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

 

List:

·        Venous admixture

·        Oxygen transport

·        Low V/Q

·        Shunt

 

Venous admixture:

Definition:

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.

Components

·   Low V/Q

·   Shunt

Equation

·   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:

Definition

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

Physiological

·   Normal V/Q scatter due to gravity

·   Average 0.8, apex 3.3, base 0.6

Pathological

·   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. at PaCO2 100mmHg: oxygenation is restored at FiO2 of only 30%)

 

Shunt:

Definition

·   Blood returns to LV without any oxygenation

Physiological

·   Normal shunt 1.3%

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

Pathological

·   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)