Tuesday, August 2, 2016

Approach to acid base disorders: Metabolic acidosis notes


I made these notes while studying acid base disturbances. Now they might not make sense to someone who has never studied this topic before.. But for those who have read about it, this should be excellent for revision.

Calculation of anion gap:
ALWAYS calculate the anion gap first.
Anion gap = [Na+]  − ([Cl-] + [HCO3−])
Normal anion gap = 8 - 16 mEq / L

Causes of HAGMA:
If anion gap is elevated, recall all causes of high anion gap metabolic acidosis (HAGMA).
Mnemonic: MUDPILES
M - Methanol
U - Uremia
P - Paraldehyde / phenformin
I - Iron / INH
L - Lactic acidosis
E - Ethylene glycol
S - Salicylates

Causes of NAGMA:
If anion gap is not elevated, recall all causes of non anion gap metabolic acidosis (NAGMA).
NAGMA happens either when bicarbonate is being excreted or when chloride is being produced.
Mnemonic: NAG the CRAB
Chloride excess
Acetazolamide / Addisons disease
Bicarbonate loss due to GI causes like diarrhea

HAGMA can present as NAGMA if:
Albumin is low, very commonly seen in patients with liver cirrhosis (Decreased unmeasured anions)
Multiple myeloma (Increased unmeasured cations)
Hypercalcemia, high magnesium levels, lithium use.
Lactic acidosis (Movement of lactate intracellularly in exchange of chloride)

Recall that the major unmeasured cations are calcium, magnesium, gamma globulins and potassium. The major unmeasured anions are negatively charged plasma proteins (albumin), sulphate, phosphates, lactate and other organic anions.

Calculating adjusted anion gap for low serum albumin:
The approximate correction is a reduction in the normal anion gap of 2.5 meq/l for every 1 g/dl decline in the plasma albumin concentration (Normal serum albumin value = 4 g/dl).

Osmolar gap:
Osmolar Gap can also be used in differentiating the causes of elevated anion gap metabolic acidosis.
Plasma osmolarity = 2 (Na) + Glucose/18 + BUN/2.8 (I round it off to 2 Na + Glu / 20 + BUN / 3)
Osmolar Gap = Measured Posm – Calculated Posm
The normal osmolar gap is  10-15 mmol/L H20 .
In a patient suspected of poisoning, a high osmolar gap (particularly if ≥ 25) with an otherwise unexplained high anion gap metabolic acidosis is suggestive of either methanol or ethylene glycol intoxication.

Delta ratio:
The delta ratio is sometimes used in the assessment of elevated anion gap metabolic acidosis to determine if a mixed acid base disorder is present.
Delta ratio
= Measured anion gap – Normal anion gap / Normal [HCO3-] – Measured [HCO3-]
= AG - 12 / 24 - Bicarb
If delta < 1 : HAGMA + NAGMA
If delta 1-2 : Pure HAGMA
If delta > 2 : HAGMA + Metabolic alkalosis (DKA + vomiting, MUDPILES + COPD, high HCO3)

Calculation of urine anion gap: 
A urine anion gap helps to distinguish these two causes of normal anion gap acidosis (NAGMA):
Loss of HCO3- from Gastrointestinal tract (diarrhea)
Loss of HCO3- from the Kidneys (RTAs)
Urine anion gap = [Na+] + [K+] - [Cl-]
In a patient with a hyperchloremic metabolic acidosis (NAGMA): A negative UAG suggests GI loss of bicarbonate (eg diarrhea), a positive UAG suggests impaired renal acidification (ie renal tubular acidosis).
Mnemonic: neGUTive - Negative UAG in bowel causes.

Calculation of compensation:
Winter's Formula PCO2 = (1.5 × [HCO3-]) + 8 ± 2

That's all!

PS: Many of you ask me where to study acid base and electrolytes disorders from, I think this Acid base online tutorial by Timur Graham and Steven Agnus, University of Connecticut is the best free resource for everyone.

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