Blood Gases

Question 1

What is type 1 respiratory failure?

Answer 1

Hypoxaemia (PaO2 <8 kPa) with normocapnia (PaCO2 <6.0 kPa)

Question 2

What is type 2 respiratory failure?

Answer 2

Hypoxaemia (PaO2 <8 kPa) with hypercapnia (PaCO2 >6.0 kPa).

Question 3

What result in an ABG should you look at immediately after pH to see if the pH cause is respiratory or metabolic?

Answer 3

CO2 - if the CO2 fits with the pH (eg CO2 is high so the pH is low) then it’s respiratory. If the CO2 doesn’t make sense as the cause of the pH (e.g. normal or ↓ CO2 and ↓ pH), it’s metabolic

Question 4

What changes in CO2, pH and HCO3 indicate respiratory acidosis?

Answer 4

pH = low CO2 = high HCO3 = normal

Question 5

What changes in CO2, pH and HCO3 indicate respiratory alkalosis?

Answer 5

pH = high CO2 = low HCO3 = normal

Question 6

What changes in CO2, pH and HCO3 indicate respiratory acidosis with metabolic compensation

Answer 6

pH = low/normal CO2 = low HCO3 = high

Question 7

What changes in CO2, pH and HCO3 indicate respiratory alkalosis with metabolic compensation

Answer 7

pH = high/normal CO2 = high HCO3 = low

Question 8

What changes in CO2, pH and HCO3 indicate metabolic acidosis?

Answer 8

pH = low

CO2 = normal

HCO3- = low

Question 9

What changes in CO2, pH and HCO3 indicate metabolic alkalosis

Answer 9

pH = high

CO2 = normal

HCO3- = high

Question 10

What changes in CO2, pH and HCO3 indicate metabolic acidosis with respiratory compensation

Answer 10

pH = low

CO2 = low

HCO3- = low

Question 11

What changes in CO2, pH and HCO3 indicate etabolic alkalosis with respiratory compensation

Answer 11

pH = high

CO2 = high

HCO3- = high

Question 12

What does a high base excess mean?

Answer 12

There is a higher than normal amount of HCO3- in the blood (due to a primary metabolic alkalosis or a compensated respiratory acidosis)

Question 13

What does a low base excess mean?

Answer 13

There is a lower than normal amount of HCO3- in the blood (either a primary metabolic acidosis or a compensated respiratory alkalosis)

Question 14

What are the causes of respiratory acidosis?

Answer 14

• Respiratory depression (e.g. opiates)
• Guillain-Barre – paralysis leads to an inability to adequately ventilate
• Asthma
• Chronic obstructive pulmonary disease (COPD)
• Iatrogenic (incorrect mechanical ventilation settings)

Question 15

What are the causes of respiratory alkalosis?

Answer 15

• Anxiety
• Pain – causing an increased respiratory rate
• Hypoxia – resulting in increased alveolar ventilation in an attempt to compensate
• Pulmonary embolism
• Pneumothorax
• Iatrogenic (excessive mechanical ventilation)

Question 16

What are the causes of metabolic acidosis?

Answer 16

• Increased acid production or acid ingestion (eg DKA, aspirin overdose etc)

Decreased acid excretion / GI or renal HCO3– addisons disease

Question 17

What are the causes of metabolic alkalosis?

Answer 17

• Gastrointestinal loss of H+ ions – vomiting/diarrhoea
• Renal loss of H+ ions – loop and thiazide diuretics / heart failure / nephrotic syndrome / cirrhosis / Conn’s syndrome
• Iatrogenic – addition of alkali (e.g. milk-alkali syndrome)

Question 18

What would a mixed respiratory and metabolic acidosis look like?

Answer 18

↓ pH

↑CO2

↓HCO3–

Causes = cardiac arrest / multi-organ faliure

Question 19

What would a mixed respiratory and metabolic alkalosis look like?

Answer 19

↑ pH

↓ CO2

↑ HCO3–

Causes = Liver cirrhosis with diuretic use / Hyperemesis gravidarum / Excessive ventilation in COPD