Central and mixed venous oxygen Flashcards
Define central venous oxygen
Oxygen saturation measured at the SVC/RA
Define mixed venous oxygen
Oxygen saturation sampled from the pulmonary artery representing a mixture of all systemic venous blood post adequate mixture
What are the techniques of measuring mixed and central veinous oxygen
◦ ABG - SVO2 derivation from PO2, pH and PCO2 using the oxyhaemoglobin curve using the Clark, Severinghaus and Glass electrode to directly measure the latter.
◦ Co-oximetry - Near infrared light absorption properties of haemoglobin species utilising the Beer-Lambert law to calculate concentrations of oxyhaemoglobin and deoxyhaemoglobin
How might you continuously measure oxygen saturations intravascularly
◦ Continuous measurement via reflectance spectrophotometry - uses near infrared light to continuously assess oxygenated/deoxygenated haemoglobin fractions
‣ Catheter containing two optic fibres one transmitting near infrared light, the other carries it back for processing. Light source flickers with frequency 200-300Hz to compensate for blood flow changes to measurement. Detection unit contains beam splitter and subsequent interference filters to isolate wavelengths 640nm (reflected by oxygenated) and 920nm (reflected by deoxygenated) with signal strength measured and compared to calibration samples.
What are the pros and cons of measuring mixed veinous vs CVP
CVP - measures only CVP
Mixed venous - core temperature, RA pressure, PA pressure, PAWP and thermodilution measurements
What is a normal central veinous saturation
◦ Central venous - 75% - some discrepancy over whether central venous or mixed venous is higher, OH’s has this lower by 2-3% but other resources have it slightly higher; they seem to be more and more similar the closer to the tricuspid valve you get whereas the central venous saturations get higher the further up the SVC you go
What is a normal Mixed veinous saturation
◦ Mixed venous - 70% - Usually lower than central venous as receives coronary sinus blood (Saturations 35% however also IVC blood has a higher venous saturation under non shocked conditions)
Why is the there a difference between central and mixed veinous saturations
Mixed veinous usually 5% lower due to coronary sinus filling, this widens with shock as IVC blood contains less renal veinous return
Why do we measure central and mixed veinous oxygen saturations
Mixed venous and central venous blood saturations both relate to changes in blood oxygen content (supply) and demand and the factors which alter saturations described in the modified Fick Equation
What is the Fick equation
CO = VO2 / CaO2 - CvO2
If rearranged CVO2 = CaO2 - VO2/CO
What factors according to the Fick equation is mixed veinous oxygen prioportional to?
CO = VO2 / CaO2 - CvO2
If rearranged CVO2 = CaO2 - VO2/CO
Therefore Venous oxygen content is changes if:
* Increased CaO2 (related to oxygen carrying capacity and FiO2) = increased CVO2
◦ E.g. High FiO2, high Hb. Conversely anaemia reduces CaO2
* Increased VO2 = reduced CVO2
◦ Sepsis, malignant hyperthermia, exercise
* Increased cardiac output reduces VO2/CO therefore increasing CVO2
◦ E.g. Septic shock (early), high output cardiac failure. Conversely reduced cardiac output in cardiac failure/cardiogenic shock has the opposite effect
CVO2 refers to what? How is this related to measured oxygen saturation?
CVO2 is merely the oxygen content - to relate this to saturations requires utilising the oxyhaemoglobin dissociation curve. Therefore factors which prevent oxygen utilisation e.g. cyanide, methaemoglobinaemia, severe hypothermia, severe alkalosis, low CO2 will reduce oxygen availability but the blood will have a higher oxygen content.
How might CVO2 be reduced
Overall CVO2 may be low in: cardiogenic shock, septic shock, malignant hyperthermia, hypoxia
CVO2 may be high in early septic shock, cyanide toxicity, high output cardiac failure, hypothermia, anaesthesia and paralysis
What is a normal PO2 of mixed vineous blood
40mmHg and saturations of 75%
What is a normal coronary sinus PO2 and saturations
‣ Coronary sinus PaO2 20mmHg —> saturations 35%
What is a normal SVC saturations and Jugular
‣ SVC - 79% saturations
* Jugular 55%
What is a normal IVC saturations? Renal vien saturations? Hepatic vein saturations?
‣ IVC - 71% venous saturations
* Renal vein 81%
* Hepatic vein 66%
What factors influence VO2 in the body?
◦ VO2, the oxygen consumption rate: decreased VO2 will produce an increased PvO2. Factors which influce VO2 include:
‣ Factors which influence metabolic rate,
* Increased metabolic rate due to e.g. hyperthermia will decrease mixed venous oxygen saturations, and vice versa for factors decreasing metabolic rate e.g. hypothermia, paralysis, anaesthesia
‣ Factors which influence oxygen utilisation, eg. mitochondrial toxins (cyanide), microvascular shunting in sepsis
What pathological abnormalities are not accounted for in Ficks equation
Left to right shunts
PO2 in mixed veinous blood is related to SVO2 how?
◦ In situations in which the oxyhaemoglobin dissociation curve is heavily right shifted, or pathology reduces effective haemoglobin availability dissolved oxygen may be a more important method of O2 delivery
‣ PO2 describes the amount of dissolved oxygen via: PO2 x 0.03
‣ It has a non linear relationship with blood oxygen content as seen by its relationship int he above equation
◦ PO2 also determines the SVO2 according to the shape of the oxyhaemoglobin dissociation curve - this is right shifted in venous blood because of the Bohr effect
‣ The SVO2 will then determine the oxygen carriage by haemoglobin which is the major contributor to mixed venous oxygen content
Mixed veinous CO2 is of what relevance?
VCO2 = CO x k x (PVCO2 - PaCO2)
* VCO2 - the rate of CO2 production
* CO = cardiac output
* Arteriovenous CO2 difference
* K is the coefficient used to discribe the near linear relationship between CO2 content and the partial pressure in the blood
What is a normal mixed veinous CO2
46mmHg
Dtermined by total CO2 content and CO2 dissociation curve
Total CO2 content of mixed veinous blood usually 46mmHg
How is CO2 different to O2 in mixed veinous calculations
Linear relationship between pressure and content
CO2 is more influenced by ventilation reflexes
What is the VCO2 equation
VCO2 = CO x k x (PVCO2 - PaCO2)
* VCO2 - the rate of CO2 production
* CO = cardiac output
* Arteriovenous CO2 difference
* K is the coefficient used to discribe the near linear relationship between CO2 content and the partial pressure in the blood
Draw a curve explaining the relationship between CO2 content and PCO2
PCO2 is related to VCO2 how
- pCO2 = Vco2/MV
How is PO2 in mixed veinous blood related to oxygen content
The PO2 in mixed venous blood, usually 40 mmHg, is a major determinant of its oxygen content:
* The PO2 describes the proportion of dissolved oxygen (PO2 × 0.03)
* The PO2 also determines the SvO2 (usually 70-75%) according to the shape of the oxygen-haemoglobin dissociation curve in mixed venous blood
◦ This curve is slightly right-shifted (compared to arterial blood) because of the Bohr effect
* The SvO2 then determines the oxygen carriage by haemoglobin in mixed venous blood, and therefore the mixed venous oxygen content
How to work out oxygen content
Mixed venous oxygen content depends on:
* Total blood oxygen content = (SvO2 × ceHb × BO2) + (PvO2 × 0.03)
◦ ceHb = the effective haemoglobin concentration
◦ PvO2 = the partial pressure of oxygen in mixed venous blood
◦ 0.03 = the content, in ml/L/mmHg, of dissolved oxygen in blood
◦ BO2 = the maximum amount of Hb-bound O2 per unit volume of blood (normally 1.39)
◦ SvO2 = oxygen saturation of mixed venous blood
Describe the factors influence oxygen content vs oxygen saturation
◦ Factors that influence the affinity of haemoglobin for oxygen:
‣ The partial pressure of O2 in mixed venous blood
‣ The partial pressure of CO2 in mixed venous blood
* Increasing CO2 shifts the curve to the right
‣ pH of mixed venous blood, independent of CO2
* Decreasing pH (acidosis) shifts the curve to the right
‣ The concentration of 2,3-DPG inside the erythrocytes
* Increased 2,3-DPG (eg. in response to hypoxia or erythropoietin) shifts the curve to the right
‣ The presence of unusual haemoglobin species
* Methaemoglobin, carboxyhaemoglobin and foetal haemoglobin shift the curve to the left; sulfhaemoglobin shifts the curve to the right
‣ Temperature
* Hyperthermia shifts the curve right
Demonstrate the Haldane effect using a CO2 content/PCO2 curve
What is the equation for extraction ratio?
VO2/DO2
ERCrit or the critical oxygen extraction ratio is generally seen to be what point? Why is it of significance?
ER Crit is 70%
With 70% oxygen extraction SVO2 is 30%, and beyond this point (i..e with reducing oxygen delivery) there is a nearly 1:1 relationship between DO2 and VO2 i.e. 100% extraction
How do you figure out the SVO2 from VO2 and DO2?
SVO2 = (1-ER) x SaO2
ER = VO2/DO2
(This is a combination of solving CO = VO2/CaO2 - CvO2)
* Extraction ratio = [CaO2 x CO] - VO2 / DO2
◦ DO2 = CO x CaO2
Draaw a Do2 VO2 curve and explain the significant markers on it
What is the critical partial pressure of oxygen? What does this correlate to in veinous saturations?
0.8mmHg at the mitochondria
This is generally held to correlate to a PVO2 of 26mmHg or 50%
At what veinous oxygen saturations can you infer tissue oxygen extraction is low?
> 80% SVO2 is decreased tissue oxygen extraction
◦ Either some from of shunt
◦ Tissues unable to extract oxygen
◦ Low oxygen demand
‣ Anaesthesia/paralysis
‣ Hypothermia
◦ Increased cardiac output
What is normal SVO2
70-80%
What is increased oxygen consumption in the cnotext of SVO2
50-70%
What is a critical SVO2 value
<50%
