Central venous oxygen saturation (ScvO2)

Reviewed and revised 15th November 2013

USES

• surrogate for SvO2 thus provides a surrogate measure of oxygen flux, reflecting the balance between oxygen delivery (DO2) and consumption (VO2).
• was (controversially) advocated by the Surviving Sepsis Guidelines as part of early goal directed therapy for septic shock (target ScvO2 >70%)
• may have a role in the management of postoperative patients

DESCRIPTION

• can be measured intermittently (venous gas) or continuously via a spectrophotometer
• continuous systems typically have either:
  • a standard CVC with specialised fiberoptics on the tip (e.g. PreSep CVC, attaches to a Vigileo monitor), or
  • a fiberoptic line that can be fed down a lumen of a pre-existing CVC (e.g. CeVox, attaches to a PiCCO monitor)

METHOD OF INSERTION AND/OR USE

Insertion and use

• as per central line insertion
• SvO2 monitor is either built in to the CVC or can be fed down a lumen in an already sited CVC
• perform calibration

O2 flux = oxygen delivery (DO2) – oxygen consumption (VO2)

O2 flux = (cardiac output x (Haemoglobin concentration x SpO2 x 1.34) + (PaO2 x 0.003)) – VO2

Interpretation of scvO2

• Normal oxygen extraction is 25–30% corresponding to a ScvO2 >65%
• < 65% = Impaired tissue oxygenation
• >80% = High PaO2; or suspect:
  • Cytotoxic dysoxia (e.g. cyanide poisoning, mitochrondial disease, severe sepsis)
  • Microcirculatory shunting (e.g. severe sepsis, liver failure, hyperthyroidism)
  • Left to right shunts

OTHER INFORMATION

Utility

• rarely used in Australasia (except as part of the ARISE trial)
• response to therapy can be quickly observed when connected to a continuous spectrophotometer
• studies in critical illness show that ScVO2 is consistently higher than SvO2 by approximately 5% (when averaged over multiple measurements) but parallels changes in response (e.g. to volume loading). i.e. changes in ScVO2 and SvO2 track one another, but the absolute values of one cannot be inferred from the other.
• ScvO2 > 70% forms part of the strategy adopted in early goal directed therapy which showed a significant mortality reduction in sepsis (16%)
• these patients often already have a CVL in situ (venous access, CVP monitoring)

Problems

• recent data showing that lactate clearance in sepsis is non-inferior to continuous ScVO2 monitoring (Jones, JAMA, 2010)
• titration of end of resuscitation to ScvO2 may not be required (ICU Monitor, 2010 – summary of Jones, JAMA, 2010)
• ScvO2 does not reflect myocardial perfusion (upstream from the opening of the coronary venous sinus)

Difference between ScvO2 and SvO2

• ScvO2 in anormal person is expected to be 2-3% less than SvO2 because it contains predominantly SVC blood from the upper body (ie. ScVO2 < SvO2)
  • blood from the upper body has a higher oxygen extraction ratio (OER), and thus a lower SO2 than IVC blood
  • the brain has high oxygen extraction ratio and the kidneys have the lowest, of the major organs at rest
• However, the reverse relationship is often seen in critical illness. Situations where ScvO2 > SvO2:
  • decreased OER in SVC distribution
    • anaesthesia – because of increase in CBF and depression of metabolism
    • TBI or cerebral cooling – when cerebral metabolism depressed
  • decreased OER in IVC distribution
    • shock states – because of diversion of blood from splanchnic circulation and increased oxygen extraction and therefore IVC saturation decreases.

COMPLICATIONS

• same as those associated with central line insertion
• equipment failure
• CeVox can block a CVC lumen completely and is prone to drift
• Potential misinterpretation of the measured values if devices are incorrectly calibrated or malpositioned

References and Links

LITFL

• CCC — Lactate Clearance vs ScvO2 Monitoring in Severe Sepsis
• CCC — SvO2 versus ScvO2
• CCC — Mixed venous oxygen saturation (SvO2) monitoring

Journal articles

• Jones AE, Shapiro NI, Trzeciak S, Arnold RC, Claremont HA, Kline JA; Emergency Medicine Shock Research Network (EMShockNet) Investigators. Lactate clearance vs central venous oxygen saturation as goals of early sepsis therapy: a randomized clinical trial. JAMA. 2010 Feb 24;303(8):739-46. doi: 10.1001/jama.2010.158. PubMed PMID: 20179283; PubMed Central PMCID: PMC2918907.
• Nebout S, Pirracchio R. Should We Monitor ScVO2 in Critically Ill Patients? Cardiol Res Pract. 2012;2012:370697. doi: 10.1155/2012/370697. Epub 2011 Sep 21. PubMed PMID: 21941671; PubMed Central PMCID: PMC3177360.
• van Beest P, Wietasch G, Scheeren T, Spronk P, Kuiper M. Clinical review: use of venous oxygen saturations as a goal – a yet unfinished puzzle. Crit Care. 2011;15(5):232. doi: 10.1186/cc10351. Epub 2011 Oct 24. Review. PubMed PMID: 22047813; PubMed Central PMCID: PMC3334733.
• Walley KR. Use of central venous oxygen saturation to guide therapy. Am J Respir Crit Care Med. 2011 Sep 1;184(5):514-20. doi: 10.1164/rccm.201010-1584CI. Review. PubMed PMID: 21177882. [Free Fulltext]