Normally pulse rate and the heart rate are equal. When the pulse rate is less than the heart rate, the difference is known as pulse deficit. It usually occurs in a fast irregular rhythm like atrial fibrillation. In atrial fibrillation, atria (upper chambers of the heart) are electrically activated at a very fast rate of the order of 450 to 600 per minute. Proper organized contraction of the atria cannot occur at such fast rate and the electrical activity of the atria in atrial fibrillation is highly disorganized. Hence atria are practically at stand still. When the very fast electrical from the atria get conducted to the ventricles, a good number of them are blocked at the AV node (atrioventricular node) situated at the junction between the atria and the ventricles, in the lower part of the right atrium. Still a good number of electrical signals reach the ventricle so that they contract at a fast and irregular rate. When the ventricular rhythm is irregular, during some beats, ventricles get very little time to fill. The following contraction will be less forceful and not enough to open the aortic valve. Hence a pulse will not be felt during some heart beats, leading to the pulse deficit.