Anaesthetic calculations - help required

Re: Anaesthetic calculations - help required

Nick Shackleton — Thu, 06 Aug 2009 10:45:43 GMT

[quote user="Jody Duddy"]

Does anyone know the CF for a to and fro circuit?

[/quote]

The best place for a to and fro is the bin! They are several major disadvantages to this system!

the canister is far to close to the patient and therefore they may breath in either dust or granules from the canister
It cumbersome and easily knocked off!
less eggective at remove carbon dioxide due to potential tracking of gases over the soda lime
increased dead space as soda lime becomes exhausted

Anyway rant over!! If you can get you boss to invest in a circle circuit this would be a better option in my opinion!

but the CF for the to and fro is the same as a circle circuit, so start with an initially high flow rate (4-8l/min) depending on the size of the patient, reducing to a low flow rate (1-2l/min) again depending on the size of the patient. You can take the flow rates lower, but its is best to have capnography so that rebreathing can be checked and monitored. Remember at the end of anaesthesia flow rates must be increased back up to between 4-8l/mins. Also like with a circle circuit it is safe to use nitrous provided that you have capnography. you sould use 50/50 mixes.

hope this is helpful

Re: Anaesthetic calculations - help required

Jody Barry — Thu, 06 Aug 2009 10:05:36 GMT

Does anyone know the CF for a to and fro circuit?

Re: Anaesthetic calculations - help required

Sandra Taylor RVN, MBVNA — Wed, 13 May 2009 20:28:41 GMT

I have not long attended an Anaesthesia CPd, and they reckon you should not be using rebreathing circuits at anything less than a litre/min

They also said that the FGF were a source of considerable confusion and that

oxygen consumption was 10ml/kg/min

TV-is 10-20ml/kg

and that 2x MV is sufficient

It was an interesting and eye opening CPD.

Most anaesthesia deaths occur in recovery, and that cats on IVFT are 4 x more likely to die under anaesthesia due to over perfusion.

Re: Anaesthetic calculations - help required

Gillian Mostyn — Wed, 13 May 2009 17:01:05 GMT

[quote user="emvn80"]ie a 20kg dog would have a fresh gas flow requirement of 20 x 15 or 20 = 300-400ml/min (0.3-0.4L/min) [/quote]

I was taught that most vaporisers are not efficient at such a low flow rate so be careful how low you go - but it was a while ago I was taught! We tend not to go below 600 l/min flow.

Re: Anaesthetic calculations - help required

Claire Cameron — Tue, 12 May 2009 22:43:39 GMT

I remember them by

BT are the most expensive phone company

Bain and T-Piece have highest circuit factors

silly but helped me x

Re: Anaesthetic calculations - help required

emvn80 — Tue, 12 May 2009 18:51:19 GMT

the calculations you are referring to are for non-rebreathing systems, where you require freah gas flow to flush out/remove carbondioxide, hence the need for higher fresh gas flows.

Circles are rebreathing circuits and the soda lime absorbs the carbon dioxide therefore the fresh gas flow is not dependent on the patients tidal volume or minute volume but rather the patients metabolic oxygen demand, which is normally around 10ml/kg/minute, this is obvoiusly very low and you have to have flow meters that can accurately go that low and also add on the amount your capnograph removes for sampling if you are using one and also often people add on a small safety factor for not knowing each indicidual patients exact metabolic oxygen requiremen so you may prefer to use 15-20ml/min to be safe. ie a 20kg dog would have a fresh gas flow requirement of 20 x 15 or 20 = 300-400ml/min (0.3-0.4L/min) this is after using a higher fresh gas flow for first few mins to denitrogenate system ie 2L/min.

Re: Anaesthetic calculations - help required

Anonymous — Tue, 12 May 2009 14:37:18 GMT

i remeber them by saying

BALM

Bain 2.5 - 3

Ayres ( t - piece) 2.5 - 3

lack 1 - 1.5

magil 1 - 1.5.

the word BALM just helps me keep them in order and it is easy.

again hope helpful x x x

Re: Anaesthetic calculations - help required

Nick Shackleton — Tue, 12 May 2009 14:20:29 GMT

Different circuits have different circuit factors depending on their classification

Magill: 1 x MV

Lack: 1 x MV

T-Piece: 2,5-3 x MV

Bain: 2.5-3 x MV

Circle: 1 x MV (following inital high fresh gas flow rate)

These are all minimum volumes, if your patient has increased inspired CO2 then you must increase the fresh gas flow rates

Re: Anaesthetic calculations - help required

sarahnurse — Tue, 12 May 2009 14:12:59 GMT

Hi, thanks for that - really useful.

Just one query......is CF always 1 or 1.5? How do you know? Sorry - still a novice!!

Re: Anaesthetic calculations - help required

Anonymous — Tue, 12 May 2009 12:46:35 GMT

dont know site but

Tidal Volume (TV)= air exchanged each breath

Minute volume (MV)= air exchanged each minute

TV :15ml/kg for animals < 10 kg

OR 10ml/kg for animals > 10kg x

decide which one you need to use then x by body weigh(bw).

take this answer and x by resp rate(rr)( assume 20 if not stated)

take that answer and x by circiut factor(cf)

eg a 45kg dog on a lack

tv = 45(bw) x 10 = 450ml

450ml x 20(rr) = 9000ml(9litres)

flow rate = 9 x 1(cf) or 9 x 1.5 (cf).

ranges = 9 - 13.5l/min.

hope this makes sense. x x x

tv x bw x mv x cf