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". 
 
Instead of a change in temperature, the change you measure is of concentration. This resembles the dye-indicator dilution method.

lithium dilution diagram

Why Lithium? Why not, say, Thorium?

  • In small doses, lithium is non-toxic
  • There is no significant first-pass loss: this satisfies the “critical boundary condition”

Advantages over Thermodilution PiCCO

  • The concentration stays the same throughout the arterial circulation, and thus
    • You don’t need a big central artery to sample the lithium.
    • You don’t need to inject the lithium through a central vein
  • The technique shows good agreement with PA catheter thermodilution measurement.

Limitations

  • Same as all dilution methods, you get inaccurate results if there are shunts in the heart.
  • If you are already on lithium, this background lithium concentration will cause the machine to overestimate your cardiac output.
  • “electrode drift” can occur if there are high doses of muscle relaxants present
  • You do end up disposing of some blood each time you sample.

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.