60-100 beats per minute.
• Irregular Ventricular Contraction: The ventricles contract irregularly, impairing the heart's ability to pump blood effectively.
• Increased Stroke Risk: Blood clots (emboli) may form in the atria due to stagnant blood flow and can travel to the brain, causing an ischemic stroke.
• Heart Failure: If AF is not controlled, it can eventually lead to heart failure due to the heart's inability to meet the body's demands.

Risk Factors

Several factors increase the risk of developing AF:

• Age: The risk of AF increases with age.
• Heart Disease: Conditions such as previous MI, heart surgery, congenital heart defects, and heart failure.
• Hypertension: High blood pressure is a major risk factor.
• Thyroid Disease: Hyperthyroidism, in particular, increases the risk.
• Obesity: Excess weight can strain the heart and lead to AF.
• Family History: A family history of AF can increase the risk.
• Alcohol Consumption: Excessive alcohol intake can trigger episodes of AF.
• Other Medical Conditions: Diabetes, metabolic disorders, chronic kidney disease, and lung diseases are associated with a higher risk of AF.

Signs and Symptoms

AF is often asymptomatic and may be detected incidentally during examinations for other conditions. However, common symptoms include:

• Palpitations: Sensation of a rapid, fluttering, or pounding heart.
• Chest Pain: Discomfort or pain in the chest, which may indicate compromised blood flow.
• Shortness of Breath: Difficulty breathing, especially during exertion.
• Dizziness: Feeling lightheaded or faint.
• Tiredness: Fatigue due to reduced cardiac output.

Investigations

Several diagnostic investigations are used to confirm AF and assess its impact:

• 12-Lead ECG: The primary diagnostic tool for AF, showing fibrillation waves, absent P waves, and an irregular rhythm (see Figure 2).
• Blood Tests: To check for underlying conditions such as thyroid disease or electrolyte imbalances.
• 24-Hour ECG (Holter Monitor): Continuous ECG monitoring to detect intermittent AF episodes.
• Echocardiogram: To assess heart structure and function, including checking for heart failure or valve issues.
• Chest X-ray: To evaluate heart size and rule out heart failure or other lung-related causes of symptoms.

Figure 2: This ECG image shows the classic features of Atrial Fibrillation (AF). Notice the absence of distinct P waves and the presence of fibrillation waves, along with an irregularly irregular rhythm. These features are indicative of the disorganized electrical activity in the atria.

Diagnosis

The diagnosis of AF is made through a combination of physical examination and diagnostic tests:

• Physical Examination: Detection of an irregular pulse rate, which is further confirmed with an ECG.
• Patient History: Reviewing the patient's history of symptoms, risk factors, and any underlying conditions.

Management

Management of AF is focused on preventing stroke and controlling heart rate or rhythm:

Rate Control

• Beta-Blockers: Medications like bisoprolol or atenolol are commonly used to control the heart rate.
• Calcium-Channel Blockers: Diltiazem or verapamil can be used as alternatives to beta-blockers.
• Digoxin: Used primarily in sedentary patients or as a second-line option when beta-blockers or calcium-channel blockers are maximized.

Rhythm Control

Rhythm control is often preferred in patients with new-onset AF (lasting less than 48 hours) and involves the following approaches:

• Cardioversion:
  • Pharmacological Cardioversion: Using medications like flecainide (oral or IV) or amiodarone to restore sinus rhythm.
  • Electrical Cardioversion: Delivering controlled shocks through a defibrillator to reset the heart rhythm, typically under sedation or general anaesthesia.
• Long-Term Rhythm Control:
  • First-line: Beta-blockers.
  • Second-line: Dronedarone.
  • In patients with heart failure: Amiodarone.

Paroxysmal Atrial Fibrillation

Paroxysmal AF is characterized by intermittent episodes of AF, usually lasting no more than 48 hours. Management includes:

• Anticoagulation based on the CHA2DS2VASc score.
• Medications like flecainide, which are taken only during AF episodes, provided the patient has no underlying heart disease.

Anticoagulation

Anticoagulation is critical in reducing the risk of stroke in AF. Options include:

• Direct-Acting Oral Anticoagulants (DOACs): Apixaban, edoxaban, rivaroxaban, and dabigatran are commonly used to inhibit factor Xa, preventing clot formation.
• Warfarin: A vitamin K antagonist that prolongs prothrombin time, measured by the International Normalised Ratio (INR). The target INR for AF is typically 2-3.

Bleeding Risk

The HAS-BLED score can be used to assess bleeding risk in patients on anticoagulation therapy:

• H: Hypertension (BP >160 mmHg)
• A: Abnormal renal and liver function
• S: Stroke history
• B: Bleeding history
• L: Labile INRs (while on warfarin)
• E: Elderly (age >65 years)
• D: Drugs or alcohol use

References

1. National Institute for Health and Care Excellence (NICE) (2024) Guidelines for the Management of Atrial Fibrillation. Available at: https://www.nice.org.uk/guidance/ng196 (Accessed: 26 August 2024).
2. British Heart Foundation (2024) Understanding Atrial Fibrillation. Available at: https://www.bhf.org.uk (Accessed: 26 August 2024).
3. British National Formulary (BNF) (2024) Cardiovascular System: Atrial Fibrillation. Available at: https://bnf.nice.org.uk/ (Accessed: 26 August 2024).

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