Briefly describe the factors that influence the partial pressure of oxygen in mixed venous blood.
The main points candidates were expected to cover included:
· A discussion of the non-linear relationship between O2 content and partial pressure and
the factors which affect this relationship. No candidate included this.
· Modification of the Fick equation as it relates mixed-venous oxygen to delivery and
· The components of delivery should have been described and use of the O2 flux equation
would have been helpful. Additional marks were available for describing how these
might change in physiological and pathological states.
Candidates frequently interchanged content and partial pressure, without clearly displaying
how these are related. Normal values were not provided. The O2 flux equation, when
included, was often written incorrectly. No consideration was given to normal variations,
such as pregnancy or exercise
Reference: Nunn 5th edition pages 267 to 269, page 493
Syllabus: B1h Gas transport in the blood 2a
What follows is probably unreasonably long for a 10-mark answer, but it was designed to cover at least most of what was mentioned in the college comments. It would, on reflection, be impossible to answer this question in ten minutes in a way which covers every point.
Rivers, Emanuel P., Douglas S. Ander, and Doris Powell. "Central venous oxygen saturation monitoring in the critically ill patient." Current opinion in critical care 7.3 (2001): 204-211.
Pearse, R. M., and A. Rhodes. "Mixed and central venous oxygen saturation." Yearbook of Intensive Care and Emergency Medicine 2005. Springer, New York, NY, 2005. 592-602.
Kandel, Gabor, and Arnold Aberman. "Mixed venous oxygen saturation: its role in the assessment of the critically ill patient." Archives of internal medicine 143.7 (1983): 1400-1402.