Question 5

Compare and contrast the information generated by and the usefulness of mixed venous oxygen saturation (SvO2) and central venous oxygen saturation (ScvO2) monitors.

[Click here to toggle visibility of the answers]

College Answer

	SvO2	ScvO2
Measurement	Pulmonary artery	Superior vena cava
Invasiveness	Invasive	Less invasive than SvO2
Physiology	SvO2 is > than ScvO2 as it contains blood from both SVC and IVC	ScvO2 is < SvO2 because it contains predominantly SVC blood which is lower than IVC blood saturation
Situations where SCVO2 > SvO2	a) Anaesthesia – because of increase in CBF & depression of metabolism b) Patients with head injury where cerebral metab is depressed c) Shock: because of diversion of blood from splanchnic circulation, there is increased O2 extraction and therefore IVC saturation decreases. *** Both track each other well during shock states
Other data generated from monitoring devices	Qt, PA pressures, derived indices and body temperature measurements may be obtained	CVP,
Evidence from clinical trials	Study by Gattinoni – only RCT as far as SvO2 is concerned showed no benefit from SVO2 monitoring	Study by Rivers- early goal directed therapy improved outcome in septic shock
Other data	In general no benefit from PACs.
Complications:	More risk from PACs	Less invasive and therefore fewer complications.

Discussion

The disparity between central venous and mixed venous saturation measurements is discussed in greater detail in the chapter on ScVO₂ physiology. These measurements are means of assessment of the adequacy of oxygen delivery.

A slight adjustment to the college answer is probably called for.

A Comparison of Central Venous and Mixed Venous Saturation Measurements
	SvO2: mixed venous saturation	ScvO2: central venous saturation
Measurement	Pulmonary artery	Superior vena cava
Invasiveness	Invasive	Less invasive than SvO2
Blood content	Mixed right atrial blood with blood from the coronary sinus,	Mixed blood from the head and
Higher measurements	Normal conditions: Oh's Manual specifies that under normal physiological conditions central venous saturation (ScvO₂) is 2-3% lower than mixed venous oxygen saturation (SvO₂).	Pathological states: ScvO2 can be abnormally elevated under the following conditions: Decreased cerebral metabolism: Hypothermia Anaesthesia Decreased upper body metabolism Paralysis
Lower measurements	Pathological states: SvO2 can be abnormally depressed under the following circumstances: Increased myocardial oxygen extraction Hyperdynamic cardiac failure Shock (decreased ScvO₂ in the IVC, mixing with the right ventricular blood)	Normal conditions: ScvO₂ is usually 2-3% lower than SvO₂.
Other data generated from monitoring devices	The PA catheter can measure the following variables directly: Core temperature RA pressure PA pressure PAWP On top of that, thermodilution measurements can be performed, with numeorus dreived variables including cardiac output.	CVP. Only CVP.
Evidence from clinical trials	Study by Gattinoni – only RCT as far as SvO2 is concerned showed no benefit from SVO2 monitoring	Study by Rivers- early goal directed therapy improved outcome in septic shock
Other benefits	In general no benefit from PACs.	CVCs are required for drug administration
Complications:	More risk from PACs	Less invasive and therefore fewer complications.

References

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

[Submit a comment or correction]