need help about circle system and dead space

[ketap]

hello everyone, i am quiet new in anesthesia and really need help about the anesthesia circle system...
if u don't mind, i have several questions,especially about the dead space in this system:
1. i have read many books that said that "because of the unidirectional valves, the dead space is generally localized only at the Y connector piece"..i don't really understand, how can those unidirectional valves determine the dead space only to the y connector?
2. the breathing tubes have also compliance that will subtract the Tidal volume delivered to the alveoli, so, why is there no book mentioned it as dead space?
3. my teacher said that the ETT will help counter the dead space , is it true? why?

those are my several questions, please forgive me if those are foolish questions..i 'm new to this...please help..thx a lot 🙂

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[pgg]

1. i have read many books that said that "because of the unidirectional valves, the dead space is generally localized only at the Y connector piece"..i don't really understand, how can those unidirectional valves determine the dead space only to the y connector?

Dead space is only possible in areas where bidirectional flow can occur. In a circle system, this means distal to the Y, because the valves prevent backwards flow in the inspiratory limb of the circuit.

2. the breathing tubes have also compliance that will subtract the Tidal volume delivered to the alveoli, so, why is there no book mentioned it as dead space?

Because it's negligible.

3. my teacher said that the ETT will help counter the dead space , is it true? why?

I'm not sure exactly what you mean here. Compared to mask ventilation, sure, because the dead space is reduced from the larger volume of the oropharynx, nasopharynx, trachea, and bronchi/bronchioles, to just what's inside the ETT + bronchi/bronchioles. This is not likely to be clinically significant though.

 

[SleepIsGood]

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[ketap]

hi,PGG : thx u so much for helping me 🙂 but i am still confuse...you said:

dead space is only possible in areas where bidirectional flow can occur.

i'm very sorry, i don't really understand this..if u don't mind, please help explain me more about this...

Quote:
Originally Posted by ketap
2. the breathing tubes have also compliance that will subtract the Tidal volume delivered to the alveoli, so, why is there no book mentioned it as dead space?
Because it's negligible.

i don't understand... if that so, why some textbooks told us to count also the compliance of the breathing circuit when determining FGF and setting the TV on the ventilator like this:

the TV set on the ventilator ( 6-10ml/kg)+ TV coupling from the fresh gas flow(according to FGF, I:E ratio, f of the patient) - dead space (2ml/kg) - compliance ( pressure x compliance) ?

sorry if i may have some misunderstanding...please help enlighten me...thx u so much

 

[pgg]

hi,PGG : thx u so much for helping me 🙂 but i am still confuse...you said:

dead space is only possible in areas where bidirectional flow can occur.

i'm very sorry, i don't really understand this..if u don't mind, please help explain me more about this...

Dead space is defined as an area that is ventilated but not perfused.

As they say, air goes in and out, blood goes round and round. Air that only flows in one direction isn't "ventilating" anything.

• The gas in the hose from the wall goes in one direction, it isn't ventilating anything, so the wall O2 line isn't dead space.
• The gas in the O2 flowmeter goes in one direction, it isn't ventilating anything, so the flowmeter isn't dead space.
• The gas in the inspiratory limb of the circuit only moves toward the patient, it isn't ventilating anything, so the inspiratory limb isn't dead space.
• Past the Y piece, gas goes into the endotracheal tube and then comes back out of the endotracheal tube. The ETT is ventilated but not perfused; it is dead space.
• The gas in the expiratory limb of the circuit only moves away from the patient, it isn't ventilating anything, so the expiratory limb isn't dead space.

It doesn't matter how long the hose from the wall is, or the volume of the flowmeter, or the length/volume of inspiratory/expiratory limbs of the circuit.

i don't understand... if that so, why some textbooks told us to count also the compliance of the breathing circuit when determining FGF and setting the TV on the ventilator like this:

the TV set on the ventilator ( 6-10ml/kg)+ TV coupling from the fresh gas flow(according to FGF, I:E ratio, f of the patient) - dead space (2ml/kg) - compliance ( pressure x compliance) ?

Circuit compliance is so small that no one really does the math and counts it.

But if you wanted to be precise, you could count it. You can think of the circuit expansion as ventilation of a potential space. As the circuit pressure rises during inspiration, the circuit gets a tiny bit larger, and gas moves into that newly created space. During expiration, the pressure drops, the circuit shrinks to is original size, and the air is forced out that space. Since you're ventilating a temporarily created volume, it's dead space.

Also, a couple other things.

Be aware that the TV coupling you mention in that equation isn't an issue with newer ventilators. I'm certain the newer Dragers compensate for fresh gas flow and don't vary TV much with higher flows.

And that equation is more of a bit of textbook theory trivia. It's not used in day-to-day practice. Eg, if you look at the ARDSNet recs for low volume ventilation in patients with ARDS, the investigators didn't deliver X cc/kg tidal volumes based on set TV + coupling - dead space - compliance; they just set the vents to X cc/kg and got on with their lives.

 

[rsgillmd]

Nicely written post pgg. 👍👍

 

[ketap]

Pgg: thx u so much , i understand about it right now..thx u so much for giving your time to answer my questions..thx for the concern too 🙂 btw, actually , i'm thinking of precisely counting it because in my country ,those traditional ventilators are still the most available ventilators in every hospital in my country...and because i am new to this stuff, i am trying to learn by "textbooks way "😛, especially for the closed circle system ....

thx a lot,Pgg 🙂

 

[Sergio99]

Hello,

I hope this will not offend you, Ketap, but I think you have some serious issues with respiratory physiology and definitions of terms. You need to go back to a good physiology textbook and study in great depth the section on respiration. I am saying this based on the fact that when pgg first explained it, you didn't understand it.

I recommend you do this urgently because your knowledge of the specialty depends on it. Further, I recommend that periodically you reread it in order to refresh your memory. It is that important.

I am not exaggerating: some day you will thank me for this piece of advice.

And please, don't get offended!

 

[ketap]

hello, Sergio99: i am not offended, i even pleased of having those input..thx u so much...🙂
more inputs are welcome 🙂