Blood Gas Analysis

Question 1

What does a normal PaCO2 in a hypoxic asthmatic patient suggest?

Answer 1

Sign they are tiring and need ITU intervention

Question 2

What does a very low PaO2 in a patient who looks completely well, is not short of breath and has normal O2 saturation suggest?

Answer 2

Likely a venous sample and not arterial

Question 3

What should the PaO2 be on air in a healthy patient?

Answer 3

> 10 kPa

Question 4

If a patient is receiving oxygen therapy, what should their PaO2 be?

Answer 4

PaO2 should be approximately 10kPa less than the % inspired concentration / FiO2 (i.e. 40% oxygen would be expected to have a PaO2 of approximately 30kPa)

Question 5

What does it mean if a patient has PaO2 <10 kPa on air?

Answer 5

Hypoxaemic

Question 6

What does it mean if a patient has PaO2 <8 kPa on air?

Answer 6

Severely hypoxaemic & in respiratory failure. Next look at the PaCO2 to determine if it type 1 or 2

Question 7

Describe the ABG in type 1 respiratory failure?

Answer 7

• Hypoxaemic (<8 kPa)

- Normocapnic (PaCO2 <6.0 kPa)

Question 8

Describe the ABG in type 2 respiratory failure?

Answer 8

• Hypoxaemia (PaO2 <8 kPa)

- Hypercapnia (PaCO2 >6.0 kPa)

Question 9

How does type 1 respiratory failure occur?

Answer 9

Ventilation/perfusion (V/Q) mismatch

Question 10

What are 2 types of V/Q mismatches leading to type 1 respiratory failure & give examples of each?

Answer 10

1. Reduced ventilation & normal perfusion: pulmonary oedema, bronchoconstriction
2. Reduced perfusion with normal ventilation: pulmonary embolism

Question 11

How does type 2 respiratory failure occur?

Answer 11

Alveolar hypoventilation

Question 12

Give 4 examples of when hypoventilation occurs leading to type 2 respiratory failure?

Answer 12

1. Increased resistance as a result of airway obstruction (COPD)
2. Reduced compliance of the lung tissue/chest wall (pneumonia/rib fractures/obesity)
3. Reduced strength of the respiratory muscles (Guillain–Barré / motor neurone disease)
4. Drugs acting on the respiratory centre reducing overall ventilation (opiates)

Question 13

Describe an ABG in respiratory acidosis?

Answer 13

• pH: decreased
• CO2: increased
• HCO3-: normal

Question 14

Describe an ABG in respiratory alkalosis?

Answer 14

• pH: increased
• CO2: decreased
• HCO3-: normal

Question 15

Describe an ABG in respiratory acidosis with metabolic compensation?

Answer 15

• pH: normal/decreased
• CO2: increased
• HCO3-: increased

Question 16

Describe an ABG in respiratory alkalosis with metabolic compensation?

Answer 16

• pH: normal/increased
• CO2: decreased
• HCO3-: decreased

Question 17

What is the carbonic acid equation?

Answer 17

CO2 + H2O H2CO3 (carbonic acid) HCO3- + H+

Question 18

Describe an ABG in metabolic acidosis?

Answer 18

• pH: decreased
• HCO3-: decreased
• CO2: normal

Question 19

Describe an ABG in metabolic alkalosis?

Answer 19

• pH: increased
• HCO3-: decreased
• CO2: normal

Question 20

Describe an ABG in metabolic acidosis with respiratory compensation?

Answer 20

• pH: decreased
• HCO3-: decreased
• CO2: decreased

Question 21

Describe an ABG in metabolic alkalosis with respiratory compensation?

Answer 21

• pH: increased
• HCO3-: increased
• CO2: increased

Question 22

What does a high base excess (> +2mmol/L) indicate?

Answer 22

Higher than normal amount of HCO3- in the blood, which may be due to a primary metabolic alkalosis or a compensated respiratory acidosis

Question 23

What does a low base excess (< -2mmol/L) indicate?

Answer 23

Lower than normal amount of HCO3- in the blood, suggesting either a primary metabolic acidosis or a compensated respiratory alkalosis

Question 24

Describe the ABG in a mixed acidosis/alkalosis?

Answer 24

CO2 & HCO3- will be moving in opposite directions (↑ CO2 & ↓ HCO3- in mixed respiratory & metabolic acidosis)

Question 25

List 5 causes of respiratory acidosis?

Answer 25

1. Respiratory depression (opiates) 2. Guillain-Barre: paralysis leads to an inability to adequately ventilate 3. Asthma 4. Chronic obstructive pulmonary disease (COPD) 5. Iatrogenic (incorrect mechanical ventilation settings)

Question 26

What is a definition of respiratory acidosis?

Answer 26

Inadequate alveolar ventilation leading to CO2 retention

Question 27

What is a definition of respiratory alkalosis?

Answer 27

- Excessive alveolar ventilation (hyperventilation) resulting in more CO2 than normal being exhaled - PaCO2 is reduced & pH increases causing alkalosis

Question 28

List 6 causes of respiratory alkalosis?

Answer 28

1. Anxiety: often referred to as a panic attack 2. Pain: causing an increased respiratory rate 3. Hypoxia: resulting in increased alveolar ventilation in an attempt to compensate 4. Pulmonary embolism 5. Pneumothorax 6. Iatrogenic (excessive mechanical ventilation)

Question 29

What is an anion gap used for?

Answer 29

To work out if the metabolic acidosis is due to increased acid production or ingestion vs decreased acid excretion or loss of HCO3–

Question 30

List 3 causes of a metabolic acidosis with increased anion gap?

Answer 30

1. Diabetic ketoacidosis (↑ production) 2. Lactic acidosis (↑ production) 3. Aspirin overdose (ingestion of acid)

Question 31

List 3 causes of a metabolic acidosis with a decreased anion gap?

Answer 31

1. GI loss of HCO3- (diarrhoea, ileostomy, proximal colostomy) 2. Renal tubular acidosis (retaining H+) 3. Addison’s disease (retaining H+)

Question 32

How do you work out the anion gap?

Answer 32

Anion Gap = Na+ – (Cl- + HCO3-)

Question 33

What is the definition of metabolic alkalosis?

Answer 33

Decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations

Question 34

List 3 causes of a metabolic alkalosis?

Answer 34

1. GI loss of H+ ions (vomiting/diarrhoea) 2. Renal loss of H+ ions (loop & thiazide diuretics/heart failure/nephrotic syndrome/cirrhosis/Conn’s syndrome) 3. Iatrogenic: addition of alkali (milk-alkali syndrome)

Question 35

List 2 causes of a mixed respiratory & metabolic acidosis?

Answer 35

1. Cardiac arrest | 2. Multi-organ failure

Question 36

List 3 causes of a mixed respiratory & metabolic alkalosis?

Answer 36

1. Liver cirrhosis with diuretic use 2. Hyperemesis gravidarum 3. Excessive ventilation in COPD