Transpulmonary Thermodilution for Cardiac Output Measurement

Cold fluid is poured into the venous circultation, and it mixes with blood in the cardiac chambers before making its way into the arterial circulation. A thermistor is dangling in the arterial circulation, and it measures the change in blood temperature. The resulting temperature over time curve is used to make all sorts of comments.

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

diagram of picco thermodilution

GEDV and ITBV

Thermodilution temperature change curve

You measure the curve of the temperature change, as seen below.

Picco thrmodilution curv

From this curve, we can arrive at two important measurements:

the Mean Transit Time (MTt) and the Downslope Time (DSt)

mean transit time and downslope time

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.