Question 5

Pass rate: 18%

Highest mark: ?

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

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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 conditionsOh'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.

 

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References

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