Venous Admixture

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2024A Q02

Exam question

(a) Describe venous admixture and the sources contributing to it in an adult (60% of marks). (b) Explain the effects of supplemental oxygen on arterial hypoxaemia (40% of marks).

CICMWrecks answer

Master answer

Venous admixture

“Theoretical volume of mixed venous blood added to pulmonary end-capillary blood to produce the observed C_aO₂ “

Causes of venous admixture

Shunt

Physiological
- Thebesian vessels – drain venous blood from coronary circulation directly into cardiac chambers (including L ventricle)
- Bronchial circulation – Fracture of blood from bronchial circulation drains into pulmonary veins
Pathological
Blood from pulmonary capillaries supplying unventilated alveoli (V/Q = 0)
- Obstruction within bronchial tree
- Atelectasis
- Usually mitigated by hypoxic pulmonary vasoconstriction

V/Q mismatch

Variation in ventilation and perfusion within the lung
- V/Q > 1 – Areas of lung with high relative ventilation to perfusion
  - West Zone 1
  - Lung apices
  - In pathology
    - Positive pressure ventilation
    - Shock and hypoperfusion
    - PE
- V/Q < 1 – Areas of lung with low relative ventilation to perfusion
  - Lung bases
  - West zone 4
  - In pathology: Left heart failure, Pulmonary oedema
Areas of high V/Q cannot compensate for areas of low V/Q due to the shape of the oxyHb dissociation curve and the sharp desaturation of deoxygenated blood. Also there may be decreased flow through the highly ventilated areas of lung
V/Q mismatch in pathology
- Obstructive lung diseases: Asthma, COPD
- Restrictive lung diseases: ILD, ARDS

Sakurai 2016

Diagnosis of venous admixture

Estimated using A-a gradient using alveolar gas equation

P_AO_2 \; = \; (f_iO_2 \; \times \; (P_{atm} \; – P_{H_2O})) \; – \; ({{P_aCO_2} \over {R}})

R – Respiratory quotient, usually 0.8

Normal A-a gradient = 2.5 + (0.21 x Age)

Quantification of venous admixture

Shunt equation

Q_t (C_aO_2) \; = \; Q_{ns} (C_cO_2) \; + \; Q_s (C_{mv}O_2)

Where

Q_t = Total cardiac output
Q_ns = Flow through non-shunt areas
Q_s = Flow through shunt
C_aO₂ = arterial content of O₂
C_cO₂ = pulmonary end-capillary O₂ (presumed 100% SpO₂)
C_mvO₂ = mixed venous content of O₂
- Blood sampled from R atrium

V/A quantification

MIGET (Multiple inert gas elimination technique)
- IV injection of a volume of liquid with known quantities of 6 inert gases
- V/Q calculated from the elimination spectra of these gases
PET
- Xe¹³³ gas and Tc⁹⁹ labelled RBC
MRI

Sakurai 2016

Reusable content

Formulae used in this answer

Changes in Inspired and Alveolar PO2

CWF-0043

P_AO_2 \; = \; FiO_2 (P_{ATM} \; – \; SVP_{H2O}) \; – \; {{P_aCO_2} \over {RER}}

Shunt equation

CWF-0075

Q_t (C_aO_2) \; = \; Q_{ns} (C_cO_2) \; + \; Q_s (C_{mv}O_2)

Answer by Sakurai

Past papers

Exam appearances

2 appearances

Exam	Exact exam wording	Candidate success
2024A Q02	(a) Describe venous admixture and the sources contributing to it in an adult (60% of marks). (b) Explain the effects of supplemental oxygen on arterial hypoxaemia (40% of marks).	43%
2009B Q06	Define venous admixture and list its causes. (50% of marks) How is it diagnosed and how is it quantified? (50% of marks)	56%