ABGs Interpretation

Question 1

Describe the normal values on an ABG. Consider:

a) Normal [H+]
b) Normal pH
c) Normal pCO2
d) Normal pO2
e. Normal HCO3-

Answer 1

pH: 7.35-7.45

pCO2: 4.6-6.0

pO2: 10.5-13.5

HCO3-: 23-30 mmol/L

Question 2

What is the Henderson-Hasselbalch equation?

Answer 2

Relationship between [H+], pCO2 and [HCO3-]

pH = 6.1 + log[HCO3-] / (0.23 x pCO2)

Question 3

Describe the systematic approach to interpretation of ABGs. (6)

Answer 3

1. Look at [H+]
2, Look at pCO2
3. If pCO2 normal, look at HCO3-
4. Look at anion gap
5. Look for compensation
6. Look for osmolal gap

Question 4

When looking at [H+] on ABGs, what are you looking for?

What are the normal values?

Answer 4

Acidosis
Alkalosis

Normal: 36-43 nmol/L

Question 5

When looking at the pCO2 on ABGs, what are you looking for?

Answer 5

Whether H+ derangement is metabolic or respiratory

Question 6

In acidosis, what does:

a) High pCO2 indicate?
b) Low pCO2 indicate?

In alkalosis, what does:

a) High pCO2 indicate?
b) Low pCO2 indicate?

Answer 6

IN ACIDOSIS:
High pCO2 - respiratory acidosis
Low pCO2 - metabolic acidosis

IN ALKALOSIS:
High pCO2 - metabolic alkalosis
Low pCO2 - respiratory alkalosis

Question 7

In acidosis, what does:

a) High HCO3- indicate?
b) Low HCO3- indiacte?

In alkalosis, what does:

a) High HCO3- indicate?
b) Low HCO3- indicate?

Answer 7

IN ACIDOSIS:
High HCO3-: respiratory acidosis
Low HCO3-: metabolic acidosis

IN ALKALOSIS:
High HCO3-: metabolic alkalosis
Low HCO3-: respiratory alkalosis

Question 8

How do you calculate the anion gap?

What is the function?

Answer 8

Anion gap = [Na+] - ([Cl-] + [HCO3-])

Function: determines cause of metabolic acidosis

Question 9

What is the normal anion gap?

Answer 9

8-12 mmol/L

Question 10

List 5 causes of metabolic acidosis with a high anion gap.

Answer 10

Renal failure
Diabetic ketoacidosis
Lactic acidosis
Toxins (e.g. salicylate)
Inborn errors of metabolism

Question 11

List 4 causes of metabolic acidosis with a normal anion gap.

Answer 11

Renal tubular acidosis
Severe diarrhoea
Carbonic anhydrase inhibitors
Ureteric diversion

Question 12

In terms of compensation for acid-base disorders, what does each of the following indicate?

a) No compensation
b) Partial compensation
c) Complete compensation (i.e. normal H+)
d) Over-compensation

Answer 12

No compensation - acute H+ derangement

Partial compensation - acute H+ derangement

Complete compensation - chronic H+ derangement

Over-compensation - not possible; indicates mixed disorders (acidosis AND alkalosis)

Question 13

How fast do each of the 3 compensatory systems (buffers, respiratory and renal) act to compensate H+ derangement?

Answer 13

Buffers - immediate
Respiratory compensation - minutes-hours
Renal compensation - days

Question 14

How do you calculate the osmolal gap?

What other calculation is needed to do this?

Answer 14

Osmolal gap = (measured osmolality) - (calculated osmolality)

Calculated osmolality = 2([Na+] + [K+]) + urea + glucose

Question 15

What is the normal osmolal gap?

Answer 15

<10 mOsm/kg

Question 16

What is the normal serum osmolality?

Answer 16

275-295 mOsm/kg

Question 17

What does a raised osmolal gap indicate? (2)

Answer 17

Something else dissolved in the serum (a possible cause of metabolic acidosis)

Ethanol

Question 18

When would you suspect MIXED acid-base disorders when looking at ABGs? (3)

Answer 18

Inadequate or too extreme compensation (over-compensation not possible!)

If pCO2 and HCO3- are abnormal in opposite directions (i.e. one increased, one decreased)

If pH is normal but pCO2 or HCO3- is abnormal

Question 19

Apart from ABGs, which 2 additional biochemistry might you order in acid base disorders?

Answer 19

Lactate levels
Glucose levels (may indicate ketoacidosis)