Pericardial sinus

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

Structure

There are three pericardial sinuses: superior, transverse and oblique.

The superior 'sinus is anterior to the ascending aorta and the pulmonary trunk. It cannot be assessed in electrophysiology procedures.
The oblique sinus is an inverted Glossary of [shapes with metaphorical names|J-shaped] reflection of the venae cavae and pulmonary veins. It lies behind the atria, 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. 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.
Clinical significance

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