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Metabolic acidosis

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Analysis of Metabolic acidosis Blood gas

pH < 7.35 unless corrected
HCO₃ < 22 mmol/L
PaCO₂ is low due to hyperventilation to blow off CO₂ to less than 5.33 KPa
PaO₂ usually normal
Base Excess < -2

Aetiology

Accumulation of Organic and Inorganic Acids or Loss of Bicarbonate HCO₃
Anion gap = ([Na⁺ + K⁺] - [HCO₃⁻ + Cl⁻]) more than 12 mmol/L.

High and Normal AG causes

High Anion Gap Metabolic Acidosis	Normal Anion Gap Metabolic Acidosis
Lactic Acidosis: Caused by tissue hypoxia (e.g., sepsis, shock, hypoxia) or certain drugs (e.g., metformin). Ketoacidosis: Seen in diabetic ketoacidosis (DKA), alcoholic ketoacidosis, or starvation. Renal Failure: Impaired excretion of acids due to chronic or acute kidney disease. Toxins: Ingestion of substances like methanol, ethylene glycol, or salicylates. Rhabdomyolysis: Muscle breakdown leads to the release of myoglobin, causing kidney damage and acidosis. Propylene Glycol: A component in some IV medications (e.g., lorazepam) can cause acidosis in large amounts.	Diarrhoea: Bicarbonate loss from the gastrointestinal tract causes acidosis. Renal Tubular Acidosis (RTA): Defects in the kidney's ability to excrete acid or reabsorb bicarbonate. Hyperchloremia: Often seen with saline infusion, leading to chloride-driven acidosis. Carbonic Anhydrase Inhibitors: Medications like acetazolamide cause bicarbonate wasting in the kidneys. Ureterosigmoidostomy: Surgery connecting the ureters to the colon leads to bicarbonate loss. Early Renal Failure: Impaired acid excretion in early kidney dysfunction without a high anion gap.

High Anion Gap Metabolic Acidosis

Normal Anion Gap Metabolic Acidosis

Lactic Acidosis: Caused by tissue hypoxia (e.g., sepsis, shock, hypoxia) or certain drugs (e.g., metformin).
Ketoacidosis: Seen in diabetic ketoacidosis (DKA), alcoholic ketoacidosis, or starvation.
Renal Failure: Impaired excretion of acids due to chronic or acute kidney disease.
Toxins: Ingestion of substances like methanol, ethylene glycol, or salicylates.
Rhabdomyolysis: Muscle breakdown leads to the release of myoglobin, causing kidney damage and acidosis.
Propylene Glycol: A component in some IV medications (e.g., lorazepam) can cause acidosis in large amounts.

Diarrhoea: Bicarbonate loss from the gastrointestinal tract causes acidosis.
Renal Tubular Acidosis (RTA): Defects in the kidney's ability to excrete acid or reabsorb bicarbonate.
Hyperchloremia: Often seen with saline infusion, leading to chloride-driven acidosis.
Carbonic Anhydrase Inhibitors: Medications like acetazolamide cause bicarbonate wasting in the kidneys.
Ureterosigmoidostomy: Surgery connecting the ureters to the colon leads to bicarbonate loss.
Early Renal Failure: Impaired acid excretion in early kidney dysfunction without a high anion gap.

Clinical

Hyperventilation with Kussmaul's breathing
The cause is often evident but the AG helps eliminate some causes

Anion	Clinical case
L-Lactate	Anaerobic metabolism
Beta-hydroxybutyrate	Diabetic ketoacidosis
Hippurate	Toluene poisoning (Glue sniffers)
Glycolate and oxalate	Methanol poisoning
D-Lactate	Gut bacterial fermentation e.g. blind loop syndrome

Management

Cause	Clinical Features	Diagnostic Tests	Management
Lactic Acidosis	- Signs of shock or hypoxia (e.g., sepsis, hypotension, cyanosis). - Tachypnoea, confusion, fatigue.	- Arterial blood gas (ABG): Low pH, low bicarbonate, increased lactate. - Serum lactate: Elevated.	- Treat underlying cause (e.g., sepsis, hypoxia). - IV fluids for shock. - Oxygen supplementation. - Inotropes if needed (e.g., norepinephrine).
Ketoacidosis (Diabetic, Alcoholic, Starvation)	- Polyuria, polydipsia, nausea, vomiting. - Kussmaul breathing (deep, labored), fruity breath odor (in DKA). - Confusion, dehydration.	- ABG: Low pH, low bicarbonate, low CO₂. - Serum ketones: Elevated (e.g., beta-hydroxybutyrate). - Blood glucose: Elevated in diabetic ketoacidosis (DKA).	- IV fluids (isotonic saline initially). - IV insulin for DKA. - Correct electrolytes (potassium) as needed. - Monitor blood glucose and ketones.
Renal Failure (Acute or Chronic)	- Oedema, oliguria, fatigue, confusion. - Uremic symptoms (nausea, itching, metallic taste).	- ABG: Metabolic acidosis with normal or elevated anion gap. - Serum creatinine and blood urea nitrogen (BUN): Elevated. - Electrolytes: Hyperkalemia, hypocalcemia.	- Dialysis in severe cases or for chronic renal failure. - Correct underlying cause (e.g., dehydration, infection). - Treat hyperkalemia (calcium, insulin-glucose).
Toxin Ingestion (Methanol, Ethylene Glycol, Salicylates)	- Symptoms depend on the toxin (e.g., visual disturbances in methanol, kidney failure in ethylene glycol, tinnitus in salicylate poisoning). - Confusion, tachypnoea, vomiting.	- ABG: Low pH, low bicarbonate, high anion gap. - Serum specific toxins: Methanol, ethylene glycol, or salicylate levels. - Osmolar gap: May be elevated in methanol or ethylene glycol poisoning.	- Specific antidotes (e.g., fomepizole for methanol or ethylene glycol). - Sodium bicarbonate for salicylate toxicity. - Hemodialysis in severe cases.
Diarrhoea	- Frequent loose stools, dehydration. - Weakness, lethargy, abdominal cramps.	- ABG: Normal anion gap metabolic acidosis. - Serum electrolytes: Hypokalemia, hyponatremia.	- Oral rehydration solutions or IV fluids. - Replace electrolytes (e.g., potassium, sodium). - Treat underlying cause (e.g., infection, malabsorption).
Renal Tubular Acidosis (RTA)	- Failure to thrive (in children), dehydration. - Polyuria, bone pain, muscle weakness.	- ABG: Normal anion gap metabolic acidosis. - Urine pH: Alkaline in type 1 RTA (distal), low in type 2 RTA (proximal). - Serum electrolytes: Hypokalemia in type 1 and 2 RTA, hyperkalemia in type 4 RTA.	- Alkali therapy (e.g., sodium bicarbonate, potassium citrate). - Correct underlying electrolyte imbalances. - Treat any associated conditions (e.g., autoimmune disease).
Hyperchloremic Acidosis (Saline Infusion)	- Often asymptomatic but may cause weakness, confusion. - Symptoms of volume overload (e.g., oedema) if excessive saline is given.	- ABG: Normal anion gap metabolic acidosis. - Serum electrolytes: Elevated chloride, low bicarbonate.	- Discontinue or reduce saline infusion. - Use balanced fluids (e.g., lactated Ringer's solution). - Correct electrolyte imbalances.

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