Metabolic acidosis is characterized by a decrease in serum pH caused by a decrease in bicarbonate or increase in hydrogen ions. It can be caused by increased acid production, decreased acid excretion, or alkali loss. The kidneys help eliminate daily acid load from metabolism by reclaiming bicarbonate and removing acids. Metabolic acidosis is categorized as normal or elevated anion gap based on the difference between measured cations and anions. Elevated anion gap acidosis involves unmeasured anions like in ketoacidosis, while normal anion gap involves chloride replacing depleted bicarbonate. Common causes of each type are represented by the mnemonics MUDPILES and USEDC

1. Approach to
Metabolic
Acidosis

2. Metabolic acidosis is an acid-base
disorder characterized by a decrease in
serum pH that results from either a
primary decrease in plasma
bicarbonate concentration ([HCO3-]) or
an increase in hydrogen ion
concentration ([H+])

3. A primary metabolic acidosis is a
pathophysiologic state characterized
by an arterial pH of less than 7.35
in the absence of an elevated PaCO2

4. It is created by
one of three
mechanisms:
(1) increased
production of
acids
(2) decreased
excretion of
acids
(3) loss of alkali.

5. The kidneys are responsible for
reclaiming filtered bicarbonate (HCO3-)
and eliminating the daily acid load
generated from nitrogen (protein)
metabolism

6. Anion gap
To achieve electrochemical balance, ionic
elements in the extracellular fluid must
equal a net charge of zero. Therefore, the
number of negatively charged ions (anions)
should equal the number of positively
charged ions (cations).
Anion Gap = (Sodium) – (Chloride +
Bicarbonate)

7. Practically, a metabolic acidosis is
divided into processes that are
associated with a normal anion gap (8-
12 mEq/L) or an elevated anion gap
(>12 mEq/L)

8. A normal anion gap metabolic acidosis
involves no gain of unmeasured anions;
however, because of the need for electrical
neutrality, serum chloride replaces the
depleted bicarbonate, and hyperchloremia
develops.
In contrast, an elevated anion gap
metabolic acidosis is caused when extra
unmeasured anions are added to the blood.

9. Frequent
causes of an
elevated
anion gap
metabolic
acidosis are
represented
by the
mnemonic
MUDPILES:
Methanol
Uremia
Diabetic ketoacidosis
Paraldehyde
Iron, isoniazid (INH)
Lactic acid
Ethanol, ethylene glycol
Salicylates

10. Causes of
Normal
anion gap
metabolic
acidosis can
be
represented
by the
mnemonic
USEDCARP:
Ureterostomy
Small bowel fistula
Extra chloride
Diarrhea
Carbonic anhydrase inhibitors (eg, acetazolamide)
Adrenal insufficiency
Renal tubular acidosis (RTA)
Pancreatic fistula