Question 13

College Answer

Mixed venous oxygen saturation is used as a surrogate marker for the overall balance between oxygen delivery and oxygen consumption. A good answer stated this, described the importance of where it is measured and went on to describe the various factors that affect oxygen delivery and consumption. Descriptions of the factors that affect oxygen saturation of haemoglobin, partial pressure of oxygen in the blood and position of oxygen-haemoglobin dissociation curve were necessary to score well. Important omissions were factors that increased and decreased oxygen consumption. While many candidates were able to correctly write the equations for oxygen content and oxygen flux, they then failed to describe how the variables within these equations were related to mixed venous oxygen saturation.

Discussion

• Mixed venous blood is:
  • sampled from the pulmonary artery
  • mixed in the right ventricle out of multiple venous sources 
  • representative of the oxygen extraction for the entire body
• Mixed venous oxygen saturation depends on:
  • Factors that influence the affinity of haemoglobin for oxygen:
    • The partial pressure of O₂ in mixed venous blood
    • The partial pressure of CO₂ in mixed venous blood
      • Increasing CO₂ shifts the curve to the right
    • pH of mixed venous blood, independent of CO₂
      • Decreasing pH (acidosis) shifts the curve to the right
    • The concentration of 2,3-DPG inside the erythrocytes
      • Increased 2,3-DPG (eg. in response to hypoxia or erythropoietin) shifts the curve to the right
    • The presence of unusual haemoglobin species
      • Methaemoglobin, carboxyhaemoglobin and foetal haemoglobin shift the curve to the left; sulfhaemoglobin shifts the curve to the right
    • Temperature
      • Hyperthermia shifts the curve right
  • 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 SvO_2. Factors which influce VO₂ include:
      • Factors which influence metabolic rate, eg. hypothermia, hyperthermia, paralysis, anaesthesia
      • Factors which influence oxygen utilisation, eg. mitochondrial toxins, microvascular shunting in sepsis
    • Cardiac output: a decreased cardiac output will produce a reduced SvO₂
  • Pathological abnormalities of the arterial-venous blood flow
    • Left to right shunts

Leach, R. M., and D. F. Treacher. "The pulmonary physician in critical care• 2: Oxygen delivery and consumption in the critically ill." Thorax 57.2 (2002): 170-177.

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.

Reinhart, K. "Mixed venous oxygen saturation." Applied Cardiopulmonary Pathophysiology 2 (1989): 315-325.