Respiratory and Metabolic Acidosis

Acid-Base Fundamentals - pH Puzzlers

• Core Concepts:
  • Normal blood pH: 7.35-7.45.
  • Acidosis: pH < 7.35. Alkalosis: pH > 7.45.
• Key Parameters:
  • PaCO2 (Respiratory): 35-45 mmHg; reflects alveolar ventilation.
  • HCO3- (Metabolic): 22-26 mEq/L; regulated by kidneys.
• Henderson-Hasselbalch Equation:
  • Governs pH: $pH = 6.1 + log([HCO_3^-] / (0.03 \times PaCO_2))$.
• Body's Buffer Systems (resist pH change):
  • Bicarbonate-carbonic acid: Primary ECF buffer.
  • Phosphate: Important ICF and renal tubular buffer.
  • Proteins: Hemoglobin (RBCs), plasma proteins.

  ⭐ The bicarbonate buffer system is the most important extracellular buffer in the human body, crucial for immediate pH stabilization. oka

Respiratory Acidosis - CO2 Calamity

• Definition: Primary ↑PaCO2 leading to ↓pH. $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$.

• Pathophysiology: Alveolar hypoventilation causes $CO_2$ retention.

• Etiology & Compensation:

  Feature	Acute Respiratory Acidosis	Chronic Respiratory Acidosis
  Onset	Sudden hypoventilation	Prolonged hypoventilation
  Causes	Opioid overdose, GBS, airway obstruction, CNS depression	COPD, obesity hypoventilation, neuromuscular disorders
  Renal Comp.	HCO3⁻ ↑ 1 mEq/L per 10 mmHg ↑PaCO2	HCO3⁻ ↑ 3-4 mEq/L per 10 mmHg ↑PaCO2 (max in 3-5 days)

• Clinical Features: Headache, anxiety, blurred vision, asterixis (flapping tremor), delirium, somnolence (CO2 narcosis).

• Diagnosis: ABG shows pH < 7.35, PaCO2 > 45 mmHg.

• Management: Treat underlying cause; improve ventilation (non-invasive or mechanical).

⭐ In chronic respiratory acidosis, renal compensation takes 3-5 days to become maximal.

Metabolic Acidosis - Bicarb Blues

• Definition & Key Tool: Primary ↓HCO3- leading to ↓pH. Anion Gap is crucial for diagnosis.
• Anion Gap (AG): Calculate as $Na^+ - (Cl^- + HCO_3^-)$. Normal range: 8-12 mEq/L.
• Diagnostic Flow & Types:

• Etiologies (Differential Diagnosis based on AG):

  Type	Common Causes (Mnemonics)
  HAGMA (High AG > 12 mEq/L)	📌 MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates OR 📌 GOLDMARK: Glycols, Oxoproline, L-Lactate, D-Lactate, Methanol, Aspirin, Renal failure, Ketoacidosis
  NAGMA (Normal AG 8-12 mEq/L)	📌 HARDUPS: Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureteroenteric fistula, Pancreatic fistula, Saline administration

• Pathophysiology: ↑Acid production, loss of HCO3-, or ↓renal acid excretion.
• Clinical Features: Kussmaul breathing (deep, rapid), nausea, vomiting, abdominal pain, altered mental status.
• Compensation (Respiratory): Hyperventilation to ↓PaCO2. Expected PaCO2 (Winter's formula): $PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$.
• Management Principles: Treat underlying cause. Bicarbonate therapy considered if pH < 7.1-7.2 (controversial).

⭐ Diarrhea is a common cause of Normal Anion Gap Metabolic Acidosis (NAGMA) due to loss of bicarbonate.

Acidosis Diagnosis - ABG Detective

• 1. pH < 7.35 → Acidosis.
• 2. Primary: PaCO2 > 45 mmHg (Resp) / $HCO_3^-$ < 22 mEq/L (Metab).
• 3. Compensation:
  • Metabolic: Winter's (Exp. PaCO2 = $1.5 \times HCO_3^- + 8 \pm 2$).
  • Respiratory: Acute/Chronic rules.
• 4. Mixed: Abnormal comp. / Normal pH + abnormal gases.
• HAGMA: Delta-Delta Gap $(\text{Measured AG} - \text{Normal AG}) / (\text{Normal } HCO_3^- - \text{Measured } HCO_3^-)$.
• NAGMA: Urine AG (renal vs GI $HCO_3^-$ loss).

⭐ The body never overcompensates for a primary acid-base disorder; a pH moving past normal (7.40) in the opposite direction indicates a mixed disorder.

High‑Yield Points - ⚡ Biggest Takeaways

• Respiratory acidosis: Primary ↑ pCO₂ from hypoventilation; slow renal compensation (↑ HCO₃⁻).
• Metabolic acidosis: Primary ↓ HCO₃⁻ or ↑ H⁺; rapid respiratory compensation (↓ pCO₂ via hyperventilation).
• Anion Gap (AG) is crucial for metabolic acidosis types: AG = Na⁺ - (Cl⁻ + HCO₃⁻).
• Common High AG causes: DKA, Lactic acidosis, Salicylates, Methanol.
• Common Normal AG causes: Diarrhea, Renal Tubular Acidosis (RTA).
• Key triggers for respiratory acidosis: COPD, Opioid overdose, Neuromuscular disease.