ITTV: the intra-thoracic 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". 

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.

mean transit time and downslope time


PiCCO claims to estimate volumes of the cardiac chambers from the thermodilution curve. How, precisely, can you arrive at a volume from these measured variables?


Lets look at what cardiac output actually is.

cardiac output equation

So it would be sensible to say that

volume equation

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

ITTv equation

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.

ITTV diagram


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.