Arterial Blood Gases Flashcards
define oxygen “off-loading”
dynamic unbinding of O2 from Hb at tissues so freely dissolved O2 is available for use
tissues can only use ___ oxygen
freely dissolved oxygen
although both binding and unbinding from Hb are fast, which is faster
binding rates faster so more O2 bound to Hb than freely dissolved
what are factors that shift oxy-Hb curve to right?
1) decr pH (BOHR)
2) incr pCO2 (CO2 binding decr O2 affinity for Hb)
3) incr temp
4) incr [2,3 DPG]
what are factors that shift oxy-Hb curve to right?
1) decr pH (BOHR)
2) incr pCO2
3) incr temp
4) incr [2,3 DPG]
what is the effect of right shift in oxy-Hb curve on O2 binding
1) O2 binds less tightly to Hb
2) O2 undergoes more rapid off-loading to tissues
what is effect of right shift physiologically in terms of incr 2,3-DPG and exercise
chronic hypoxia at altitude
incr 2,3 DPG
more O2 to tissues
exercise
incr temp and incr pCO2
decr pH
more O2 to exercising muscle
what are factors that shift oxy-Hb curve to left?
1) incr pH
2) decr pCO2
3) decr temp
4) decr [2,3 DPG]
Define DO2 dot
Equation
DO2dot = Volume of O2 delivered to tissues in one minute
DO2dot = Qdot (cardiac output now) x CaO2 (arterial O2 content)
Typical Value for DO2dot
DO2dot = 1000 mL O2
Equation for CaO2 in terms of SaO2 and O2 carrying capacity
CaO2 = Hb-boundO2 + freely dissolved O2
~ CaO2 = Hb-bound O2
CaO2 = SaO2 x [Hb] x 1.39 mL O2/gm Hb CaO2 = SaO2 x O2 carrying capacity
Define O2 carrying capacity
max O2 that can be carried by a particular amount of Hb
assuming all O2 binding sites occupied
calculate O2 consumption from CO based on equations for CaO2 and CvO2 and difference in arterial and venous blood
CaO2 = SaO2 x [Hb] x 1.39 mL O2/gm Hb CvO2 = SvO2 x [Hb] x 1.39 mL O2/gm Hb
CaO2 - CvO2 = (SaO2 - SvO2) x [Hb] x 1.39 mL O2/gm Hb
VO2dot = Volume of O2 consumed per minute
VO2dot = Qdot (cardiac output) x (SaO2 - SvO2) x [Hb] x 1.39 mL O2/gm Hb VO2dot = 240 mL O2 typically
Typical [Hb]
15 gm/100 mL blood
calculate O2 consumption from CO and difference in arterial and venous blood
CaO2 = SaO2 x [Hb] x 1.39 mL O2/gm Hb CvO2 = SvO2 x [Hb] x 1.39 mL O2/gm Hb
CaO2 - CvO2 = (SaO2 - SvO2) x [Hb] x 1.39 mL O2/gm Hb
VO2dot = Volume of O2 consumed per minute
VO2dot = Qdot (cardiac output) x (SaO2 - SvO2) x [Hb] x 1.39 mL O2/gm Hb VO2dot = 240 mL O2 typically
Typical values for
SaO2
SvO2
VO2 dot
SaO2 = 98
SvO2 = 75%
VO2 dot = 240 mL O2
Significance of VO2dot
at rest more O2 delivered than being consumed so at rest you can engage in activity without incr O2 delivery (HR, etc)
O2 cascade from air to mitochondria
1) O2 diluted by water vapor as inspired
2) further diluted through gas exchange with blood and introduce CO2
3) additional drop in tension btwn alveolar and arterial blood due to venous mixing
4) large drop in PO2 between capillaries and mitochondria
where does venous mixing occur
1) shunted venous blood
2) V/Q mismatch
at level of inspired air what does PO2 depend on (2)
1) barometric pressure
2) fraction of O2 in atmosphere
what does O2 tension in alveoli depend on? (2)
1) alveolar ventilation
2) O2 consumption
what does O2 tension in capillaries depend on? (3)
1) Hb concentration
2) blood flow
3) oxygen off-loading
where is lowest level of PO2
values typically of PO2
in mitochondria
between 4-23 Torr
what is importance in maintaining O2 gradient that drives delivery to mitochondria
oxidative phosphorylation in mitochondria –> continues until level of 1-2 Torr
what is Pasteur Point
when PO2 too low and oxid phosphorylation and O2 consumption drops
what helps determine cause of hypoxemia
ranges
A-a gradient (alveolar - arterial pressure gradient for O2)
normally = 5-10 Torr from gravity
which causes of hypoxemia would show normal A-a gradient
which show a widened A-a gradient
normal gradient b/c O2 decr to all alveoli
1) high altitude
2) hypoventilation
widened A-a gradient
1) diffusion problems
2) V/Q mismatch
3) shunt
what defines hypoxemia (values for PaO2)
at sea level
at Denver
How do you measure
PaO2
PAO2
PaO2 = directly
PAO2 = Estimated from PaCO2 (inverse relationship btwn PAO2 and PACO2 in alveoli)
PACO2 = PaCO2 PAO2 = PIO2 - (PACO2/R)
Causes of hypoxemia
- Low PO2 in the inspired air (ex. altitude)
- Low PAO2 –> Hypoventilation (reduced VA) —> reduces alveolar oxygen by increasing alveolar PACO2
- Diffusion problem between the alveoli and capillaries
- V/Q mismatch
- Shunt
what helps determine cause of hypoxemia
ranges
A-a gradient (alveolar - arterial pressure gradient for O2)
which causes of hypoxemia would show normal A-a gradient
which show a widened A-a gradient
normal gradient b/c O2 decr to all alveoli
1) high altitude
2) hypoventilation
widened A-a gradient
1) diffusion problems
2) V/Q mismatch
3) shunt
how do you distinguish shunts, V/Q mismatch and diffusion problems as causes of hypoxemia
have patient breathe 100% O2 to separate diffusion vs. V/Q mistmatch
measure DLCO for diffusion limitations
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define hypoxia
Low O2 at tissue
more general term that causes low O2 in tissue
Causes of hypoxia
1) Low Qdot (low cardiac output)
2) Low SaO2 assoc with Low PaO2 (hypoxemia)
3) Delivery problems
- anemia
- carbon monoxide
define hypoxemia
more specific term
Low SaO2 assoc with low PaO2
can be in hypoxia without hypoxemic if you have low cardiac output
lying at lower points on Oxyhemoglobin dissociation curve
How does anemia cause hypoxia
anemia decr Hb concentration decr O2 per Hb decr CaO2 decr DO2
forms in which CO2 carried in blood
typical concentrations of each
1) CO2
freely dissolved gas = 1.2 mM = alphaCO2 x PaCO2 = 0.03 x 40
2) bicarbonate ion (HCO3-) = 24 mM
hydration of gas CO2 (CO2 + H2O H2CO3 HCO3- + H
3) Carbamino compounds = 1.2 mM
bound to proteins (mainly Hb)
How does carbon monoxide cause hypoxia
CO compete out O2 on Hb and bind more tightly and slow unbinding of CO from O2 and causes O2 bind more tightly to other existing binding sites decr O2 unloading decr SaO2 decr CaO2 decr DO2
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forms in which CO2 carried in blood
typical concentrations of each
1) CO2
freely dissolved gas = 1.2 mM = alphaCO2 x PaCO2 = 0.03 x 40
2) bicarbonate ion (HCO3-) = 24 mM
hydration of gas CO2 (CO2 + H2O H2CO3 HCO3- + H
3) Carbamino compounds = 1.2 mM
bound to proteins (mainly Hb)
which is more soluble CO2 vs. O2
values of solubility coefficient
alphaCO2 = 0.03 mM/torr
alpha O2 = 0.0013 mM/Torr
what is concentration of free CO2 for typical arterial PaCO2 of 40 Torr
alphaCO2 x PaCO2 = 0.03 x 40 = 1.2 mM
what is haldane effect
O2 binding decr CO2 affinity for Hb
what accounts for majority of CO2 carriage
bicarbonate ion (HCO3-)
how much of CO2 in arterial blood carried in bicarb form?
24 mM compared to 1.2 mM of other modes
what is almost all carbamino carried on
hemoglobin
what is haldane effect
O2 binding decr CO2 affinity for Hb
what is bohr effect
CO2 binding decr O2 affinity for Hb
what is bohr effect
CO2 binding decr O2 affinity for Hb
Hypoxemia
Problem:
Low PIO2 (high altitude)
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2= decr b/c hypoxemia means low PaO2
SaO2 = decr b/c hypoxemia also means low SaO2 (shift down on Oxy Hb curve)
PaCO2 = decr b/c hyperventilation (decr PaO2, decr PaCO2)
A-a gradient = normal b/c problem is level of PIO2, lower PAO2 and then lower PaO2 by similar amounts
Special Tests= measure PaCO2
Hypoxemia
Problem:
Hypoventilation (normal PIO2, decr PAO2)
such as severe COPD
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = decr (hypoxemia)
SaO2 = decr (hypoxemia)
PaCO2 = incr b/c hypoventilaton (as CO2 diffuse into alveoli due to hypoventilating, O2 gets pushed out and decr)
A-a gradient = normal (happening within alveoli which will have similar effect on both PAO2 and PaO2)
Special Tests= measure PaCO2
Hypoxemia
Problem:
Hypoventilation (normal PIO2, decr PAO2)
such as severe COPD
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = decr (hypoxemia)
SaO2 = decr (hypoxemia)
PaCO2 = incr b/c hypoventilaton (as CO2 diffuse into alveoli due to hypoventilating, O2 gets pushed out and decr)
A-a gradient = normal (happening within alveoli which will have similar effect on both PAO2 and PaO2)
Special Tests= measure PaCO2
Hypoxemia
Problem:
Diffusion (interstitial disease)
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = decr (hypoxemia)
SaO2 = decr (hypoxemia)
PaCO2 = normal (diffusion problems don’t affect CO2 levels)
A-a gradient = incr b/c problem between alveoli and arterial circulation (affects PaO2 without affecting PAO2)
Special Tests= CO single breath
Hypoxemia
Problem:
V/Q mismatch (moderate COPD)
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = decr (hypoxemia)
SaO2 = decr (hypoxemia)
PaCO2 = normal
as long as total ventilation level is normal, CO2 levels would be normal
no hypoventilation, just V/Q mismatch because moderate
A-a gradient = incr
Special Tests
What is PAO2 in V/Q mismatch?
high level of PaCO2, low PAO2
normal PAO2 because PaCO2 is normal
Hypoxemia
Problem:
Shunt (pneumonia) = extreme version of low V/Q region
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
similar to V/Q mismatch
PaO2 = decr
SaO2 = decr
PaCO2 = normal
A-a gradient = incr
Special Tests
Breathe 100% O2
Delivery Problem
Problem:
Low [Hb}
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = normal b/c freely diffusing O2 offset by low [Hb] impedes diffusion process
Hb binds free O2 to maintain large pressure gradient for O2 diffusion
SaO2 = normal
PaCO2 = normal
A-a gradient = normal
Special Tests
Measure [Hb]
Delivery Problem
Problem:
CO poisoning
Effect on PaO2 SaO2 PaCO2 A-a gradient
Special Tests
PaO2 = normal b/c freely diffusing O2 offset by low [Hb] impedes diffusion process
Hb binds free O2 to maintain large pressure gradient for O2 diffusion
SaO2 = decr
PaCO2 = normal
A-a gradient = normal
Special tests
Measure [CO-Hb]
