Low Flow Anesthesia Question

[Iso4ane]

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When utilizing low flow (<1L/min) with 100% O2, the measured inspiratory concentration of O2 is never near 100%. Now obviously O2 is being used up, but if inspired is only 60% what makes up the difference? It obviously no CO2 nor is it volition anesthetic. Is it just expired nitrogen or something else entirely.

 

[Urzuz]

When utilizing low flow (<1L/min) with 100% O2, the measured inspiratory concentration of O2 is never near 100%. Now obviously O2 is being used up, but if inspired is only 60% what makes up the difference? It obviously no CO2 nor is it volition anesthetic. Is it just expired nitrogen or something else entirely.

A question for you to think about to help you get your answer: where is the gas being sampled from?

 

[Iso4ane]

A question for you to think about to help you get your answer: where is the gas being sampled from?

Well, it taken from the circuit. But, if it's a closed circuit and you had flushed it with pure O2 then the only thing in the circuit should be oxygen, CO2 and volatile anesthetic. So, that why I guessed it's just exhaled dissolved nitrogen from the patient.

 

[dhb]

Expire CO2 is 16%, gas at low flow has to be higher % like 4% sevo + the nitrogzn that you didn't eliminate at induction so 75% is about right when you give 100%O2.

 

[Urzuz]

Well, it taken from the circuit. But, if it's a closed circuit and you had flushed it with pure O2 then the only thing in the circuit should be oxygen, CO2 and volatile anesthetic. So, that why I guessed it's just exhaled dissolved nitrogen from the patient.

 

[Iso4ane]

Expire CO2 is 16%, gas at low flow has to be higher % like 4% sevo + the nitrogzn that you didn't eliminate at induction so 75% is about right when you give 100%O2.

Where is it ever mentioned (Miller? M&M? Big Blue?) that Expired CO2 is 16%?

 

[WholeLottaGame7]

Expire CO2 is 16%, gas at low flow has to be higher % like 4% sevo + the nitrogzn that you didn't eliminate at induction so 75% is about right when you give 100%O2.

Nooo, expired FiO2 is 16%. Expired CO2 is like 5%. And inspired CO2 is (should be) 0%. It's nitrogen, O2, and volatile.

 

[Aether2000]

Just a few thoughts- may not be helpful......

The total amount of dissolved nitrogen gas in blood and tissues is around 1000 ml (from old studies Campbell and Hill). Unless you start the anesthetic in low flow, some of the N2 would have been washed out during the higher flow induction with O2, but overall the amount of nitrogen washed out from the tissues and lungs averages about 50ml/min for the first three minutes then 10ml/min thereafter when using 100% O2. So- even if you have a case lasting up to an hour, there will still be some N2 stored in the tissues and coming into the expired gases. If the circuit flow rate is at 500ml, the nitrogen is only 2% of the expired gas.

The 6.5% water vapor is usually filtered out before measurement.

Sevo vaporizer accuracy is +/- 20% of the dial setting but this is maintained in low flow states but measured values are irrelevant compared to dial values anyway. However sevo using 200ml/min fresh gas flow rate will have only 9% of the sevo concentrations as a 5000 ml fresh gas flow rate at 5 minutes.

Some inaccuracies measurement (or more accurately in circuit volumes in low flow anesthesia) due to aspirated side stream volumes of up to 250ml/min measured. If one is running 1000 ml/min and 1/4 of the volume is being removed by the measuring device and an additional volume in the circuit being reduced by the CO2 absorber is around 200ml/min. Therefore the minimum inspired gas volumes are 500ml/min unless a closed circuit is being used to recycle the measured gases.

 

[propadope]

Nooo, expired FiO2 is 16%. Expired CO2 is like 5%. And inspired CO2 is (should be) 0%. It's nitrogen, O2, and volatile.

Yes. 40mmHg/760mmHg is around 5%. Don't forget about partial pressure.