Metabolic acidosis

Learning objectives

Metabolic acidosis is a disturbance in the homeostasis of plasma acidity
Any process that increases the serum hydrogen ion concentration is a distinct acidosis
A patient can have multiple acidoses contributing to the decrease of serum pH
Adicosis can be either respiratory (changes in CO2) or metabolic (changes in bicarbonate)
Metabolic acidosis is characterized by an increase in serum hydrogen ion concentration resulting in serum bicarbonate (HCO3) <24 mEq/L
May be associated with organ failure, especially respiratory and cardiovascular
Can be acute or chronic

Classification of metabolic acidosis is based on the presence or absence of an anion gap (concentration of unmeasured serum anions)
Sodium, the main plasma cation, is balanced by the sum of the anions bicarbonate and chloride in addition to the unmeasured anions (e.g., lactate, acetoacetate), which represent the anion gap
Anion gap metabolic acidosis is often caused by anaerobic metabolism and lactic acid accumulation
Non-gap metabolic acidosis is primarily caused by the loss of bicarbonate (e.g., diarrhea, renal tubular acidosis)
Causes:

Anion gap metabolic acidosis	Non-gap metabolic acidosis associated with normal or high serum K+	Non-gap metabolic acidosis associated with low serum K+
Acute kidney injury	Administration of HCl or precursors	Diarrhea
Chronic kidney disease	Administration of cationic amino acids	intestinal, pancreatic, or biliary fistula
Diabetic ketoacidosis	Chronic kidney disease	Proximal renal tubular acidosis
Alcoholic ketoacidosis	Adrenal insufficiency (primary or secondary)	Distal renal tubular acidosis
Lactic acidosis	Hyporeninemic hypoaldosteronism	Ureterosigmoidostomy
Salicylate intoxication	Hyperkalemic distal renal tubular acidosis	Ureteroileostomy
Toxic alcohol intoxication (methanol, ethylene glycol, diethylene glycol, propylene glycol)	Pseudoaldosteronism type I	Diabetic ketoacidosis
Pyroglutamic acidosis	Pseudoaldosteronism type II (Gordon’s syndrome)	Toluene intoxication
Fasting ketoacidosis	Drugs (spironolactone, prostaglandin inhibitors, triamterene, amiloride, trimethoprim, pentamidine, ciclosporin)	Lactic acidosis
Toluene intoxication

Acute metabolic acidosis	Chronic metabolic acidosis
Impaired leukocyte function	Generation or exacerbation of bone disease
Predisposition to ventricular arrhythmias	Growth retardation (in children)
Arterial vasodilation and hypotension	Impaired glucose tolerance
Resistance to action of infused catecholamines	Acceleration of progression of kidney disease
Resistance to action of insulin	Increased muscle wasting
Suppression of lymphocyte function	Reduced albumin synthesis
Impaired cellular energy production	Enhanced production of β2-microglobulin
Stimulation of apoptosis
Changes in mental status
Stimulation of interleukin production
Alteration in oxygen binding to hemoglobin
Venoconstriction
Decreased cardiac contractility and cardiac output

History: Identify potential causes (vomiting, diarrhea, medications, possible overdoses, chronic conditions such as diabetes mellitus)
Physical examination: dry mucus membranes in diabetic ketoacidosis, compensatory hyperventilation
Lab tests;
- Blood pH <7.35
- pCO2:
  - >40-45: respiratory acidosis
  - <40: metabolic acidosis
- Anion gap
  - Anion gap = Na+ – (Cl- + HCO3-)
  - Normal anion gap = 12
  - Anion gap >12: Anion gap metabolic acidosis
- Respiratory compensation
  - Winter’s formula: Expected CO2 = (HCO3- x 1.5) + 8 +/- 2
  - If pCO2 is within the predicted range, there is no additional respiratory disturbance
  - If pCO2 is greater than expected, there is an additional respiratory acidosis
  - If pCO2 is less than expected, there is an additional respiratory alkalosis
- Additional metabolic disturbances
  - If anion gap is present, determine delta gap
  - Delta gap = Delta anion gap – Delta HCO3- = (anion gap – 12) – (24 – HCO3-)
  - Delta gap < -6: Non anion gap metabolic acidosis
  - Delta gap >6: underlying metabolic alkalosis
  - Delta gap between -6 and 6: only anion gap metabolic acidosis

Address the cause of acidosis
Fluid resuscitation and electrolyte imbalance correction for sepsis and diabetic ketoacidosis
Antidotes for poisoning, dialysis, antibiotics, bicarbonate administration