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Spontaneously Breathing Pneumothorax - Re-thinking Hypotension

When an opening between the lung and pleural cavity becomes established, a patient will begin to accumulate air within that cavity. The amount of air that accumulates will depend on the size of the hole and whether or not the patient is being ventilated with positive pressure. If we define a tension physiology as one that causes hypotension from obstructive impedance of the great vessels and heart, then a tension pneumothorax in a spontaneously breathing patient is rare. This makes sense if you think about the pressure gradients that are needed in order for air to move into the lungs.  At the end of exhalation our alveolar pressure is equal to atmospheric pressure.

In order to generate a gradient that promotes flow into the lungs, the alveolar pressure must drop below atmospheric pressure. This is why we call it a negative pressure breath. In the illustration below we drop the pleural pressure and alveolar pressure below atmopsheric to inflate the lung.

Now imagine a patient who has a pneumothorax. The positive pressure accumulating in the pleural cavity will make our end alveolar pressure positive. This means in order to move air into the alveoli on the next breath, we need to drop our alveolar pressure not only back down to atmospheric pressure, but then below atmospheric pressure. This is why patients become dyspneic and fatigued. When the atmosphere is your driving pressure, and your alveoli are ending with a net positive pressure, the inspiratory swings needed to reverse that gradient are extreme. While different in cause, this is very similar to the asthmatic who increases functional residual capacity and attempts to inflate lungs with inspiratory swings that can reach up to -100 cmh20. It is this limitation that makes it is very hard for a spontaenous breathing patient to develop hypotension from tension physiology. 

Once a patient is transitioned to positive pressure ventilation, they will no longer rely on dropping alveolar pressure to generate air flow into the lung. The air is now blown into the lung by the ventilator. This takes away the self-limiting effect of negative pressure breathing and a tension pneumothorax can occur within seconds. 

I want to be very clear wen I say this is not to say it can't occur in a spontaneously breathing patient, and it without a doubt will cause shortness of breath - but hypotension should be further investigated and not contributed merely to tension physiology.


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