EDUBRIEFS
in CCTC
SvO2
Monitoring (mixed venous oxygen saturation)
| What is
it? |
-
Mixed
venous oxygen saturation (SVO2) is the percentage of oxygen bound
to hemoglobin in blood returning to the right side of the heart.
This refects the amount of oxygen "left over" after the tissues remove
what they need.
|
| What does
it tell us? |
-
Mixed
venous oxygen saturation (SVO2) can help to determine whether the cardiac
output and oxygen delivery is high enough to meet a patient's needs.
It can be very useful if measured before and after changes are made to
cardiac medications or mechanical ventilation, particularly in unstable
patients.
|
| What are
the normal values? |
-
Normal
SvO2 60-80%.
|
| How do
I use this information clinically? |
-
ATP
(energy) is needed for all cell function and survival. Tissues require
oxygen in order to make ATP (energy). If the amount of oxygen being
received by the tissues falls below the amount of oxygen required (because
of an increased need, or decreased supply), the body attempts to compensate
as follows:
-
First
- Cardiac Output is increased
-
The
cardiac output is increased in an effort to increase the amount of oxygen
being delivered to the tissues as shown below.
-
Oxygen
Delivery is the amount of oxygen being sent to the tissues, and is determined
by the following:
|
Oxygen
Delivery (DO2) =
Cardiac
Output (HR X Stroke Volume)
X
Oxygen
Content (Hb X SaO2) |
If
this is not sufficient to meet tissue energy needs;
Second
-
Tissues Extract an Increased Amount of Oxygen.
Tissues
begin to remove or extract a higher percentage of oxygen from the
arterial blood. This results in a reduced amount of oxygen remaining
in the blood as it returns to the right side of the heart (decreased SVO2).
If
this is not sufficient to meet tissue energy needs;
Third
- Anaerobic Metabolism Increases.
-
If
the tissues fail to receive an adequate supply of oxygen, anaerobic metabolism
becomes the only mechanism to produce tissue ATP. Anaerobic metabolism
is inefficient, producing a large amount of metabolic waste (e.g. lactic
acid) that is difficult for the body to eliminate quickly. It also
produces a relatively poor supply of ATP. Prolonged anaerobic metabolism
leads to energy depletion and metabolic acidosis.
|
| Why measure
it? |
-
If
SvO2 decreases, it indicates that the tissues are extracting a higher percentage
of oxygen from the blood than normal. In otherwords, a decreased
SvO2 indicates that the cardiac output is not high enough to meet tissue
oxygen needs. Thus, SvO2 can indicate whether an individual's
cardiac output is
high enough to meet their needs.
-
A rise
in SvO2 demonstrates a decrease in oxygen extraction, and usually indiates
that the cardiac output is meeting the tissue oxygen need. A return
of the SvO2 to normal suggests patient improvement. However, a rise
in SvO2 in the presence of a rising lactate is inappropriate - the patient
who has resorted to anaerobic metabolism (third compensation) should have
evidence of a high cardiac output and increased extraction. This
is an ominous finding, suggesting that the tissues are unable to extract.
It can be seen in late septic shock, or in cell poisoning such as cyanide.
-
Cardiac
output is routinely measured to evaluate drug effectiveness. Unfortunately,
the cardiac output measurement only gives us a value, it does not indicate
whether the measured cardiac output is meeting the patient's needs.
For some individuals, a cardiac output that falls below the normal range
may be adequte, whereas for others, a normal or elevated cardiac output
value may be too low. An SvO2 in the normal range, along with
a normal lactate, suggests that the cardiac output is adequate.
SVO2
can be very helpful when attempting to determine whether a change in therapy
is beneficial. Measuring SvO2 before and after a change can assist
in determining whether the therapy made the patient better or worse.
SvO2
can also be useful when evaluating changes to ventilator therapy, especially
in unstable patients. Changes may be made to the ventilator to increase
the oxygen content of the blood, which is important to the total oxygen
delivery (cardiac ouptut X oxygen content). Increased PEEP may be
required to increase the oxygen content, however, increased levels of PEEP
can decrease the cardiac output. By measuring the SVO2 before and
after a change in PEEP, the optimal level of PEEP can be determined.
The "best" PEEP is the level that improves the SaO2 without causing the
SvO2 to fall.
Tissue
oxygen need is met when the amount of oxygen being delivered to the tissues
is sufficient to meet the amount of oxygen being consumed (VO2).
When the oxygen delivery falls below oxygen consumption needs, lactic acidosis
develops.
VO2
(Oxygen Consumption) = Cardiac Output X Hb X (SaO2 - SvO2)
There
are 4 causes for a drop in SvO2:
1.
The cardiac output is not high enough to meet tissue oxygen needs
2.
The Hb is too low
3.
The SaO2 is too low
-
4.
The oxygen consumption has increased without an increase in oxgyen delivery
-
|
Brenda Morgan
Clinical Educator, CCTC
September, 1999
Reviewd May 12, 2001
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