Thermodilution measurement of cardiac output by PA catheter

This chapter is relevant to Section G7(iv) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the methods of measurement of cardiac output including calibration, sources of errors and limitations". It covers only a fraction of the material. Generic principles related to the measurement of cardiac output by indicator dilution are covered in a separate chapter, as are the limitations or sources of error. Thus, this can remain a brief entry which covers the practical aspects of measuring cardiac output using the PA catheter.

In summary:

• A bolus of 5-10ml cold 5% dextrose into the right atrium should decrease the temperature of the blood in the pulmonary artery.
• The rate of blood flow is  inversely proportional to the change in temperature over time
• Thus, the mean decrease in temperature is inversely proportional to the cardiac output.
• The Stewart-Hamilton Equation, which describes this relationship, is modified for the purpose of using temperature as an indicator:
  
  where:
  • V̇ = flow, or cardiac output, if you will
  • V = volume
  • T_b = temperature of the blood,
  • T_i = temperature of the injectate,
  • k₁ = "density constant", a fudge factor measured from specific heat and specific gravity of both the indicator and the blood, i.e. this will be different for any given substance used as thermodilution injectate, and will be affected by the haematocrit of the blood.
  • k₂ = "calibration constant", another fudge factor which represents the temperature change in °C corresponding to each height interval of the thermodilution curve amplitude, eg. 1°C per 1mm height difference. 
  • The integral of ∆Tdt is the area under the extrapolated dilution curve

These variables are derived from the measured thermodilution curves. These curves are visibly different in different cardiovascular pathological states. The higher the cardiac output, the faster the blood flow and the shorter smaller and steeper the thermodilution curve. In low cardiac output, the curve is slurred and lazy, with a large area under the curve (AUC) because there is less blood available for the injectate to mix with.

Some catheter manufacturers have a heating filament near the tip, which means automated measurements can be continuously repeated. 

Limitations of thermodilution for measuring cardiac output

This method shows good agreement with the Fick method and with the indocyanine green dye dilution method. However, there are numerous possible sources of error.

In summary:

• 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

The technique plays a major role:

• Too much injected cold stuff causes underestimation of cardiac output.
• Too little injected cold stuff causes overestimation of cardiac output.
• A small volume also increases the signal-to-noise ratio
• 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.

Other sources of error include:

• Catheter being 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