Relationship of downslope time and 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".
Change in temperature in the chambers of the heart
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
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.
References
From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition, as well as http://www.pulsion.com/ 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.