ABGs

Question 1

What occurs in respiratory acidosis and alkalosis?

Answer 1

Check pH, then CO2 (simples!)
pH CO2 HCO3-
Respiratory acidosis ↓ ↑ Normal
Respiratory alkalosis ↑ ↓ Normal

Question 2

What causes respiratory acidosis?

Answer 2

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)
Respiratory alkalosis

Question 3

What causes respiratory alkalosis?

Answer 3

Anxiety (i.e. panic attack)
Pain: causing an increased respiratory rate.
Hypoxia: resulting in increased alveolar ventilation in an attempt to compensate.
Pulmonary embolism
Pneumothorax
Iatrogenic (e.g. excessive mechanical ventilation)

Question 4

What does respiratory acidosis and alkalosis with compensation appear like on an ABG?

Answer 4

Respiratory acidosis with metabolic compensation
pH CO2 HCO3-
↓ / ↔ ↑ ↑
Respiratory alkalosis with metabolic compensation
↑ / ↔ ↓ ↓

Question 5

What happens in metabolic acidosis and alkalosis?

Answer 5

pH HCO3– CO2
Metabolic acidosis ↓ ↓ Normal
Metabolic alkalosis ↑ ↑ Normal

Question 6

What causes metabolic acidosis?

Answer 6

Metabolic acidosis can occur as a result of either:

Increased acid production or acid ingestion.
Decreased acid excretion or rate of gastrointestinal and renal HCO3– loss.

Question 7

What is the anion gap? What does it tell you?

Answer 7

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.

Anion gap formula: Anion gap = Na+ – (Cl- + HCO3-)

Question 8

What are some causes of increased anion gap?

Answer 8

Lactic acidosis
Aspirin overdose
Diabetic ketoacidosis

Question 9

What are the causes of a decrease anion gap?

Answer 9

Renal tubular acidosis
Addisons
Gastrointestinal losses

Question 10

What causes metabolic alkalosis?

Answer 10

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

Question 11

What causes mixed acidosis and alkalosis?

Answer 11

A mixed respiratory and metabolic acidosis would have the following characteristics on an ABG:

↓ pH
↑CO2
↓HCO3–
Causes of mixed respiratory and metabolic acidosis include:

Cardiac arrest
Multi-organ failure

Question 12

Causes of mixed acidosis and resp acidosis

Answer 12