Acid Base Regulation

Question 1

What are the main arterial blood gas measurements and what do they measure?

Answer 1

• PO2: how much oxygen is dissolved in the arterial blood
• PCO2: how much CO2is dissolved in arterial blood
• pH
• HCO3-:concentration of bicarbonate dissolved in arterial blood.
• Base excess (BE): 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

Question 2

What is the normal pH of arterial and venous blood?

Answer 2

• Arterial blood: 7.4

* Venous blood: 7.36

Question 3

What is the importance of carbonic acid?

Answer 3

evaluating acid-base status because of the rate it is produced in cellular respiration
Aka respiratroy acid

Question 4

What is acidaemia or alkalaemia?

Answer 4

If pH is low and CO2is elevated, this is termed respiratory acidosis.

Question 5

What is metabolic acidosis?

Answer 5

Acidosis caused by other acids 
Metabolic acid (e.g. lactic acid and pyruvic acid) is produced is much lower volume and is mostly eliminated by the kidney or the liver.

Question 6

What is the Sørensen equation?

Answer 6

calculate pH from proton concentration (or vice versa)

Question 7

What is the Henderson equation?

Answer 7

calculate the dissociation constant (Ka)

Question 8

What is the Henderson-Hasselbalch equation?

Answer 8

see notes

Question 9

What is the main human buffer?

Answer 9

Bicarb
Phosphate
Protein chains (plasma and Hb)

Question 10

Why do buffers buffer?

Answer 10

• N- and C-terminal ends are charged
• Can either bind an additional proton in acidotic environments or to liberate a proton in alkalotic environments
• certain amino acids in the protein sequence are capable of reversibly binding protons and are referred to as proton acceptors (e.g. Histidine residues).

see equation

Question 11

What acids mostly affect blood pH?

Answer 11

Respiratory acids

Question 12

What stimulates compensatory mechanisms?

Answer 12

• Changes in ventilation to change CO2 elimination and alter pH
• Changes in HCO3- and H+ retention/secretion in the kidneys can stimulate a SLOW compensatory response to increase/decrease pH

Question 13

What a physiological response to CO2 production?

Answer 13

Increased ventilation

Question 14

How is base excess calculated?

Answer 14

Amount of bicarbonate measured compared to amount of bicarbonate expected
Will be close to zero providing there is no metabolic acid base disturbance and no change in the renal excretion of acid

Question 15

What are normal ranges of base excess?

Answer 15

-2-+2

Question 16

What causes a rise in base excess?

Answer 16

increase in renal excretion of acid or ingestion or the administration of base.
loss of acid from vomiting (lost HCl from the stomach – the highest concentration of acid in the body).
result is a metabolic alkalosis.

Question 17

What causes a fall in base excess?

Answer 17

overproduction of metabolic acids (e.g. lactic acid) or the ingestion of acid.
reduction in, or failure of, acid excretion by the kidney or to excessive loss of alkali from the intestine with diarrhoea
result is a metabolic acidosis

Question 18

How is an ABG interpreted?

Answer 18

1. Type of imbalance
   
   • Alkalosis
   • Acidosis
   • Normal
2. Imbalance aetiology
   
   • Compare the PaCO2with the reference range.

Question 19

What are normal ABG values?

Answer 19

see notes

Question 20

If acidosis what does a high PaCO2 mean?

Answer 20

respiratory acidosis

Question 21

If acidosis what does a normal or low PaCO2 mean?

Answer 21

metabolic acidosis

Question 22

What does a low BE suggest, with acidosis?

Answer 22

• Coupled with low PaCO2suggests a partially compensated metabolic acidosis
• Coupled with a normal PaCO2confirms an uncompensated metabolic acidosis
• Coupled with a high PaCO2suggests an uncompensated mixed acidosis

Question 23

What does a normal BE suggest with with established acidosis ?

Answer 23

associated with a high PaCO2and indicates an uncompensated respiratory acidosis

Question 24

What does a high BE suggest with acidosis ?

Answer 24

partially compensated respiratory acidosis

Question 25

If alkalosis what does a low PaCO2 mean?

Answer 25

respiratory alkalosis

Question 26

If alkalosis what does a normal or high PaCO2 mean?

Answer 26

metabolic alkalosis

Question 27

What does a high BE suggest with alkalosis?

Answer 27

Metabolic acidosis
• Coupled with high PaCO2suggests a partially compensated metabolic alkalosis
• Coupled with a normal PaCO2confirms an uncompensated metabolic alkalosis
• Coupled with a low PaCO2suggests an uncompensated mixed alkalosis

Question 28

What does a normal BE with alkalosis suggest?

Answer 28

uncompensated respiratory alkalosis

Question 29

What does a low BE suggest, with alkalosis?

Answer 29

partially compensated respiratory alkalosis

Question 30

What is done if a normal pH has been established?

Answer 30

• Assess the PaCO2 and BE together

* Are both within range? If so, excellent news, your patient is normal

Question 31

What if a normal pH with low PaCO2 and BE

Answer 31

• Fully compensated respiratory alkalosis

* Fully compensated metabolic acidosis

Question 32

What if a normal pH with high PaCO2 and BE

Answer 32

• Fully compensated respiratory acidosis

* Fully compensated metabolic alkalosis

Question 33

Summary of AGB interpretation?

Answer 33

1. Type of imbalance?Acidosis (or acidaemia) / Alkalosis (or alkalaemia) / Normal
2. Aetiology of imbalance?Respiratory (acidosis or alkalosis) / Metabolic (acidosis or alkalosis) / Mixed (respiratory and metabolic) / Normal
3. Any homeostatic compensation?Uncompensated / Partially compensated / Fully compensated
4. Oxygenation?
5. Look at pH
   - Will tell what happened
   - Will give idea of type of imbalance and if compensated
6. Assess PaCO2
   - Tells you if metabolic or respiratory
7. Assess BE
   Assess PaO2

see table