A mixing chamber is simply an enclosed box whereby expiratory gases collect,
mix and are then exhausted. These systems allow for the gathering of expiratory
gases for one or more breaths, where they mix and effectively average their contents.
So what is the point of this practice? Well two confounding phenomenon are occurring
not obvious to the uninformed observer:
1. The ventilation is not happening evenly but rather as a two peak curve
2. Oxygen concentration is being expired a highly chaotic (noisy) concentrations
The Mixing Chamber puts manners on these curves by effectively smoothing out these
outputs to an average value. In fact Mixing Chambers effectively eliminate these problems.
So if you dont use a Mixing Chamber, what is the alternative?
The answer is
a.Breath by Breath or
b. Douglas Bags…
Douglas bags method allows us to collect data and process from first principles.
They are an accepted standard for this reason. They still have issues of dead space
and lost data, collecting half a breath, and not being able to eliminate aberrant breaths
with say coughing data and the like. Unfortunately, they are relatively slow and no commercial
systems use this method.
Breath by Breath measurement was though to be a revolution many decades ago when
technology evolved and made it possible. It was thought that one could look at respiratory
dynamics in real time and decipher some of the complicated questions in our physiology.
In theory, this is still possible. The problem is that in order to calculate VO2 we must multiply
the ventilation and oxygen and carbon dioxide concentrations in a multifactorial complex
equation. So what? Well if the ventilation is coming out as a polynomial and the oxygen
concentration value is coming out very noisy from low to high back and forth, then the resultant
multiplied value is going to be an extremely noisy value indeed.
So how do manufacturers cope with this major data dilemma? What do they do with all this noise?
Well they smooth it. So that is OK right? Well, no actually. If you see how noisy the data is once
multiplied, you would wonder how it could ever be smoothed. Nonetheless, this is done and to add insult
, they do not publish (or make available) the algorithms used to smooth this, making the information
non-traceable from a scientific perspective.
This is an excellent question. (to be continued)