In atrial fibrillation, the patient’s atrial frequency is about 350-600 beats/min and the ventricular rate is usually 100-160 beats/min. The ventricular rate can be fast or slow, and the severity of atrial fibrillation symptoms is affected by the fast or slow ventricular rate. Patients may experience angina and congestive heart failure if the ventricular rate exceeds 150 beats/min. Patients may be asymptomatic when the ventricular rate is not fast, and in atrial fibrillation, the effective contraction of the atria is lost and the volume of cardiac output is 20% or more less than in sinus rhythm. When the ventricular rate is fast, pulse dystocia, where the pulse rate is less than the ventricular rate, can occur because many ventricular beats are so weak that they fail to open the aortic valve or because fluctuations in aortic blood pressure are too small to reach the peripheral arteries to be measured, and therefore the pulse rate can be less than the ventricular rate.