Question 6

College Answer

This question related to an area of basic respiratory physiology. A good answer necessitated
the precise meaning of venous admixture, being that amount of mixed venous blood which
would have to be added to ideal pulmonary end-capillary blood to explain the observed
pulmonary end-capillary to arterial PO2 difference. Diagnosis required the candidate to
mention, a demonstrated increased in the Aa-DO2, what are normal values. For quantification
mention and description of the shunt equation was required. Candidates lacked a definition
for venous admixture, were inaccurate with their description of the shunt equation and often
overlooked mentioning calculation of arterial and mixed venous blood oxygen content.
Syllabus - B1a
Reference: Nunn’s Applied Respiratory Physiology 6th Ed p122

Discussion:

The college answer here is informative with regards to what was expected, notwithstanding sections which appear to have been generated by Google Translate ("Diagnosis required the candidate to
mention, a demonstrated increased in the Aa-DO2, what are normal values"). As far as one can tell, they expected something like this:

• Shunt is the volume of blood which enters the systemic arterial circulation without participating in gas exchange
• Venous admixture is that amount of mixed venous blood which would have to be added to ideal pulmonary end-capillary blood to explain the observed  difference between pulmonary end-capillary PO₂ and arterial PO₂ 
• Shunt fraction is the calculated ratio of venous admixture to total cardiac output
• The shunt equation, otherwise known as the Berggren equation, is used to calculate the shunt fraction:
  Qs/Qt = (Cc_O2 - Ca_O2) / (Cc_O2 - Cv_O2)
  where
      Qs/Qt = shunt fraction
      Cc_O2 = pulmonary end-capillary O₂ content, same as alveolar O₂ content
      Ca_O2 = arterial O₂ content
      Cv_O2 = mixed venous O₂ content
• Sources of venous admixture include:
  • "True" intrapulmonary shunt, blood which passes through lung regions where V/Q = 0
  • V/Q scatter, blood which passes through lung regions where V/Q < 1.0
  • Thebesian veins, which contribute myocardial venous blood with low oxygen content 
  • Bronchial veins,  which drain the bronchial walls 
  • Intracardiac right-to-left shunts
• Normal shunt fraction in healthy adults breathing room air is 0%
• Normal venous admixture is around 3%

Bigeleisen, Paul E. "Models of venous admixture." Advances in physiology education 25.3 (2001): 159-166.

Cruz, Julio C., and Patricia J. Metting. "Understanding the meaning of the shunt fraction calculation." Journal of clinical monitoring 3.2 (1987): 124-134.

Berggren SM. The oxygen deficit of arterial blood caused by nonventilating parts of the lung. Acta Physiol Scand 1942; 4:Suppl 11:1-92

Araos, Joaquin D., et al. "Use of the oxygen content–based index, Fshunt, as an indicator of pulmonary venous admixture at various inspired oxygen fractions in anesthetized sheep." American journal of veterinary research 73.12 (2012): 2013-2020.

SIGGAARD‐ANDERSEN, Ole, and Ivar H. Gøthgen. "Oxygen and acid‐base parameters of arterial and mixed venous blood, relevant versus redundant."Acta Anaesthesiologica Scandinavica 39.s107 (1995): 21-27.