So does it matter where I inject the cold fluid?
One would think so. Lets say you inject the cold water into a peripheral cannula- who knows what tissues it exchanges heat with on the way to the heart; who knows what its temperature will be at the time it reaches the right atrium; and so on and so forth. It is after all the cardiac chambers we are interested in.
Thus, a jugular central line is the vehicle of choice for your cold saline.
However, a study has been done to investigate whether or not it matters whether you are using a femoral CVC or a jugular CVC. Surprisingly, those people found that IJ and femoral thermodilution measurements agree very well, provided one uses a correction factor. One issue raised by the study was that the femoral CVC should not be in the same groin as the PiCCO- because there will be "crosstalk" between the cold fluid in the catheter and the PiCCO thermistor in the neighbouring artery.
A diagram of the thermodilution
Thermodilution temperature change curve
You measure the curve of the temperature change, as seen below.
From this curve, we can arrive at two important measurements:
the Mean Transit Time (MTt) and the Downslope Time (DSt)