Fresh gas flow rates

Re: Fresh gas flow rates

Honeybadger — Mon, 11 Apr 2011 22:32:06 GMT

[quote user="emvn80"]

You should calculate before based on the patients pre-GA resp rate but remember that for NON-rebreathing systems if the resp rate changes dramatically then your FGF requirement will change. if the resp rate decreasees you don't need to adjust 9other than for economy!) but if the resp rate increases significantly you will need to. ... In simple terms if the resp rate doubles (eg due to light anaesthesia) then your FGF requirements will double...remember the calculations are based on the patients minute volume which is tidal volume x resp rate, therefore if resp rate doubles, minute volume doubles (assumming TV stays roughly constant)

15kg dog on a Lack RR preop is 10 but increases to 20 under anaesthesia halfway through...

TV = 10 x 15 = 150mL

MV= TV X RR = 150 x 10 = 1500mL (1.5 L/min) but increases to 150 x 20 = 3000 (3L/min)

[/quote]

This is what I have always gone by. I now work in a practice which has a capnograph and it is scary how often the Inspired Co2 goes up and I think about all the GA's I have monitored in the past and not had a clue.

Re: Fresh gas flow rates

emvn80 — Mon, 11 Apr 2011 21:03:42 GMT

15kg dog on a Lack RR preop is 10 but increases to 20 under anaesthesia halfway through...

TV = 10 x 15 = 150mL

MV= TV X RR = 150 x 10 = 1500mL (1.5 L/min) but increases to 150 x 20 = 3000 (3L/min)

circuit factor for lack = 1 x MV

Because I have a capnograph you do notice that you have to increase the FGF when the resp rate changes dramatically (not just by a couple of breaths/min) otherwise you do get rebreathing of CO2

Re: Fresh gas flow rates

Phrin Vernon RVN — Mon, 04 Apr 2011 22:35:41 GMT

I shouldnt think you should ever have to decrease - as long as your APL or PEEP valve is open - surely all the waste O2 will just go out via the scavenging?

I do increase O2 levels occasionally - pulse ox is the only monitor we have (apart from VN's ), so I'll up the O2 rate if I get an SpO2 below 96% - after of course wetting the tongue, repositioning etc..

Re: Fresh gas flow rates

BengalcatRVN — Mon, 04 Apr 2011 22:30:51 GMT

would you need to increase/decrease if the resps change?

Re: Fresh gas flow rates

Phrin Vernon RVN — Mon, 04 Apr 2011 22:28:23 GMT

Plus the fact, how is the nurse going to monitor the patient effectively, if they are busy working out new FGF rates.... especially just after induction!?

But then flow rates should be higher after induction anyway, to allow for denitrogination

I just dislike the thought of spending time calculating flow rates when you have a patient asleep!

Re: Fresh gas flow rates

Phrin Vernon RVN — Mon, 04 Apr 2011 22:25:16 GMT

Yes it was taught that the only safe way to lower FGF rates to anything less than pre ga RR, was with capnography!

Its not worth the risk without IMHO!

Re: Fresh gas flow rates

Juliet Drummond DAVN (med) RVN D32/33 — Mon, 04 Apr 2011 22:11:46 GMT

[quote user="Phrin Vernon"]

This is what I was taught, but there was emphasis put on the fact that it was a bit academic to recalculate after induction, and that by far the safest method was to go with was on pre premed resp rate.

[/quote]

hmmm interesting phrin- i assume this is with capnography tho.

the original poster does not state if they have a capnograph, if they do they can go as low as they like as long as the Et C02 is ok, if they don't then it is surely a dangerous practice for the patient.

i would also say that anaesthetic volatile agents are significantly cheaper than they once were, as is oxygen, is it really worth a patients sight, hearing, brain function or death to save a few quid?

Re: Fresh gas flow rates

Phrin Vernon RVN — Mon, 04 Apr 2011 21:50:08 GMT

[quote user="Juliet Drummond DAVN (med) RVN D32/33"]

[quote user="diverdippy"]

you should calculate it afterwards as well, but you need something to start with - you can't attach the patient to the system and let it breathe nothing until you have calculated it.

[/quote]

i have never heard of this?

[/quote]

[quote user="Juliet Drummond DAVN (med) RVN D32/33"]

[quote user="sazzle RVN"]

well the majority of anaesthetic agents/ opioids/ premeds effect respiratory rate of an animal, usually making them bradypnoeic, even apnoeic so calculating flow rates of an animal after it has been anaesthetised can mean that you can put it at risk of hypoxia by using too low flow rates.

[/quote]

i agree with this statement.

without a capnograph you cannot be sure if your patient is becoming hypoxic or not, so you should use the rate calculated based on the patients TPR exam before any drugs are administered.

[/quote]

I agree fully

Re: Fresh gas flow rates

Juliet Drummond DAVN (med) RVN D32/33 — Sun, 03 Apr 2011 21:12:11 GMT

[quote user="Juliet Drummond DAVN (med) RVN D32/33"]

without a capnograph you cannot be sure if your patient is becoming hypoxic or not, so you should use the rate calculated based on the patients TPR exam before any drugs are administered.

[/quote]

unles the patient has respiratory compromise, then use 'normal' resp rates.

Re: Fresh gas flow rates

Juliet Drummond DAVN (med) RVN D32/33 — Sun, 03 Apr 2011 21:09:09 GMT

[quote user="diverdippy"]

you should calculate it afterwards as well, but you need something to start with - you can't attach the patient to the system and let it breathe nothing until you have calculated it.

[/quote]

i have never heard of this?

[quote user="sazzle RVN"]

[/quote]

i agree with this statement.

without a capnograph you cannot be sure if your patient is becoming hypoxic or not, so you should use the rate calculated based on the patients TPR exam before any drugs are administered.

Re: Fresh gas flow rates

Nessie_W — Sat, 02 Apr 2011 13:09:33 GMT

He says the oxygen is just going to waste and going straight out of the circuit if we set it at the rate we believe it should be. It's all about £££.

Re: Fresh gas flow rates

bongo — Fri, 01 Apr 2011 22:47:16 GMT

Re: Fresh gas flow rates

Geniverger — Fri, 01 Apr 2011 22:38:33 GMT

Point out that brachycephalic breeds often have a slower resp rate under GA than when conscious due to vagal stimulation caused by some premeds...

Re: Fresh gas flow rates

Catherine Goulding — Fri, 01 Apr 2011 15:48:12 GMT

You are both correct, as you should calculate it afterwards as well, but you need something to start with - you can't attach the patient to the system and let it breathe nothing until you have calculated it. This is why they are called initial gas flow rates. We assume its twenty as thats what animals average breathing rate it without an anaesthetic. The RCVS OSCE exams are based on this principle so for these exams you should calculate it prior to the anesthetic at least. Maybe contact them as they will have it written somewhere.