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Spr2015_Week5 > Arterial Blood Gases > Flashcards

Arterial Blood Gases Flashcards

1
Q

define oxygen “off-loading”

A

dynamic unbinding of O2 from Hb at tissues so freely dissolved O2 is available for use

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2
Q

tissues can only use ___ oxygen

A

freely dissolved oxygen

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3
Q

although both binding and unbinding from Hb are fast, which is faster

A

binding rates faster so more O2 bound to Hb than freely dissolved

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4
Q

what are factors that shift oxy-Hb curve to right?

A

1) decr pH (BOHR)
2) incr pCO2 (CO2 binding decr O2 affinity for Hb)
3) incr temp
4) incr [2,3 DPG]

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5
Q

what are factors that shift oxy-Hb curve to right?

A

1) decr pH (BOHR)
2) incr pCO2
3) incr temp
4) incr [2,3 DPG]

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6
Q

what is the effect of right shift in oxy-Hb curve on O2 binding

A

1) O2 binds less tightly to Hb

2) O2 undergoes more rapid off-loading to tissues

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7
Q

what is effect of right shift physiologically in terms of incr 2,3-DPG and exercise

A

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

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8
Q

what are factors that shift oxy-Hb curve to left?

A

1) incr pH
2) decr pCO2
3) decr temp
4) decr [2,3 DPG]

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9
Q

Define DO2 dot

Equation

A

DO2dot = Volume of O2 delivered to tissues in one minute

DO2dot = Qdot (cardiac output now) x CaO2 (arterial O2 content)

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10
Q

Typical Value for DO2dot

A

DO2dot = 1000 mL O2

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11
Q

Equation for CaO2 in terms of SaO2 and O2 carrying capacity

A

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
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12
Q

Define O2 carrying capacity

A

max O2 that can be carried by a particular amount of Hb

assuming all O2 binding sites occupied

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13
Q

calculate O2 consumption from CO based on equations for CaO2 and CvO2 and difference in arterial and venous blood

A 
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
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14
Q

Typical [Hb]

A

15 gm/100 mL blood

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15
Q

calculate O2 consumption from CO and difference in arterial and venous blood

A 
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
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16
Q

Typical values for
SaO2
SvO2
VO2 dot

A

SaO2 = 98
SvO2 = 75%
VO2 dot = 240 mL O2

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17
Q

Significance of VO2dot

A

at rest more O2 delivered than being consumed so at rest you can engage in activity without incr O2 delivery (HR, etc)

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18
Q

O2 cascade from air to mitochondria

A

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

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19
Q

where does venous mixing occur

A

1) shunted venous blood

2) V/Q mismatch

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20
Q

at level of inspired air what does PO2 depend on (2)

A

1) barometric pressure

2) fraction of O2 in atmosphere

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21
Q

what does O2 tension in alveoli depend on? (2)

A

1) alveolar ventilation

2) O2 consumption

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22
Q

what does O2 tension in capillaries depend on? (3)

A

1) Hb concentration
2) blood flow
3) oxygen off-loading

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23
Q

where is lowest level of PO2

values typically of PO2

A

in mitochondria

between 4-23 Torr

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24
Q

what is importance in maintaining O2 gradient that drives delivery to mitochondria

A

oxidative phosphorylation in mitochondria –> continues until level of 1-2 Torr

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25
Q

what is Pasteur Point

A

when PO2 too low and oxid phosphorylation and O2 consumption drops

26
Q

what helps determine cause of hypoxemia

ranges

A

A-a gradient (alveolar - arterial pressure gradient for O2)

normally = 5-10 Torr from gravity

27
Q

which causes of hypoxemia would show normal A-a gradient

which show a widened A-a gradient

A

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

28
Q

what defines hypoxemia (values for PaO2)
at sea level
at Denver

A
29
Q

How do you measure
PaO2

PAO2

A

PaO2 = directly

PAO2 = Estimated from PaCO2 (inverse relationship btwn PAO2 and PACO2 in alveoli)

PACO2 = PaCO2
PAO2 = PIO2 - (PACO2/R)
30
Q

Causes of hypoxemia

A
  1. Low PO2 in the inspired air (ex. altitude)
  2. Low PAO2 –> Hypoventilation (reduced VA) —> reduces alveolar oxygen by increasing alveolar PACO2
  3. Diffusion problem between the alveoli and capillaries
  4. V/Q mismatch
  5. Shunt
31
Q

what helps determine cause of hypoxemia

ranges

A

A-a gradient (alveolar - arterial pressure gradient for O2)

32
Q

which causes of hypoxemia would show normal A-a gradient

which show a widened A-a gradient

A

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

33
Q

how do you distinguish shunts, V/Q mismatch and diffusion problems as causes of hypoxemia

A

have patient breathe 100% O2 to separate diffusion vs. V/Q mistmatch

measure DLCO for diffusion limitations

34
Q

a

A

a

35
Q

a

A

a

36
Q

a

A

a

37
Q

define hypoxia

A

Low O2 at tissue

more general term that causes low O2 in tissue

38
Q

Causes of hypoxia

A

1) Low Qdot (low cardiac output)
2) Low SaO2 assoc with Low PaO2 (hypoxemia)

3) Delivery problems
- anemia
- carbon monoxide

39
Q

define hypoxemia

A

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

40
Q

How does anemia cause hypoxia

A 
anemia
decr Hb concentration
decr O2 per Hb
decr CaO2
decr DO2
41
Q

forms in which CO2 carried in blood

typical concentrations of each

A

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)

42
Q

How does carbon monoxide cause hypoxia

A 
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
43
Q

a

A

a

44
Q

forms in which CO2 carried in blood

typical concentrations of each

A

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)

45
Q

which is more soluble CO2 vs. O2

values of solubility coefficient

A

alphaCO2 = 0.03 mM/torr

alpha O2 = 0.0013 mM/Torr

46
Q

what is concentration of free CO2 for typical arterial PaCO2 of 40 Torr

A

alphaCO2 x PaCO2 = 0.03 x 40 = 1.2 mM

47
Q

what is haldane effect

A

O2 binding decr CO2 affinity for Hb

48
Q

what accounts for majority of CO2 carriage

A

bicarbonate ion (HCO3-)

49
Q

how much of CO2 in arterial blood carried in bicarb form?

A

24 mM compared to 1.2 mM of other modes

50
Q

what is almost all carbamino carried on

A

hemoglobin

51
Q

what is haldane effect

A

O2 binding decr CO2 affinity for Hb

52
Q

what is bohr effect

A

CO2 binding decr O2 affinity for Hb

53
Q

what is bohr effect

A

CO2 binding decr O2 affinity for Hb

54
Q

Hypoxemia
Problem:
Low PIO2 (high altitude)

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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

55
Q

Hypoxemia
Problem:
Hypoventilation (normal PIO2, decr PAO2)
such as severe COPD

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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

56
Q

Hypoxemia
Problem:
Hypoventilation (normal PIO2, decr PAO2)
such as severe COPD

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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

57
Q

Hypoxemia
Problem:
Diffusion (interstitial disease)

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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

58
Q

Hypoxemia
Problem:
V/Q mismatch (moderate COPD)

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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

59
Q

What is PAO2 in V/Q mismatch?

A

high level of PaCO2, low PAO2

normal PAO2 because PaCO2 is normal

60
Q

Hypoxemia
Problem:
Shunt (pneumonia) = extreme version of low V/Q region

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

similar to V/Q mismatch

PaO2 = decr
SaO2 = decr
PaCO2 = normal
A-a gradient = incr

Special Tests
Breathe 100% O2

61
Q

Delivery Problem
Problem:
Low [Hb}

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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]

62
Q

Delivery Problem
Problem:
CO poisoning

Effect on 
PaO2
SaO2
PaCO2
A-a gradient

Special Tests

A

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]

Spr2015_Week5 (19 decks)

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