Venous O2 Saturation. Walton & Hansen. 2018. JVECC

Question 1

What is dysoxia?

Answer 1

impaired O2 utilization

Question 2

How does the body respond to increased tissue O2 needs?

Answer 2

• local increase of arterial blood flow
• increase in CO
• increased minute ventilation
• increased oxygen extraction (= greater O2 consumption at given blood flow rate

in summary = increased DO2 and increased O2 extraction

Question 3

Explain SvO2

Answer 3

SvO2 is the oxygen saturation of the mixed venous/pulmonary arterial blood

SvO2 is a function of the the O2 delivery to and consumption by all tissues of the body

Question 4

How do you calculate DO2?

Answer 4

DO2 = CO x CaO2

Question 5

How do you change Fick’s equation to calculate VO2?

Answer 5

VO2 = CO x (CaO2 - CvO2)

Question 6

What is the difference between SvO2 and ScvO2?

Answer 6

SvO2 is the mixed venous hemoglobin O2 saturation, ScvO2 is the central venous hemoglobin O2 saturation

SvO2 is measured from the pulmonary artery

ScvO2 is measured from the right atrium or a central vein, e.g., cranial vena cava

Question 7

How do you calculate CaO2?

Answer 7

(1.34 x Hb x SaO2) + (0.003 x PaO2)

Question 8

What is the Huefner Constant?

Answer 8

the Huefner constant stands for the Hb binding capacity in mL/g

Question 9

What are typical SO2 and PO2 values of the mixed venous blood?

Answer 9

• SO2 of 75%
• PO2 of 44 mm Hg

Question 10

How does the SO2 compare between the cranial and caudal vena cava?

Answer 10

SO2 from the cranial vena cava is typically lower (70-75%) than the caudal vena cava (75-80%).

This is due to the high O2 extraction by the brain and the high non-nutrient blood flow to the kidneys

Question 11

How do you calculate O2ER?

Answer 11

oxygen extraction rate

O2ER = VO2/DO2

• will increase during decreased supply or increased demand > will decrease the regional or mixed venous O2 saturation

Question 12

What could decrease O2ER and increase venous oxygenation?

Answer 12

• decreased O2 demand
• impaired O2 utilization, e.g, mitochondrial dysfunction in sepsis

Question 13

Explain what the vertical arrows indicate

Answer 13

critical extraction ratio

• the point of DO2 where the VO2 becomes DO2-dependent
• supply-dependent oxygen consumption beyond the critical extraction ratio/critical DO2

Question 14

Explain how optical oximetry works

Answer 14

oxyhemoglobin and deoxyhemoglobin absorb a different spectrum of light between the wavelength 650 and 1000 nm

Question 15

what are the 2 most important techniques of measuring venous O2 saturation

Answer 15

• continuous in viva measurement with indwelling fiber optic catheter
• co-oximeter: ex vivo analysis of a blood sample

Question 16

How much do SvO2 and ScvO2 usually differ and why do they differ

Answer 16

2-5%

• SvO2 is measured from blood from the pulmoanry artery, representing venous oxygenation of the whole body
• ScvO2 is measured from a central vein, usually the cranial vena cava, representing venous oxygenation of only the blood returning from the cranial part of the body
• the cerebral O2 consumption is proportionally higher
• the renal blood flow is largely non-nutrient blood flow (i.e., won’t use up much O2)
• therefore ScvO2 will be lower than SvO2

Question 17

How does a state of shock affect the difference between SvO2 and ScvO2?

Answer 17

shock > redistribution of perfusion to the cranial circulation at the expense of renal and splanchnic perfusion

> blood coming from the caudal part of the body will contain less O2

> reverses the relationship between SvO2 and ScvO2 found in health

> SvO2 may be up to 20% lower than ScvO2 (shown to be the case in humans, in hemorrhagic dog models, SvO2 remained higher and changed in parralel with ScvO2)

Question 18

What technique could be utilized to make ScvO2 correlate better with SvO2?

What complications are associated with this technique?

Answer 18

the central venous catheter may be advanced further into the right atrium so that measurements include also blood from the caudal vena cava

advancing the catheter into the RA may increase risk of right atrial thrombus formation or rupture

Question 19

What are normal ScvO2 and SvO2 measurements of healthy animals and people at rest?

Answer 19

ScvO2 65-70%

SvO2 70-75%

Question 20

What are the 4 determinants of venous oxygen saturation?

Answer 20

• arterial oxygen saturation
• hemoglobin concentration
• cardiac output
• tissue oxygen consumption

Question 21

What are the 4 causes leading to decreased venous oxygen saturation from decreased O2 delivery?

Answer 21

• hypoxemia
• anemia
• hypovolemia
• cardiac failure

Question 22

What are the 3 causes leading to decreased venous oxygen saturation from increased O2 consumption?

Answer 22

• exercise
• shivering
• increased metabolic demands

Question 23

What can lead to increased venous oxygen saturation from increased oxygen delivery?

Answer 23

• increased FiO2 ratio
• increased cardiac output

Question 24

What can lead to increased venous oxygen saturation from decreased oxygen extraction?

Answer 24

• microcirculatory failure
• mitochondrial dysfunction

Question 25

what microcirculatory alterations lead to microcirculatory failure in sepsis?

Answer 25

• decreased functional capillary density
• increased microvascular permeability
• increased perfusion heterogeneity
• impaired vasomotor tone and functional shunting of oxygen

Question 26

What are the 2 ways in which mitochondria may be affected in sepsis?

Answer 26

• decreased PO2 of mitochondria from impaired perfusion
• direct inhibition of mitochondrial respiration secondary to ROS (i.e., cytopathic hypoxia)

Question 27

Explain why measuring ScvO2 in hemorrhagic shock may have additional benefits to traditional perfusion parameters and the venous-arterial pCO2 gap.

Answer 27

in human trauma patients, after resuscitation and stabilization of vital signs, some patients had an ScvO2 < 65%, and these patients were associated with more severe underlying injuries

in swine hemorrhagic shock models, decreased ScvO2 persisted after normalization of vitals and venous-arterial pCO2 gap, indicating other issues beyond hemodynamic performance, e.g., hemodilution etc.

Question 28

In reference to the Rivers study and the PROCESS trial, what is the proof for utilizing ScvO2 for EGDT in septic shock>

Answer 28

It has been suggested that a ScvO2 of 70% or higher should be targeted during resuscitation of patients with septic shock.

• Rivers study (2001) showed improved mortality when using ScvO2 of 70% or higher for EGDT
• These PROCESS trial did not find differences in all-cause mortality in patients with EGDT or without

Question 29

What changes in ScvO2 are associated with increased mortality in sepsis

Answer 29

decreased AND increased values

Question 30

What have recent veterinary studies shown in regards of using ScvO2 to guide prognosis and care of critically ill patients

Answer 30

The results of both of these studies suggest that dogs with ScvO2 values that are low after initial resuscitation and remain low despite ongoing care have a higher mortality risk than dogs with ScvO2 values that normalize.