VQ – Venous admixture

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

R – Respiratory quotient, usually 0.8

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

Quantification of venous admixture

Shunt equation

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

Answer by Sakurai