Nitrous pharmacokinetics

[Renald]

I want to analyze nitrous pharmacokinetics in-depth. Imagine, you are inhaling pure nitrous, which you inhale after a maximum exhalation. For a typical person it is about 1 liter of residual lung volume and he can inhale about 4 liters additionally, what means the concentration of nitrous in lungs after this would be approx. 80%. If we needed no oxygen for breathing, we would be able in several breaths achieve 100% nitrous concentration in lungs. Concentration in alveolar blood is proportional to the gas concentration in alveoli by a 0.47 proportionality blood/gas partition coefficient, and letting pure nitrous remain in lungs for about 30 seconds (full circulatory time) we would be able to reach maximum available BNC (Blood Nitrous Concentration). However, this is not possible it practice, as part of breathing gases must be oxygen.
My questions are:
* It is written in Wiki than nitrous T1/2 is about 5 minutes. Is it an elimination halflife? Would it mean that if such a person with maximum available BNC is stopped receiving nitrous and begin breathing air, his BNC would be 1/2 of maximum in 5 minutes?
* Can we calculate nitrous concentrations and doses in blood using available data on this drug? Lets say I have 100% nitrous in my lungs, does it mean BNC in alveoli would be 0.47*nitrous density (2 g/l), and after a short time the same concentration would be achieved in all the blood?

 

[sekio]

5 mins sounds like an elimination half life.

The kinetics of N2O usage are a little more complex than what you've described, but you get the basic idea. Remember that nitrous will also dissovle into fatty tissues & partition into any gas spaces like sinuses or the inner ear.

 

[polymath]

It is written in Wiki than nitrous T1/2 is about 5 minutes. Is it an elimination halflife? Would it mean that if such a person with maximum available BNC is stopped receiving nitrous and begin breathing air, his BNC would be 1/2 of maximum in 5 minutes?

When someone starts to inhale nitrous, its concentration in the blood and in those organs that have a lot of vasculature (brain, liver, etc.) rises quickly until the partial pressure of nitrous in those organs is the same as its partial pressure in inhaled air. The partial pressure in fatty tissues and other parts of the body that have little circulation through them rises much slower.

The elimination half life of 5 min applies for the situation where there's an equal partial pressure of N2O in the whole body. If the equilibrium between N2O concentrations in all organs has not yet been achieved (as is the case after inhaling a single balloon of nitrous), the effect weans off a lot faster than that, because there is not only excretion of nitrous in exhaled air but also redistribution into fatty tissue (biexponential kinetics).

Because of the same redistribution phenomenon, the effects of a single injection of fentanyl lasts for only 15-30 min, even though its terminal half-life is about 5 hours.

 

[Kittycat5]

You generally measure, rather than calculate concentration in blood in PK.