Dr. Samuel Garcia and Dr. Bergthor and Jonsson

Dr. Garcia is a PGY-2 in Emergency Medicine at Mayo Clinic.

Dr. Jonsson is a PGY-2 in Emergency Medicine at Mayo Clinic.

In the ED we resuscitate critically ill patients every day and initiate high quality critical care that is then continued in the ICU. The Emergency Critical Care Series is a series of blog posts where we cover critical care topics that we find interesting, confusing or clinically important. Our hope is that this can be a useful resource to improve critical care in the ED and for preparing EM residents for their ICU rotations.

Oxygen Delivery (DO₂) = Cardiac Output x Oxygen Content in the blood

• It is all about oxygen supply and demand. There must always be a balance between oxygen delivery and its consumption by the tissues
• Cardiac Output (CO) = (Heart Rate) x (Stroke Volume)
  • Stroke volume is determined by preload, afterload, and contractility
• Oxygen content in the blood = (Oxygen bound to hemoglobin) + (Oxygen dissolved in blood)
  • Oxygen bound to hemoglobin = (Hgb X Arterial O₂ Saturation [SaO₂] x 1.34)
    • Major contributor to oxygen content
  • Oxygen dissolved in blood= (PaO₂ X 0.003)
    • Minor contributor to oxygen content

Central Venous Oxygen Saturation (ScV0₂)

• Central venous oxygen saturation is measured from blood in the superior vena cava (SVC) (ideally superior cavo-atrial junction)
• It reflects the balance between oxygen delivery and oxygen consumption
• Oxygen consumption varies between different organs or body parts
• Mixed venous oxygen saturation (SvO2) is measured at the pulmonary artery since blood from the whole body has mixed in the right heart
  • This requires a pulmonary artery catheter (Swan-Ganz catheter) which is more invasive than a normal central venous catheter (CVC)
• ScV0₂ is a good and more practical surrogate for SvO₂

If you begin with 100% arterial oxygen saturation and end up with a 75% venous oxygen saturation (ScV0₂), then oxygen consumption or “oxygen extraction” was 25%.

Normal range of oxygen extraction = 25-30%, corresponding to a ScV0₂ > 70%.

How to obtain ScV02

• ScV0₂ is usually measured in the SVC/right atrium
• ScV0₂ can be measured intermittently by drawing a venous blood gas from a CVC
  • Examples: Central line, PICC, Hemodialysis line, ECMO cannula, Implanted port (port-a-cath)
• ScV0₂ can also be measured continuously by using a specialized CVC with a built-in sensor or by using a sensor that can be threaded through a traditional CVC

Pitfalls

• Do not try to estimate ScV0₂ from a peripheral blood draw.
  • Venous blood gas obtained from the arm will only give you an idea of oxygen extraction in the tissues of that extremity and can be very different from the rest of the body.
• Do not use a femoral CVC for ScV0₂.
  • Studies and treatment algorithms have used blood from the SVC to measure ScV0₂ which can be different from the IVC so femoral CVC measurements are not reliable.
• Do not place a central line just to measure ScV0₂.
  • If a central line is otherwise indicated, consider measuring ScV0₂.
• The tip of the CVC should be positioned close to the junction of the SVC and the right atrium.
  • If the CVC tip is far away from the right atrium the ScV0₂ will be less reliable
• Know that patients with congestive heart failure can have low ScV0₂ (e.g. 50%) at baseline.
• Looking at trends in ScV0₂ especially in response to interventions is more valuable than just a single ScV0₂ value.
• Higher is not always better.
  • ScV0_{2 >}90% indicates poor oxygen extraction and is associated with increased mortality.
• Do not rely solely on the ScV0₂, it is only one piece of the puzzle.
  • Make sure you also assess vital signs, capillary refill, skin temperature, mental status, urine output, volume status, central venous pressure, lactate etc.

Summary and how ScV0₂ can be used in vasodilatory or cardiogenic shock:

• If you insert a central line, consider measuring ScV0₂ if clinically indicated
• Normal tissue oxygen extraction is around 25%.
• Targeting ScV0₂ >70% is often recommended in septic shock patients although targeting ScV0₂ has not been proven to decrease mortality.
• Assess the volume status of the patient and give fluids if indicated.
• Start vasopressors (usually norepinephrine) if the patient is hypotensive, despite fluid resuscitation, to achieve a MAP >65.
• ScV0₂ can help you decide if a patient with anemia (but hemoglobin above the typical transfusion threshold of 7 g/dL) would benefit from transfusion.
• If the hemoglobin and volume status is reasonable but you suspect low stroke volume (low ejection fraction) or inappropriately low heart rate, ScV0₂ could help you decide if you should add an inotropic agent to increase the cardiac output.
• Low ScV0₂ can indicate that the patient would benefit from increased tissue oxygen delivery despite adequate volume status and having a MAP >65.
• In practice, tissue oxygen delivery can be improved by:
  • Administering fluid if the patient is hypovolemic or at least volume responsive
  • Transfusing blood in the anemic patient
  • Adding inotropes in the patient you suspect has low cardiac output.

References:

De Backer, D., & Vincent, J. (2018). Should we measure the central venous pressure to Guide fluid management? Ten answers to 10 questions. Critical Care, 22(1). doi:10.1186/s13054-018-1959-3

Hartog, C., & Bloos, F. (2014). Venous oxygen saturation. Best Practice & Research Clinical Anaesthesiology, 28(4), 419-428. doi:10.1016/j.bpa.2014.09.006

Nickson, C. (2020, November 03). Mixed venous oxygen saturation (SvO2) monitoring • LITFL • ccc. Retrieved April 14, 2021, from https://litfl.com/mixed-venous-oxygen-saturation-svo2-monitoring/

Vincent, J., Rhodes, A., Perel, A., Martin, G. S., Rocca, G., Vallet, B., . . . Singer, M. (2011). Clinical review: Update On hemodynamic monitoring - a consensus of 16. Critical Care, 15(4), 229. doi:10.1186/cc10291

Whitener, S., Konoske, R., & Mark, J. B. (2014). Pulmonary artery catheter. Best Practice & Research Clinical Anaesthesiology, 28(4), 323-335. doi:10.1016/j.bpa.2014.08.003