Compare and contrast the measurement (40% of marks) and interpretation (60% of marks) of both central venous and mixed venous oxygen saturations.

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College Answer

Many candidates did not appreciate that ScvO2 refers to SVC / RA junction venous oximetry and not femoral or peripheral venous oximetry. Methods of measurement such as co-oximetry and reflectance spectrophotometry needed to be explained. Marks were awarded for the normal values. Discussion of the relationship between ScvO2 and SmvO2 and changes during shock attracted marks. Better answers quoted the modified Fick equation and related this to cardiac output and factors affecting oxygen consumption versus delivery.

Discussion

It's hard to "compare and contrast" the interpretation of these parameters, because they are interpreted in much the same way, albeit with slightly differen normal values; and both have essentially the same determinants. The merger of columns has slightly derailed the author's normal practice of tabulating such answers, but the wreckage of this attempt will still be offered here, mostly as a cautionary tale. One consoles oneself with the knowledge that this cannot possibly be worse than what most of the candidates wrote for this paper, as only 8% of them passed this question.

Measurement and Interepretation
of Mixed Venous and Central Venous Oxygen Saturation
Domain Central venous Mixed venous
Site of sampling SVC or right atrium Pulmonary artery
Measurement
  • Intermittent sampling: 
    • ABG: derivation of the SvO2 value from the PO2, pH and pCO2, using the oxygen-haemoglobin dissociation curve.
    • Co-oximetry: relies on measuring the absoprtion of near-IR light by haemoglobin species, and the use of the Beer-Lambert law to calculate the concentrations of oxyhaemoglobin and deoxyhaemoglobin
  • Continuous monitoring:
    • Reflectance spectrophotometry: relies on measuing the reflected wavelengths of near-IR light, and calculating the concentrations of oxyhaemoglobin and deoxyhaemoglobin from the log ratio of the signal strength (as each Hb species reflects a different wavelength)
Normal values 75% 70%
Relationship between them Usually, higher than mixed venous Usually, lower than central venous
(incorporates blood from the coronary sinus, which has sats of ~ 35%)
How this is diffent in shock Some authors report that in states of normal health, central venous oxygenation is lower than mixed venous by 2-3%, and that this relationship is reversed in states of shock.
Determinants
  • Mixed venous oxygen content
    • Oxygen carrying capacity of blood
    • Oxyhameoglbin dissociation curve shape
    • PO2
  • Components of the modified Fick equation: 
     (CO = VO2 / CaO2 - CvO2):
    • Arterial oxygen content
    • VO2 (oxygen consumption)
    • Cardiac output
Interepretation
  • Low in:
    • Cardiogenic shock
    • Septic shock
    • Malignant hyperthermia
    • High FiO2
  • Elevated in:
    • Also septic shock
    • Cyanide toxicity
    • High output cardiac failure
    • Hypothermia
    • Anaesthesia and paralysis

References

Chawla, Lakhmir S., et al. "Lack of equivalence between central and mixed venous oxygen saturation." CHEST Journal 126.6 (2004): 1891-1896.

Connett, R. J., et al. "Defining hypoxia: a systems view of VO2, glycolysis, energetics, and intracellular PO2." Journal of Applied Physiology 68.3 (1990): 833-842.

Walley, Keith R. "Use of central venous oxygen saturation to guide therapy."American journal of respiratory and critical care medicine 184.5 (2011): 514-520.

Siggaard-Andersen, Ole, et al. "Oxygen status of arterial and mixed venous blood." Critical care medicine 23.7 (1995): 1284-1293.

Ronco, Juan J., et al. "Identification of the critical oxygen delivery for anaerobic metabolism in critically III septic and nonseptic humans." JAMA: the journal of the American Medical Association 270.14 (1993): 1724-1730.

Chawla, Lakhmir S., et al. "Lack of equivalence between central and mixed venous oxygen saturation." CHEST Journal 126.6 (2004): 1891-1896.

Connett, R. J., et al. "Defining hypoxia: a systems view of VO2, glycolysis, energetics, and intracellular PO2." Journal of Applied Physiology 68.3 (1990): 833-842.

Walley, Keith R. "Use of central venous oxygen saturation to guide therapy."American journal of respiratory and critical care medicine 184.5 (2011): 514-520.

Siggaard-Andersen, Ole, et al. "Oxygen status of arterial and mixed venous blood." Critical care medicine 23.7 (1995): 1284-1293.

Nelson, DAVID P., et al. "Systemic and intestinal limits of O2 extraction in the dog." Journal of Applied Physiology 63.1 (1987): 387-394.

Ronco, Juan J., et al. "Identification of the critical oxygen delivery for anaerobic metabolism in critically III septic and nonseptic humans." JAMA: the journal of the American Medical Association 270.14 (1993): 1724-1730.

Rivers, Emanuel, et al. "Early goal-directed therapy in the treatment of severe sepsis and septic shock." New England Journal of Medicine 345.19 (2001): 1368-1377.

Teixeira, Cassiano, et al. "Central venous saturation is a predictor of reintubation in difficult-to-wean patients*." Critical care medicine 38.2 (2010): 491-496.

Gowardman, John R., David Huntington, and Joy Whiting. "The effect of extubation failure on outcome in a multidisciplinary Australian intensive care unit." Crit Care Resusc 8.4 (2006): 328-33.

Esteban A, Alia I. Clinical management of weaning from mechanical ventilation. Intens Care Med 1998; 24: 999–1008.

He, Huaiwu, et al. "The prognostic value of central venous-to-arterial CO2 difference/arterial-central venous O2 difference ratio in septic shock patients with central venous O2 saturation≥ 80%." Shock: Injury, Inflammation, and Sepsis: Laboratory and Clinical Approaches 48.5 (2017): 551-557.

Wochenschr, Wien Klin. "Bioethics, the Surviving Sepsis Campaign, and the industry." Wien Klin Wochenschr 117 (2005): 13-14.

Subramanian, Gayathri, V. P. Anitha, and Suchitra Ranjit. "Comparison of central venous saturation by standard ABG machine versus co-oximeter: Is 18 carat as good as the 24 carat gold standard?." Indian journal of critical care medicine: peer-reviewed, official publication of Indian Society of Critical Care Medicine 17.2 (2013): 82.

Landsman, M_ LJ, et al. "A fiberoptic reflection oximeter." Pflügers Archiv 373.3 (1978): 273-282.

Nierman, David M., and Clyde B. Schechter. "Mixed venous O2 saturation: Measured by co-oximetry versus calculated from PvO2." Journal of clinical monitoring 10.1 (1994): 39-44.

Mathews Jr, P. J. "Co-oximetry." Respiratory care clinics of North America 1.1 (1995): 47-68.

Mekontso-Dessap, Armand, et al. "Combination of venoarterial PCO 2 difference with arteriovenous O 2 content difference to detect anaerobic metabolism in patients." Intensive care medicine 28.3 (2002): 272-277.