Flow Rates

Re: Flow Rates

emvn80 — Mon, 05 Sep 2011 14:36:15 GMT

yep, as previously stated the flow rate for a circle is about 7-10mL/kg. This is the metabolic oxygen requirement.(you only need to supply the oxygen required for metabolism as the sodalime is removing the CO2 `- non-rebreathing systems use the fresh gas flow to remove the CO2 hence the higher rates required)...

however, as previously stated you should denitrogenate the system by using a higher FGF for the first 5 min or so, then you can lower it. Most people generally advise unless you are very confident at low flow not to go below 1 L, however you can go lower as long as you know what you are doing and know how low your vaporiser is calibrated to...this is just a safety thing to prevent inadequate oxygen supply as you never know the exact metabolic oxygen requirement of each patient, and some vaporisers are not calibrated below 0.5L/min. Also, importantly if you are using capnpography (sidestream) they sample between 50 and 500mL/min !!!! So you should know how much the one you use samples and add this to your FGF calculation (older machines can be 250mL/min but new microstream technology is only 50mL/min).

I think the pop-off valve should NEVER be closed as it is unlikely/and very tricky to deliver only the metabolic oxygen requirement, therefore excess oxygen administered will fill up the bag and if left un-noticed can cause lung damage and even death. You can leave the pop off valve fully open or partly open even when using low flow or semi low flow anaesthesia allowing any excess oxygen/FGF to be expelled out of the system.

To ensure you are delivering enough oxygen, just keep an eye on the rebreathing bag to ceck it is not fully empty and being sucked dry. Remember that unlike nonrebreathing systems suchs as T-pieces or Bains you cannot use capnography to check the FGF rate as you won't see rebreathing of CO2 with a circle if the FGF is inadequate like you do non-rebreathing systems as the sodalime will still continue to remove the CO2.

Re: Flow Rates

Siobhan Steven — Mon, 05 Sep 2011 09:27:42 GMT

Oh, and I forgot, total O2 flow shouldn't be below 500mLs as most vaporizers are not calibrated for flows below this. If you have too high a O2 flow you waste a lot of gas and agent. Use this formula to work out cost:

3 x fresh gas flow rate (O2) x vaporizer setting % = Ml/hour. So for example, if you are using 2L/min O2 and your vaporizer is set at 2% this is the equation, 3 x 2 x 2 = 12mL/inhalant an hour. Then you just divide the cost of your inhalant by mL in the bottle and you will get the price per mL.

If your Iso cost $200 (excuse the $ but you get my drift) per 250mL bottle thats 80 cents a mL, so the result for the above equation would be $9.60 an hour of inhalent (not including O2 cost). If you decrease your O2 to 0.8L/minute, 3 x 0.8 x 2 = 4.8 mL/hour or 4.8 x .80 = $3.84 per hour.

Re: Flow Rates

Mrs Dot Dot — Mon, 05 Sep 2011 08:50:42 GMT

[quote user="Siobhan Steven"]

10mL O2 per kg is correct when it comes to what the patient needs with normal respiration. This amount can be used in a circle system when the circuit is 'closed', i.e the pop off valve is closed, but it is recommended this isn't done without capnography to measure ETCO2. Therefore 15-20mLs is 'safer' to ensure effective O2 flow. On induction the O2 should be delivered at a higher rate for a few minutes (2-3 L/min) to enable nitrogen to be removed and the mechanical dead space of the circuit and machine to contain adequate levels of gas (O2) and agent (Iso etc). Of course depth should be assessed regularly and O2 flow adjusted to maintenance levels on an individual basis, its virtually impossible to stamp a time on this as anaesthetic is such a varied 'science'. Essentially the circuit has to be correct for the weight of the patient, the ET tube has to be the correct diameter and length, the re-breath bag has to be the correct size (60 x BW and rounded up to the nearest size). You can then choose to operate on a closed system, semi-closed or open. Like I say you need multi-modal monitoring for closed, plus if you don't have all you factors wrong you run the risk of patient injury so a semi closed system should suffice, pop off open enough to relieve excess pressure, but closed enough to retain gas in the bag for re-breathing and to minimise O2 & volatile agent use = money saved. And use the flow meter to fill the bag for IPPV, not the 'flush' button, as using the flush bypasses the vaporizer and you are effectively going to wake your patient up (and explode its lungs if its and a non rebreath system.).

Too much information? Sorry, once I get going on this I can't stop!

[/quote]

Never too much information! So I've quoted it...! Thanks fot that Siobhan

Re: Flow Rates

Siobhan Steven — Sun, 04 Sep 2011 23:21:14 GMT

Too much information? Sorry, once I get going on this I can't stop!

Re: Flow Rates

Rachael_24 — Sun, 04 Sep 2011 18:49:02 GMT

10ml/kg is what is required by the patient

BUT

there will be nitrogen build up in the breathing system (i.e. the nitrogen tht is present in the air), therefore a flow rate of at least 500ml is usually recommended. It is advised that you should use a higher flow rate of about 3L for the first 5-10minutes (to get rid of the nitrogen) then reduce this.

Re: Flow Rates

Cloudy Weather — Sun, 04 Sep 2011 18:33:35 GMT

I too had read it was 10ml/kg but then read it was a minimum of 1litre so surely you never actually end up using the calculation because you will never get an animal in a small animal practice where the caluculation will take you over 1000ml!? Im sorry if I am being silly just want to make sure I fully understand it!

Re: Flow Rates

Rachael_24 — Sun, 04 Sep 2011 16:24:03 GMT

[quote user="NuttyNu"]

10ml/kg/min

e.g. 20kg dog would require 200ml o2 in 1 minute therefore your flow meter should read 0.2.

[/quote]

Yep, that is correct!

Re: Flow Rates

Eira Davies — Sun, 04 Sep 2011 12:13:47 GMT

There is no flow rate for a circle, just halve it after 10mins to a minimum of 1L :) In practice we always start on 4L and halve to 2L after 10-15mins

Hope this helps xx

Re: Flow Rates

NuttyNu — Sun, 04 Sep 2011 11:53:51 GMT

I always got taught for a to-fro or a circle it was:

10ml/kg/min

e.g. 20kg dog would require 200ml o2 in 1 minute therefore your flow meter should read 0.2.

I don't know if other people have a different way to work it out though!