Relationship of downslope time and 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". 

Change in temperature in the chambers of the heart

The drop in temperature occurs in all the chambers, to some degree. How fast it drops depends on how much warm blood the cold bolus is mixing with.
downslope time and chamber volumes

Obviously, it would mix the most rapidly in the chamber with the greatest volume.

Let us say the cold bolus travels through 5 chambers of different volume.

Let us assume it takes the same amount of time to travel through them all. Let us suppose that one of the chambers is particularly large.

Relationship of chamber temperature change to downslope time

dst curve and chamber volumes

The actual thermodilution curve is less jagged, as the chambers are all connected. In order to capture the steepest part of the curve, the downslope time is taken as the time between a point at 85% of temperature response and 45% of temperature response.

That is why DSt multiplied by the cardac output gives you the volume of the largest chamber.


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.