Anaesthetics: ABGs

Question 1

State which pts may need an ABG

Answer 1

• Any sick pt
• Those ventilated using NIV
• Those in hypercapnic respiratory failure
• Septic pts
• High FiO2 (once established correlationbetween sats and PO2 than sats can be used)

Question 2

What values are given on an ABG

Answer 2

Question 3

Remind yourself of some reasons for a low pH on ABG

Answer 3

• Increased CO2 e.g.hypoventilation, type 2 resp failure
• Increased lactate e.g. in sepsis
• Other acids e.g. DKA, methanol poisoning, salicyclate
• Loss of HCO3-

Question 4

Remind yourself of some reasons for a high pH on ABG

Answer 4

• Low CO2 due to hyperventilation
• Decreased H+
• Excess HCO3-

Question 5

What is base excess?

What is the normal range?

Answer 5

Base excess tells you whether amount of HCO3- in blood is high or low:

The base excess is another surrogate marker of metabolic acidosis or alkalosis:

• A high base excess (> +2mmol/L) indicates that there is a higher than normal amount of HCO3– in the blood, which may be due to a primary metabolic alkalosis or a compensated respiratory acidosis.
• A low base excess (< -2mmol/L) indicates that there is a lower than normal amount of HCO3– in the blood, suggesting either a primary metabolic acidosis or a compensated respiratory

Question 6

HCO3- can be elevated in chronic CO2 retainers; true or false?

Answer 6

True

Question 8

Discuss how you should interpret an ABG

Answer 8

Question 9

What are normal values for the following on room air:

• PaO2
• PaCO2

Answer 9

• PaO2: 10.5-13.5kPa
• PaCO2: 4.7-6kPa

*REMEMBER: must always interpret PaO2 with FiO2

Question 10

Remind yourself of the difference between hypoxia and hypoxaemia

Answer 10

• Hypoxia: tissues do not receive adequete supply of O2 to support aerobic respiration
• Hypoxaemia: O2 content of arterial blood is reduced

Question 11

What should you calculate when you have a metabolic acidosis to help you determine the cause?

Why do you calculate this?

Answer 11

Anion gap

To work out if the metabolic acidosis is due to increased acid production or ingestion vs decreased acid excretion or loss of HCO3– you can calculate the anion gap. The normal anion gap varies with different assays but is typically between 4 to 12 mmol/L.

Question 12

How do you calculate anion gap?

Answer 12

([Na+] + [K+]) - ([Cl-] + [HCO3-])

Anion gap is due to presence of unmeasured anions e.g. phosphate, sulphate and negatively charged proteins

Question 13

State some potential causes of a metabolic acidosis with normal anion gap

Answer 13

• Gastrointestinal loss of HCO3– (e.g. diarrhoea, ileostomy, proximal colostomy)
• Renal tubular acidosis (retaining H+)
• Addison’s disease (retaining H+)

Question 14

State some potential causes of metabolic acidosis with raised anion gap

*THINK MUDPILES CATS

Answer 14

Question 15

State some caues of memtabolic alkalosis

Answer 15

• Vomitting
• Diuretic use
• Conn’s syndrome
• Cushing’s syndrome
• Laxative abuse
• Massive blood transfusion
• Milk alkali syndrome