F6vi: Explain venous admixture, its relationship to shunt and V/Q mismatch

Definition

  • Venous admixture is a concept → it is the amount of mixed venous blood which needs to be added to end pulm. capillary blood to get the observed ↓PO2 between pulmn end cap blood & arterial blood

Aetiology

1. Shunt

  • Blood that enters arterial circulation without passing through ventilated lung
  • 2 types:
Shunt

2. V/Q Mismatch

  • NORMAL V/Q SCATTER: due to regional differences of ventilation & perfusion
    • Normal contribution to venous admixture
  • PATHOLOGICAL: due to disease i.e. pneumonia

Quantification of Venous Admixture

  • Because venous admixture includes physiological shunt, it’s actually not possible to truly calculate how much of this is contributing

So you never actually know its contribution to ‘mixed venous blood’

  • But you can calculate the SHUNT EQUATION

Normally expressed as a % → 2 – 3%

Diagnosis

↑FiO­2­

  • Calculated by A – a gradient
  • Normally A – a gradient is 1 – 5mmHg & attributed to normal V/Q scatter
  • ↑V/Q mismatching causing ↑venous admixture can be almost resolved by ↑FiO2 > 35%
  • However, if the high venous admixture is due to a real shunt (physiological, or non-ventilated alveoli) → doesn’t matter how much you ↑FiO2, the blood won’t ever reach it
  • So you can test this by giving 100% FiO2

NB: on induction, you immediately get ↑ venous admixture due to atelectasis → so anaesthesia causes ↑ venous admixture to 10% of CO (that’s without any lung disease!)

The Isoshunt Diagram

  • Again, true shunt = not much effect by ↑FiO2
  • Due to Hb already saturated & small addition of O2 in form of dissolved O2 in blood

Diagram

  • 10% shunt requires FiO2 30% to restore PaO2
  • 20% shunt requires FiO2 57% to restore PaO2
  • 30% shunt requires FiO2 97% to restore PaO2
  • Shunt >30% → correction not really possible