🔍 Metabolic Acidosis & Acid-Base Analysis

Systematic approach to acid-base disorders with anion gap interpretation

🚨 Emergency Management Priority

Critical pH Thresholds

  • pH < 7.1: Life-threatening - immediate intervention
  • pH < 7.2: Severe - aggressive treatment
  • pH 7.2-7.35: Moderate - treat underlying cause
Immediate Actions:
  1. Ensure adequate ventilation (respiratory compensation)
  2. IV access and volume status assessment
  3. Calculate anion gap and assess for mixed disorders
  4. Treat underlying cause (DKA, lactic acidosis, toxins)
  5. Consider bicarbonate ONLY if pH <7.15 with shock

🔬 The ABC Method: Systematic Acid-Base Analysis

Step A: Assess pH

  • Acidemia: pH < 7.40
  • Alkalemia: pH > 7.40
  • Remember: Acidosis ≠ Acidemia
  • Note: Can have acidosis with normal/high pH due to compensation

Step B: Primary Disorder

  • Metabolic: Primary HCO₃⁻ change
  • Respiratory: Primary pCO₂ change
  • Mixed: Both systems affected
  • Key: Which change explains the pH?

Step C: Check Compensation

  • Never complete: pH always abnormal
  • Never overcompensates: pH doesn't cross 7.40
  • Winter's Formula: Expected pCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2
  • If overcompensation: Mixed disorder present

🧮 Anion Gap: The Diagnostic Key

Anion Gap Calculation

AG = Na⁺ - (Cl⁻ + HCO₃⁻)

Normal AG: 8-12 mEq/L (modern analyzers)

Traditional AG: 12±4 mEq/L (older labs)

⚠️ Know your lab's normal range!

High Anion Gap Acidosis (MUDPILES)

  • M: Methanol
  • U: Uremia (BUN >100)
  • D: DKA, alcoholic ketoacidosis
  • P: Propylene glycol (IV Ativan!)
  • I: Isoniazid, Iron
  • L: Lactic acidosis (most common)
  • E: Ethylene glycol
  • S: Salicylates
Clinical Pearl: Lactic acidosis is the most common cause of high AG metabolic acidosis in hospitalized patients.

Normal AG Acidosis (USED CARP)

Normal Kidney Function (UAG < 0):

  • U: Ureterosigmoidostomy
  • S: Small bowel fistulae
  • E: Extra Cl⁻ (normal saline)
  • D: Diarrhea (most common)

Impaired Kidney Function (UAG > 0):

  • C: Chronic kidney disease
  • A: Acetazolamide, Addison's
  • R: Renal tubular acidosis
  • P: Protein overfeeding

Urinary Anion Gap (UAG) for Normal AG Acidosis

UAG = (Urinary Na⁺ + K⁺) - Cl⁻

UAG < 0 (Negative)

Meaning: Appropriate NH₄⁺ production

Cause: Extra-renal HCO₃⁻ loss

Examples: Diarrhea, fistulae, saline administration

UAG > 0 (Positive)

Meaning: Impaired NH₄⁺ production

Cause: Renal acid excretion defect

Examples: RTA, CKD, hypoaldosteronism

🔄 Delta-Delta Ratio: Detecting Mixed Disorders

The Delta-Delta Calculation

Δ AG = Measured AG - Normal AG (10)

Δ HCO₃⁻ = Normal HCO₃⁻ (24) - Measured HCO₃⁻

Delta/Delta Ratio = Δ AG / Δ HCO₃⁻

Ratio < 1.0

Interpretation: Mixed disorder

  • High AG metabolic acidosis
  • PLUS normal AG metabolic acidosis
Example: DKA + diarrhea

Ratio 1.0-2.0

Interpretation: Pure disorder

  • High AG metabolic acidosis only
  • Expected compensation pattern
Example: Isolated DKA or lactic acidosis

Ratio > 2.0

Interpretation: Mixed disorder

  • High AG metabolic acidosis
  • PLUS metabolic alkalosis
Example: DKA + vomiting/diuretics

Alternative: Potential Bicarbonate Method

Potential HCO₃⁻ = Measured HCO₃⁻ + Δ AG
< 22 mEq/L
Concurrent NAG acidosis
22-26 mEq/L
Pure AG acidosis
> 26 mEq/L
Concurrent alkalosis

🧮 Interactive Acid-Base Calculator

📋 Clinical Case Applications

Case 1: ICU Patient with Sepsis

Clinical: 55M, severe sepsis, received 15L normal saline, on lorazepam drip

Labs: pH 7.10, pCO₂ 19, HCO₃⁻ 6, Na 135, Cl 110, UAG -55

Analysis:

  1. Severe acidemia (pH 7.10)
  2. Primary metabolic acidosis (HCO₃⁻ 6)
  3. Appropriate respiratory compensation (expected pCO₂ 17±2)
  4. AG = 135-110-6 = 19 (elevated)
  5. Delta/Delta = 9/18 = 0.5 (<1 = mixed)

Diagnosis: AG acidosis (propylene glycol from lorazepam) + NAG acidosis (NS-induced)

Case 2: DKA with Fungal Sepsis

Clinical: 60M, DM, fungal sepsis

Labs: pH 7.01, pCO₂ 39, HCO₃⁻ 10, glucose 480, β-hydroxybutyrate 3, lactate 8

Analysis:

  1. Severe acidemia with inadequate respiratory compensation
  2. Expected pCO₂ = 1.5×10 + 8 = 23 ± 2 (actual 39 = respiratory acidosis too)
  3. Multiple causes: DKA + lactic acidosis + respiratory failure
  4. Triple acid-base disorder

Management: Insulin, fluids, ventilatory support, treat sepsis

Case 3: C. diff Colitis vs RTA

Clinical: 67F, C. diff colitis

Labs: pH 6.91, pCO₂ 40, HCO₃⁻ 8, Na 135, Cl 110, UAG -40

Analysis:

  1. Severe acidemia with minimal respiratory compensation (concerning)
  2. AG = 17 (elevated, likely from uremia/dehydration)
  3. UAG = -40 (negative = appropriate NH₄⁺ production)
  4. Negative UAG confirms diarrheal losses

Diagnosis: Mixed uremic acidosis + diarrhea-induced NAG acidosis

💊 Treatment Principles

Primary Rule: Treat the Underlying Cause

Correcting pH without addressing the underlying pathophysiology is ineffective and potentially harmful.

High AG Metabolic Acidosis

  • DKA: Insulin, fluids, electrolyte replacement
  • Lactic acidosis: Improve tissue perfusion
  • Uremia: Dialysis for severe cases
  • Toxic ingestions: Specific antidotes, dialysis
  • Ketoacidosis: Thiamine, glucose, insulin
Pearl: Bicarbonate therapy rarely indicated; may worsen intracellular acidosis.

Normal AG Metabolic Acidosis

  • Diarrhea: Volume and electrolyte replacement
  • RTA: Alkali therapy (K-citrate preferred)
  • Saline-induced: Switch to balanced crystalloids
  • Hypoaldosteronism: Mineralocorticoid replacement
Pearl: Always correct hypokalemia and hypomagnesemia first.

⚠️ Bicarbonate Therapy: Use Sparingly

Consider Only When:

  • pH < 7.15 with hemodynamic instability
  • Severe hyperkalemia with ECG changes
  • Certain toxic ingestions (salicylates, methanol, TCA)
  • Severe RTA with failure to thrive

Risks of Bicarbonate Therapy:

  • Paradoxical CSF acidosis
  • Hypokalemia and hypocalcemia
  • Volume overload
  • Rebound alkalosis
  • Shift of O₂-hemoglobin curve
Dosing (if absolutely necessary):
HCO₃⁻ deficit = 0.5 × weight (kg) × (desired HCO₃⁻ - current HCO₃⁻)
Give 1/2 to 1/3 of calculated deficit over 3-4 hours

🎯 Key Learning Points

Systematic Approach

  • Always use the ABC method for acid-base analysis
  • Compensation is never complete or overcompensating
  • Check for mixed disorders in all complex cases
  • Use Winter's formula to assess appropriate compensation

Anion Gap Mastery

  • Know your lab's normal AG range (8-12 vs 12±4)
  • Always correct for albumin in critically ill patients
  • Use UAG to differentiate renal vs GI causes of NAG acidosis
  • Delta-delta ratio reveals mixed metabolic disorders

Treatment Priority

  • Always treat the underlying cause, not just the pH
  • Bicarbonate therapy is rarely indicated and often harmful
  • Support ventilation for respiratory compensation
  • Address volume status and electrolyte abnormalities

📚 For Educational Purposes Only

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