Venous O2 Saturation. Walton & Hansen. 2018. JVECC Flashcards
What is dysoxia?
impaired O2 utilization
How does the body respond to increased tissue O2 needs?
- 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
Explain SvO2
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
How do you calculate DO2?
DO2 = CO x CaO2
How do you change Fick’s equation to calculate VO2?
VO2 = CO x (CaO2 - CvO2)
What is the difference between SvO2 and ScvO2?
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
How do you calculate CaO2?
(1.34 x Hb x SaO2) + (0.003 x PaO2)
What is the Huefner Constant?
the Huefner constant stands for the Hb binding capacity in mL/g
What are typical SO2 and PO2 values of the mixed venous blood?
- SO2 of 75%
- PO2 of 44 mm Hg
How does the SO2 compare between the cranial and caudal vena cava?
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
How do you calculate O2ER?
oxygen extraction rate
O2ER = VO2/DO2
- will increase during decreased supply or increased demand > will decrease the regional or mixed venous O2 saturation
What could decrease O2ER and increase venous oxygenation?
- decreased O2 demand
- impaired O2 utilization, e.g, mitochondrial dysfunction in sepsis
Explain what the vertical arrows indicate
critical extraction ratio
- the point of DO2 where the VO2 becomes DO2-dependent
- supply-dependent oxygen consumption beyond the critical extraction ratio/critical DO2
Explain how optical oximetry works
oxyhemoglobin and deoxyhemoglobin absorb a different spectrum of light between the wavelength 650 and 1000 nm
what are the 2 most important techniques of measuring venous O2 saturation
- continuous in viva measurement with indwelling fiber optic catheter
- co-oximeter: ex vivo analysis of a blood sample
How much do SvO2 and ScvO2 usually differ and why do they differ
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
How does a state of shock affect the difference between SvO2 and ScvO2?
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)
What technique could be utilized to make ScvO2 correlate better with SvO2?
What complications are associated with this technique?
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
What are normal ScvO2 and SvO2 measurements of healthy animals and people at rest?
ScvO2 65-70%
SvO2 70-75%
What are the 4 determinants of venous oxygen saturation?
- arterial oxygen saturation
- hemoglobin concentration
- cardiac output
- tissue oxygen consumption
What are the 4 causes leading to decreased venous oxygen saturation from decreased O2 delivery?
- hypoxemia
- anemia
- hypovolemia
- cardiac failure
What are the 3 causes leading to decreased venous oxygen saturation from increased O2 consumption?
- exercise
- shivering
- increased metabolic demands
What can lead to increased venous oxygen saturation from increased oxygen delivery?
- increased FiO2 ratio
- increased cardiac output
What can lead to increased venous oxygen saturation from decreased oxygen extraction?
- microcirculatory failure
- mitochondrial dysfunction
what microcirculatory alterations lead to microcirculatory failure in sepsis?
- decreased functional capillary density
- increased microvascular permeability
- increased perfusion heterogeneity
- impaired vasomotor tone and functional shunting of oxygen
What are the 2 ways in which mitochondria may be affected in sepsis?
- decreased PO2 of mitochondria from impaired perfusion
- direct inhibition of mitochondrial respiration secondary to ROS (i.e., cytopathic hypoxia)
Explain why measuring ScvO2 in hemorrhagic shock may have additional benefits to traditional perfusion parameters and the venous-arterial pCO2 gap.
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.
In reference to the Rivers study and the PROCESS trial, what is the proof for utilizing ScvO2 for EGDT in septic shock>
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
What changes in ScvO2 are associated with increased mortality in sepsis
decreased AND increased values
What have recent veterinary studies shown in regards of using ScvO2 to guide prognosis and care of critically ill patients
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.
