about compression volume and dead space

[ketap]

hello..i just want to ask a topic that has been quiet hard to understand for me..i have been learning about anesthesia machine from few books ( "understanding anesthesia equipment" and "anesthesia equipment: principles and applications") ..and i found it hard to understand about several things..so,i decided to ask it here:
it is about the compressed volume :
well, i do understand that it is the volume lost inside the circuit because of the compliance n expansion of the tubing when pressurized..but i am confuse about how it is related to the aparatus dead space..so ,here are my questions:
1. apparatus (mechanical) dead space is commonly known as where the circuit has bidirectional flow( and contains fresh gas or "used" alveolar gas or mixed gas)..,right?
so, if the breathing tube accomodates fresh gas, expired gases or mixed (like in mapleson) ,can i call compressed volume as an apparatus dead space too?many literatures are not clear about this..

2. in the circle system,does the fresh gas can never meet with the exhaled gas if the unidirectional valves are intact and everything were properly positioned?
if the answer is "they can not meet" ..then i have another question:

3.Don't the breathing circuit in the circle system has a compressed volume too? if it do, can we relate it as a dead space too?

i have been thinking about it and become confused .. in many literature, it has been said that "because of the unidirectional valves, the apparatus dead space in circle system are only between the Ypiece and the patient"..i believe this is very true if the fresh gas indeed can never meet the exhaled gas in the tubing and of course, we can't call the "compressed volume" in the circle system as the apparatus dead space..am i right?

BUT one of the book clearly said "in the past, the unidirectional valves have been incorporated inside the y piece to decrease the apparatus dead space effect of compliance volume" (quote from "Anesthesia Equipment : Principles and Practice")...so, the compressed volume in circle system does contribute to apparatus dead space..?

..i don't get it...this statement really contradicts the previous statement..which one is true?
i am sorry for the long post..i am very confuse about this...please help..i really want to understand this concept...

if i am misunderstanding something, please just say it to me..it may help me too
thank you,

best regards,Ketap

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[jwk]

I've never heard anyone use the expression "compressed volume" when talking about an anesthesia circuit. For some reason I think you're confusing that and compliance.

 

[Stank811]

Compression volume is accurate. Important in lab situations and potentially neonates when using very small TV for ventilation in a controlled manner. Cant have a circuit with high compliance bc you will have a hard time knowing the exact TV you delivered. It will also result in increased dead space in your circuit. It is why we use pediatric circuits. If you are delivering a TV of 50cc with an adult circuit using VCV most some of that volume will be lost to the compliance of the circuit.

http://www.ncbi.nlm.nih.gov/pubmed/6579868

 

[pgg]

You're overthinking it.

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

Compression volume due to circuit compliance is an area that is ventilated but not perfused. It is dead space. It is bidirectional flow into a space that is created by the pressure increase of a vent cycle that has distended compliant tubing.

 

[ketap]

hi,Stank811 and PGG, thank you for the clarification about the compliance as a dead space..but i am sorry,,i still don't get the answer for the number 2 and 3 question..

how about in circle system? if the valves are functioning properly, the fresh gas from the inspiratory tube never goes to the expiratory side and the exhaled gas from the patient never reaches the inspiratory tube, right? hence, no bidirectional flow (and no dead space) inside those tubes ..?
please help me understand this...thx u very much

regards,Ketap

 

[ketap]

hi,Stank811 and PGG, thank you for the clarification about the compliance as a dead space..but i am sorry,,i still don't get the answer for the number 2 and 3 question..

how about in circle system? if the valves are functioning properly, the fresh gas from the inspiratory tube never goes to the expiratory side and the exhaled gas from the patient never reaches the inspiratory tube, right? hence, no bidirectional flow (and no dead space) inside those tubes ..?
please help me understand this...thx u very much

regards,Ketap

 

[deleted162650]

hi,Stank811 and PGG, thank you for the clarification about the compliance as a dead space..but i am sorry,,i still don't get the answer for the number 2 and 3 question..

how about in circle system? if the valves are functioning properly, the fresh gas from the inspiratory tube never goes to the expiratory side and the exhaled gas from the patient never reaches the inspiratory tube, right? hence, no bidirectional flow (and no dead space) inside those tubes ..?
please help me understand this...thx u very much

regards,Ketap

Correct, but there is bidirectional flow from the Y-piece forward (including the ETT itself).

 

[ketap]

hi,SaltyDog...thx for the reply..but why some of the book said that the y piece used to have unidirectional valves to minimize the compliance- dead space effect?

i will write it down again.. "in the past, the unidirectional valves have been incorporated inside the y piece to decrease the apparatus dead space effect of compliance volume" (quote from "Anesthesia Equipment : Principles and Practice")
this is confusing..if i got it right, this book actually told us (indirectly) that the circle breathing tubes do have compliance and it is also considered as the apparatus dead space??

as u can see..that statement quiet contradicts my understanding about the circle system's dead space (which is the same as yours --restricted only between the y piece and the patient)..

the only thing i can think of as the explanation for that statement is that :
the inspiratory gas do enter the expiratory limb at some moment and the expiratory gas do enter the inspiratory limb at some time, hence dead space effect of compliance volume..
but if this is true..we cannot say that the app dead space in circle system is just between the y piece and the patient anymore..right? i can't find any literature said about this....
so,any thoughts?
thx u

sorry for bothering all of you...
best regards,Ketap

 

[ketap]

hi, no one answer here..so, i did more digging from the book and after reading few pages more..i did find what i think might be the answer to it??(i am just guessing here..please do let me know if i am right..) and i quote: "when exhalation or inhalation starts, the gases in the breathing tubes move in the opposite direction from their usual flow until stopped by a closed unidirectional valves.. this is referred to as backlash and causes slightly increase in the dead space. if the unidirectional valves are competent,however, blacklash will be clinically insignificant"(from "a practical approach to anesthesia", page 154)
...so, the dead space in the circle system is indeed not only between the patient and the y piece, but also appears in the breathing tubes (with its compliance) too..but because it is clinically negligible, we can just ignore it unless the unidirectional valves aren't working..am i right?

please help confirm me if i am right...and thank you .

best regards,
Ketap

 

[pgg]

Yes.

Although I would imagine that the dead space could be clinically significant if one was using adult size circuit tubing with a neonate. Dead space from circuit compliance could be a non trivial fraction of your intended volumes.

But mostly this is hair splitting. If it's a useful line of thought it's because it forces you to understand exactly how the machine works.

 

[ketap]

hi,PGG..thx u so much for replying...
yeah, u are right...reviewing these machines deeply has been driving me into several headaches...

thx again,PGG..sorry for bothering you

regards,Ketap