ACID-BASE in a nutshell

Question 1

What causes respiratory acidosis?

Answer 1

• CNS depression (head injury, stroke, drugs)
• Respiratory depression (myopathy, spinal cord injury, drugs)
• Hypoventilation (pain, chest wall injury/deformity, raised intra-abdominal pressures)
• Respiratory failure (pneumonia, pneumothorax, edema, bronchial obstruction)
• Airway obstruction
• Chronic respiratory acidosis (e.g., COPD, restrictive lung disease)

Question 2

What is the formula for corrected anion gap?

Answer 2

AG (Alb. correct) = AG + 0.25 (40 - Alb.)

Question 3

What does an osmolar gap indicate?

Answer 3

Difference between measured osmolality & calculated osmolarity

Question 4

What is the normal osmolar gap value?

Answer 4

<10

Question 5

What are some causes of an increased osmolar gap?

Answer 5

• Mannitol
• Glycine
• Methanol
• Ethylene glycol
• Ethanol
• Non-metabolised glycols
• Maltose

Question 6

What is the formula for calculating osmolarity?

Answer 6

Osmolarity = 2x [Na*] + Urea + Glucose + (EtOH x 1.25)

Question 7

What is the normal anion gap formula?

Answer 7

AG = [Na*] - ([Cl-] + [HCO3-])

Question 8

What does ‘CHAMPS’ stand for in the context of causes of hyperventilation?

Answer 8

• CNS disease (stroke, hemorrhage, psychogenic)
• Hypoxia (Pneumonia, PE, asthma, altitude)
• Anxiety
• Pain
• Mechanical or excessive ventilation

Question 9

What does ‘USED CRAP’ refer to in high anion gap metabolic acidosis?

Answer 9

• Ureterostomy
• Salicylates and sepsis
• Lactate, Toxins, Ketones, Renal
• Diabetic ketoacidosis
• Progesterone, pregnancy
• Small bowel fistula

Question 10

What does ‘CAT MUD PILES’ stand for in high anion gap metabolic acidosis?

Answer 10

• Carbon monoxide, cyanide
• Alcoholic ketoacidosis
• Toluene
• Renal tubular acidosis
• Methanol, metformin (phenformin)
• Uraemia
• Pancreatic duodenal fistula
• Extra chloride
• Diarrhea
• Addison’s disease

Question 11

What is the expected change in HCO3- for acute respiratory acidosis?

Answer 11

For every 10mmHg CO2 rises above 40mmHg, expect HCO3- to increase by 1mmol/L

Question 12

What is the expected change in HCO3- for chronic respiratory acidosis?

Answer 12

For every 10mmHg CO2 rises above 40mmHg, expect HCO3- to increase by 4mmol/L

Question 13

What is Winter’s formula used for?

Answer 13

Expected pCO2 in metabolic acidosis

Question 14

What does the delta ratio indicate?

Answer 14

Ratio of change in AG from normal, compared to change in HCO3- from normal

Question 15

What are the causes of metabolic alkalosis represented by ‘CLEVER PD’?

Answer 15

• Contraction (volume contraction)
• Liquorice, laxative abuse
• Endocrine (Conn’s, Cushing’s)
• Vomiting, GI losses
• Excess alkali (antacids)
• Renal (Bartter’s)
• Post-hypercapnia
• Diuretics

Question 16

How does acidaemia affect serum potassium levels?

Answer 16

Raises serum K+ (shifts intravascularly)

Question 17

What is the formula for correcting sodium in hyperglycaemia?

Answer 17

Corrected Na = [Na] + glucose/4

Question 18

What does a low/negative (<3) AGMA indicate?

Answer 18

• Paracetamol
• Pyroglutamic acid
• Paraldehyde
• Propylene glycol

Question 19

For acute respiratory alkalosis, what is the expected change in HCO3-?

Answer 19

For every 10mmHg CO2 gets below 40mmHg, expect HCO3- to reduce by 2mmol/L

Question 20

What happens to potassium levels with changes in pH?

Answer 20

Every 0.1 unit change in pH = 0.5mEq/L change in K+

Question 21

What is the expected compensation for chronic respiratory alkalosis?

Answer 21

Expected pCO2 = (0.7 x HCO3-) + 20 (+5)