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)
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