Positive pressure ventilation

[ferroportin]

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Physiology question: In PEEP, the alveoli are kept expanded by external positive pressure. So, if gas of any sort is being blown in to achieve this, doesn't this prevent any gas (eg, CO2) from being blown out?

 

[SpecterGT260]

Physiology question: In PEEP, the alveoli are kept expanded by external positive pressure. So, if gas of any sort is being blown in to achieve this, doesn't this prevent any gas (eg, CO2) from being blown out?

No, the gas will still mix as it would anyways due to passive diffusion of the gasses in the lung spaces. You can't "cap" a gas system with gas pressure.

PEEP is fairly slight and you still blow against it, you just have to work a little harder to do so. This mimics the closed mouth breathing that those with emphysema do and for similar reasons. As long as you are moving net air volume larger than your dead space you are good to go.

 

[ferroportin]

No, the gas will still mix as it would anyways due to passive diffusion of the gasses in the lung spaces. You can't "cap" a gas system with gas pressure.

PEEP is fairly slight and you still blow against it, you just have to work a little harder to do so. This mimics the closed mouth breathing that those with emphysema do and for similar reasons. As long as you are moving net air volume larger than your dead space you are good to go.

Ah, right, gases will mix and molecules still move at a decent pace, but there is a slight decrease in the rate of gas effusion. Emphysema patients purse their lips to increase resistance to flow in order to increase intra-luminal pressure, thus preventing airways from collapsing; here, the size of the outlet itself remains the same, but there is more matter inside the tube, thus greater intra-luminal pressure. That sound right?

I guess the other issue that comes up is what to do if CO2 starts to accumulate in alveoli anyway. Since the rate of ventilation is more important in this sense than the positive pressure, one could increase the rate to blow off more CO2.

 

[SpecterGT260]

Ah, right, gases will mix and molecules still move at a decent pace, but there is a slight decrease in the rate of gas effusion. Emphysema patients purse their lips to increase resistance to flow in order to increase intra-luminal pressure, thus preventing airways from collapsing; here, the size of the outlet itself remains the same, but there is more matter inside the tube, thus greater intra-luminal pressure. That sound right?

I guess the other issue that comes up is what to do if CO2 starts to accumulate in alveoli anyway. Since the rate of ventilation is more important in this sense than the positive pressure, one could increase the rate to blow off more CO2.

Yeah that is right enough. If you turned the CPAP machine or whatever it is they are hooked to up to "leaf blower", then yes, you would have issues. With a slight bit of pressure increase you just slow down the expiratory phase a little and since the airways can all be thought of as elastic bands, the air pressure keeps them from "snapping closed", but you still move a large volume of air with each breath.

So, PEEP only increases the resistance at the end of expiration (when you would expect the biggest problem with airway collapse), but this doesn't impact the rate. Remember, expiration is a passive process usually and you will slow the rate of flow slightly at the end of the cycle, but the active parts of breathing dictate your resp rate. If the system is matching RR (which it should) you can still increase your resps to compensate for extra CO2 production without any issue. Furthermore, the vast majority of air movement happens within the first second of expiration (look at a flow diagram and note the peak that occurs very early in the cycle).

 

[FutureInternist]

Physiology question: In PEEP, the alveoli are kept expanded by external positive pressure. So, if gas of any sort is being blown in to achieve this, doesn't this prevent any gas (eg, CO2) from being blown out?

The ventilator is programmed to shut off the outflow valve when the intra-thoracic pressure reaches a certain number (the PEEP)
If you waited till the alveoli collapsed (full expiration) & then tried to blow in air, it would defeat the whole purpose