ABG

Question 1

What is the normal range for PaCO2

Answer 1

4.6-6.0 kPa

Question 2

What is the normal range for PaO2

Answer 2

10-13 kPa

Question 3

What is the normal range for bicarbonate?

Answer 3

22-26 mmol/L

Question 4

What is the normal anion gap?

Answer 4

8-16 mmol/L

Question 5

What is the normal A-a gradient

Answer 5

<2.6 kPa

Question 6

How do you calculate A-a gradient?

Answer 6

PAO2 (alveolar pressure O2) - PaO2 (arterial pressure O2)

The difference should be less than 2.6 kPa

Question 7

Under what circumstances might a patient be hypoxic with a NORMAL A-a gradient?

Answer 7

Alveolar hypoventilation

Low PiO2 (either due to reduced air pressure, or due to reduced FiO2)

Question 8

What might be the cause of a patient’s hypoxia with a RAISED A-a gradient?

Answer 8

V/Q mismatch
Right to left shunt
Increased O2 consumption

Diffusion defect (rare)

Question 9

What does a reduced PaO2 usually suggest?

Answer 9

A problem with gas exchange

Question 10

How do you interpret PaO2 in the context of the FiO2?

Answer 10

Calculate the P:F ratio:
PaO2 / FiO2

Normal P:F ratio is approximately 60 (in kPa).

e.g. healthy patient:
PaO2 of 12, divided by FiO2 of 0.21 (room air) = 57

Question 11

What determines oxygen content?

Answer 11

Haemoglobin concentration and saturation.

Question 12

What is the timeframe for chronic compensation?

Answer 12

Hours to days only.

Evidence of a “chronic compensation” on a blood gas doesn’t necessarily imply a longstanding disorder.

Question 13

What are the two different types of metabolic acidosis?

Answer 13

Increased anion gap metabolic acidosis

Normal anion gap metabolic acidosis

Question 14

How is anion gap calculated?

Answer 14

Na - Cl - HCO3

Question 15

What is a normal anion gap?

Answer 15

8-16

e.g. Na of 140 - Cl of 100 - HCO3 of 24
= 16

Question 16

What makes up most of the anion gap?

Answer 16

Albumin (negatively charged protein)

Question 17

What causes increased anion gap metabolic acidosis?

Answer 17

Lactic acidosis
Ketoacidosis (diabetes, alcohol)
Renal failure
Poisonin (e.g. salicylate)

Question 18

What causes normal anion gap metabolic acidosis?

Answer 18

GI bicarbonate loss (diarrhoea)
Renal bicarbonate loss

Question 19

What is the most common cause of lactic acidosis?

Answer 19

Inadequate tissue perfusion.

Question 20

How much can the respiratory system compensate for a metabolic acidosis?

Answer 20

Complete compensation does not occur.

Therefore a normal pH in a pateitn with a metabolic acidosis suggests a concomitant alkalosis.

Question 21

What is the typical cause for a metabolic alkalosis?

Answer 21

Volume (and therefore chloride) depletion: vomiting, diuresis

Hyperadrenocoticoidism (Cushing’s, Conns, steroid therapy)

Severe K depletion

Question 22

What should be corrected first: alkalosis or volume depletion?

Answer 22

Volume depletion.

Alkalosis will sustain until volume depletion is corrected with NaCl

Question 23

How do you distinguish between causes of metabolic alkalosis?

Answer 23

Measure urinary chloride

Urinary chloride is low when alkalosis is due to volume contraction.

Urinary chloride is high when alkalosis is due to hyperadrenocorticoidism or severe K depletion.

Question 24

What can cause respiratory alkalosis?

Answer 24

Non-hypoxic: Anxiety, sepsis, salicylate intoxication, pregnancy

Hypoxic: pneumonia, asthma, oedema, fibrosis, high altitude