This came up in Question 19 from the second paper of 2007.

The awesomeness of the PA catheter is discussed in greater detail elsewehere.

So, why might the cold thermodilution measurement be wrong?

In summary:

  • Catheter is in the wrong position
  • The thermistor tip is up against the wall
  • The respiration is erratic
  • There is an intracardiac shunt
  • Tricuspid regurgitation
  • Cardiac arrhythmia
  • Rapid infusion happening via the IJ line
  • Abnormal hematocrit
  • Slow injectate delivery
  • Injectate not cold enough, or not enough of it

Limitations of thermodilution as a method for measuring cardiac output

This shows good agreement with the Fick method and with the indocyanine green dye dilution method. However, there is a lot of variability.

    • You should take measurements in expiration.
    • You have to take a mean of 3 measurements.
    • The mean has to be 15% different to the previous mean, otherwise it is within the margin of error.
    • The thermodilution cardiac output can vary by 10% from measurement to measurement without any change in the condition of the patient

Too much injected cold stuff causes underestimation of cardiac output.
Too little injected cold stuff causes overestimation of cardiac output.
Room temperature injectate  produces less accurate readings, but is safer.
Very cold injectate (0-4 degrees) is more accurate, but can induce bradycardia and decreased cardiac output.

References

An excellent online resource is available, which treats this subject with a massive amount of detail.

This a full-text version of the seminal paper from 1970:

Swan HJ, Ganz W, Forrester J, Marcus H, Diamond G, Chonette D (August 1970). "Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter"N. Engl. J. Med. 283 (9): 447–51.

A manufacturer (Edwards) offers some free information about the PA catheter on their product page.

The PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007) is a valuable read.