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 congenital 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:
    ${\dot{Q}_S \over \dot{Q}_T }= {C_cO_2 - C_aO_2 \over C_cO_2 - C_vO_2 }$, where:
    • $\dot{Q}_S$ = Shunt blood flow
    • $\dot{Q}_T$ = Cardiac output
    • $C_cO_2$ = Pulmonary end-capillary oxygen content, assumed to have an oxygen tension equal to PAO₂ (with the corresponding oxygen saturation)
    • $C_aO_2$ = Arterial oxygen content
    • $C_vO_2$ = 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 alveolus 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 alveolus 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.