2. A. MIXED VENOUS BLOOD
Mixed venous blood - mixture of all the systemic
venous blood draining from all the tissue capillary
beds of the body, excluding shunted blood (i.e.
central or peripheral shunt). Pulmonary venous blood
is not included.
H 2
3. It Has 3 major component: SVC ;IVC ;coronary
sinus.
Sustained tissue hypoxia is one of the most
important cofactors in the development of
multiorgan failure
Oxygen delivery – tissue demand mismatch can be
measured by venous oximetry.
H 3
4. Oxygen Delivery (DO2) =
Cardiac Output (HR X Stroke
Volume)
X
Oxygen Content (Hb X SaO2)
H 4
5. Second Compensation: Tissue oxygen extraction
increases. (decreased SvO2).
Third Compensation: Anaerobic Metabolism
iIncreases:Prolonged anaerobic metabolism leads
to energy depletion and metabolic acidosis.
H 5
6.
7. If SvO2 decreases, it indicates that the tissues are
extracting a higher o2.
In otherwords, a decreased SvO2 indicates that the
cardiac output is not high enough to meet tissue
oxygen needs.
A rise in SvO2 demonstrates a decrease in oxygen
extraction
A return of the SvO2 to normal suggests patient
improvement.
H 7
8. A rise in SvO2 in the presence of a rising lactate =
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.(Desoxyia)
H 8
9. 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.
The "best" PEEP is the level that improves the SaO2
without causing the SvO2 to fall.(PEEP affect COP)
H 9
10. VO2 (Oxygen Consumption) = Cardiac Output X Hb X
(SaO2 - SvO2)
There are 4 fundamental 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(ERO2)
H 10
11. In healthy individuals, anerobic metabolism may occur when SvO2 drops below
its normal value of 75% to 30–40%
12. Normal oxygen extraction is 25–30% corresponding to a
ScvO2 >65%
< 65% = Impaired tissue oxygenation
>80% = High PaO2; or suspect:
1-Cytotoxic dysoxia (e.g. cyanide poisoning, mitochrondial
disease, severe sepsis)
2-Microcirculatory shunting (e.g. severe sepsis, liver failure,
hyperthyroidism)
3-Left to right shunts
H 12
14. Mixed venous blood is best taken from
pulmonary artery as adequate mixing has
occurred. Sometimes right ventricle can be
used. In right atrium, the bloods are not
adequately mixed.
H 14
15. Patients with injury suggestive of blood loss.
HR, BP, Urine output, CVP and SCVO2 measured.
Blood loss estimated.
SCVO2 most sensitive indicator blood loss
SCVO2 <65% associated with increased injury, blood
loss and transfusion requirements.
H 15
16. In a retrospective study, Varpula et al. showed
the most important hemodynamic variables
relevant to outcome are mean arterial pressure
and lactate levels in the first 6 h and mean
arterial pressure, SvO2, and central venous
pressure in the first 48 h.
17. 1-In patients with severe sepsis or septic shock a goal of
70% for central venous oxygen saturation corresponds to a
mixed venous oxygen saturation between 60 and 65% !!!
2-Reflecting the balance between oxygen delivery (DO2) and
consumption (VO2).
3-May have a role in the management of postoperative patients
18. Early goal-directed therapy in the treatment
of severe sepsis and septic shock.
N Engl J Med 2001
19.
20. ScvO2 is usually less
than SvO2 by about 2–
3%
In septic shock ScvO2
often exceeds SvO2 by
about 8%.
During anesthesia,
ScvO2 may exceed
SvO2 by up to 6%
21. 1• Coronary Sinus
The heart has a High O2 extraction ratio (55-65%) The PO2
of coronary sinus blood is thus Typically low > 20mmHg
Increase in myocardial oxygen consumption can only be
met by increasing coronary blood flow
2• PO2 of SVC is higher than IVC
PO2 of SVC and IVC: PO2 from IVC is normally higher (SO2
77%) than from SVC (SO2 71%)
H 21
22. because kidney takes 25% of cardiac output but use
only 7-8% of body's O2 consumption
; IVC receives blood more oxygen rich.
Important
With severe haemorrhage,PO2 from SVC may be
higher because of renal vasoconstriction.
H 22
23. The PvO2 and SvO2 of mixed venous blood has a
typical value of 40mmHg or 75% oxygen saturation
This mixed venous blood point does NOT lie on the
standard oxygen dissociation curve because at mixed
venous blood level, the curve is right-shifted because
of increased PvCO2 and decreased pH(CvO2 =
15mL/100mL)
H 23
24. Typical value: 46mmHg O2 per 100mL blood
(assuming SvO2 of 75%)
If SvO2 is 97%, > at PvCO2 of 46mmHg, CvCO2 =
50mLs/100mL > due to Haldane effect
NB
PaCO2 = 40mmHg
CaCO2 = 48mLs/100mL
H 24
25. "It is the Amount of O2 extracted from respired gases equals the
amount added to the blood that flows through the lung"
i.e. "O2 consumption per unit time = O2 taken up by pulmonary blood
flow per unit time"
Fick equation VO2 = Q (CaO2 - CvO2)
VO2 = O2 consumption per minute (mL O2/time)
Q = pulmonary blood flow (mL/time)
CaO2 = O2 concentration in blood leaving lung (mL/100mL)
CvO2 = O2 concentration in mixed venous blood (mL/100mL)
H 25
26. Factors affecting mixed venous O2 tension
From Fick equation
VO2 = Q x (CaO2 - CvO2)
CvO2 = CaO2 - VO2/Q
SvO2 = SaO2 - VO2/(Q x 1.34 x [Hb])
NB:SvO2 is derived so O2 dissociation curve (which is
SpO2 vs PO2) can be used
H 26
27. Firstly When O2 dissociation curve is fixed:
SvO2 = SaO2 - VO2/(Qx1.34x[Hb])
SvO2 is increased when:
• SaO2 is increased
• O2 consumption (VO2) is decreased
• cardiac output (Q) is increased
• Hb concentration is increased
As SvO2 increase, PO2 is increased.
H 27
28. However, at mixed venous blood level of PO2
(40mmHg), changes in SvO2 doesn't have as great
an effect on PvO2 as it would at higher level of PO2.
PvO2 will increase when ODC moves to the right
due to:• increased PvCO2;• increased [H+] (i.e. drop
in pH);• increased temperature;increased red cell
2,3 DPG
H 28
29. The Bohr effect denotes CO2 loading assisting in O2
unloading from Hb (for comparison, Haldane effect
is when O2 unloading from Hb helps with CO2
loading)
Majority of Bohr effect is due to pH change caused
by changes in PO2)
What are the Factors affecting mixed CO2 tension?
According to Fick's principle
Production of CO2 = Elimination of CO2
H 29
30. Mixed venous O2 tension is increased by:
increased SaO2
decreased O2 consumption
increased cardiac output
increased Hb concentration
right shift in ODC, due to:
* increased PvCO2
* increased [H+]
* increased temperature
* increased red cell 2,3DPG
6/7/2015 30
31. Difference between SvO2 and ScvO2
Early goal-directed therapy in the treatment of
severe sepsis and septic shock is important to
reduce mortality and morbidity.
In major surgery, whether reductions in ScvO2 are
independently associated with post-operative
complications still needs a large interventional
multi-center study
32. ScvO2 and SvO2 are superior to conventional
hemodynamic monitoring parameters in the
assessment of the adequacy of global tissue
oxygenation
Continuous monitoring of ScvO2 and SvO2 in the
framework of hemodynamic goals and treatment
algorithms have resulted im improved patient
outcome H 32
33. ScvO2 closely parallels SvO2 saturation
In patients with shock ScvO2 is 7 – 10% (mean) higher than
SvO2
These differences between ScvO2 and SvO2 saturation result
from changes in the regional blood flow and oxygen
supply/demand ratio
Normal or high ScvO2 and SvO2 do not rule out tissue
hypoxia on the organ or regional level
6/7/2015 33
