During cardiopulmonary bypass, the surgeon's finger is passed behind the pulmonary trunk and ascending aorta into a transverse passage within the pericardial cavity. Which chamber forms the posterior wall of this passage?

• A Left atrium ✓
• B Right atrium
• C Right ventricle
• D Superior vena cava

Explanation

The transverse sinus of the pericardium lies between the arterial mesocardium in front, formed by the ascending aorta and pulmonary trunk, and the venous mesocardium behind, formed mainly by the left atrium. Its posterior wall is therefore the left atrium, which allows the surgeon to pass a tape around the aorta and pulmonary trunk. The right atrium and superior vena cava form parts of the oblique sinus region, not the transverse sinus.

Reference: Gray's Anatomy for Students, 4th ed.

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Written and medically reviewed by the StethoPrep medical team.