The ventricular rate in atrial fibrillation is usually quite rapid unless there is underlying AV nodal disease or the patient is being treated with drugs that slow conduction through the AV node. The ECG shown here is from a 72 year old male who was seen in the emergency department because of symptoms of congestive heart failure. He was not receiving any medications. Note the absence of identifiable P waves and the irregularly irregular ventricular rhythm. The RR intervals range from 0.30 to 0.68 seconds and the overall ventricular rate is 150.