Why tension pneumothorax in penetrating chest trauma?

[mhco]

From my understanding tension PTx is due to penetrating trauma or blunt trauma, infx, invasive ventilation, iatrogenic ⇒ a flap like tear in visceral pleura tf as you breathe (diaphram goes down increasing negative intrathoracic pressure) air comes into the plueral cavity but can't go back through the flap, it (the flap) seals itself on inspiration building up intra thoracic pressure

the thing i dont understand is that if there is penetratng trauma (one of the causes of tension pneumothorax) there is definitely a tear in the chest wall & probably the parietal pleura tf any air that enters into the pleural space via the lung and the flap like tear in the visceral pleura in a tension PTx, surely air can exit through the tear in the chest wall? Thus there shouldnt really be a be a build up of pressure right?

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[surgonc2017]

What you get is a "one way valve" where when you breathe and increases negative intrathoracic pressure, you suck in air from the outside into the pleural space. This air builds up progressively collapsing the lung.


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[chromuffin]

Think of the flap as having an angle. I would think theoretically a penetrating injury could allow air to flow out if the injury is exactly at a 90 angle from the chest wall, but that would require a lot of luck. Even still, the pressure difference would equilibrate out to cause a more positive intrathoracic pressure, causing the lung to collapse. Think of the injury as a 45 angle (or any non-90 angle). With that in mind, I think it is easier to see why the flap would only be able to open in one direction.