Ventilation and Positive vs. Negative Pressure

[sarjasy]

Not quite grasping this. Why does "positive" ventilation pressure lead to hypotension? Per my instructor (and book) the positive pressure of ventilation leads to compression of the vena cava. I'm cool with the idea of increased thoracic pressure and vena cava compression (assuming that is what they are getting at), but how does that differ from a normal, deep, inspiration on a non-ventilated person (negative pressure)?

It seems to me that it is all about thoracic volume. Less thoracic volume (from expanded lungs - either by ventilation or normal inspiration) increases thoracic pressure. I don't see the relevance of whether the lungs expand via negative pressure (normal inspiration) or positive pressure (mechanical ventilation).

The example I would give is a balloon with a tube to the atmosphere inside a sealed glass box. Whether that balloon is pumped with air (positive pressure) or filled with air via vacuum (negative pressure), it has the same effect on the partial pressure of gas inside the box. How the balloon increases its volume (via negative or positive pressure) is irrelevant. How am I wrong here?

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[GeneValgene]

Not quite grasping this. Why does "positive" ventilation pressure lead to hypotension? Per my instructor (and book) the positive pressure of ventilation leads to compression of the vena cava. I'm cool with the idea of increased thoracic pressure and vena cava compression (assuming that is what they are getting at), but how does that differ from a normal, deep, inspiration on a non-ventilated person (negative pressure)?

It seems to me that it is all about thoracic volume. Less thoracic volume (from expanded lungs - either by ventilation or normal inspiration) increases thoracic pressure. I don't see the relevance of whether the lungs expand via negative pressure (normal inspiration) or positive pressure (mechanical ventilation).

The example I would give is a balloon with a tube to the atmosphere inside a sealed glass box. Whether that balloon is pumped with air (positive pressure) or filled with air via vacuum (negative pressure), it has the same effect on the partial pressure of gas inside the box. How the balloon increases its volume (via negative or positive pressure) is irrelevant. How am I wrong here?

using your example with the box and the balloon...think of the pressure inside the box as X. when you use positive pressure, you are forcing air in at some pressure A, which must be > X and > atm. the balloon will expand until A=X. when you use the vacuum, you decrease the pressure of X until X < atm. the balloon will expand until A=X=atm. you can see that X in the first case is greater than X in the second case. of course this is not exactly how your lungs work, but might help see why your illustration might be confusing you...

remember in regular ventilation, you're creating negative pressure by increasing your thoracic volume by diaphragm contraction

 

[Paseo Del Norte]

Your balloon analogy is not completely correct. In "normal" breathing the lungs fill due to the pressure difference in the thoracic cavity. The pressure difference can be seen as negative compared to the atmosphere. If you remember back to chemistry and physics, this is essentially Boyle's law applied.

However,positive pressure ventilation is completely opposite. You take a negative pressure system and turn it into a positive pressure system. You in essence have a ventilator pushing and forcing air into to the lungs. So, instead of having sub-atmospheric pressures in the thoracic cavity during inhalation, you have positive pressure.

 

[sarjasy]

remember in regular ventilation, you're creating negative pressure by increasing your thoracic volume by diaphragm contraction

So, instead of having sub-atmospheric pressures in the thoracic cavity during inhalation, you have positive pressure.

...and that's what I wasn't factoring into my analogy. Chest cavity volume increases during normal inspiration increasing negative pressure and my "glass box" doesn't. Makes perfect sense. Thanks.

 

[Paseo Del Norte]

...and that's what I wasn't factoring into my analogy. Chest cavity volume increases during normal inspiration increasing negative pressure and my "glass box" doesn't. Makes perfect sense. Thanks.

No problem. I've received good advice here and I'm glad that I've been able to return the favour.

 

[Disinence2]

Don't forget that the negative pressure generated to suck air into the lungs also sucks blood into the vena cava.

This is pretty cool to see on ultrasound.