Define mixed venous PO2 (20% of marks).
Outline the factors that affect this value (80% of marks).
This question was in two parts – the first part was worth 20% and candidates were expected to
provide a definition of mixed venous blood as well as the partial pressure of oxygen in mixed
venous blood (including normal range). Good answers also provided the varying PO2 from
different tissue beds that make up mixed venous blood, where the ‘mixing’ occurs (the right
ventricle) and where it is sampled (pulmonary artery).
For the second part of the question, worth 80% of the marks, good answers included the
relationship between mixed venous PO2 and mixed venous O2 content (including the shape and
position of the HbO2 dissociation curve); the variables encompassed in the modified Fick
equation; arterial oxygen content and its determinants; oxygen consumption (VO2); and cardiac
output (CO). Including an outline of how each affects the value of mixed venous PO2.
A number of candidates wrote about mixed venous oxygen saturation. Other common errors
were: missing a number of key factors that affect PO2; and using an incorrect form and/or
content of the modified Fick equation.