Shunt

Explain the concept of shunt and its measurement

Shunt is blood reaching the systemic circulation without being oxygenated via passage through the lungs.

Factors Contributing to Shunt

  • Normal shunt
    • Anatomical shunt
      • Thebesian veins, which drain directly into the left cardiac chambers
      • Bronchial circulations, which drain into the pulmonary veins
    • Functional shunt
      Blood draining through alveoli with a V/Q between 0 and 1.
      • This may not be true shunt, as blood may have some oxygen content but not be maximally oxygenated
  • Pathological shunt
    Pathological shunting can be anatomical (e.g congential cardiac malformations), or physiological (e.g. pneumonia causing alveolar consolidation).
    • Intra-cardiac e.g. VSD
    • Extra-cardiac
      e.g. Pulmonary AVM, PDA

Calculation of Shunt

  • Shunt cannot be directly measured
  • This is because we cannot separate true shunt (V = 0) from V/Q scatter (V/Q < 1) when sampling blood entering the left heart
  • Venous admixture is used instead
    Venous admixture is the amount of mixed venous blood that must be added to pulmonary end-capillary blood to give the observed arterial oxygen content. Venous admixture:
    • Is a calculated, theoretical value
    • Assumes that alveoli have either complete shunt (no ventilation at all, i.e. V/Q = 0) or no shunt (V/Q = 1)
    • Is expressed as a ratio, or shunt fraction:
      , where:
      • = Shunt blood flow
      • = Cardiac output
      • = Pulmonary end-capillary oxygen content, assumed to have an oxygen tension equal to PAO2 (with the corresponding oxygen saturation)
      • = Arterial oxygen content
      • = Mixed venous oxygen content

Physiological Consequences of Shunt

Effect on Carbon Dioxide

  • No CO2 can diffuse from shunted blood
  • Therefore PaCO2 might be expected to rise, however:
    • In a spontaneously breathing patient the increased PaCO2 increases respiratory drive, and alveolar ventilation increases
      • Therefore, shunt does not tend to increase PaCO2 unless:
        • The shunt fraction is large and
        • The patient is unable to increase their alveolar ventilation to compensate
    • Additionally, the steepness of the CO2 dissociation curve at the arterial point means that although CO2 content increases, the increase in PaCO2 is small

Effect on Oxygen

  • PaO2 falls proportionally to shunt fraction
  • As shunted alveoli are perfused but not ventilated, true shunt is said to be unresponsive to an increase in FiO2
    This is where technical definitions become important to avoid confusion.
    • For an alveoli with a V/Q between 0-1 (V/Q mismatch or V/Q scatter, but not true shunt):
      • There is perfusion, but relatively less ventilation
      • Therefore blood passing through this alveoli will be partially oxygenated
      • Increasing PAO2 will improve oxygenation (assuming no diffusion limitation):
        • Administration of supplemental oxygen
        • Hyperventilation
        • As per the alveolar gas equation
    • For an alveoli with a V/Q of 0 (true shunt)
      There is no ventilation. Regardless of the increase in PAO2, PaO2 will not improve.

The Isoshunt Diagram

  • Isoshunt diagram plots the relationship between FiO2 and PaO2 against a set of 'virtual shunt lines'
  • These 'shunt fractions' are calculated from the above equation and so are actually V/Q admixture fractions


References

  1. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
  2. West J. Respiratory Physiology: The Essentials. 9th Edition. Lippincott Williams and Wilkins. 2011.
  3. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
Last updated 2018-09-21

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