ITTV: the intra-thoracic thermal volume

This chapter is relevant to Section G7(iv)  of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the methods of measurement of cardiac output including calibration,
sources of errors and limitations". 

To recap:

Cardiac Output  is calculated from the area under the temperature change over time curve.

MTt is the Mean Transit Time, time it takes for half of the cold indicator to pass.

DSt is the Down Slope Time, time it takes for the temperature to return to baseline.

Now;

Well.

Lets look at what cardiac output actually is.

So it would be sensible to say that

You have two different "time" parameters to choose from: MTt and DSt.

And when we say Total Volume, that is all the volume between the injection site (CVC) and the measurement site. So, in realistic terms, it is RAEDV + RVEDV + PTV + LAEDV + LVEDV

…otherwise known as the ITTV (intrathoracic thermal volume).

I.e. all the red boxes below are encompassed in this variable.