PTV: the pulmonary 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". 

Mathematics underlying the calculation of the PTV

MTt (Mean Transit Time) multiplied by cardiac output(CO) determines your ITTV (intrathoracic thermal volume)
The other time parameter is your Down Slope Time (DSt)

Pulmonary thermal volume is both the pulmonary blood volume (PBV) and the fluid which bathes the pulmonary vessels, the ExtraVascular Lung Water (EVLW).

This water participates in the dissipation of heat from the cold fluid bolus. The calculation of this is presented elsewhere.

Suffice to say, it is calculated from the predictable relationship of the global end-diastolic volume (all 4 chambers) to the experimentally measured total volume of blood in the thorax.

Because the Pulmonary Thermal Volume (PTV) is the largest volume in the series of intrathoracic thermal volumes, this is what you get if you multiply your cardiac output by your downslope time.