Central and mixed venous saturation monitoring

This topic is vaguely touched upon in Question 27.1 from the first paper of 2008, and the comparison of the two measurement techniques was explored in Question 5 from the second paper of 2006. The usual tricks seems to be asking the candidate to come up with an x number of causes of ridiculously high SvO2. Central venous saturation measurements are discussed in greater detail elsewhere, as is the calculation of the oxygen extraction ratio.

Here is a nice tabulated answer.

A Comparison of Central Venous 
and Mixed Venous Saturation Measurements

SvO2: mixed venous saturation

ScvO2: central venous saturation


Pulmonary artery

Superior vena cava



Less invasive than SvO2

Blood content

Mixed right atrial blood with blood from the coronary sinus,

Mixed blood from the head and upper extremities

Higher measurements

ScvO2 is usually stated to be higher than SvO2, and the only place this is inverted is in this college answer, and in Oh's manual. SvO2 should be lower because of the relatively oxygen-poor blood added by the IVC and the coronary sinus.

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

CVCs are required for drug administration


More risk from PACs

Less invasive and therefore
fewer complications.

Influence of catheter position on ScvO2 measurments

In brief summary, the further up you move away from the pulmonary artery, the greater your SvO2becomes; whereas if you go further down, the very hypoxic splanchnic and coronary sinus venous blood will decrease the SvO2.

a comparison of venous saturation measurments

Of course, this is going to vary from patient to patient, and within a given patient it will vary andpotentially even invert between states of health and disease. It is therefore not surprising that the literature disagrees. Oh's manual, and the college answer to  Question 5 from the second paper of 2006, are just about the only places where the SvO2 is said to be higher than the ScvO2. Most other literature seems to disagree. A lengthy digression on this topic is available in the chapter on mixed venous oxygen and carbon dioxide content, tucked safely away into the Primary Exam section.


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