METABOLIC ACIDOSIS

✓ Medically ReviewedDISEASEID: MDBT0221🚨 TRIAGE: EMERGENCYICD-10: E87.2

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Definition

CLINICAL DEFINITION

A primary acid-base disorder characterised by a decrease in serum bicarbonate concentration and a primary reduction in blood pH, typically accompanied by a compensatory decrease in arterial partial pressure of carbon dioxide (PaCO2). It is diagnosed when arterial pH is less than 7.35 and serum bicarbonate is below the normal reference range.

IN PLAIN ENGLISH

A condition where the body's blood and tissues become too acidic, either because the kidneys cannot remove enough acid, the body produces too much acid, or too much base (bicarbonate) is lost.

COMMON MISCONCEPTION / WHAT IT DOES NOT MEAN

Metabolic Acidosis is NOT respiratory acidosis, which is caused by inadequate ventilation and carbon dioxide retention. It is also NOT a standalone disease but a biochemical manifestation of an underlying systemic, renal, or toxicological disorder.

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Word Breakdown

meta-

=change or transformation (referring to metabolism)

acid

=sour or sharp (referring to the chemical property of acids)

-osis

=abnormal condition or process

Literal Meaning: An abnormal condition of metabolic acidity

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Abbreviations

AGMA=Anion Gap Metabolic Acidosis — high anion gap subtype

NAGMA=Normal Anion Gap Metabolic Acidosis — hyperchloraemic subtype

HCO3-=Bicarbonate — primary buffer depleted in this condition

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Synonyms & Variants

Acidemia, metabolic›Non-respiratory acidosis›

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Category-Specific Context

As an acid-base disorder, metabolic acidosis represents a disruption in systemic physiological homeostasis. It is classified into two main subtypes based on the serum anion gap: high anion gap metabolic acidosis (HAGMA), driven by the accumulation of unmeasured organic acids, and normal anion gap metabolic acidosis (NAGMA), driven by bicarbonate loss or impaired renal acid excretion.

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Clinical Context & Usage

CLINICAL SETTINGS

Intensive Care UnitEmergency DepartmentNephrology WardEndocrinology ClinicGeneral Medicine Ward

USAGE & DOCUMENTATION NOTES

In clinical notes, it is often documented alongside the anion gap and delta ratio. Arterial blood gas (ABG) and venous blood gas (VBG) results are referenced to confirm the pH and bicarbonate levels, while the Winter's formula is frequently cited to assess the adequacy of respiratory compensation.

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Primary Causes & Pathophysiology

PATHOPHYSIOLOGY

Metabolic acidosis occurs when there is an overproduction of organic acids, an inability of the kidneys to excrete daily acid loads, or an excessive loss of bicarbonate. The primary drop in serum bicarbonate reduces the blood's buffering capacity, leading to a fall in pH. Chemoreceptors detect this acidaemia and stimulate the respiratory centre to increase alveolar ventilation, causing a compensatory drop in PaCO2. The expected respiratory compensation can be estimated using Winter's formula. If the acid load exceeds the buffering and respiratory compensation capacity, severe acidaemia ensues, leading to decreased myocardial contractility, altered mental status, and potentially fatal arrhythmias.

PRIMARY CAUSES

Lactic Acidosis

Tissue hypoperfusion or mitochondrial dysfunction leads to anaerobic metabolism and accumulation of L-lactate.

Diabetic Ketoacidosis (DKA)

Insulin deficiency and counter-regulatory hormone excess drive lipolysis and hepatic ketogenesis, accumulating beta-hydroxybutyrate and acetoacetate.

Renal Failure (Uraemia)

Decreased glomerular filtration rate impairs the excretion of sulphates, phosphates, and other organic acids.

Toxins (Methanol, Ethylene Glycol, Salicylates)

Hepatic metabolism of these toxic alcohols or drugs produces strong unmeasured organic acids.

Severe Diarrhoea

Gastrointestinal loss of bicarbonate-rich fluids leads to a net deficit of base, with chloride retained to maintain electroneutrality.

Renal Tubular Acidosis (RTA)

Defects in proximal bicarbonate reabsorption (Type 2) or distal hydrogen ion secretion (Type 1) impair net acid excretion.

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Drug-Induced Causes

💊 Metformin ↗

Mechanism: Inhibits mitochondrial complex I, potentially increasing lactate production, especially in patients with acute kidney injury or tissue hypoxia.

→ Discontinue metformin. Initiate supportive care and consider haemodialysis in severe cases with pH < 7.0 or renal failure.

💊 Carbonic Anhydrase Inhibitors (Acetazolamide) ↗

Mechanism: Inhibit proximal tubular bicarbonate reabsorption, inducing a Type 2 (proximal) renal tubular acidosis.

→ Stop the offending agent. Correct volume depletion and supplement potassium and bicarbonate if symptomatic.

💊 Topiramate and Zonisamide ↗

Mechanism: Weak carbonic anhydrase inhibition can cause mixed distal and proximal RTA, leading to normal anion gap acidosis and nephrolithiasis.

→ Discontinue or reduce dose. Alkalinise urine if stones are present; monitor serum electrolytes.

💊 Salicylates (Aspirin toxicity) ↗

Mechanism: Uncouples oxidative phosphorylation and interferes with the Krebs cycle, leading to accumulation of organic acids (lactic and ketoacids).

→ Aggressive fluid resuscitation, urinary alkalinisation with IV sodium bicarbonate, and haemodialysis for severe toxicity.

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Epidemiology

Prevalence:Metabolic acidosis is highly prevalent in hospitalised patients, particularly in the intensive care unit, where it is observed in up to 40–60% of critically ill patients. Lactic acidosis is the most common cause of high anion gap metabolic acidosis in the ICU setting.

Age Group:All age groups are susceptible. Neonates are prone to inborn errors of metabolism; young adults frequently present with DKA or toxic ingestions; older adults more commonly present with lactic acidosis (sepsis, heart failure) or uraemic acidosis (chronic kidney disease).

Sex Predilection:Equal

Geographic Distribution:Globally distributed. Specific toxic ingestions (e.g., diethylene glycol, methanol) or endemic causes of severe diarrhoea (e.g., cholera) may cause regional spikes in incidence.

Red Flags & Triage ★

TRIAGE LEVEL

🚨 EMERGENCY

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

Toxic alcohol ingestion (methanol, ethylene glycol) — suspect in unexplained high anion gap acidosis with an osmolar gap; requires immediate fomepizole and haemodialysis.
Mesenteric ischaemia — suspect in older patients with severe high anion gap lactic acidosis and out-of-proportion abdominal pain; requires emergent surgical evaluation.
Sepsis and tissue hypoperfusion — unrecognised shock state driving lactic acidosis; requires immediate fluid resuscitation and vasopressors.

URGENT EVALUATION TRIGGERS

Arterial pH < 7.10 indicating severe acidaemia with risk of haemodynamic instability.
High anion gap metabolic acidosis of unknown origin, requiring immediate toxicological and metabolic workup.
Concurrent hyperkalaemia (K+ > 6.0 mmol/L) or acute kidney injury with oliguria.

Contraindications & Precautions

CONTRAINDICATIONS

Routine use of intravenous sodium bicarbonate is contraindicated in lactic acidosis without severe acidaemia (pH < 7.1) or specific indications
as it may paradoxically worsen intracellular acidosis and cause volume overload.

SPECIAL PRECAUTIONS

Rapid correction of chronic metabolic acidosis can lead to hypokalaemia and hypocalcaemia; electrolytes must be monitored and repleted cautiously.
When administering IV sodium bicarbonate
monitor for hypernatraemia
volume overload
and overshoot metabolic alkalosis.

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Cardiovascular collapse — severe acidaemia (pH < 7.1) decreases myocardial contractility
induces vasodilation
and blunts the response to catecholamines.
Hyperkalaemia — extracellular shift of potassium in exchange for hydrogen ions
increasing the risk of life-threatening cardiac arrhythmias.
Altered mental status and coma — profound central nervous system depression secondary to intracellular acidaemia.
Bone demineralisation — chronic metabolic acidosis (e.g.
in CKD or RTA) buffers acid using bone carbonate
leading to osteomalacia and growth retardation in children.

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Reference Ranges

NORMAL RANGEArterial pH: 7.35–7.45. Serum bicarbonate (HCO3-): 22–26 mmol/L. Anion Gap: 8–12 mmol/L (without potassium).

CRITICAL (PANIC) VALUEArterial pH < 7.10 or Serum bicarbonate < 8 mmol/L — high risk of cardiovascular collapse, requiring urgent intensive care evaluation and potential bicarbonate therapy or renal replacement therapy.

UNIT OF MEASUREMENTpH (dimensionless), mmol/L (for HCO3- and Anion Gap), mmHg (for PaCO2)

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Confused With (LASA & DDx) ★

LEXICAL (LASA) — SIMILAR NAMES

Respiratory Acidosis

Both present with acidaemia (low pH), but respiratory acidosis is driven by primary CO2 retention (high PaCO2) rather than bicarbonate depletion.

Metabolic Alkalosis

Shares the 'metabolic' prefix but represents the opposite acid-base disturbance, characterised by elevated serum bicarbonate and high pH.

CLINICAL DIFFERENTIAL DIAGNOSES (DDx)

Respiratory Alkalosis with compensatory metabolic acidosis

Key Distinguishing Feature: Primary disturbance is low PaCO2 with elevated pH; the low bicarbonate is a compensatory renal response, not the primary disorder.

Mixed Acid-Base Disorder

Key Distinguishing Feature: Winter's formula reveals that the measured PaCO2 does not match the expected compensatory PaCO2, indicating a concurrent primary respiratory disorder.

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Clinical Pearls

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Always apply Winter's formula (Expected PaCO2 = 1.5 × HCO3- + 8 ± 2) to determine if respiratory compensation is adequate. A measured PaCO2 higher than expected indicates a concurrent primary respiratory acidosis, while a lower PaCO2 indicates a concurrent primary respiratory alkalosis.

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The delta-delta ratio (Delta Anion Gap / Delta Bicarbonate) helps identify mixed acid-base disorders in patients with a high anion gap. A ratio < 1 suggests a concurrent normal anion gap acidosis, while a ratio > 2 suggests a concurrent metabolic alkalosis.

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In lactic acidosis, the clearance of lactate is primarily hepatic. Severe liver dysfunction can impair lactate clearance, exacerbating the acidosis even if production has ceased.

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Urine anion gap (Urine Na+ + Urine K+ - Urine Cl-) is a useful surrogate for urine ammonium excretion in normal anion gap acidosis. A negative urine anion gap indicates appropriate renal ammonium excretion (e.g., diarrhoea), while a positive gap suggests impaired renal acid excretion (e.g., RTA).

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Example Usage & SOAP

CLINICAL SENTENCE

"The patient presented with a severe high anion gap metabolic acidosis secondary to diabetic ketoacidosis, with a pH of 7.12, bicarbonate of 6 mmol/L, and appropriate respiratory compensation."

IN A SOAP NOTE

S: 45yo M with T1DM presents with 2 days of nausea, vomiting, and abdominal pain. Missed insulin for 3 days. O: Vitals: HR 115, BP 100/60, RR 28 (Kussmaul respirations). Labs: pH 7.15, PaCO2 18 mmHg, HCO3- 6 mmol/L, Na+ 135, Cl- 100, Glucose 450 mg/dL. Anion gap 29. Ketones positive. A: Severe high anion gap metabolic acidosis secondary to DKA. Appropriate respiratory compensation per Winter's formula. P: Admit to ICU. Start aggressive IV normal saline. Initiate IV insulin infusion at 0.1 U/kg/hr. Monitor K+ closely and replete as needed. Hourly neuro and glucose checks.

DIFFERENTIAL DIAGNOSIS CONTEXT

Differential for High Anion Gap Metabolic Acidosis: (1) Ketoacidosis (diabetic, alcoholic, starvation) — check glucose, beta-hydroxybutyrate, and history; (2) Lactic acidosis (Type A from hypoperfusion, Type B from malignancy/toxins) — check serum lactate and assess perfusion; (3) Renal failure — check BUN/Creatinine; (4) Toxins (methanol, ethylene glycol, salicylates) — check osmolar gap, toxicology screen, and history.

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Prognosis

SHORT-TERM OUTCOME

Highly dependent on the underlying aetiology and the severity of the acidaemia. Reversible causes like DKA or mild lactic acidosis have a good short-term prognosis with prompt resuscitation, whereas severe lactic acidosis from cardiogenic shock carries a mortality rate exceeding 50%.

LONG-TERM PROGNOSIS

Chronic metabolic acidosis (e.g., in advanced CKD) accelerates muscle wasting, bone disease, and progression of renal failure. Correcting chronic acidosis with oral bicarbonate can slow CKD progression and improve nutritional status.

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References & Coding

ICD-10 DIAGNOSTIC CODE

E87.2

LITERATURE CITATIONS & GUIDELINES

[1]Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445.

[2]Hamm LL, Nakhoul N, Hering-Smith KS. Acid-Base Homeostasis. Clin J Am Soc Nephrol. 2015;10(12):2232-2242.

[3]Kraut JA, Madias NE. Metabolic acidosis: pathophysiology, diagnosis and management. Nat Rev Nephrol. 2010;6(5):274-285.

MEDICAL DISCLAIMER — This content is intended for educational purposes and qualified healthcare professionals only. Always refer to current clinical diagnostic criteria, institutional protocols, and official guidelines. Medobo is not responsible for clinical decisions made based on this content.