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Atrial fibrillation

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	- Introduction - Causes - Diagnosis - Treatment
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Causes

• Ischaemic heart disease
• Hypertensive heart disease
• Rheumatic heart disease
• Thyrotoxicosis
• Cardiomyopathy (dilated or hypertrophic)
• Alcohol misuse (acute or chronic)
• Sick sinus syndrome
• Post-cardiac surgery
• Chronic pulmonary disease
• Idiopathic (lone)

It may be precipitated by an atrial extrasystole or result from degeneration of other supraventricular tachycardias, particularly atrial tachycardia and/or flutter.

Atrial flutter

Atrial flutter is due to a re-entry circuit in the right atrium with secondary activation of the left atrium. This produces atrial contractions at a rate of about 300 beats/min seen on the electrocardiogram as flutter (F) waves. These are broad and appear saw-toothed and are best seen in the inferior leads and in lead V1.

The ventricular rate depends on conduction through the atrioventricular node. Typically 2:1 block (atrial rate to ventricular rate) occurs, giving a ventricular rate of 150 beats/min. Identification of a regular tachycardia with this rate should prompt the diagnosis of atrial flutter. The non-conducting flutter waves are often mistaken for or merged with T waves and become apparent only if the block is increased. Manoeuvres that induce transient atrioventricular block may allow identification of flutter waves.

The causes of atrial flutter are similar to those of atrial fibrillation, although idiopathic atrial flutter is uncommon. It may convert into atrial fibrillation over time or, after administration of drugs such as digoxin.

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Risk factors

Risk factors for development of AF include:

1. Those who have had coronary heart disease, heart attack or heart failure.
2. It's also found in people with heart valve disease, an inflamed heart muscle or lining (endocarditis) or
3. Recent heart surgery
4. People with atherosclerosis and angina
5. Congenital heart defects
6. People with chronic lung disease, emphysema and asthma
7. Thyroid disorders
8. Diabetes
9. High blood pressure
10. Excessive consumption of alcohol, cigarette or stimulant drugs, including caffeine.

Pathogenesis

Atrial fibrillation is caused by multiple re-entrant circuits or "wavelets" of activation sweeping around the atrial myocardium. These are often triggered by rapid firing foci. Atrial fibrillation is seen on the electrocardiogram as a wavy, irregular baseline made up of f (fibrillation) waves discharging at a frequency of 350 to 600 beats/min. The amplitude of these waves varies between leads but may be so coarse that they are mistaken for flutter waves.

Conduction of atrial impulses to the ventricles is variable and unpredictable. Only a few of the impulses transmit through the atrioventricular node to produce an irregular ventricular response. This combination of absent P waves, fine baseline f wave oscillations, and irregular ventricular complexes is characteristic of atrial fibrillation. The ventricular rate depends on the degree of atrioventricular conduction, and with normal conduction it varies between 100 and 180 beats/min. Slower rates suggest a higher degree of atrioventricular block or the patient may be taking medication such as digoxin.

Atrial fibrillation has three stages

1. Paroxysmal AF is characterized by brief episodes of the arrhythmia, which can resolve by themselves.
2. In persistent AF, the episodes require some form of intervention to return the heart rhythm back to normal.
3. For those with permanent AF, intervention (if successful at all) only restores normal heart rhythm for a brief time.

As the uncoordinated atrial depolarizations from various places within the atria in AF causes blood in the upper chambers of the heart not to be carried through in a regular manner, there is a tendency for blood clots to form in these chambers. These clots may then be swept into the ventricles and pumped into the lungs from the right side of the heart and into the general circulation from the left ventricle. Sometimes, clotted blood dislodges from the atria and results in a stroke.

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