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Metabolic acidosis pathophysiology causes type etiology management with special focus to Anaesthesia

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Metabolic Acidosis: Pathophysiology, Causes, Etiology, Management - with Anaesthesia Focus

1. Definition and Basic Concepts

Metabolic acidosis is a primary decrease in serum bicarbonate (HCO3-) concentration that, per the Henderson-Hasselbalch equation, leads to a decrease in blood pH. It results from:
  • Gain of fixed H+ (overproduction, ingestion, or decreased excretion)
  • Loss of HCO3- (gastrointestinal or renal)
Key values:
  • pH < 7.35 = acidemia
  • Primary disturbance: ↓ HCO3-
  • Respiratory compensation: hyperventilation (↓ PaCO2)
  • Compensation formula: Expected PaCO2 = 1.5 × [HCO3-] + 8 ± 2 (Winter's formula)
  • Costanzo Physiology 7th Ed., p.330; Harrison's Principles of Internal Medicine 22E (2025), p.413

2. Pathophysiology

Buffering Systems

The body uses three buffering systems:
SystemSpeedLocation
Chemical (HCO3-, proteins, phosphate)Seconds-minutesExtracellular & intracellular
Respiratory (↑ ventilation → ↓ PaCO2)Minutes-hoursLungs
Renal (↑ H+ secretion, ↑ HCO3- reabsorption)Hours-daysKidneys
The HCO3-/CO2 buffer is central: H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
When fixed acid accumulates, H+ is buffered by HCO3-, consuming it. The medulla senses rising H+ and drives hyperventilation (Kussmaul breathing) to blow off CO2. Simultaneously, the kidney increases H+ secretion and HCO3- reabsorption over hours to days.

Stewart (Strong Ion) Model - Perioperative Perspective

Miller's Anesthesia highlights that from the physicochemical standpoint, only three independent variables determine acid-base status:
  1. PaCO2
  2. Strong ion difference (SID) = [Na+ + K+ + Ca2+ + Mg2+] - [Cl- + lactate-]
  3. Total weak acid concentration (ATOT - mainly albumin and phosphate)
Metabolic acidosis results from:
  • Decreased SID - due to accumulation of metabolic anions (lactate, ketoacids), hyperchloremia, or free water excess
  • Increased ATOT - hyperphosphatemia
This is particularly relevant intraoperatively, where large-volume crystalloid infusion (especially 0.9% NaCl) reduces SID by raising Cl-, causing dilutional/hyperchloremic acidosis.
  • Miller's Anesthesia, 10e, Chapter 44 - Perioperative Acid-Base Balance

3. Systemic Effects of Metabolic Acidosis

SystemEffect
CardiovascularDepressed myocardial contractility, peripheral vasodilation, central venoconstriction, predisposition to pulmonary edema; catecholamine release may partially compensate
RespiratoryHyperventilation (Kussmaul breathing); increased work of breathing
CNSHeadache, lethargy, stupor, coma
MetabolicGlucose intolerance, muscle catabolism, bone demineralization (chronic)
ElectrolytesHyperkalemia (H+ shifts intracellularly, K+ shifts out)
  • Harrison's Principles of Internal Medicine 22E, p.413-414

4. Classification and Causes (Etiology)

The Anion Gap (AG) = Na+ - (Cl- + HCO3-) is the cornerstone of classification.
  • Normal AG: 6-12 mEq/L (average ~10); correct for hypoalbuminemia: add 2.5 mEq/L per 1 g/dL decrease in albumin below 4.5 g/dL

A. High Anion Gap Metabolic Acidosis (HAGMA)

Mnemonic: MUDPILES / GOLDMARK
CategoryCauses
L - Lactic acidosisSeptic shock, cardiac failure, mesenteric ischemia, liver failure, metformin toxicity, cyanide, CO poisoning
K - KetoacidosisDiabetic (DKA), alcoholic, starvation
U - UremiaChronic kidney disease (stages 4-5), acute kidney injury
D - Drug toxinsEthylene glycol (oxalic acid), methanol (formic acid), salicylates, propylene glycol, isoniazid, cyanide
P - Pyroglutamic acidAcetaminophen overdose (chronic), malnourished patients
OtherRhabdomyolysis, massive hemolysis

B. Normal Anion Gap (Hyperchloremic) Metabolic Acidosis (NAGMA)

Mnemonic: DURHAM / HARD UP
MechanismExamples
GI HCO3- lossDiarrhea, small bowel/pancreatic fistulas, ileostomy, ureterosigmoidostomy
Renal tubular acidosis (RTA)Type 1 (distal) - can't acidify urine; Type 2 (proximal) - HCO3- wasting; Type 4 - hypoaldosteronism
IatrogenicLarge-volume 0.9% NaCl infusion (hyperchloremic/dilutional acidosis)
Early renal failureModerate CKD (stages 3B-4)
OtherCarbonic anhydrase inhibitors (acetazolamide), ammonium chloride ingestion, adrenal insufficiency
  • Harrison's Principles of Internal Medicine 22E, Tables 58-4 and 58-5; Current Surgical Therapy 14e

5. Diagnosis and Approach

Stepwise Approach (Harrison's 22E)

  1. Draw ABG and serum electrolytes simultaneously
  2. Confirm ABG HCO3- matches calculated value (within ±2 mmol/L)
  3. Calculate AG; correct for albumin if hypoalbuminemia present
  4. Identify HAGMA vs NAGMA
  5. Calculate compensation: Expected PaCO2 = 1.5 × HCO3- + 8 ± 2
    • If actual PaCO2 < expected → concurrent respiratory alkalosis
    • If actual PaCO2 > expected → concurrent respiratory acidosis
  6. If HAGMA: calculate delta-delta (Δ/Δ) ratio = ΔAG / ΔHCO3-
    • Δ/Δ = 1-2: pure HAGMA
    • Δ/Δ > 2: HAGMA + concurrent metabolic alkalosis
    • Δ/Δ < 1: HAGMA + concurrent NAGMA
  7. For NAGMA: calculate urine anion gap = Na+ + K+ - Cl-
    • Negative UAG → GI HCO3- loss (diarrhea) - appropriate NH4+ excretion
    • Positive UAG → RTA or renal failure - impaired NH4+ excretion

6. Treatment / Management

General Principles

  • Treat the underlying cause - this is the most important step
  • Alkali therapy is generally reserved for severe acidemia (pH < 7.1-7.2)
  • Avoid sodium bicarbonate when "potential HCO3-" exists (metabolizable anions - ketoacids, lactate) - correcting the cause regenerates HCO3-

Condition-Specific Management

ConditionManagement
Lactic acidosis (type A - hypoperfusion)Restore perfusion (fluids, vasopressors, treat sepsis/cardiac cause); avoid NaHCO3 unless pH < 7.1
DKAIV insulin + fluids; potassium replacement; HCO3- only if pH < 6.9
Alcoholic ketoacidosisIV dextrose + thiamine; fluids
Toxic alcohols (ethylene glycol, methanol)Fomepizole (4-MP); hemodialysis; ethanol if fomepizole unavailable
Salicylate toxicityUrinary alkalinization (NaHCO3 IV), forced diuresis, hemodialysis
CKD metabolic acidosisOral NaHCO3 (1-1.5 mmol/kg/day) or sodium citrate to maintain HCO3- > 22-24 mEq/L
RTAAlkali replacement (NaHCO3 or K-citrate depending on type)
Hyperchloremic/dilutionalSwitch to balanced crystalloids (Lactated Ringer's, PlasmaLyte); restrict NaCl

NaHCO3 Administration

  • IV NaHCO3 is considered when pH < 7.1 and HCO3- < 10 mEq/L
  • Amount to give: NaHCO3 (mEq) = 0.5 × body weight (kg) × (target HCO3- - current HCO3-)
  • Give half the calculated amount, then reassess
  • Risks: volume overload, hypernatremia, paradoxical CSF acidosis, "overshoot" alkalosis, hypokalemia
  • Washington Manual of Medical Therapeutics; Rosen's Emergency Medicine; Harrison's 22E

7. Special Focus: Anaesthesia

7.1 Perioperative Causes of Metabolic Acidosis

CauseMechanismNotes
Tissue hypoperfusion / lactic acidosisAnaesthetic-induced cardiac depression, hypovolemia, inadequate resuscitationType A lactic acidosis - most common intraoperative cause
Large-volume 0.9% NaCl infusion↑ Cl- → ↓ SID → hyperchloremic acidosisUse balanced crystalloids to prevent
Dilutional acidosisFree water excess dilutes HCO3-Common with large-volume resuscitation
Propofol Infusion Syndrome (PRIS)Inhibits mitochondrial respiratory chain complex II-IV and beta-oxidationSee below
Malignant hyperthermiaHypermetabolism → profound lactic and mixed metabolic acidosisTriggered by volatile agents + succinylcholine
Tourniquet-related ischemia-reperfusionLactic acid washout on tourniquet releaseTransient, usually self-resolving
Cardiopulmonary bypass (CPB)Hemodilution, non-pulsatile flow, hypothermiaHalothane anesthesia worsens; opioid-based anesthesia reduces postoperative metabolic acidosis and lactate compared to volatile agents
Fasting/starvationKetogenesisProlonged NPO in pediatric and metabolically vulnerable patients

7.2 Propofol Infusion Syndrome (PRIS)

A rare but potentially fatal complication, first described by Parke et al. (1992).
Mechanism: Propofol inhibits mitochondrial respiratory chain (complexes II-IV) and fatty acid beta-oxidation, leading to impaired cellular energy production.
Predisposing factors:
  • Doses > 4-5 mg/kg/hr for > 48 hours
  • Low carbohydrate supply / high fat load
  • Pre-existing mitochondrial or fatty acid oxidation disorders (MCAD deficiency)
  • Catecholamine/corticosteroid infusions (increase metabolic demand)
  • Children especially vulnerable
Clinical features (PRIS triad):
  • Unexplained metabolic acidosis (high AG)
  • Cardiac failure - new onset bradycardia, right bundle branch block, ST changes, arrhythmia
  • Rhabdomyolysis - elevated CK, myoglobinuria
  • Lipaemia, hepatomegaly
Management:
  • Immediately discontinue propofol
  • Switch to alternative sedation (midazolam, dexmedetomidine, ketamine)
  • Hemodynamic support; consider ECMO in refractory cardiac failure
  • Hemodialysis for severe cases
  • Give dextrose-containing fluids to prevent fatty acid dominance
Note: Unexpected tachycardia during propofol anesthesia should prompt ABG evaluation for metabolic acidosis. - Katzung's Basic and Clinical Pharmacology 16th Ed., p.2350
  • Barash's Clinical Anesthesia 9e; Miller's Anesthesia 10e; Katzung's Basic and Clinical Pharmacology 16e

7.3 Malignant Hyperthermia (MH) and Metabolic Acidosis

MH is a pharmacogenetic hypermetabolic crisis triggered by volatile anesthetic agents (halothane, isoflurane, sevoflurane, desflurane) and/or succinylcholine in genetically susceptible individuals (RYR1 mutations most common).
  • Massive uncontrolled Ca2+ release from sarcoplasmic reticulum → hypermetabolism
  • Results in: profound mixed metabolic and respiratory acidosis, hyperthermia, muscle rigidity, rhabdomyolysis, hyperkalemia
  • Management: Dantrolene sodium (2.5 mg/kg IV, repeat as needed), cooling, NaHCO3 for severe acidosis, correct hyperkalemia, avoid triggers

7.4 Acid-Base Assessment in Anaesthesia

Methods used perioperatively:
MethodToolClinical utility
TraditionalpH, PaCO2, HCO3-, base excess/deficitSimple, universal
Anion gapAG = Na - (Cl + HCO3-)Detects HAGMA even when pH is normal
Base deficitBE < -2 mEq/L indicates metabolic acidosisUseful to guide resuscitation endpoints
Stewart-FenclSID, ATOT, UMA, SIGBetter for complex ICU/perioperative patients
Lactate> 2 mmol/L abnormal; > 4 mmol/L = shockBest marker of tissue hypoperfusion
Key points from Miller's Anesthesia (Ch. 44):
  • Most acid-base disorders should be treated by reversing the cause
  • The SIG (Strong Ion Gap) identifies unmeasured anions but its advantage over corrected AG is not clearly established for most clinical settings
  • In previously healthy patients presenting to the OR, base deficit and AG are usually sufficient
  • For complex critically ill patients (ICU, major surgery), the Fencl-Stewart approach helps disentangle multiple simultaneous disorders

7.5 Fluid Choice and Metabolic Acidosis

FluidCl- contentEffect on acid-base
0.9% NaCl154 mEq/L (supraphysiologic)Hyperchloremic metabolic acidosis - ↓ SID
Lactated Ringer's109 mEq/LNear-neutral; lactate metabolized to HCO3-
PlasmaLyte98 mEq/LMost physiologically balanced; no acidosis
Colloids (albumin in saline)VariableCan also cause hyperchloremia
Clinical relevance: Massive resuscitation with 0.9% NaCl is a well-recognized intraoperative cause of NAGMA. Switching to balanced crystalloids (LR, PlasmaLyte) reduces this risk.

7.6 Mitochondrial Disease and Anaesthesia

Patients with mitochondrial disorders have baseline metabolic acidosis (often lactic acidosis) that can be precipitated or worsened perioperatively.
Key anaesthetic precautions (Miller's Anesthesia, Box 31.6):
  1. Thorough preoperative assessment of organ involvement (cardiac, neurologic)
  2. Minimize fasting - give glucose-containing maintenance fluids
  3. Avoid lactated Ringer's in patients prone to lactic acidosis
  4. Maintain normothermia - shivering, hypoxia, hypotension worsen lactic acidosis
  5. Avoid succinylcholine (hyperkalemia risk with myopathy); cautious use of NMBAs with monitoring
  6. Both volatile anesthetics and propofol can worsen mitochondrial function; use the lowest effective dose
  7. Propofol: single bolus only for induction; avoid infusions in mitochondrial disease (extreme PRIS sensitivity)
  8. Consider regional/neuraxial techniques to minimize systemic anesthetic exposure

7.7 Specific Anaesthetic Drug Interactions with Metabolic Acidosis

Drug/SituationEffect / Consideration
OpioidsUse with caution - respiratory depression worsens underlying metabolic acidosis by adding respiratory component; remifentanil is preferred for titratable use
Volatile agentsDepress myocardial function more severely in acidotic patients; primary opioid techniques reduce postoperative metabolic acidosis vs halothane
Local anestheticsCardiotoxicity enhanced by acidosis (↓ protein binding, ion trapping increases intracellular LA concentration); toxicity threshold lowered
Non-depolarizing NMBAsProlonged action in severe acidosis (↓ protein binding, ↓ hepatic/renal clearance)
SuccinylcholineContraindicated if pre-existing hyperkalemia (hyperkalemia worsens in metabolic acidosis)
NaHCO3 + catecholaminesAlkalinization may improve catecholamine efficacy; do not mix in same line (precipitation)
MetforminMust be withheld 24-48h preoperatively - perioperative lactic acidosis risk (especially with contrast agents or renal hypoperfusion)

7.8 Postoperative Metabolic Acidosis - Differential

In the PACU/ICU, metabolic acidosis should prompt systematic evaluation:
  1. Lactic acidosis - rule out hypoperfusion, sepsis, bowel ischemia
  2. PRIS - if propofol infusion > 48h or high dose
  3. Hyperchloremic acidosis - review intraoperative fluid record
  4. Residual effects - acidosis from surgical stress (CPB, tourniquet, major hemorrhage)
  5. Drug toxicity - salicylates, alcohols, cyanide (consider in patients with burns/smoke inhalation)
  6. Ketoacidosis - diabetic patients, prolonged fasting, alcoholics

8. Summary Table

FeatureHigh AG Metabolic AcidosisNormal AG (Hyperchloremic) Metabolic Acidosis
AG> 12 mEq/LNormal (8-12)
Cl-NormalHigh
CausesLactic acidosis, DKA, uremia, toxinsDiarrhea, RTA, excess NaCl, early CKD
TreatmentTreat cause; NaHCO3 if pH < 7.1Treat cause; balanced fluids; alkali if needed
Periop exampleSeptic shock, PRIS, MH, CPBLarge-volume NaCl resuscitation
Key References (Textbooks):
  • Harrison's Principles of Internal Medicine 22E (2025), Chapter 58
  • Miller's Anesthesia, 10th Ed. (2023), Chapter 44 - Perioperative Acid-Base Balance
  • Barash's Clinical Anesthesia, 9th Ed.
  • Costanzo Physiology, 7th Ed., Chapter 7
  • Current Surgical Therapy, 14th Ed. - Acid-Base Disorders
  • Katzung's Basic and Clinical Pharmacology, 16th Ed.
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