Diagnosis

Metabolic acidosis

Etiology

Cause [csa]

Anion gap = (Na + K) - (Cl + HCO3). Normal AG = 12-16

1. Anion gap acidosis --> increased production or decreased excretion of acids:

   CAT MUDPILES [csa]

   • Cyanide and carbon monoxide poisoning

   • Arsenic

   • Toluene

   • Methanol, Metformin, Metabolism inborn errors

   • Uremia/renal failure

   • Diabetic ketoacidosis (other ketoacidosis: alcoholism, starvation)

   • Paraldehyde, phenformin

   • Iron/Isoniazid

   • Lactic acidosis >4 mmol/L:

     • Type A: impaired tissue oxygenation: hypoxia, carbon monoxide poisoning, anemia, cardiogenic/hemorrhagic/septic shock

     • Type B: liver failure, malignancy, and drugs or toxins (alcohols, salicylates, isoniazid, metformin, propofol, linezolid, albuterol, intravenous epinephrine, nucleoside reverse transcriptase inhibitors for HIV-treatment, iron, paraldehyde, sulfur, toluene, ammonium chloride, hyperalimentation fluids/TPN)

   • Ethylene glycol

   • Salicylates/Strychnine

2. Non-anion gap (hyperchloraemic) acidosis --> increased loss of alkali (HCO3) from:

   the GI tract or kidneys or reduced elimination of acids due to renal failure

   Urine anion gap = Urine Na + Urine K – Urine Cl, if urine pH is <6.5 [iou]

   Urine anion gap = Urine Na + Urine K - (Urine Cl + Urine HCO3), if urine pH is >6.5

   USEDCARP [csa]

   • Ureteral diversion (ureterosigmoidostomy)

   • Fistula (pancreatic, biliary, intestinal)

   • Extra chloride, saline infusion, hyperalimentation (TPN)

   • Diarrhea

   • Carbonic anhydrase inhibitors (acetazolamide), Cholestyramine

   • Adrenal insufficiency (Hypoaldosteronism, Addison disease)

   • Renal tubular acidosis (reduced acid secretion)

   • Spironolactone

Pathophysiology [iou]

1. Anion gap: the number of cations must equal the number of anions

2. The anion gap measures the amount of unmeasured anions (normally albumin)

3. High anion gap --> excess acids:

   • Lactate

   • Ketones

   • Toxic metabolites

4. Compensation:

   • Lungs --> respiratory alkalosis: Tachypnea and hyperpnea reduces the PaCO2 in an attempt to increase the pH back toward normal

   • Kidneys: Responsible for reabdorption of filtered bicarbonate (HCO3-) and elimination of acid generated from nitrogen (protein) metabolism. Metabolic compensation with increased acid elimination.

   • Hypoalbuminemia: The albumin level must also be taken into account because albumin is a negatively charged protein:

     • [anion gap](corrected) = [anion gap](measured) + 2.5 × (4.0 - [albumin])

Complications of severe metabolic acidosis:

• Myocardial depression

• Seizure

• Shock

• Multiorgan failure

Epidemiology

Incidence per 100.000 [t9s][afd][47s][tvs][4zg][a2s][nma][zds][gqm]

Approach

• Blood test: glucose, anion gap, ketone levels, lactate, salicylate, iron, ethanol, methanol, ethylene glycol, isopropyl, hCG

• Blood gas: pH, HCO3, pO2, pCO2, CO

• Urine stix

• Urine microscopy

• Urinalysis: pH > 5.5 + acidemia --> type I renal tubular acidosis

• ECG

1. What is the pH? Acidemia or alkalemia? [csa]

2. Is the primary disturbance metabolic or respiratory?

   • Acute respiratory acidosis: elevated pCO2 >40-45 indicating decreased ventilation

   • Chronic respiratory acidosis: elevated pCO2 and elevated bicarbonate indicating renal compensation with increased hydrogen elimination and bicarbonate retention

   • Metabolic acidosis: decreased pCO2 <40 due to compensatory respiratory alkalosis

3. Is there an AG or non-AG metabolic acidosis?

4. Is there an appropriate respiratory compensation?

   • Winter formula: Expected pCO2 = 1.5 x (HCO3) + 8 ± 2

5. Are there additional metabolic disturbances?

   • Delta/delta concept: For every increase in the AG of 1 mmol/L above normal (12 mmol), serum HCO3- will drop by an equal amount (ΔAG / ΔHCO3-)

   • Ratio <1: additional metabolic acidosis is present

   • Ratio >1: additional metabolic alkalosis is present

Treatment

1. Inadequate respiratory compensation --> Address respiratory failure:

   • Airways

   • Breathing

2. General therapy: [a2s]

   • Sodium bicarbonate IV (NaHCO3) [etv]

   • Tris(hydroxymethyl)aminomethane (THAM)

   • Carbicarb IV

   • Dialysis

3. Treat the underlying conditions: [iou]

   • Ketoacidosis: insulin and fluids

   • Alcohol toxicity: fomepizole and/or dialysis

   • Adrenal insufficiency: mineralocorticoid

   • Lactic acidosis: Thiamine, IV fluid and respiratory support

   • Metformin-induced lactic acidosis: hemodialysis

   • Sepsis: antibiotics

   • Salicylate toxicity: GI decontamination with charcoal, IV bicarbonate, hemodialysis

Differential diagnoses

Acidosis, Acute renal failure, Adrenal insufficiency, Alcoholic ketoacidosis, Alcohol intoxication, Anemia, Colovesical fistula, Diabetes mellitus, Diabetic ketoacidosis, Drug side effects, Hemolysis, Hyperkalemia, Hyperparathyroidism, Iron toxicity, Lactic acidosis, Methanol toxicity, Necrotizing enterocolitis, Renal failure, Respiratory alkalosis, Salicylate intoxication, Sepsis