LiDCO: lithium dilution cardiac output measurement

Principles of LiDCO

• The basic premise is an indicator dilution method of cardiac output monitoring
• Instead of a change in temperature, the change you measure is of concentration. This resembles the dye-indicator dilution method. 
• A small amount of lithium chloride (0.002 to 0.004 mmol/kg) is injected into a vein
• A lithium-sensitive electrode then senses the lithium in the arterial circulation
• The resulting dilution curve is used to derive cardiac output by the Fick method, as well as several calculated variables.
• The resulting variables are used to calibrate the analysis of the pulse contour
• Ths calibration should happen every 8 hours or so

Why lithium? Why not, say, thorium? In small doses, lithium is non-toxic. The dose used here is about 1/240th of the effective mood stabiliser dose. There is also no significant first-pass loss, which satisfies the “critical boundary condition”, i.e no indicator disappears during measurement. 

Calibration of LiDCO

• To be precise, LiDCO is actually the lithium-based calibration method, PulseCO is the thing that is being calibrated, and the whole system together is LiDCOplus.
• PulseCO is "a beat-to-beat cardiac output monitor that calculates stroke volume from the arterial pressure waveform using an autocorrelation algorithm" (Sundar & Panzica, 2010)
• Pulse power calculation (LiDCO) may be more accurate than other waveform analysis for continuous cardiac output
• The algorithm used by the manufacturer is proprietary and  is not dependent on waveform morphology, i.e. it should not be confused by AF

Advantages of LiDCO

• 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.
• The indicator is nontoxic and unlikely to cause arrhythmias (unlike the cold injectate)
• There is  no risk of large vessel damage or PA rupture

Limitations of LiDCO

• 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 (the quaternary ammonium residues interfere with the lithium sensor)
• You do end up disposing of some blood each time you sample.
• The inputs into the equations that run the cardiac output monitoring require you to input serum sodium and haematocrit measurements
• The system does not report intrathoracic blood volume index
• The support in the literature is smaller in terms of the number of studies as compared to PiCCO or the PA catheter