Question 7

College Answer

Candidates were expected to provide a definition of important terms such as mixed
venous. Many candidates provided much information about the partial pressure of
carbon dioxide in arterial blood without discussing the factors which alter the mixed
venous pressure.
Partial pressure of CO2 in mixed venous blood depends on the CO2 content of the
mixed venous blood, which in turn represents a balance between CO2 production in
the tissues and the CO2 content in arterial blood. Good answers demonstrated an
understanding of this and provided relevant details about these aspects. The partial
pressure of CO2 is related to the CO2 content by the CO2 dissociation curve, the
position of which is determined by the state of oxygenation of haemoglobin, the
Haldane effect. CO2 production is related to aerobic metabolism in cells and total
production is defined by the metabolic rate. Examples of increased and decreased
CO2 production gained additional marks. The partial pressure of CO2 in mixed
venous blood is related to the partial pressure or content of CO2 in arterial blood.
This is determined mainly by alveolar ventilation under the control of chemoreceptors
and the brainstem respiratory centre.
Syllabus: B1h, 2c
Recommended sources: Applied Respiratory Physiology, Nunn 5th edition, Chp 10
pages 222 to 239

Discussion

• Mixed venous PCO₂ is usually about 46 mmHg, and is determined by the total oxygen content of mixed venous blood and the shape of the CO₂ dissociation curve
• The total CO₂ content of mixed venous blood, which is usually about 520 ml/L, is described by the modified Fick equation:

  VCO₂ = CO × k × (PvCO₂ - PaCO₂)

  where

  • VCO₂  is the rate of CO₂ production, 
  • CO is the cardiac output,
  • PvCO₂ - PaCO₂ is the arteriovenous CO₂ difference, and
  • k is a coefficient used to describe the near-linear relationship between CO₂ content and partial pressure in the blood.

  • The CO₂ content of arterial blood - any increase in arterial CO₂ will be inherited by the mixed venous CO₂. This is controlled by the central ventilation reflexes.
  • CO₂ production in the tissues, which is related to the rate of aerobic metabolism and oxygen consumption (VO₂). A low metabolic rate will cause a decrease in mixed venous CO₂ (eg. hypothermia). 
  • Cardiac output, which determines the rate of tissue CO₂ removal.
    • Poor cardiac output (eg. in cardiogenic shock) will cause an increased mixed venous CO₂ by a "stagnation phenomenon"
      • I.e. an abnormally large amount of CO₂  will be added to capillary blood per unit volume if the transit time is increased (i.e. flow is decreased)
• The CO₂-carrying capacity of blood, which is described by the CO₂ dissociation curve:
  • The curve is left-shifted because deoxygenated haemoglobin has a higher affinity for CO₂ (the Haldane effect).

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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.

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