The a wave is caused by right atrial contraction (systole) and is followed by the x descent, which is caused by atrial relaxation. The c wave, an interruption of the x descent, is caused by the transmitted carotid pulse; it is seldom discerned clinically. The v wave is caused by right atrial filling during ventricular systole (tricuspid valve is closed). The y descent is caused by rapid filling of the right ventricle during ventricular diastole before atrial contraction.