The mediastinum is the space between the two pleural cavities.

It is divided into four compartments by relationship to the pericardium (Fig 1):

• Anterior
• Middle (containing the pericardium)
• Posterior
• Superior

A mass in the anterior mediastinum is a major risk factor for anaesthesia. Loss of muscle tone after induction may allow the mass to fall back to compress the heart, great vessels and major airways.