Transpulmonary thermodilution for cardiac output measurement

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". This chapter refers specifically to the method of measuring cardiac output by transpulmonary dilution, which can be thermodilution (PiCCO) or lithium dilution (LiDCO). 

From the exam preparation perspective, so little attention has been paid to this topic that the time-poor candidate can safely abandon it in favour of studying higher-yield SAQ answers. For example, Question 10 from the second paper of 2017 asked the candidates to choose two methods of measuring cardiac output to compare and contrast, opening the possibility of using PiCCO as one and perhaps the PA catheter as the other. Question 19 from the first paper of 2014 also vaguely mentioned cardiac output measurement, where for 30% of the mark the trainees were expected to "briefly describe the thermodilution method of measuring cardiac output" without specific reference to transpulmonary methods. Clearly, becoming a master of this topic is not going to convert a tragic exam failure into a shining success.

In summary:

• Transpulmonary dilution cardiac output measurement techniques differ from dilutional techniques which use the pulmonary artery catheter
  • Cold fluid is poured into the venous circulation
  • 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.
• This method has advantages over PA catheter thermodilution, as well as unique disadvantages.

The single best peer-reviewed resource for this specific topic is probably the article by Monnet & Teboul (2017), made more attractive by virtue of being free to read. 

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)