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?
- 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.