2007A09 Define ‘venous admixture’. Briefly explain how venous admixture influences arterial
oxygen tension and how an increase in inspired oxygen concentration may affect this.



·         Venous admixture

·         Oxygen transport

·         Low V/Q

·         Shunt


Venous admixture:


The amount of mixed venous (pulmonary arterial) blood that would need to be added to end-pulmonary capillary blood to account for the observed ↓ pO2 from end-pulmonary capillary to systemic artery.


·   Low V/Q

·   Shunt


·   Shunt equation: calculates venous admixture as a proportion

·   Qs/Qt = (CcO2 – CaO2) / (CcO2 – CvO2)

Normal values

·   Qs: shunt flow (normal 0.04)

·   Qt: total cardiac output (5L/min)

·   CcO2: endpulmonary capillary (19.9mL/100mL)

·   CaO2: systemic arterial (19.6mL/100mL)

·   CvO2: mixed venous (15.1mL/100mL)


Oxygen transport:

Oxygen content equation

CxO2 = [Hb] x SxO2 x 1.34 + 0.003 x PxO2 (per 100mL blood)

Oxyhaemoglobin dissociation curve


·   Hb-bound O2: large amount

·   Dissolved O2: small amount

·   Sigmoid shape: due to 1) co-operative binding 2) law of mass action

·   Flat upper part: once Hb saturated, minimal further ↑CxO2 possible


Low V/Q:


·   Where a region of lung is hypoventilated relative to its perfusion


·   Normal V/Q scatter due to gravity

·   Average 0.8, apex 3.3, base 0.6


·   Lung disease: e.g. COPD

·   Vasodilators impairing HPV hence V/Q matching: e.g. sevoflurane > propofol

Effect of 100% FiO2

·   ↑FiO2 compensates effectively for both local and global hypoventilation

·   (e.g. FiO2 30% restores SaO2 when hypoventilation is sufficient to produce PaCO2 100mmHg)




·   Blood returns to LV without any oxygenation


·   Normal shunt 1.3%

·   Thebesian veins 0.3%, (some) bronchial and pleural veins ~1%


·   Intra-pulmonary: e.g. AVM, or pneumonia where regional V/Q = 0

·   Intracardiac: e.g. PFO

Effect of 100% FiO2

·   Normal blood: PcO2 660mmHg, CcO2 ~22mL/100mL (i.e. extra 2mL/100mL)

·   Shunt blood: PvO2 40mmHg, CvO2 ~15mL/100mL (i.e. loss of 5mL/100mL)

·   Unable to add any O2 to blood with V/Q = 0

·   Cannot achieve SaO2 100% if Qs/Qt > 0.3, even if FiO2 100% (at 1 atmosphere)



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