Question 10

Briefly describe the factors that influence the partial pressure of oxygen in mixed venous blood.

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College Answer

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
consumption.
· 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

Discussion

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. 

  • Mixed venous blood is:
    • Blood sampled from the pulmonary artery which is mixed in the RV and which represents a weighted average of venous blood from all tissues and organs:
    • Varying SvO2s from different tissue beds include: 
      • Jugular vein (55%)
      • Renal vein (81%)
      • Hepatic vein (66%)
      • IVC (71%)
      • SVC (79%)
      • Muscles (72%)
  • The PO2 in mixed venous blood, usually 40 mmHg, 
    is a major determinant of its oxygen content:
    • The PO2 describes the proportion of dissolved oxygen (PO2 × 0.03)
    • The PO2 also determines the SvO2 (usually 70-75%) according to the shape of the oxygen-haemoglobin dissociation curve in mixed venous blood
      • This curve is slightly right-shifted (compared to arterial blood) because of the Bohr effect
    • The SvOthen determines the oxygen carriage by haemoglobin in mixed venous blood, and therefore the mixed venous oxygen content
  • Mixed venous oxygen content depends on:
    • Total blood oxygen content = (SvO2 × ceHb × BO2) + (PvO2 × 0.03)
      • ceHb = the effective haemoglobin concentration
      • PvO2 = the partial pressure of oxygen in mixed venous blood
      • 0.03 = the content, in ml/L/mmHg, of dissolved oxygen in blood
      • BO2 =  the maximum amount of Hb-bound O2 per unit volume of blood (normally 1.39)
      • SvO2 = oxygen saturation of mixed venous blood
    • Balance of total body oxygen delivery and consumption,  expressed in terms of the modified Fick equation (CO = VO2 / CaO2 - CvO2):
      • Arterial oxygen content: decreased arterial oxygenation will produce a decreased SvO2 
      • VO2, the oxygen consumption rate: decreased VO2  will produce an increased SvO2 
      • Cardiac output: a decreased cardiac output will produce a reduced SvO2 

References

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.