Atrial Fibrillation (AFib) (Last Updated - 3/2/2005)

-most common sustained arrythmia
-increased mortality rate -> 2x that of people w/ NSR (normal sinus rhythm)
-affex 6% > 80 y.o. -> prevalence increases w/ age
-risk factors:
   -valvular HD
   -heart failure
   -HTN
   -diabetes
   -previous MI (in men)
   -alcohol ingestion
   -thyrotoxicosis
   -increased vagal tone (causes postprandial Afib)
   -increased sympathetic tone (causes exercise-related Afib)
-"lone Afib" -> 17% of cases -> no clinical, EKG or echo evidence of risk factors
-familial cases -> rare, assoc. w/ chromosome 10 abnormalities
-fibrosis and inflammation -> decreased conduction
-thyrotoxicosis -> decreased refractoriness
-ischemia and changed autonomic tone -> both decreased conduction and refractoriness
-atrial enlargement helps to sustain Afib by accomodating more Afib wavelets
-some episodes initiated by firing of atrial myocytes in muscle sleeves located in pulm. veins
-produces:
   -myocyte degeneration
   -focal accumulation of ER and mitochondria
   -widening of some intercalated disks
   -replacement of myofibrils
   -above contribute to patchy fibrosis and dilation
   -shortened refractory period
   -loss of normal adaptation of refractoriness to changes in HR
   -above 2 contribute to recurrence
-most assoc. symptoms caused by poorly controlled ventricular rates
-assoc. w/:
   -palpitations
   -decreased exercise tolerance
   -dyspnea
-serious complications:
   -thromboembolus
   -stroke (Afib accounts for ~15% of strokes)
-may induce a hypercoagulable state:
   -increased plasma fibrin D-dimer concentration
   -increased plasma beta-thromboglobulin concentration
-2 approaches to therapy:
   1) Ventricular Rate Control Approach:
      -allowing Afib to persist while controlling HR
      -avoidance of anti-arrhythmic drugs is desirable
      -risk of stroke reduced w/ anti-coags
      -drugs commonly used:
         -beta-blockers
         -Ca channel blockers (verapamil and diltiazem)
         -digoxin
      -done via AV nodal blocking agents or ablation of the AV junction w/ pacemaker implantation
      -advantage:
         -decreased drug toxicity w/ absence of anti-arrythmics
   2) Rhythm Control Approach:
      -goal is to maintain sinus rhythm
      -cardioversion
      -escalating doses of increasingly potent anti-arrhythmic drugs administered until Afib is abolished or drug toxicities develop
      -anti-arryhthmics commonly used:
         -amiodarone
         -disopyramide
         -flecainide
         -moricizine
         -procainamide
         -propafenone
         -quinidine
         -sotalol
         -dofetilide
      -advantages:
         -possibly fewer symptoms
         -better exercise tolerance
         -decreased risk of stroke
         -eventual discontinuation of anti-coag's
         -better quality of life
         -better survival (if NSR is reinstated)
      -disadvantage:
         -Afib often unresponsive to anti-arrhythmics
      -Anti-coags (warfarin) are recommended for both approaches
      -"A Comparison of Rate Control and Rhythm Control in Patients with Atrial Fibrillation" - NEJM, Vol. 347, Number 23, 12/5/2002 shows that rhythm
         control provides no advantage over rate control with respect to survival and that rate control can now be considered as primary approach to Afib
         treatment
      -increased rates of mortality (23.8% vs. 21.3%, though not statistically significant 95% confidence interval 0.99 to 1.34), hospitalizations (80.1%
         vs. 73.0%, p < 0.001) and adverse drug effex in rhythm-controlled subjects vs. rate-controlled subjects
      -uncontrolled symptoms due to Afib and CHF -> switch to rhythm control if using rate control

Chronic Afib:
-pts. must be on chronic anti-coagulation to prevent left-sided emboli inducing strokes or acute MI --> Warfarin (Coumadin)
-target INR's in pts. taking oral anti-coagulation with Warfarin (Coumadin) --> 2.0 - 3.0 (3)

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References:

1) Cain, Michael E., M.D. "Perspective: Atrial Fibrillation - Rhythm or Rate Control". NEJM, Vol. 347, Number 23, pgs. 1822-1823. 12/5/2002.
2) Massachusetts Medical Society. "A Comparison of Rate Control and Rhythm Control in Patients with Atrial Fibrillation" - NEJM, Vol. 347, Number 23,
   pgs. 1825-1833. 12/5/2002.
3) Journal of the American College of Cardiology - May 7, 2003 - Vol. 41 (#9) - Pages 1646-1647.