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
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