Pathophysiology of Atrial Fibrillation
1) atrial fibrillation is a disorganized electrical activity in the atria with an increased number of atrial impulses reaching the AV node (350-600/min)
2) this causes a subsequent partial refractoriness at the AV node with a “concealed conduction” that can result in a relatively slow ventricular response to the atrial fibrillation
3) one variant is “lone” atrial fibrillation in patients with no other heart disease; this may be the tachycardic phase in the bradycardia-tachycardia syndrome
Signs and Symptoms
1) “irregularly irregular” pulse
2) hypotension
3) angina
4) pulmonary edema (especially if atrial fibrillation is in setting of mitral stenosis)
5) syncope (owing to pause after the atrial fibrillation ventricular response)
6) fatigue (secondary to decreased cardiac output from loss of atrial kick in ventricular filling
7) anxiety
8) palpitations
Characteristic Test Findings
ECG
1) absent P waves
2) irregular QRS intervals
3) absence of alpha waves in jugular venous pulse
Associated Conditions
1) increased incidence in mitral stenosis, hypertrophic cardiomyopathy, hyperthyroidism, cardiac and thoracic surgery, hypoxia, hypercapnia, stress and infection, alcohol intoxication, increased vasovagal response, hypertension, rheumatic heart disease
2) increased incidence of cerebral embolism and atrial flutter
Echocardiogram
enlarged left atrium
Treatment
1) for chronic atrial fibrillation, must compute CHADS score to determine if antithrombotic prophylaxis is warranted (aspirin, warfarin or newer pharmaceutical agent) to decrease the risk of cerebral embolism
2) treatment of underlying cause
3) cardioversion if acute onset (48h) and patient is hypotensive
4) select patients may receive cardioversion after 48 hour window if echocardiogram rules out thrombus formation
5) if patient is stable, drugs are used to try to slow ventricular rate (e.g., beta blockers and calcium channel blockers) and then to convert to sinus rhythm (e.g., quinidine [type IA antiarrhythmic or flecainide [type IC antiarrhythmic])
6) amiodarone is being used with increasing frequency to convert to and then maintain sinus rhythm
Tips for USMLE
1) sometimes in an attempt to convert atrial fibrillation, atrial flutter occurs with a paradoxial increase in ventricular rate
2) if a-fib is not converted to sinus rhythm in the first several days, the chances decrease that this will ever be accomplished
3) if there are no P waves on the ECG, think atrial fibrillation
4) if irregularly irregular beats are mentioned, think a-fib
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