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Year 2 - BRS > 5.17 - Arterial blood gases & acid base regulation > Flashcards

5.17 - Arterial blood gases & acid base regulation Flashcards

1
Q

What is PO2?

A
  • partial pressure of oxygen
  • indicates how much oxygen is dissolved in the arterial blood
  • if it is particularly low it can suggest inadequate gas exchange in the lungs
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2
Q

What is PCO2?

A
  • partial pressure of carbon dioxide
  • indicates how much CO2 is dissolved in arterial blood
  • if particularly high it can suggest inadequate gas exchange in the lungs
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3
Q

What is pH?

A
  • the ‘power of hydrogen’
  • describes the acidity, neutrality or alkalinity of the blood
  • pH of arterial blood is finely tuned and small deviations can affect oxygen transport and delivery
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4
Q

What is HCO3-?

A
  • plasma bicarbonate
  • describes the concentration of bicarbonate dissolved in arterial blood
  • if higher or lower than normal this could be evidence of gas exchange imbalance
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5
Q

What is base excess (BE)?

A
  • describes the concentration of bases (predominantly bicarbonate) compared with the ‘expected concentration’
  • an exact match is 0, an excess of base is positive and a base deficit is negative
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6
Q

What is PaO2, SaO2 and PaCO2 like in arterial blood?

A
  • PaO2: >10 kPa
  • SaO2: >95%
  • PaCO2: 4.7-6.0 kPa
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7
Q

What is PaO2, SaO2 and PaCO2 like in venous blood?

A
  • PaO2: 4.0-5.3kPa
  • SaO2: ~75%
  • PaCO2: 5.3-6.7 kPa
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8
Q

What is pulmonary transit time and gas exchange time, and what do these mean?

A
  • pulmonary transit time: 0.75s - erythrocytes in contact with gas exchange surface for 0.75s
  • gas exchange time: 0.25s (O2 moves slowest and takes 0.25s for exchange)
  • CO2 is more soluble and is exchanged faster
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9
Q

What are the equations for pH and [H+]?

A
  • pH = -log10[H+]
  • [H+] = 10^(-pH)
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10
Q

What is an acid?

A

Any molecule that has a loosely bound H+ ion that it can donate

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11
Q

What are H+ ions also known as?

A

Protons (because a H atom with a +1 valency has no electrons or neutrons)

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12
Q

What is the relationship between [H+] and pH?

A

A greater concentration of H+ ions refers to a lower pH

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13
Q

What is a base?

A
  • an anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’)
  • H+A- <–> H+ and A-
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14
Q

What is the carbonic acid equilibrium?

A
  • H2O + CO2 <-(carbonic anhydrase)-> H2CO3 <–> H+ + HCO3-
  • this relationship is in an equilibrium - increasing something on one side will push the equation in the opposite direction
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15
Q

What is the balance of respiratory acids to metabolic acids in the body?

A
  • 99% respiratory acids (carbonic acid - CO2 cleared by lungs)
  • 1% metabolic acids (lactic acid, fatty acids etc from rest of body)
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16
Q

What is the difference in arterial and venous pH?

A
  • arterial pH = 7.4
  • venous pH = 7.36
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17
Q

How does the blood react to imbalances?

A

The blood has enormous buffering capacity that can react almost immediately to imbalances

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18
Q

What are the two methods of corrective compensation for acidosis/alkalosis?

A
  • rapid - changes in ventilation can stimulate a rapid compensatory response to change CO2 elimination and therefore alter pH
  • slow - changes in HCO3- and H+ retention/secretion in the kidneys can stimulate a slow compensatory response to increase/decrease pH
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19
Q

What is alkalaemia and acidaemia?

A
  • alkalaemia - higher than normal blood pH
  • acidaemia - lower than normal blood pH
20
Q

What is acidosis and alkalosis?

A
  • acidosis - circumstances that increase [H+] and decrease pH
  • alkalosis - circumstances that decrease [H+] and increase pH
21
Q

How do we get alkalaemia/acidaemia?

A
  • an alkalaemia requires an alkalosis where either acid is lost or increased production/retention of a base
  • an acidaemia requires an acidosis where either acid is overproduced/retained or base is lost
22
Q

How is an acidosis and alkalosis corrected by the body?

A
  • acidosis causes acidaemia which will need an alkalosis to correct
  • alkalosis causes alkalaemia which will need an acidosis to correct
23
Q

What four things do we look at in an ABG interpretation?

A
  • type of imbalance?
    • acidosis (or acidaemia) / alkalosis (or alkalaemia) / normal
  • aetiology of imbalance?
    • respiratory (acidosis or alkalosis) / metabolic (acidosis or alkalosis) / mixed (respiratory and metabolic) / normal
  • any homeostatic compensation?
    • uncompensated / partially compensated / fully compensated
  • oxygenation?
    • hypoxaemia / normoxaemia / hyperoxaemia
24
Q

How do you report an ABG measurement?

A

CADO

  • compensation?
  • aetiology?
  • disturbance?
  • oxygenation?

e.g. uncompensated respiratory alkalosis with moderate hypoxia

25
Q

What are the ranges for normal base excess (BE)?

A
  • 0 = perfect (observed HCO3- = HCO3- needed for pO2)
  • -2 to 2 is the normal range
26
Q

What are the ranges for high, normal and low pH?

A
  • high: >7.45
  • normal: 7.35 to 7.45
  • low: <7.35
27
Q

What are the ranges for high, normal and low PaCO2?

A
  • low: <4.7kPa
  • normal: 4.7-6.4kPa
  • high: >6.4kPa
28
Q

What are the ranges for high, normal and low BE?

A
  • low: <-2
  • normal: -2 to 2
  • high: >2
29
Q

What are the ranges for high, normal and low PaO2?

A
  • low: <10kPa (mild hypoxia 8-10, moderate hypoxia 6-8, severe hypoxia <6)
  • normal: 10 to 13.5kPa
  • high: >13.5kPa (rare)
30
Q

What is the ABG like for uncompensated respiratory acidosis and alkalosis?

A
  • uncompensated respiratory acidosis: pH low, PaCO2 high, BE normal
  • uncompensated respiratory alkalosis: pH high, PaCO2 low, BE normal
31
Q

What is the ABG like for uncompensated metabolic acidosis and alkalosis?

A
  • uncompensated metabolic acidosis: pH low, PaCO2 normal, BE low
  • uncompensated metabolic alkalosis: pH high, PaCO2 normal, BE high
32
Q

What is the ABG like for uncompensated mixed acidosis and alkalosis?

A
  • uncompensated mixed acidosis: pH low, PaCO2 high, BE low
  • uncompensated mixed alkalosis: pH high, PaCO2 low, BE high
33
Q

What is the ABG like for partially compensated respiratory acidosis and alkalosis?

A
  • partially compensated respiratory acidosis: pH low, PaCO2 high, BE high
  • partially compensated respiratory alkalosis: pH high, PaCO2 low, BE low
34
Q

What is the ABG like for partially compensated metabolic acidosis and alkalosis?

A
  • partially compensated metabolic acidosis: pH low, PaCO2 low, BE low
  • partially compensated metabolic alkalosis: pH high, PaCO2 high, BE high
35
Q

What is the ABG like for fully compensated respiratory acidosis / metabolic alkalosis?

A

pH normal, PaCO2 high, BE high

36
Q

What is the ABG like for fully compensated respiratory alkalosis / metabolic acidosis?

A

pH normal, PaCO2 low, BE low

37
Q

How does uncompensated respiratory acidosis occur?

A
  • sub-optimal ventilation
  • less minute ventilation = less fresh air in alveoli
  • increase in CO2 in alveoli reduces diffusion gradient = less CO2 blood–>alveoli = more CO2 in blood
  • increases CO2+H2O –> H2CO3 –> dissociates into protons which accumulate
  • lower pH, increased PaCO2 and normal BE (since correct for the PaCO2)
38
Q

How is respiratory acidosis compensated for?

A
  • body tries to reduce [H+] by increasing HCO3- to bind H+
    • acute phase - CO2 moving into RBC combines with H2O in presence of carbonic anhydrase to form HCO3-, which moves out of cell via AE1 transporter into plasma
    • chronic phase - increase HCO3- reabsorption in kidneys
  • pH still low but closer to normal, PaCO2 high as long as hypoventilating, BE high since plasma HCO3- higher than expected for the PaCO2
  • partially compensated respiratory acidosis
  • eventually pH will normalise, PCO2 and BE will remain high (fully compensated)
39
Q

How does uncompensated respiratory alkalosis occur?

A
  • hyperventilation - increased minute ventilation (e.g. through increase in tidal volume with same breathing frequency) → increases alveolar ventilation
  • reduces alveolar PaCO2 and increases concentration gradient for CO2 diffusion out of blood so post-capillary blood has lower than normal CO2
  • leftward shift in carbonic acid equilibrium meaning less H+
  • higher pH, lower PaCO2, same BE
40
Q

How is respiratory alkalosis corrected?

A
  • body tries to increase [H+] in blood
  • no acute phase, only chronic - reduces amount of HCO3- reabsorbed and more HCO3- secretion in collecting duct
  • reduces plasma HCO3- meaning more dissociation of carbonic acid into H+ and HCO3-
  • higher pH than normal, PaCO2 low and BE will be low (due to increased HCO3- excretion) - partially compensated
  • eventually pH will normalise (fully compensated)
41
Q

What happens to acid-base homeostasis in diarrhoea?

A
  • lots of HCO3- lost in excretions
  • increases H2CO3 dissociated to release more HCO3-
  • also increases [H+] which decreases pH, PaCO2 same, BE decreased
42
Q

What are some other causes of uncompensated metabolic acidosis (apart from diarrhoea)?

A
  • other HCO3- losing conditions
  • H+ gaining conditions e.g. increased lactic acid production
43
Q

How do we compensate for metabolic acidosis?

A
  • H+ conc needs to be reduced
  • manipulation of ventilation helps with this- increases in ventilation reduces alveolar PCO2 and increases diffusion gradient of CO2 from blood so reduces systemic arterial PCO2
  • this causes carbonic acid equation to shift left to correct the lower PCO2
  • causes more H+ and HCO3- to combine to form carbonic acid which will be converted into H2O and CO2
  • at this point pH will be low, PCO2 will be low and BE will be low
    • this partially compensated metabolic acidosis
  • eventually pH will normalise but PCO2 and BE will be lower than normal
    • this is fully compensated metabolic acidosis
44
Q

What happens to acid-base homeostasis in vomiting?

A
  • HCl loss occurs which causes H+ loss
  • HCO3- increases as there are less H+ to bind
  • ABG shows high pH, normal PaCO2 and high BE (as HCO3- is disproportionately high for the PaCO2)
  • uncompensated metabolic alkalosis
45
Q

What else can cause uncompensated metabolic alkalosis (apart from vomiting)?

A
  • other H+ losing conditions
  • HCO3- gaining conditions
46
Q

How is metabolic alkalosis corrected?

A
  • need to increase H+ conc
  • reducing ventilation increases PCO2 conc in alveoli, decreasing diffusion gradient from blood so increasing PCO2 in arterial blood
  • this shifts carbonic acid equation to right to correct higher CO2 → this produces more H+ and further increasing HCO3-
  • blood gas reads high pH, high PCO2 and high BE
  • this is partially compensated metabolic alkalosis
  • at one point, the pH normalises, PCO2 and BE are higher than normal
  • this is fully compensated metabolic alkalosis

Year 2 - BRS (84 decks)

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