Calculations

Question 1

Expected CO2 for metabolic acidosis

Answer 1

(1.5 x HCO3) + 8

Question 2

Expected CO2 for primary metabolic alkalosis

Answer 2

(0.7 x HCO3) + 20

Question 3

Expected HCO3 for respiratory acidosis

Answer 3

Increases by 1 per 10mmHg CO2 above 40 acute

Increases by 4 if chronic

Question 4

Expected HCO3 for primary respiratory alkalosis

Answer 4

Decreases by 2 per 10mmHg CO2 for acute

Decreases by 5 for chronic

Question 5

Sodium correction for high glucose

Answer 5

Sodium + (glucose - 5)/3

Question 6

Corrected potassium for pH

Answer 6

Increase K by 0.5 (over 5) for every drop in pH by 0.1

Question 7

Osmolar gap (measured - calculated)

Answer 7

Normal osmolality 270-290
Gap (2 x Na) + urea + glucose + alcohol
Normal gap -4 to +10

Question 8

Causes of raised osmolar gap

Answer 8

Alcohol
Toxic alcohols
Ketones
Sugars, mannitol
Lactate
Proteins, lipids
Mag, phos, calcium

Question 9

A- a gradient

Answer 9

(760 - vapour pressure) - FiO2 - (1.25 x PaCO2)

Cheats method
FiO2 21% then first half is 150
FiO2 40% then first half 300

Question 10

A-a normality is based on age

Answer 10

Should be less than (age/4) + 4

Question 11

Causes of elevated A-a gradient

Answer 11

V/Q mismatch
PE
APO
ARDS
LRTI

Question 12

Causes of RAGMA 1

Answer 12

CATMUDPILES
carbon monoxide, cyanide
Alcoholic ketoacidosis, starvation ketoacidosis,
Theophylline, tuluene
Methanol, metformin
Uraemia
Diabetic ketoacidosis
Paracetamol, paraldehyde, Paraquat
Iron, isoniazid, inborn errors of metabolism
Lactic acidosis
Ethanol, ethylene glycol
Salicylate

Question 13

Causes of RAGMA2

Answer 13

1. Lactic acid
    A - tissue hypoperfusion
    B1 - sepsis, liver failure, paracetamol
    B2 - drugs
    B3 - inborn errors of metabolism
2. Ketoacids 
3. Salicylate
4. Uraemia renal failure

Question 14

Causes of NAGMA

Answer 14

1. Excess HCO3 loss - diarrhoea, fistulas, renal tubular acidosis
2. Excess NaCl
3. Endocrinopathies - adrenal crisis, addisons
4. Drugs - spironolactone, acetazolamide

Question 15

Causes of metabolic alkalosis

Answer 15

1. HCl loss - vomiting
2. Drugs - furosemide, laxatives, milk alkali
3. Endocrinopathies - Cushing, conns, bartters

Question 16

Causes of respiratory acidosis

Answer 16

1. Airway - asthma, airway obstruction
2. COPD
3. Lung insult
   
   • trauma - PTX, HTX, pulmonary contusion
     Lung membrane disease - LRTI, ARDS, APO, aspiration
4. Depression of respiratory drive
   
   • head/CNS injury
   • drugs
5. Inadequate response effort - GBS, myopathies, drugs

Question 17

Causes of respiratory alkalosis

Answer 17

Central stimulation of respiratory drive - anxiety, pain, head injury, salicylate, sympathomimetics
Profound hypoxia causing peripheral respiratory stimulation