ICU Primary PrepCast: Mixed Venous Oxygen Saturation

Introduction

• Hosts: Maddy and Swapnil.
• Topic discussion: Mixed venous oxygen saturation (SvO2).
• Importance: Many candidates struggle with this topic.

What is Mixed Venous Oxygen Saturation (SvO2)?

• Measured from blood in the pulmonary artery through a PA catheter.
• Represents oxygen extraction for the whole body.
• Serves as a surrogate marker for balance between oxygen delivery and consumption.

Factors Affecting Mixed Venous Oxygen Saturation

Oxygen Delivery Factors

• Oxygen Delivery Equation:

  • Formula: Cardiac output × Oxygen content of blood.
  • Oxygen Content: Maximal oxygen-carrying capacity (1.34 or 1.39) × Hemoglobin concentration × Hemoglobin oxygen saturation + Solubility constant (0.03) × PaO2.

• Cardiac Output:

  • Depends on heart rate, stroke volume (affected by preload, afterload, contractility).
  • Factors affecting these will affect SvO2.

• Hemoglobin Concentration:

  • Anemia decreases SvO2.

• Oxygen Saturation of Hemoglobin:

  • Right Shift in Oxygen-Hemoglobin Dissociation Curve: Decreases affinity for oxygen (e.g., acidosis, increased CO2, 2,3-DPG concentration, temperature) → Decreased SvO2.
  • Left Shift: Increases affinity for oxygen (e.g., alkalosis, decreased CO2) → Increased SvO2.

• Partial Pressure of Oxygen (PaO2):

  • Increased PaO2 → Increased SvO2 (extreme conditions).
  • Decreased PaO2 → Decreased SvO2 (hypoxia causes).

Oxygen Consumption Factors

• Equation: Cardiac output × (Oxygen content in arterial blood - Oxygen content in venous blood).

• Metabolic Demand:

  • Increase in demand (exercise, sepsis, hyperthyroidism, fever, shivering, pain, stress) → Increased consumption → Decreased SvO2.
  • Decrease in demand (paralysis, analgesia, hypothermia) → Decreased consumption → Increased SvO2.

• Oxygen Utilization:

  • Conditions like histiotoxic hypoxia (e.g., cyanide poisoning, beriberi) → Decreased consumption → Increased SvO2 with tissue-level hypoxia.
  • Microvascular shunting and sepsis → Increased SvO2 due to abnormal utilization.

Conclusion

• Mixed venous oxygen saturation is influenced by both oxygen delivery and consumption.
• Understanding these factors is crucial for managing patient conditions.
• Future topics to be discussed in upcoming episodes.

Thank you for listening!