Question 19

Define mixed venous PO2 (20% of marks).

Outline the factors that affect this value (80% of marks).

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College Answer

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.

Discussion

This question is essentially the same as Question 10 from the first paper of 2008, but the examiner's expectations were lower. The pass marks were still 37% and 33%, respectively. 

  • 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.