pH Basics

• Acidemia: pH <7.40
• Alkalemia: pH >7.40
• Acidosis: The underlying PROCESS causing acidemia
• Alkalosis: The underlying PROCESS causing alkalemia

Critical: You can have acidosis WITHOUT acidemia (if something else compensates)

The Bicarbonate System (Foundation of Everything)

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Key insight: This is an OPEN system - Lungs eliminate CO₂ (respiratory component) - Kidneys excrete/retain HCO₃⁻ (metabolic component) - When system becomes CLOSED (respiratory failure), severe acidosis develops rapidly

Normal Values

• pH: 7.35-7.45
• PCO₂: 35-45 mmHg (respiratory control)
• HCO₃⁻: 22-26 mEq/L (metabolic control)

Step A: Assess pH

• Is it <7.40 (acidemia) or >7.40 (alkalemia)?
• This tells you the DIRECTION of the primary problem

Step B: Identify Primary Disorder

Check HCO₃⁻ and PCO₂ together to determine what matches the pH direction:

If ACIDEMIA (pH <7.40), look for: - ↓ HCO₃⁻ (<22) = Metabolic acidosis (primary) - ↑ PCO₂ (>45) = Respiratory acidosis (primary)

If ALKALEMIA (pH >7.40), look for: - ↑ HCO₃⁻ (>26) = Metabolic alkalosis (primary) - ↓ PCO₂ (<35) = Respiratory alkalosis (primary)

Critical rule: Compensation is ALWAYS INCOMPLETE. If compensation appears complete or excessive, suspect a SECOND disorder.

Step C: Assess Compensation (Use Formulas!)

For Metabolic Acidosis → Respiratory Compensation (Winter’s Formula):

Expected PCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2

Example: HCO₃⁻ = 10 - Expected PCO₂ = 1.5(10) + 8 ± 2 = 23 ± 2 → 21-25 mmHg - If actual PCO₂ is 28 → inadequate respiratory compensation (concurrent respiratory problem) - If actual PCO₂ is 18 → appropriate compensation ✓

For Metabolic Alkalosis → Respiratory Compensation:

PCO₂ increases by 0.7 mmHg per 1 mEq/L rise in HCO₃⁻

For Respiratory Acidosis (↑PCO₂) → Metabolic Compensation: - Acute: HCO₃⁻ rises by 1 mEq/L per 10 mmHg rise in PCO₂ - Chronic: HCO₃⁻ rises by 3.5 mEq/L per 10 mmHg rise in PCO₂

For Respiratory Alkalosis (↓PCO₂) → Metabolic Compensation: - Acute: HCO₃⁻ drops by 2 mEq/L per 10 mmHg drop in PCO₂ - Chronic: HCO₃⁻ drops by 5 mEq/L per 10 mmHg drop in PCO₂

Remember: Respiratory changes happen in MINUTES. Renal compensation takes DAYS.

Step D: Calculate Anion Gap

Formula:

AG = Na⁺ - (Cl⁻ + HCO₃⁻)

Normal: 8-12 mEq/L (varies by lab; check YOUR lab’s normal)

Elevated AG (>12): Means unmeasured anions present (organic acids accumulating)

Narrow AG (<8): Unusual; suggests hypoalbuminemia, myeloma, or specific toxins

Clinical pearl: Albumin correction - Each 1 g/dL drop in albumin below 4 g/dL → AG decreases by ~2.5 mEq/L - In ICU (hypoalbumemic patients) → ALWAYS correct AG for albumin

MUDPILES Mnemonic

• M: Methanol
• U: Uremia (kidney failure)
• D: DKA (Diabetic ketoacidosis)
• P: Propylene glycol (from IV Ativan!)
• I: Isoniazid, Iron
• L: Lactic acidosis
• E: Ethylene glycol
• S: Salicylates (aspirin overdose)

Most Common in Practice

1. Lactic acidosis (shock, sepsis, hypoxemia)
2. DKA (diabetes out of control)
3. Uremia (kidney failure)
4. Methanol/ethylene glycol (toxic ingestions)

USED CARP Mnemonic

Negative UAG (kidney works fine, GI problem): - U: Ureterosigmoidostomy - S: Small bowel fistula - E: Extra chloride (NS administration) - D: Diarrhea (most common)

Positive UAG (kidney problem): - C: Chronic renal failure - A: Acetazolamide, Addison’s disease - R: Renal tubular acidosis (RTA) - P: Protein overfeeding

The Delta/Delta Ratio

Used when you suspect MULTIPLE metabolic problems in anion gap acidosis:

Δ AG = Measured AG - Normal AG (10)
Δ HCO₃⁻ = Normal HCO₃⁻ (24) - Measured HCO₃⁻
Ratio = Δ AG / Δ HCO₃⁻

Interpretation: - Ratio <1: HCO₃⁻ dropped MORE than expected → concurrent non-AG acidosis - Ratio 1-2: Pure AG acidosis (appropriate) - Ratio >2: HCO₃⁻ didn’t drop as much as expected → concurrent metabolic alkalosis

Potential Bicarbonate Method

Alternative approach:

Potential HCO₃⁻ = Measured HCO₃⁻ + (Measured AG - 10)

Interpretation: - <22: Suggests concurrent NAGMA - 22-26: Pure AG acidosis - >26: Suggests concurrent metabolic alkalosis

Case 1: Severe Sepsis with Shock

ABG: pH 7.15, PCO₂ 22, HCO₃⁻ 8, Lactate 8

Analysis: 1. pH 7.15 → Acidemia 2. HCO₃⁻ 8 (low) → Metabolic acidosis (primary) 3. Winter’s formula: Expected PCO₂ = 1.5(8) + 8 ± 2 = 20 ± 2 → 18-22 mmHg - Actual 22 → appropriate respiratory compensation ✓ 4. AG = 140 - (110 + 8) = 22 (elevated) 5. Diagnosis: High AG metabolic acidosis with appropriate respiratory response 6. Cause: Likely lactic acidosis from shock 7. Management: Aggressive fluid resuscitation, antibiotics, vasopressors

Case 2: DKA with Concurrent Infection

ABG: pH 7.10, PCO₂ 28, HCO₃⁻ 9, Glucose 450, β-hydroxybutyrate 4.2

Analysis: 1. Acidemia (pH 7.10) 2. Metabolic acidosis (HCO₃⁻ 9) 3. Winter’s formula: Expected PCO₂ = 1.5(9) + 8 ± 2 = 21.5 ± 2 → 19-24 mmHg - Actual 28 → inadequate respiratory compensation (higher than expected!) 4. Interpretation: There’s a concurrent respiratory acidosis or the respiratory center is depressed 5. Possible causes: Concurrent pneumonia, pulmonary edema, CNS depression 6. Management: Aggressive insulin, fluids, monitor respiratory status closely

Related Student Handouts

• Hyperkalemia Management
• Hypokalemia Management
• Hyponatremia Management
• Hypernatremia and SIADH
• AKI Workup and Diagnosis

Clinical Content (01-Clinical-Medicine/Nephrology)

• Electrolyte Disorders Hub
• Acid-Base Disorders Clinical Reference
• Essential Renal Laboratory Tests

Atomic Notes (ZK)

• RAAS System and Electrolyte Regulation

Butler-COM Resources

• Butler COM - Nephrology Deep Dive