Blood gas interpretation

Question 1

Normal ranges:

a) pH
b) PaCO2
c) PaO2
d) HCO3-
e) base excess

Answer 1

a) 7.35 - 7.45
b) 4.7 - 6.0
c) 11.0 - 13.0
d) 22.0 - 26.0
e) -2 to +2

Question 2

In what circumstance might a normal PaO2 be considered abnormal?

Answer 2

If the patient is receiving high flow oxygen- PaO2 should be well above the reference range

Question 3

Maximum FiO2 which can be delivered by a simple face mask at flow rate of 15L/min?

Answer 3

Variable- around 40-60%

Question 4

Venturi masks are available for what concentrations? When might a Venturi be used?

Answer 4

24, 28, 35, 40 and 60%

Particularly suited for patients at risk of CO2 retention e.g. COPD, where a 24 or 28% mask is often used

Question 5

Difference between Type I and Type II respiratory failure?

Answer 5

Type I = hypoxia with normocapnia

Type II = hypoxia with hypercapnia

Question 6

What causes Type I respiratory failure? What are examples of conditions which might cause this?

Answer 6

V/Q mismatch

Reduced ventilation and normal perfusion e.g. pulmonary oedema, bronchoconstriction

Reduced perfusion with normal ventilation e.g. pulmonary embolism

Question 7

What is the underlying cause of Type II respiratory failure? Examples of conditions which might cause this?

Answer 7

Alveolar hypoventilation due to e.g.:

increased resistance due to airway obstruction (COPD)
Reduced compliance of lung tissue/chest wall (pneumonia, rib fracture, obesity)
Reduced strength of respiratory muscles (e.g. Guillane-Barre, motor neurone disease)
Centrally-acting respiratory-depressing drugs (e.g. opiates)

Question 8

What is base excess? What does an a) high base excess (>2mmol/l) and b) a low base excess (

Answer 8

A surrogate marker of metabolic acidosis or alkalosis- tells you how much base (HCO3-) is in the system

a) either due to a primary metabolic alkalosis or a compensated respiratory acidosis
b) either due to a primary metabolic acidosis or a compensated respiratory alkalosis

Question 9

ABG characteristics of

a) primary respiratory acidosis with no compensation
b) primary respiratory alkalosis with no compensation

Answer 9

a) low pH; high PaCO2; normal HCO3-

b) high pH; low PaCO2; normal bicarbonate

Question 10

What will be seen in the ABG of a mixed respiratory acidosis?

Answer 10

The PaCO2 and HCO3- will be moving in opposite directions (i.e. PaCO2 increasing, HCO3- decreasing)

Question 11

Specific causes of respiratory alkalosis? (5)

Answer 11

Anxiety, pain, hypoxia, pulmonary embolism, pneumothorax

Question 12

Underlying cause of respiratory alkalosis?

Answer 12

Excessive alveolar ventilation (hyperventilation) resulting in more CO2 than usual being exhaled

Question 13

The causes for respiratory acidosis are the same as the causes for…?

Answer 13

Type II respiratory failure

Question 14

Metabolic acidosis has what ABG characteristics? (3)

Answer 14

Low pH
Low bicarbonate
Low base excess

Question 15

What does the anion gap tell you?

Answer 15

Whether a metabolic acidosis is due to increased production/ingestion of acid, or due to impaired acid excretion or loss of base (HCO3-)

Question 16

Increased anion gap indicates what? What are potential causes of this? (3)

Answer 16

Metabolic acidosis is due to increased production/ingestion of acid

e.g. lactic acidosis, diabetic ketoacidosis, aspirin overdose

Question 17

Decreased anion gap metabolic acidosis can be caused by what? (3)

Answer 17

Loss of HCO3- from the bowel (diarrhoea, ileostomy, colostomy)
Renal tubular acidosis (H+ retention)
Addisons disease (H+ retention)

Question 18

Metabolic alkalosis has what characteristics on ABG? (3)

Answer 18

Increased pH
Increased bicarbonate
Increased base excess

Question 19

Causes of metabolic alkalosis?

Answer 19

Loss of acid (vomiting/diarrhoea)

Renal loss of H+ ions