PTV: the pulmonary thermal volume

This chapter is relevant to Section G6(iii) of the 2023 CICM Primary Syllabus, which expects the exam candidate to "describe the methods of measurement of cardiac output, including limitations, potential sources of error, the need for calibration and the values obtained". 

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)

PTV equation
Essentially, by virtue of some foggy mathematics, the downslope time of the logarithmic thermodilution curve is proportional to the size of the largest volume in the series of volumes.

PTV diagram

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.

PTV constituents

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.

PTV heat exchange


From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition, as well as who are sadly the best source for this sort of information.

Bernd Saugel, Andreas Umgelter, Tibor Schuster, Veit Phillip, Roland M Schmid, and Wolfgang Huber Transpulmonary thermodilution using femoral indicator injection: a prospective trial in patients with a femoral and a jugular central venous catheter. Crit Care. 2010; 14(3): R95.

Additionally, I'd like to thank Dr. Kamal Parmar who has helped me understand this topic. Her input has massively increased the coherence of this page content.