ARTERIAL BLOOD GASES

Question 1

What is respiratory failure?

Answer 1

Failure of the respiratory system to maintain adequate gas exchange

Question 2

Whats the difference between hypoxia and Hypoxaemia?

Answer 2

Hypoxia is lack of oxygen in tissues whereas Hypoxaemia is lack of oxygen in blood

Question 3

What is normal PaO2?

Answer 3

11-13KPa
Should be >10 on room air
If on O2 it should be 10KPa less than %inspired concentration FiO2

Question 4

What is type 1 and type 2 respiratory failure?

Answer 4

Type 1 - Hypoxaemia
Type 2 - Hypoxaemia and hypercapnia

Question 5

What causes type 1 respiratory failure?

Answer 5

Low V/Q - Asthma, pneumothorax, pneumonia, HF
High V/Q - PE

Question 6

What causes type 2 respiratory failure?

Answer 6

Obstructive lung diseases - COPD
Restrictive lung diseases - IPF
Depression of respiratory centre - opiates or TBI
Respiratory muscle weakness. Guillan-barre syndrome or motor neurone disease
Decreased lung compliance - rib fractures or obesity

Question 7

Whats the aetiology of type 1 respiratory failure?

Answer 7

Ventilation-perfusion mismatch

Question 8

Whats the aetiology of type 2 respiratory failure?

Answer 8

Alveolar hypoventilation
The lungs fail to effectively oxygenate and blow off CO2

Question 9

What causes hypocapnia?

Answer 9

Pain, anxiety attack, iatrogenic e.g. ventilating a pt too much

Question 10

Whats the problem with hypocapnia?

Answer 10

It causes vasoconstriction and cerebral ischaemia

Question 11

Whats the problem with hypercapnia?

Answer 11

It increases cerebral blood flow and volume which increases ICP

Question 12

How do we manage type 1 respiratory failure?

Answer 12

Mechanical ventilation

Question 13

How do we manage type 2 respiratory failure?

Answer 13

Non-invasive ventilation - BiPAP

Question 14

How do type 1 and type 2 respiratory failure typically present differently?

Answer 14

Type 1 likely to be agitate
Type 2 more likely to have lowered GCS and so are barely rousable

Question 15

What are the issues with pulse oximetry?

Answer 15

Requires reasonable circulation to the finger i..e wont work if pt is cold or in shock
Foetal Hb, meth Hb and carboxy Hb wont show
Motions e.g. seizures or AF will affect results
Imaging or surgery dyes can affect it
High ambient lighting e.g. in jaundice treatment of neonates can affect it
Nail varnish can impact the results

Question 16

What causes the oxygen dissociation curve to shift to the left?

Answer 16

Low CO2, high pH and low temps

Question 17

How should you interpret an ABG?

Answer 17

1. PH
2. PaCO2 (does it make sense i..e if acidosis is it high - if so then resp cause?)\
3. paO2 - type 1 or type 2 resp failure if any?
4. HCO3- (does this make more sense to pH? Or is there compensation)
5. Base excess (should be same as HCO3-)
6. Anion gap?

Question 18

How are resp and metabolic compensation different?

Answer 18

Resp acts much faster to changes in pH (sec-mins) whereas metabolic can take hours or days to compensate

Question 19

What does it mean if CO2 and HCO3- move in different directions?

Answer 19

Likely mixed

Question 20

How is lactate a prognostic factir?

Answer 20

The higher it is the worse the prognosis

Question 21

What causes raised lactate?

Answer 21

Impaired tissue oxygenation, tissue hypoperfusion

Question 22

How do you calculate the anion gap?

Answer 22

(Concentration of Na+ and K+)- (concentration of Cl- and HCO3-)

Question 23

Whats a normal anion gap?

Answer 23

8-14mmol/L

Question 24

What can cause a normal anion gap metabolic acidosis?

Answer 24

GI bicarbonate loss e.g. diarrhoea
Renal tubular acidosis
Drugs - acetazolamide (CA inhibitor)
Ammonium chloride injection
Addisons disease

Question 25

What causes raised anion gap metabolic acidosis?

Answer 25

Lactic acidosis
Ketones - DKA and alcoholics
Urate - renal failure
Acid poisoning - salicylates and methanol
5-oxoproline overdose e.g. chronic paracetemol ingestion