Define venous admixture and list its causes. (50% of marks)
How is it diagnosed and how is it quantified? (50% of marks)
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
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:
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.