Lactic Acidosis 

πŸ”Ή Introduction

Lactic acidosis is a form of metabolic acidosis characterized by the accumulation of lactate (typically >4 mmol/L) in the blood, resulting in a low pH (<7.35). It is a common cause of acid-base disturbances in critically ill patients and is associated with high morbidity and mortality, particularly in shock states, sepsis, and multi-organ failure.

πŸ”Ή Physiology of Lactate

  • Lactate is produced during anaerobic glycolysis, mainly in skeletal muscle, red blood cells, brain, and intestines.
  • Under normal circumstances:
    • Lactate production: ~1 mmol/kg/hr
    • Lactate clearance: Primarily by the liver (60%), kidneys (30%), and heart (10%)
  • Lactate acts as an energy substrate for heart, brain, and liver during stress.
  • Accumulation occurs when production exceeds clearance or when mitochondrial dysfunction impairs aerobic metabolism.


πŸ”Ή Classification

Traditionally, lactic acidosis is classified into two major types (Cohen & Woods, 1976):

Type

Mechanism

Causes

Type A

Hypoperfusion / hypoxia β€” tissue hypoxia leads to anaerobic metabolism

Shock (septic, cardiogenic, hypovolemic), severe anemia, hypoxemia, cardiac arrest, CO poisoning

Type B

Non-hypoxic β€” normal oxygen delivery but metabolic derangements cause lactate accumulation

Diabetes (metformin), liver failure, malignancy, drugs (linezolid, propofol), toxins (cyanide, ethanol), inborn errors of metabolism, mitochondrial disorders

Additional classification (recent):

  • Type B1: Associated with underlying disease (liver failure, sepsis)
  • Type B2: Drug- or toxin-induced
  • Type B3: Inborn errors of metabolism


πŸ”Ή Pathophysiology

  1. Type A (Hypoxic)
    • Decreased tissue oxygen delivery (DO2) β†’ cells shift to anaerobic glycolysis β†’ increased pyruvate β†’ excess lactate.
    • Common in shock states and severe hypoxemia.
  1. Type B (Non-Hypoxic)
    • Impaired lactate metabolism: e.g., hepatic failure β†’ decreased lactate clearance
    • Enhanced glycolysis: drugs (Ξ²-agonists, epinephrine), malignancy (Warburg effect)
    • Mitochondrial dysfunction: sepsis, metformin toxicity
  1. Mixed Forms
    • Often, patients have both hypoperfusion and metabolic dysfunction, particularly in septic shock.


πŸ”Ή Clinical Features

Symptoms are non-specific and often overlap with underlying disease:

  • Fatigue, malaise, nausea, vomiting
  • Tachypnea / Kussmaul respiration (compensatory)
  • Hypotension, signs of shock
  • Mental status changes: confusion, lethargy, coma in severe cases
  • Evidence of underlying cause: sepsis, hypoxia, drug exposure

Key Exam Points:

  • Look for shock markers: cold extremities, delayed capillary refill
  • Signs of organ failure: jaundice, renal failure, cardiac dysfunction


πŸ”Ή Laboratory Diagnosis

Blood Tests:

  • Arterial blood gas: pH < 7.35, HCO3- low


Why is HCO₃⁻ (bicarbonate) low in lactic acidosis?

πŸ”¬ Core Concept

Lactic acidosis is a form of metabolic acidosis caused by accumulation of lactic acid in blood. The fall in HCO₃⁻ occurs because bicarbonate is consumed while buffering excess hydrogen ions (H⁺).


β€”> Step-by-Step Mechanism

1️⃣ Excess Lactate Production

  • Conditions like shock, hypoxia, sepsis, or mitochondrial dysfunction β†’ ↑ anaerobic metabolism
  • Glucose β†’ Pyruvate β†’ Lactate + H⁺

πŸ‘‰ Lactic acid dissociates:Lactic acidβ†’Lactateβˆ’+H+

2️⃣ Buffering of Hydrogen Ions by Bicarbonate

The body tries to maintain physiological pH.

Bicarbonate acts as the primary extracellular buffer:

H++HCO3βˆ’ β†’H2 CO3 β†’CO2 +H2 O

πŸ‘‰ Result:

  • Hydrogen ions are neutralized
  • Bicarbonate gets consumed
  • COβ‚‚ generated β†’ eliminated via lungs (compensatory hyperventilation)


β€”> ABG Pattern in Lactic Acidosis

  • ↓ pH
  • ↓ HCO₃⁻ (primary abnormality)
  • ↓ PaCOβ‚‚ (respiratory compensation via hyperventilation)


  • Serum lactate: >2 mmol/L is abnormal, >4 mmol/L often indicates severe lactic acidosis
  • Anion gap: usually elevated (>12 mEq/L)
  • Other labs: lactate-to-pyruvate ratio (optional, research/academic use)

Key Differentiation:

  • Type A: Usually correlates with clinical hypoperfusion
  • Type B: Lactate elevated without hypoxia, check history for drugs, liver disease, malignancy


πŸ”Ή Common Causes in Critical Care

Shock-Related (Type A)

  • Septic shock (most common in ICU)
  • Cardiogenic shock (MI, heart failure)
  • Hypovolemic shock (hemorrhage, dehydration)
  • Severe hypoxemia (ARDS, pulmonary embolism)

Non-Hypoxic (Type B)

  • Liver failure (reduced clearance)
  • Renal failure (contributes)
  • Drugs/toxins: metformin, nucleoside analogues, propofol, cyanide
  • Malignancy: high tumor burden β†’ Warburg effect
  • Endocrine: diabetic ketoacidosis, thiamine deficiency


πŸ”Ή Management

1. Treat the Underlying Cause

  • Shock: fluids, vasopressors, source control (e.g., infection)
  • Hypoxemia: oxygen supplementation, mechanical ventilation
  • Toxins/drugs: antidotes (e.g., cyanide β†’ hydroxocobalamin, metformin β†’ dialysis)

2. Supportive Measures

  • Correct electrolyte abnormalities (K+, Mg2+, phosphate)
  • Optimize hemodynamics and tissue perfusion
  • Avoid excessive fluids if causing edema

3. Role of Bicarbonate

  • Sodium bicarbonate may be considered in severe acidemia (pH < 7.1)
  • Evidence is mixed; can worsen hypernatremia and CO2 generation

4. Extracorporeal Therapies

  • Consider renal replacement therapy for drug-induced or refractory lactic acidosis
  • ECMO in profound shock when lactate remains high despite resuscitation


πŸ”Ή Prognosis

  • Mortality correlates with lactate levels:
    • Lactate >4 mmol/L β†’ mortality ~30–50% in ICU patients
    • Lactate >10 mmol/L β†’ mortality >80%
  • Serial lactate monitoring is used for resuscitation endpoints in sepsis (e.g., Surviving Sepsis Campaign guidelines)

πŸ”Ή References

  1. Kraut JA, Madias NE. Lactic Acidosis. N Engl J Med. 2014;371:2309–2319.
  2. Cohen RD, Woods HF. Lactic acidosis revisited. Diabetes. 1976;25:386–391.
  3. Levraut J, et al. Clinical review: Lactic acidosis in intensive care. Crit Care. 2003;7:212–220.
  4. Surviving Sepsis Campaign: 2021 Guideline for Management of Sepsis and Septic Shock. Intensive Care Med. 2021;47:1181–1247.