ABG and anion gap

Question 1

What acronym can you use for easy interpretation of an ABG?

Answer 1

If the pH and CO2 are going in opposite direction = RESPIRATORY

If pH and CO2 are going in the same direction = METABOLIC

If HCO3 is deranged then compensated (if the primary abnormality is the HCO3 it is usually very high or very low but this can still mean that it is compensated)

Question 2

Where do different ABG abnormalities lie on a graph of H+ against pCO2?

Answer 2

Question 3

What is the normal pH range?

Answer 3

7.35 - 7.45

Question 4

What is the normal paCO2 range?

Answer 4

4.7 -6.5 kPa

Question 5

What is the normal PaO2 range?

Answer 5

10.5-13.5 kPa

Question 6

What is the normal HCO3 and BE range?

Answer 6

HCO3 - 22-26 mEq/L

Base Excess - -2 to +2 mmol/L

Question 7

What is the acid-base equation?

Answer 7

Question 8

What is the difference between type 1 and type 2 respiratory failure?

Answer 8

Type I = hypoxia only (focal issue)

Type II - hypoxia AND hypercapnia (global issue)

Question 9

What is the cause of type I and type II respiratory failure?

Answer 9

• Type I - V/Q mismatch
• Type II - alveolar hypoventilation

Question 10

Answer 10

Respiratory alkalosis without compensation

• High pH and low CO2 so RO
• High O2 and normal H2CO3 - no compensation

Question 11

What is the biggest cause of respiratory alkalosis?

Answer 11

Anxiety

The second is hyperventilation in ITU

Question 12

What ABG abnormality may be caused by opioids?

Answer 12

Respiratory acidosis because it depresses breathing

Question 13

What ABG abnormality is caused by norovirus?

Answer 13

metabolic acidosis

Question 14

What is the difference between ABG abnormalities caused by diarrhoea and vomiting?

Answer 14

Vomiting = loss of hydrochloric acid –> metabolic alkalosis

Diarrhoea = loss of bicarbonate –> metabolic acidosis

HOWEVER in any healthy person these are compensated.

Question 15

Answer 15

• Metabolic alkalosis with compensation
• High pH and high CO2 – ME
• HCO3 is high – compensated

Question 16

What often causes a metabolic alkalosis?

Answer 16

Drugs which increase bicarbonate or vomiting

Question 17

Answer 17

• Respiratory acidosis with compensation
• Low pH and high CO2 – so RO
• High HCO3 – so compensated
• This is a person with COPD for a long time so baseline HCO3 is high

Question 18

Answer 18

• Metabolic acidosis with compensation
• If you have metabolic problem in healthy person you are likely to have compensation
• CO2 is going down causing a decrease in H+ ions so should have an alkalosis but this is not happening so this is metabolic
• Starting problem is high H+ causing low CO2 (bicarbonate is probably the problem or really strong compensation)

Question 19

Name 2 normal causes of an anion gap.

Answer 19

1. Diarrhoea - loss of bicarb
2. Renal tubular acidosis

Question 20

What are the 4 causes of large anion gaps?

Answer 20

• Ketones – diabetic ketoacidosis
• Uraemia –aka renal failure
• Lactate – rhabdomyolysis and sepsis
• Toxins – antifreeze (ethylene glycol) and paracetamol/ibuprofen

normal anion gap is between 4 and 12.

Question 21

What are the normal reference ranges for these ions? Na/ K/Cl/HCO3

Answer 21

• Na - 135-145
• K - 3.5-5
• Cl - 96-106
• HCO3 - 22-26

Question 22

What are normal Mg and Ca ranges?

Answer 22

Ca - 2.2-2.6 mmol/L

Mg – 0.7–1.0 mmol/L