<prompt> PR interval in ECG denotes ?Isovolumetric contraction of ventricleInsovolumetric relaxation of heaAtrial contractionNone  <response>: Ans. is c i.e., Atrial contractionPR interval represents atrial depolarization atrial contraction and condution throught AV nudeElectrocardiogram Normal ECG in a bipolar limb leads typically shows five waves named P, Q, R, S and T. Q, R and S waves are collectively called QRS complex. 