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What is the cause of tracheal deviation towards the same side in spontaneous pneumothorax?
The pressure inside the pleura is HIGHER than atmospheric pressure. So it PUSHES the trachea to the opposite side.

Explaination : The two forces that act to maintain the -ve IPP = The outward recoil of the chest wall (due to shape and dynamics) and inward recoil of the lung. Pneumothorax is a ball valve / one way valve, The alveolus keeps collapsing, outward recoil of chest draws air into the pleural space. Keeps happening till the lung collapses.
Pleura keeps expanding like a balloon, and pushes the trachea outward.

Fibrosis, for example on the other hand, pulls (contraction of fibrotic scar) to the same side as the lesion.
 
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The key distinction and source of confusion lies in the different kinds of pneumothorax.

The trachea supposedly deviates TOWARD the affected side in a SPONTANEOUS PTX. Now, I've seen a lot of patients with this and I've never seen appreciable tracheal deviation, but apparently it can occur on step exams. You would need to collapse a lot of lung to see this finding, but theoretically it can happen. Look for these in the tall, young males with sudden onset dyspnea and chest pain but normal CV exam.

Contrast this with a TENSION pneumothorax. This is the big high yield one!!! You will see one or more of these on your step exams. In this one, you have increasing positive pressure in the pleural space on one side. This PUSHES the whole mediastinum over to the CONTRALATERAL side and even creates tamponade physiology. Certain stab wounds can do this. I saw this happen in a patient whose tracheostomy tube was accidentally third-spaced and the nurse kept bag ventilating and forced a ton of air into the pleural space and created a tension pneumo -- scary!
 
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