Acid-Base Disorders

Acid-Base Fundamentals - pH Balancing Act

• pH: Measure of $H^+$ concentration; normal arterial range 7.35-7.45.
  • Acidemia: pH < 7.35.
  • Alkalemia: pH > 7.45.
• Buffers: Resist pH shifts.
  • Bicarbonate system ($HCO_3^-/H_2CO_3$): Key extracellular buffer. $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$.
• Henderson-Hasselbalch Equation: $pH = 6.1 + \log([HCO_3^-] / (0.03 \times PCO_2))$. Links pH, $HCO_3^-$, and $PCO_2$.
• Regulation:
  • Lungs: Control $PCO_2$ (respiratory component, rapid response).
  • Kidneys: Control $HCO_3^-$ and excrete $H^+$ (metabolic component, slower response).

⭐ The body maintains the $HCO_3^-$ to dissolved $CO_2$ (proportional to $PCO_2$) ratio at approximately 20:1 to keep pH at 7.4.

Metabolic Acidosis & Alkalosis - The Body's Chemistry Quiz

• Metabolic Acidosis
  • Primary ↓ HCO₃⁻, ↓ pH. Compensation: ↓ PaCO₂ (hyperventilation).
  • Anion Gap (AG): $AG = Na⁺ - (Cl⁻ + HCO₃⁻)$. Normal: 8-12 mEq/L.
  • High AG (HAGMA):
    • 📌 MUDPILES: Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates.
    • 📌 GOLDMARK: Glycols, Oxoproline, L/D-Lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.
  • Normal AG (NAGMA) / Hyperchloremic:
    • 📌 HARDUPS: Hyperalimentation, Acetazolamide, RTA, Diarrhea, Uretero-enteric fistula, Pancreatic fistula, Saline (excess).

⭐ Assess respiratory compensation in metabolic acidosis using Winter's Formula.

Expected PaCO₂: $PaCO₂ = (1.5 \times HCO₃⁻) + 8 \pm 2$.

• Metabolic Alkalosis
  • Primary ↑ HCO₃⁻, ↑ pH. Compensation: ↑ PaCO₂ (hypoventilation).
  • Types (based on Urine Chloride, UCl⁻):
    • Saline-Responsive: UCl⁻ < 10-20 mEq/L (e.g., Vomiting, NG suction, Diuretics - early).
    • Saline-Resistant: UCl⁻ > 20 mEq/L (e.g., Hyperaldosteronism, Cushing's, severe K⁺ depletion).

Respiratory Acidosis & Alkalosis - When Lungs Steer pH

Normal $PCO_2$: 35-45 mmHg. Primary change in respiratory disorders.

Respiratory Acidosis (↓pH, ↑$PCO_2$ > 45 mmHg)

• Cause: Hypoventilation (COPD, opioids, ARDS, neuromuscular disease).
• Compensation (Renal $HCO_3^-$ ↑):
  • Acute: ↑1 $HCO_3^-$ per 10 ↑$PCO_2$.
  • Chronic: ↑3-5 $HCO_3^-$ per 10 ↑$PCO_2$.

Respiratory Alkalosis (↑pH, ↓$PCO_2$ < 35 mmHg)

• Cause: Hyperventilation (anxiety, hypoxia, salicylates, sepsis).
• Compensation (Renal $HCO_3^-$ ↓):
  • Acute: ↓2 $HCO_3^-$ per 10 ↓$PCO_2$.
  • Chronic: ↓4-5 $HCO_3^-$ per 10 ↓$PCO_2$.

📌 ROME: Respiratory Opposite (pH & $PCO_2$ move oppositely).

⭐ > Chronic Resp. Acidosis (e.g., COPD): Kidneys compensate; for every 10 mmHg ↑$PCO_2$, $HCO_3^-$ ↑ by 3-5 mEq/L.

ABG Interpretation & Mixed Disorders - Decoding the Signals

Systematic ABG analysis:

• Compensation Formulas:
  • Metabolic Acidosis (Winter's): $PCO_2 = \mathbf{1.5} \times [HCO_3^{-}] + \mathbf{8} \pm \mathbf{2}$.
  • Metabolic Alkalosis: $\Delta PCO_2 \approx \mathbf{0.7} \times \Delta [HCO_3^{-}]$.
  • Resp. Acidosis (Acute): For $\mathbf{10} \uparrow$ PaCO2, $HCO_3^{-} \uparrow \mathbf{1}$. (Chronic: $\uparrow \mathbf{3-4}$).
  • Resp. Alkalosis (Acute): For $\mathbf{10} \downarrow$ PaCO2, $HCO_3^{-} \downarrow \mathbf{2}$. (Chronic: $\downarrow \mathbf{4-5}$).
• Mixed Disorders: Suspect if compensation inadequate/excessive or pH normal with abnormal PaCO2/HCO3.
  • Delta Gap: $(\text{AG} - \mathbf{12}) : (\mathbf{24} - [HCO_3^{-}])$. Ratio ~1: pure HAGMA.

  ⭐ In a patient with DKA (HAGMA), if the fall in bicarbonate is significantly more than the rise in anion gap (i.e., delta gap < 0.8 or delta ratio < 1), suspect a co-existing normal anion gap metabolic acidosis (NAGMA).

High‑Yield Points - ⚡ Biggest Takeaways

• Anion gap (AG) is key for metabolic acidosis (MA) differential; calculate as Na - (Cl + HCO3).
• Winter's formula predicts expected PCO2 for respiratory compensation in metabolic acidosis.
• Delta-delta gap (ΔAG / ΔHCO3) helps detect mixed acid-base disorders.
• Salicylate toxicity classically presents as mixed respiratory alkalosis and high AG metabolic acidosis.
• Diarrhea leads to Normal AG Metabolic Acidosis (NAGMA) via HCO3- loss.
• Vomiting or NG suction causes metabolic alkalosis due to H+ and Cl- loss.
• Identify primary disorder first, then assess compensation using expected changes.