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Cardiac Output

Clinical estimation of CO is generally unreliable unless in extremis. Various methods of monitoring cardiac output have therefore been developed. Methods include:

The Fick principle states that the amount of a substance taken up by an organ (or the body) per unit time is the product of:

  • The arteriovenous concentration difference of the substance across the tissue bed
  • The flow of blood to that tissue bed

This method is used in multiple methods to assess CO.

Thermodilution

The principle of thermodilution is that a injection of cold fluid into the RA will transiently ↓ PA blood temperature, and that the mean ↓ in temperature is proportional to CO. Thermodilution may be:

  • Bolus
    Injectate of known volume and pressure administered rapidly via a central vein.
  • Semi-continuous
    The PAC has a thermal coil on the RV section, which periodically “boluses” heat in a random fashion, and then monitors for the ↑ in PA temperature.

Thermodilution is an indicator dilution method, where a lower thermal energy content is the indicator.

Sources of error:

  • Malposition
    • Thermistor on vessel wall
    • Wedged
  • Abnormal respiratory patterns
  • Intracardiac shunting
    Blood bypassing thermistor (right to left), or being diluted (left to right).
  • TR
  • Arrhythmia
  • Other infusions
  • Technical
    • Wrong injection volume
    • Slow injection
    • Poor temperature difference
      Accuracy is ↑ with a colder (0-12°C) fluid, but this may lead to dysrhythmias. Room temperature is acceptable.

Transpulmonary Dilution Techniques

All of these techniques inject an indicator into a central vein, and measure the indicator in a central artery. In addition to CI/ they may also report:

  • Intrathoracic Blood Volume
    Volumetric preload index that may be more accurate than pressure-based preload indices CVP, PAWP, which are rather inaccurate).
  • Extravascular Lung Water
  • Global End-Diastolic Volume

Methods include:

  • Thermodilution
    Cold injectate into a central vein, with central arterial (femoral, axillary) temperature change measured as described above.
  • Lithium dilution
    Small, non-toxic lithium doses are injected centrally or peripherally and measured by an ion-sensitive electrode in a peripheral artery.
    • Doesn’t require central arterial access
    • Not accurate in patients receiving lithium therapy
    • No volumetric measurements

Venous Oxygen Saturation

Venous oxygen tension is an index of oxygen consumption, and can be sampled from the:

  • Pulmonary artery
    Sampling here ensures adequate admixture of IVC, SVC, and coronary sinus blood.
    • PVO2 <26mmHg suggests cellular hypoxia
    • SvO2 of 70-80% implies global oxygen supply/demand balance
      May be elevated in high-CO shock states.
  • Central Venous
    Less accurate, but more convenient, than PA sampling. Interpretation is similar to mixed venous sampling.

References

  1. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.