Cardiac Output Measurement

Describe the invasive and non-invasive measurement of blood pressure and cardiac output including calibration, sources of errors and limitations

Explain the derived values from common methods of measurement of cardiac output (i.e. measures of vascular resistance)

Cardiac output measurement can be performed:

• Invasively
  • Pulmonary Artery Catheter
    • Thermodilution
    • Fick Principle
  • TOE
  • Arterial waveform analysis
    • PiCCO
    • Vigileo
• Non-invasively
  • TTE
  • MRI
  • Thoracic impedance

Thermodilution

Thermodilution remains the gold standard of cardiac output measurement.

This technique:

• Requires a pulmonary artery catheter
  Various different designs exist. For CO measurement, they require:
  • A proximal port at the RA/SVC
  • A temperature probe at the tip
    Typically a silicon oxide thermistor.
  • A balloon at the tip
    To float it into position.
  • A distal (PA) port is required for measuring PAP and the PCWP, but is not required for CO calculation

Method for Intermittent Cardiac Output Measurement by Thermodilution

• A known volume of (typically dextrose) at a known temperature (classically cooled, but this is not required) is injected into the proximal port
• The temperature of blood is measured at the tip
  This produces a temperature-time curve.
• The area under the curve can be used to calculate cardiac output, as per the modified Stewart-Hamilton Equation:
  $Q = {V(T_B - T_I) k_1 k_2 \over \int_{t_1}^{t_2} \Delta T dt}$, where:
  • $Q$ = Cardiac output
  • $V$ = Volume of injectate
  • $T_B$ = Temperature of blood
  • $T_I$ = Temperature of injectate
  • $k_1$ = Density constant
    Relates to the specific heat and specific gravity of both injectate and blood.
  • $k_2$ = Computation constant
    Accounts for catheter dead space and heat exchange during injection.
  • ${\int_{t_1}^{t_2} \Delta T dt}$ = Area under the change in temperature-time curve

Errors in Thermodilution

Any factor that spuriously reduces the area under the thermodilution curve will result in an overestimation of CO.

• Natural variability
  Cardiac output varies up to 10% with changes in intrathoracic pressure during respiration. Therefore:
  • A mean of 3-5 measurements should be taken
  • Measurements should be taken at end-expiration
• Incorrect volume of injectate
  • Too much underestimates CO
  • Too little overestimates CO
• Warm fluid
  The closer the temperature of injectate is to blood, the greater degree of error introduced to the measurement.
  • Colder injectate is more accurate, but carries the risk of inducing bradyarrhythmias
• Poorly positioned PAC
  The PAC must be positioned in West's Zone 3 for blood flow to occur past the tip, and for the measured temperature to be accurate.
• Tricuspid regurgitation or Pulmonary regurgitation Results in retrograde ejection of injectate back past the valve, resulting in a prolonged decay time of the thermodilution curve. This usually results in underestimation of CO, but can result in overestimation depending on the severity of regurgitation and the true CO.
• Shunts
  Thermodilution measures RV CO only. LV CO and RV CO are likely to not be equal in the presence of intra- or extra-cardiac shunts. This can be corrected for by calculating shunt fraction, but is not performed by bedside CO computers.
• Arrhythmia
• Fluctuation in BP
  Rapid changes in temperature result in unreliable measurements; classically seen in the few minutes after weaning from cardiopulmonary bypass when the core is rewarming.

Fick Principle

Cardiac Output can also be measured using the Fick Principle. This technique:

• Uses the Fick Principle
  The flow of blood to an organ is equal to the uptake of a tracer substance divided by the arterio-venous concentration difference.
  • In this case, the tracer substance is oxygen
  • The 'organ' is the whole body
  • This produces the equation: $CO = {VO_2 \over C_a - C_v}$, where:
    • $CO$ is Cardiac Output
    • $VO_2$ is the patients oxygen consumption
      Typically estimated as 3.5ml.kg^-1.min^-1
    • $C_a$ is arterial oxygen content
    • $C_v$ is mixed venous oxygen content
• Relies on mixed venous blood sampled from the pulmonary artery, and arterial blood sampled from a peripheral arterial line

References

1. Moise, S. F., Sinclair, C. J. and Scott, D. H. T. (2002), Pulmonary artery blood temperature and the measurement of cardiac output by thermodilution. Anaesthesia, 57: 562–566.
2. Nishikawa, T. & Dohi, S. Errors in the measurement of cardiac output by thermodilution. Can J Anaesth (1993) 40: 142.