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[jean]

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, can we call the breathing tube as dead space also?

3. how can we calculate the FGF and TV of the ventilator in the semi close circle circuit system to compensate for dead space and compliance of the breathing tube?

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

[jafo1964]

1.   Equipment dead space in circle system is upto the Y piece and (marginally  beyond it on both limbs)  in a circle system.
If both the Undirectional valves(UDV) are fully functional as they should be
Then the functional analysis of the closed system is as follows

When the patient expires, the inspiratory UDV is closed, so expired air cannot travel into the inspiratory limb.  The expiratory UDV is open and expired air travels only down the expiratory limb towards the soda lime canister.

The reverse happens during inspiration and hence the entire inspired air comes from the inspiratory limb consisting of FGF and gas coming out of the circle absorber. No air is drawn in from the expiratory limb.

So theoretically if both the UDV are working well. Patient cannot expire into the inspiratory limb and he cannot inspire any gas from  the expiratory limb.

So that leaves the only place where the inspired and expired gases mix before going into separate limbs and that is the Y piece near the patient end. That then is the dead space , the only gas which can enter during the next inspiration that can possibly contain some expired air mixed into it.


2.   The breathing tubes have compliance and can stretch and collapse. This definitely contributes to wasted ventilation thereby reducing the delivered tidal volume due to wasting in the circuit expansion.

 It will however not contribute to dead space. Dead space must entail rebreathing of expired gases and this does not happen in compliant breathing tubes.

Just need to remember that compliant breathing tubes may decrease TV enough to decrease minute ventilation over time and lead to slow development of hyper carbia.

3.   With an anaesthetic ventilator,  the FGF delivered in compliant tubing will be less than set value. This can be detected with a respirometer at the patient end. In modern machines this is built into the machine as delivered tidal volume.

Modern machines have tried to over come this problem using various methods including AUTOMATIC TUBE COMPENSATION ( ATC) or FRESH GAS DECOUPLING (FGD)

These features overcome wasted ventilation due to  compliant tubing and ensure that the set TV is delivered to the patient



THAT DOCTOR are very relevant questions. No need to be apologetic about asking them. It took me a very long time to understand these concepts too.

I hope these explanations have been lucid. If not,  you will find all you need in DORSCH & DORSCH edition 5 chapter I think on breathing circuits

Or try the APSF website that has explanations on the anaesthesia machine which will explain these concepts to you

regs

[jean]

jafo1964: hello, thx u so much for the kind respond , thx a lot the answers have enlighten me a lot ...thx u. ...btw, actually, i am still having some trouble understanding some  questions...but, i will try to read the books you 've mentioned for now , and if it is fine, i would like to ask some again in advance if i have still some questions...thx u so much