The pericardial sinuses are impressions in the pericardial sac formed between the points where great vessels enter it.

There are three pericardial sinuses: superior, transverse and oblique.^[1]

• The superior sinus is anterior to the ascending aorta and the pulmonary trunk.^[1] It cannot be assessed in electrophysiology procedures.^[1]

• The oblique sinus is an inverted J-shaped reflection of the venae cavae and pulmonary veins.^[2] It lies behind the atria (particularly the left atrium),^[1] and in between left and right pulmonary veins.
• The transverse sinus is the tunnel-shaped passage posterior to the aorta and pulmonary trunk , and anterior to the superior vena cava.^[1][2] This sinus is clinically important because passing one end of clamp through the sinus, and the other end anterior to the aorta/pulmonary trunk will allow complete blockage of blood output. This is performed during some heart surgeries.^{[citation needed]}

During pericardial effusion, fluid may build up in the pericardial sinuses.^[3] This may be diagnosed with transoesophageal echocardiography.^[3]