You are managing an elderly patient with septic shock. The ICU team is requesting ScvO2 measurement to be used in consideration of inotropic support. You are not sure how to make decisions about inotropes based on ScvO2 and wonder: What is ScvO2? what can it tell you? what are its limitations? are there alternative measurements to use?
ScvO2: Central venous oxygen saturation.
-Measured via central venous catheter at the superior vena cava/right atrium
-proxy measure of SvO2 (Venous oxygen saturation) measured at pulmonary artery
-used to assess the relationship between oxygen delivery and oxygen utilization and cardiac output
-With exceptions, Scvo2 >70% is consistent with adequate CO and perfusion status
To help understand this, a quick review:
Oxygen delivery (DO2)
- DO2= (cardiac output) x (arterial oxygen content)
- DO2= CO x (Hb x 1.34 x SaO2) + (0.003 x PaO2)
Oxygen utilization (VO2)
- VO2= (cardiac output) x (arterial oxygen content) – (venous oxygen content)
- ScvO2 is an approximation of venous oxygen content
So how is the ICU team planning to use ScvO2 to assess if inotropes are indicated?
Reviewing what is above, you note that you can measure the determinants of arterial oxygen content (hemoglobin, saturation, PaO2) and venous oxygen content (approximated by ScvO2). Assuming stability of certain variables you can start to make conclusions about cardiac contractility.
If VO2 and patient characteristics (activity, temp, etc… be careful! these change, especially in a septic patient) are stable, changes in ScvO2 reflect changes in DO2 and therefore hemoglobin and cardiac output. so if hemoglobin is also stable, changes in Scvo2 reflect changes in CO. CO is a product of heart rate and stroke volume (preload, contractility, afterload). So at long last, if the patient is not bradycardic, and is volume resuscitated (preload addressed), (afterload is not an issue… your patient is in septic shock), the remaining abnormalities in ScvO2 indicate insufficient contractility and the need for inotropic support!
1) Drawing conclusions about cardiac contractility using ScvO2 requires an understanding of complex physiology and correct assessment of many potentially dynamic variables. You would frequently harm patients if you simply hung dobutamine every time the ScvO2 dropped below 70.
2) Because of the positioning of a central venous catheter, ScvO2 does not include venous blood coming from the coronary sinus (located in the right auricle) and therefore is not reflective of myocardial oxygenation 
3) “Cytopathic hypoxia” – septic patients (especially later in their course) have deranged metabolism, their mitochondria do not work properly, and the body is not always able to use delivered oxygen. Above we assumed changes in ScvO2 reflected changes in delivery of oxygen, but you also have to consider that the body is unable to utilize oxygen. You can pick up on this with measurements of organ function. How do we do this? Currently one of our best tools is LACTATE. Lactate metabolism is also complex, but can be a marker of failure to utilize oxygen, and failure to clear lactate despite resuscitation is an independent predictor of mortality. 
Jones et al had a recent landmark study in JAMA adressing lactate clearance as a resuscitative endpoint. They found that in patients with septic shock, resuscitated to normalize central venous and mean arterial pressure, management to normalize lactate resulted in no difference in in-hospital mortality as compared to ScvO2 management.  This is the one of the studies that form the basis of non-invasive sepsis management.
Arbo, John E., Stephen Ruoss, Geoffrey K. Lighthall, Michael P. Jones, and Joshua Stillman. Decision Making in Emergency Critical Care: An Evidence-based Handbook. N.p.: n.p., n.d. Print.
1) Nebout, Sophie, and Romain Pirracchio. “Should We Monitor ScVO2 in Critically Ill Patients?” Cardiology Research and Practice 2012 (2012): 1-7. Web.
2) Arnold, Ryan C., Nathan I. Shapiro, Alan E. Jones, Christa Schorr, Jennifer Pope, Elisabeth Casner, Joseph E. Parrillo, R. Phillip Dellinger, and Stephen Trzeciak. “Multicenter Study Of Early Lactate Clearance As A Determinant Of Survival In Patients With Presumed Sepsis.” Shock32.1 (2009): 35-39. Web.
3) Jones, Alan E. “Lactate Clearance vs Central Venous Oxygen Saturation as Goals of Early Sepsis TherapyA Randomized Clinical Trial.” Jama 303.8 (2010): 739. Web.