What is the interpretation of mixed venous blood gas (MVBG) results?

From the Guidelines

Mixed venous blood gas (MVBG) results should be interpreted with a focus on the SvO2 (mixed venous oxygen saturation) and PvO2 (mixed venous oxygen tension), considering the context of arterial blood gases to guide interventions that improve oxygen delivery and utilization. When interpreting MVBG results, it is essential to consider the SvO2 goal above 66% and the ratio of O2 delivery to consumption (DO2:VO2) goal above 3, although these parameters are hemoglobin-dependent and may not be as reliable as the arteriovenous O2 difference 1.

Key Parameters to Evaluate

• SvO2: normally ranges between 60-80%
• PvO2: normally ranges from 35-45 mmHg
• Venous pH and PCO2: reflect tissue acid-base status and CO2 production

Clinical Implications

• A low SvO2 (<60%) indicates increased oxygen extraction by tissues, suggesting either increased oxygen demand or decreased oxygen delivery, which can occur in conditions like sepsis, heart failure, or hypovolemia
• A high SvO2 (>80%) suggests decreased oxygen utilization by tissues, which may occur in sepsis (due to microcirculatory shunting), cyanide toxicity, or hypothermia
• Consider the arteriovenous O2 difference as a reliable parameter for setting ECMO flow goals, as it is not influenced by hemoglobin levels, and should be maintained between 3-5 cc O2/100ml of blood 1

Integration with Arterial Blood Gases

• Calculate parameters like oxygen extraction ratio and cardiac output using the Fick principle to differentiate between supply-dependent and demand-related oxygen consumption issues
• Guide appropriate interventions such as fluid resuscitation, inotropic support, or measures to reduce oxygen demand It is crucial to recognize the potential for differential upper and lower torso oxygenation, known as Harlequins or North-South syndrome, in peripherally cannulated patients, and to manage it by increasing ECMO flow, manipulating ventilator settings, or inserting an additional cannula 1.

From the Research

Interpretation of Mixed Venous Blood Gas (MVBG) Results

The interpretation of MVBG results is crucial in assessing the balance between oxygen supply and demand in the body. The following points highlight the key aspects of MVBG interpretation:

• MVBG results reflect the peripheral extraction of oxygen, oxygen delivery, and consumption 2.
• The value of mixed venous oxygen saturation (SvO2) is related to four determinants: oxygen consumption, cardiac flow, hemoglobin level, and oxygen saturation of arterial blood 2.
• Central venous oxygen saturation (ScvO2) is more easily measurable than SvO2 and can be used as a surrogate marker for SvO2 2, 3.
• The relationship between SvO2 and cardiac index is nonlinear and can be normalized for oxygen uptake and hemoglobin 4.
• SvO2 and ScvO2 can be used to assess the adequacy of tissue oxygenation and may be useful monitoring parameters in critically ill patients 5, 3.

Clinical Applications of MVBG Interpretation

The clinical applications of MVBG interpretation include:

• Monitoring changes in ScvO2 after major surgery to predict outcome 3.
• Using early goal-directed therapy, which includes treatment goals for mean arterial pressure, central venous pressure, and central venous oxygen saturation, to increase survival in patients with severe sepsis or septic shock 3.
• Applying measurement of central venous oxygen saturation as a useful, indirect indicator for the adequacy of tissue oxygenation in intensive care unit patients 3.

Limitations and Considerations

The following limitations and considerations should be taken into account when interpreting MVBG results:

• The correlation between mixed-venous and central-venous values decreases with increasing deterioration of the cardiovascular system, especially in patients with high cardiovascular risks 5.
• SvO2 and ScvO2 per se are only partly helpful in evaluating the hemodynamic situation, and other parameters such as arteriovenous O2 difference and cardiac output should also be considered 5.
• The ability of SvO2 to serve as a therapeutic indicator in any given patient is dependent on baseline SvO2 and cardiac index 4.