Lesson 11

Lesson 11: Mixed Venous Oxygen Saturation

  • SvO2 is influenced by four key variables:

    • Cardiac output (Q, L/min)

    • Oxygen consumption (VO2, mL O2/min)

    • Hemoglobin level (Hgb, g/dL)

    • Hemoglobin saturation (SaO2, %)

SvO2 as a Monitor

  • Constant Hgb, SaO2, and VO2 allow SvO2 to serve as an indirect monitor of cardiac output.

Factors Affecting SvO2

  • Decrease SvO2:

    • Increased O2 consumption (stress, pain, thyroid storm, shivering, fever)

    • Decreased O2 delivery (decreased SaO2, Hgb, or CO)

  • Increase SvO2:

    • Decreased O2 consumption (hypothermia, cyanide toxicity)

    • Increased O2 delivery (O2 therapy, increased Hgb, increased CO)

Normal SvO2 Values

  • Normal range: 65−75%65-75\%

Key Causes of Increased SvO2

  • Sepsis: high cardiac output state, arterial admixture, elevated SvO2 despite end-organ hypoxia.

  • Impaired oxygen uptake (e.g., cyanide poisoning).

  • Left-to-right shunt: oxygenated blood mixes with pulmonary venous blood.

Measurement of SvO2

  • Pulmonary artery catheter needed for accurate SvO2 measurement.

  • A true mixed venous sample includes blood from:

    • Superior vena cava

    • Inferior vena cava

    • Coronary sinus

  • Mixing occurs in the pulmonary artery.

Relevant Formulas

  • CVO2 = (1.34imesHgbimesSvO2)+(0.003imesPVO2)(1.34 imes Hgb imes SvO2) + (0.003 imes PVO2)

  • CaO2 = (1.34imesHgbimesSaO2)+(0.003imesPaO2)(1.34 imes Hgb imes SaO2) + (0.003 imes PaO2)

  • VO2 = COimes(CaO2−CVO2)imes10CO imes (CaO2 - CVO2) imes 10