• There is a small puncture in the visceral pleura, letting air escape from lung to pleural space. The rent is small and closes spontaneously in most patients. 
• Once the air leaks into pleural space the negative pressure decreases and the lung recoils to a new position of equilibrium established by the chest wall recoil and lung recoil forces. As the lung retracts to a smaller volume the rent in the pleura becomes smaller facilitating the closure. Depending on the extent of air escape, the lung can be permitted to assume its resting completely atelectatic status. 
• The hemi thorax assumes a larger position without the lung elastic recoil forces to hold it down.
• Ventilation to the pneumothorax side is decreased because of the high FRC position and resistance required to overcome the compliance of collapsed lung.
• In tension pneumothorax, air continue to leak from the rent in the lung into pleural space increasing the tension in pleural space. 
  • As the positive pressure builds the mediastinum gets pushed to the opposite side and the diaphragm downwards. 
  • Venous return is impeded resulting in significant compromised cardiac function. 

Physical Findings of Pneumothorax without Tension

1. Mediastinum stays in middle since there is no positive pressure in the pleural space. The mediastinum and diaphragm are not displaced.
2. Chest Expansion is dramatically decreased on the side of pneumothorax.
3. There will be hyper-resonance on the side of pneumothorax due to the presence of air in pleural space. Loss of cardiac or hepatic dullness can be noted if there is sufficient air to overlay these structures.
4. Breath Sounds are dramatically decreased secondary to decreased ventilation on the side of pneumothorax.
5. Voice Transmission is decreased.
6. There will not be any adventitious sounds.
7. Hemi thorax will be enlarged on the side of pneumothorax due to loss of negative pressure in pleural space.
8. Effort of Ventilation is increased in the acute stage, as recognized by respiratory rate and use of accessory muscles. After the acute event, patient can return to a state of comfort without a significant increased effort at breathing.
9. Effect on function: Hypoxemia is mild and may not be recognized clinically. If you have stereophonic stethoscope you can appreciate the decreased breath sounds and poor voice transmission on the side of pneumothorax by simultaneous auscultation of both sides.

Pneumothorax with Tension

The following are features unique to tension pneumothorax:

1. Mediastinum gets pushed to opposite side.
2. Diaphragm gets pushed downward resulting in decreased diaphragmatic excursion. In extreme cases it becomes concave upwards resulting in paradoxical movement with respiration.
3. Intercostal bulge can be noticed due to increased pleural pressure.
4. You may hear amphoric type of  bronchial breathing. The bronchial tree is patent in the atelectatic lung. Bronchial breathing from the atelectatic lung is transmitted by tense air. This gives the metallic quality to bronchial breath sound.
5. Voice Transmission is also increased with a characteristic metallic quality.
6. Coin Sound: Transmission of coin tapping sound is increased on the side of tension pneumothorax. You need two half dollar silver coins to demonstrate this. Have one of your colleagues place one coin in back and tap it with the other while you listen in front. Compare the sound with the normal side. You will hear increased transmission of sound on the side of tension pneumothorax with a metallic tone to it.
7. Neck Veins are distended and non-pulsatile. There is impediment to venous return because of increased pleural pressure. This results in small rapid pulse and falling blood pressure.
8. Effort of Ventilation: A significant increased effort is noted with severe shortness of breath and use of accessory muscles of respiration.
9. Effect on Function: Central cyanosis becomes evident.