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.