Anatomy

• Mediastinum is space between lungs from inlet to outlet of thorax. 
• The inlet is slanting, thus only posterior mediastinum extends into neck. 
• The mediastinum is wide anteriorly and narrows posteriorly. 
• A imaginary line passing through from manubrium angle to T3 separates superior from inferior mediastinum. 
• Heart is in middle mediastinum. 
• Space in front of heart is anterior mediastinum and behind is posterior mediastinum. 
• Posterior mediastinum is divided into pre-vertebral and paravertebral spaces.

There are important structures in mediastinum. 

• SVC, ascending aorta, thymus, are in anterior mediastinum. 
• Heart, tracheobronchial tree, pulmonary artery, pulmonary veins, nodes recurrent laryngeal nerve, phrenic nerve are in middle mediastinum. 
• Esophagus, sympathetic chain, and intercostal nerves are in posterior mediastinum.

Physiology

• Configuration and size of mediastinum varies according to breathing and position. 
  • With deep inspiration the position of mediastinum is long and narrow. 
  • In supine position the mediastinum is short in length and wide. 
  • In lateral decubitus position it sinks to the dependent side. Position of mediastinum is also determined by pleural pressures.
• When there is asymmetric pleural pressures the mediastinum shifts in position.
•  Positive pleural pressures will push mediastinum to opposite side. While negative pressures will pull mediastinum to the same side.

Pathology

• Most common pathological lesions in mediastinum are masses. 
• Tumor can originate from all of the structures located in the mediastinum. 
• Disruption of structures and inflammation are other pathological conditions that can be encountered.  
• Air can also escape into mediastinum.