Clinical Assessment of Acid-Base Status

Clinical Assessment of Acid-Base Status - ABG ABCs

⭐ Normal arterial pH range is 7.35-7.45.

• 📌 ABCs of ABG Interpretation:
  • A - Assess pH:
    • Acidemia: pH < 7.35 (↑[H+])
    • Alkalemia: pH > 7.45 (↓[H+])
    • Note: -emia refers to blood pH; -osis refers to the underlying process.
  • B - Bicarbonate (HCO3-) & PaCO2:
    • PaCO2 (Respiratory): 35-45 mmHg. Primary change if respiratory disorder.
    • HCO3- (Metabolic): 22-26 mEq/L. Primary change if metabolic disorder.
  • C - Compensation:
    • Body's attempt to normalize pH.
    • Respiratory compensation: rapid; Metabolic compensation: slower (hours-days).
• Henderson-Hasselbalch Equation: $pH = 6.1 + \log \frac{[HCO_3^-]}{0.03 \times PaCO_2}$

Anion Gap & Delta Ratio - Gap Detectives

• Anion Gap (AG): $Na^+ - (Cl^- + HCO_3^-)$. Normal: 8-12 mEq/L.
  • Albumin correction: Add 2.5 to AG for each 1 g/dL albumin < 4 g/dL.
• High AGMA (HAGMA): ↑ unmeasured anions.
  • 📌 GOLDMARK: Glycols, Oxoproline, L/D-Lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.

  ⭐ Lactic acidosis: most common HAGMA cause in hospitals.

• Normal AGMA (NAGMA): $HCO_3^-$ loss or $Cl^-$ gain (hyperchloremic).
  • 📌 HARDUPS: Hyperalimentation, Acetazolamide, RTA, Diarrhea, Uretero/Pancreatic fistulas, Saline.
• Delta Ratio (ΔRatio): $(AG_{actual} - 12) / (24 - HCO_{3,actual}^-)$. Assesses mixed disorders.
  • <0.4: Pure NAGMA
  • 0.4-0.8: HAGMA + NAGMA
  • 1-2: Pure HAGMA
  • >2: HAGMA + Metabolic Alkalosis

Compensation Rules & Mixed Disorders - Balancing Acts

• Compensation: Body's response to primary imbalance.
  • Metabolic Acidosis: Exp PaCO2 = $(1.5 \times [HCO_3^-]) + 8 \pm 2$ (Winters').
  • Metabolic Alkalosis: PaCO2 ↑ 0.7 mmHg per mEq/L ↑ $[HCO_3^-]$.
  • Resp Acidosis (Acute/Chronic): $[HCO_3^-]$ ↑ 1 / 3.5 mEq/L per 10 mmHg PaCO2 ↑.
  • Resp Alkalosis (Acute/Chronic): $[HCO_3^-]$ ↓ 2 / 4-5 mEq/L per 10 mmHg PaCO2 ↓.
• Mixed Disorders: ≥2 primary disorders.
  • Suspect: compensation outside expected; pH normal with abnormal PaCO2 & $[HCO_3^-]$.
  • Delta-Delta Gap ($\Delta AG / \Delta [HCO_3^-]$):
    • <1: HAGMA + NAGMA.
    • 1-2: Pure HAGMA.
    • >2: HAGMA + Met. Alk.

⭐ In suspected mixed disorders, if measured compensation significantly deviates from expected, or if pH is normal despite abnormal PaCO2 and HCO3-, suspect a mixed acid-base disorder.

High‑Yield Points - ⚡ Biggest Takeaways

• Arterial Blood Gas (ABG) is cornerstone for acid-base assessment.
• Always check pH (7.35-7.45) first: acidemia or alkalemia.
• PaCO₂ (35-45 mmHg) reflects respiratory; HCO₃⁻ (22-26 mEq/L) metabolic.
• Calculate Anion Gap (AG = Na⁺ - (Cl⁻ + HCO₃⁻)), normally 8-12 mEq/L; crucial in metabolic acidosis.
• Winter's formula assesses respiratory compensation in metabolic acidosis.
• Delta-Delta Gap helps identify mixed acid-base disorders.
• Urine anion gap differentiates Normal Anion Gap Metabolic Acidosis (NAGMA) causes.