# Explain this to me like I am a 3 yr old

#### urge

10+ Year Member
When you give 6% des with 2 L of O2, what is the FiO2?

100 or 94%

#### jwk

##### CAA, ASA-PAC Contributor
15+ Year Member
When you give 6% des with 2 L of O2, what is the FiO2?

100 or 94%
In a perfect world, it should be 94% at equilibrium.

If you were giving 2L O2 and 2L N20, it would be 50% (yes, I know you know).

The inspired fraction (Fi) by definition can't be more than 1 or 100%.

#### Planktonmd

Moderator Emeritus
10+ Year Member
when you give 6% des with 2 l of o2, what is the fio2?

100 or 94%
94%

OP

#### urge

10+ Year Member
Why is it everybody charts 6% des with a FiO2 of 1.0?

#### Pilot Doc

##### SDN Angel
Moderator Emeritus
15+ Year Member
convenience.

The difference between 100% and 94% in partial pressure of oxygen is within the range of difference you could see with atmospheric pressure variation. the difference isn't significant.

#### Planktonmd

Moderator Emeritus
10+ Year Member
Why is it everybody charts 6% des with a FiO2 of 1.0?
I guess it means that the 6% Des is being dissolved in 100% oxygen.
But you are right it makes no sense.

#### pgg

##### Laugh at me, will they?
10+ Year Member
urge said:
When you give 6% des with 2 L of O2, what is the FiO2?

100 or 94%
urge said:
Why is it everybody charts 6% des with a FiO2 of 1.0?
Most people I know chart what the O2 sensor or gas analyzer says, which in this case would probably be around 90% (H2O vapor, left over nitrogen accounting for the rest).

But yeah, it bugs me too when I take over a case and there's a string of 1s.

OP

#### urge

10+ Year Member
Anesthesia vaporizers deliver a constant partial pressure. Not a constant volume percent.

Can you go into details?

#### Planktonmd

Moderator Emeritus
10+ Year Member
Anesthesia vaporizers deliver a constant partial pressure. Not a constant volume percent. Also there is a significant range where a vaporizer may be within tolerance for delivery.
A constant partial pressure will produce a constant volume percent as long as the altitude remains constant.

#### Planktonmd

Moderator Emeritus
10+ Year Member
Only the newest generation of vaporizers are altitude compensated. (Except desflurane)
True, but as long as you don't change altitude during the administration of anesthesia (you don't give anesthesia on an unpressurized airplane I hope), if you maintain a constant partial pressure you should expect a constant percentage of the volume.

#### Planktonmd

Moderator Emeritus
10+ Year Member
True. But taking the same Forane vaporizer from New york City to Denver will give a different volume percent delivered.
Yes

#### periopdoc

##### Cardiac Anesthesiologist
10+ Year Member
Only the newest generation of vaporizers are altitude compensated. (Except desflurane)
What do you mean by "altitude compensated." Aren't all modern variable bypass vaporizers inherently altitude compensated by the fact that turning the dial adjusts the partial pressure output? (The dial says percent, but you are actually adjusting the partial pressure output of the vaporizer). You will deliver a higher volume percent at altitude, but you need to to maintain the desired pressure. Because of this, a setting of 1 on an isoflurane vaporizer at sea level has an identical effect as a setting of 1 at altitude. Given identical patients etc)

As you note, Desflurane is not administered from a variable bypass vaporizer. Instead the dial sets a volume percent so you need to set a higher number at altitude for the same effect. A setting of 7 at altitude will be less effective than at sea level.

For the mathematically inclined, here is the vapor output equation.

VO= (CGxSVP)/(Pb-SVP)

VO=vapor output (ml), CG= carrier gas flow(mL.min), SVP=saturated vapor pressure (mm Hg) at room temp, and Pb is barometric pressure (mm Hg)

- pod

#### jwk

##### CAA, ASA-PAC Contributor
15+ Year Member
Most people I know chart what the O2 sensor or gas analyzer says, which in this case would probably be around 90% (H2O vapor, left over nitrogen accounting for the rest).

But yeah, it bugs me too when I take over a case and there's a string of 1s.
And that's the problem.

FiO2 is a measurement for us, just like SaO2, EtCO2, temp, etc. Every anesthesia setup is required to have at least one, if not more, FiO2 monitors. The number that should be going on your anesthesia record is the number on the monitor. If you're writing down 1, or 100%, or just estimating based on your O2 and N2O concentrations only, you might as well not be writing it down at all because it's simply wrong.

#### Pilot Doc

##### SDN Angel
Moderator Emeritus
15+ Year Member
If you're writing down 1, or 100%, or just estimating based on your O2 and N2O concentrations only, you might as well not be writing it down at all because it's simply wrong.
I suspect most people watch the number bounce around between 94 and 97 and just write down 1.0 once an hour. It might be imprecise, but it is hardly wrong.

Atmospheric pressure varies over a 10% range. It's not documented at all.
That's more imprecision than the 100% rounding.

10+ Year Member

#### pgg

##### Laugh at me, will they?
10+ Year Member
And that's the problem.

FiO2 is a measurement for us, just like SaO2, EtCO2, temp, etc. Every anesthesia setup is required to have at least one, if not more, FiO2 monitors. The number that should be going on your anesthesia record is the number on the monitor. If you're writing down 1, or 100%, or just estimating based on your O2 and N2O concentrations only, you might as well not be writing it down at all because it's simply wrong.
Well put.

#### pgg

##### Laugh at me, will they?
10+ Year Member
I suspect most people watch the number bounce around between 94 and 97 and just write down 1.0 once an hour. It might be imprecise, but it is hardly wrong.

Atmospheric pressure varies over a 10% range. It's not documented at all.
That's more imprecision than the 100% rounding.
Do you also round your BP readings up to numbers that are more pleasing to the eye?

#### IlDestriero

##### Ether Man
10+ Year Member
Do you also round your BP readings up to numbers that are more pleasing to the eye?
I have found that several of "our" "colleagues" are pretty creative with the old paper record. I like the electronic record much better. It actually reflects what happened. I used to give breaks, take over, etc and wonder if they write what they wanted the VS to be, or just filled out most of the record in advance.