1B arterial blood gases Flashcards
1
Q
What is base excess (BE)?
A
- The conc of bases (predominantly bicarbonate) compared with the ‘expected conc’
- An exact match is 0, an excess of base is positive and a base deficit is negative
2
Q
How does uncompensated respiratory acidosis occur?
A
- Through sub-optimal ventilation
- Less minute ventilation means less fresh air in alveoli
- Increase in CO2 in alveoli which reduces diffusion gradient meaning less CO2 moves from blood into alveoli so more CO2 remains in blood
- This increases CO2 + H2O reaction meaning more carbonic acid which dissociates meaning more protons are accumulated
- We see lower pH, increased PCO2 and same BE (since its correct for the PCO2)
3
Q
How is respiratory acidosis compensated for?
A
- Body tries to reduce H+ conc by increasing HCO3- to bind H+
- Happens in 2 phases
- Acute phase- CO2 moving into erythrocytes 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 will still be low but closer to normal, PCO2 will be high as long as breathing is still bad, BE is high because plasma HCO3 is higher than expected for the PCO2
- This is partially compensated respiratory acidosis
- Eventually, pH will normalise, PCO2 and BE will still remain high
- Now its fully compensated respiratory acidosis
4
Q
How does uncompensated respiratory alkalosis occur?
A
- Through hyperventilation- increased minute ventilation (e.g. through increase in tidal volume with same breathing frequency) → increases alveolar ventilation
- Reduces alveolar PCO2 and increases conc 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+
- Means higher pH, lower PCO2 and same BE
- This is uncompensated respiratory alkalosis
5
Q
How is respiratory alkalosis compensated for?
A
- Body tries to increase H+ conc in blood
- There is no acute phase, only chronic- reduces amount of HCO3- reabsorbed in kidney and more HCO3- secretion in collecting duct
- This reduces plasma HCO3- meaning more dissociation of carbonic acid into H+ and HCO3-
- This means higher pH than normal, PCO2 will be low and BE will be low (bc of increased HCO3 excretion)
- This is partially compensated respiratory alkalosis
- Eventually pH will normalise
- pH will be normal, PCO2 and BE will be lower than normal
- This is fully compensated respiratory alkalosis
6
Q
What happens to acid-base homeostasis in diarrhoea?
A
- Lots of HCO3- lost
- Increases how much carbonic acid is dissociated to release more HCO3-
- But, this also increases H+ conc which decreases pH, PCO2 stays same and BE has decreased
- This uncompensated metabolic acidosis
7
Q
Other than diarrhoea, what else causes uncompensated metabolic acidosis?
A
- Other HCO3- losing conditions
- H+ gaining conditions e.g. increased lactic acid production
8
Q
How is metabolic acidosis compensated for?
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
9
Q
What happens to acid-base homeostasis in vomiting?
A
- HCl loss occurs which causes H+ loss
- HCO3- increases because there are fewer H+ to bind
- blood gas shows high pH, normal PCO2 and high BE (because HCO3- is disproportionately high for the PCO2)
- This is uncompensated metabolic alkalosis
10
Q
Other than vomiting, what else can cause uncompensated metabolic alkalosis?
A
- Other H+ losing conditions
- HCO3- gaining conditions
11
Q
How can the body correct uncompensated metabolic alkalosis?
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
12
Q
Describe the pulmonary transit time
A
13
Q
What is an acid?
A
Any molecule with a loosely bound H+ that it can donate
14
Q
What is a base?
A
An anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’)
15
Q
What is the carbonic acid equilibrium?
A
18
Q
How do we get alkalaemia/acidaemia?
A
- An alkalaemia requires an alkalosis (the arrow in the diagram) where either acid is lost or increased production/retention of a base
- Body responds with an acidosis to correct this
- An acidaemia requires an acidosis (the arrow) where either acid is overproduced/retained or base is lost
- Body responds with an alkalosis to correct this
20
Q
Describe the interpretation of this ABG
A
Uncompensated respiratory alkalosis with moderate hypoxaemia
