This is a good basic rhythm strip example of atrial fibrillation with a rapid ventricular response showing the identifying characteristics of atrial fibrillation:  no P waves, an irregularly-irregular rhythm, and a "fibrillatory" baseline.  The wavy baseline will not be seen in all leads in all patients, so it is best to use the first two findings as diagnostic criteria.  Atrial fib often appears initially as a rapid rhythm, as the AV node is being bombarded by many impulses from multiple foci (pacemakers) in the atria.  Depending upon the AV node's ability to transmit these impulses,however, we could see a slow, normal, or rapid ventricular response. 

Atrial fib has very chaotic depolarization of the atrial muscle, resulting in quivering and ineffective pumping of the atria.  This loss of "atrial kick" can severely reduce ventricular filling, and can reduce cardiac output by as much as 25%.  In patients with a very rapid rate, cardiac output can be further reduced, causing CHF.  In addition, the fibrillating atria can form blood clots due to sluggish movement of blood.  These clots can embolize and cause stroke.  For these reasons, patients with atrial fib are anticoagulated and sometimes the atrial fib is stopped by medical, surgical, or electrical therapy.  Recurrence of atrial fib is common after treatment, and for some patients, control of the ventricular rate and anticoagulation become the preferred treatment.