ICM - Acid-Base Disorders

Question 1

What are the causes of normal anion gap metabolic acidosis?

Answer 1

H - Hyperchloraemia
A - Addisons/acetazolamide
R - Renal tubular acidosis (1, 2 and 4)
D - Diarrhoea/ DKA recovery
U - Ureteroileostomy
P - Pancreatoenterostomies

Question 2

What are the causes of high anion gap metabolic acidosis?

Answer 2

M - methanol
U - Uraemia
D - DKA
P - Paracetamol
I - Isoniazid, inborn errors of metabolism
L - Lactic acidosis, Lithium
E - ethylene glycol, ethanol
S - Salicylates

Question 3

How do you calculate anion gap?

Answer 3

(Na + K) - (Cl - HCO3)

Question 4

How do you calculate albumin gap?

Answer 4

Albumin Gap = 40 - serum albumin
Corrected AG = AG + 0.25(albumin gap)

Question 5

What is strong ion difference?

Answer 5

Physicochemical approach whereby water dissociation of hydrogen ions maintains electroneutrality.

Simplified is:SID = (Na + K) - (Cl + lactate) SID = 40-44

Question 6

What is a normal anion gap?

Answer 6

12 -16

Question 7

How does albumin affect the anion Gap?

Answer 7

Albumin is the major unmeasured anion and contributes almost the whole of the value of the AG.
Low albumin will reduce the ‘normal’ gap. This should be commented on as
it is relevant here. (Hypoproteinaemia is common in critical illness, albumin has a lot of negative charge.) A high AG acidosis in a patient with hypoalbuminaemia may appear as a normal AG acidosis if the low albumin is not corrected for. This albumin gap needs to be calculated as follows:

The albumin gap= 40 - apparent albumin

The AG corrected value = AG + (albumin gap/4)

It is generally accepted that the AG should be corrected upwards by 2.5 for every 10 g/L fall
in the serum albumin. [1]
For this case, the albumin gap is 40 – 16 = 24, making corrected AG 15.8 + (24/4) = 21.8.
This has changed an apparently normal AG acidosis into an increased AG acidosis.

Question 8

Causes of a low anion Gap

Answer 8

Rare

Hypermagnesaemia, lithium toxicity, excess protein, myeloma

Waldenstrom’s macroglobulinaemia (immunoglobulins are strong cations)

Question 9

Why does a high output stoma result in a NAGMA

Answer 9

The ileal conduit secretes bicarbonate into the lumen of the bowel in exchange for chloride. This results in bicarbonate loss from the body and excess chloride reabsorbtion.

Question 10

What are the problems associated with Bicarbonate administration

Answer 10

Sodium bicarbonate necessitates a
large sodium load and is said to cause a paradoxical intracellular acidosis through increased CO2 generation.