Interpretation of Arterial Blood Gases

ABG Basics - Numbers Game

• pH: 7.35-7.45 (Acidemia < 7.35 | Alkalemia > 7.45)
• PaCO₂ (Respiratory): 35-45 mmHg
  • ↑PaCO₂: Respiratory Acidosis
  • ↓PaCO₂: Respiratory Alkalosis
• HCO₃⁻ (Metabolic): 22-26 mEq/L
  • ↓HCO₃⁻: Metabolic Acidosis
  • ↑HCO₃⁻: Metabolic Alkalosis
• PaO₂: 80-100 mmHg (Oxygenation status)
• SaO₂: >95% (Oxygen saturation)
• Anion Gap (AG): $Na^+ - (Cl^- + HCO_3^-)$
  • Normal: 8-12 mEq/L (Key in metabolic acidosis)

⭐ PaCO₂ is regulated by ventilation; HCO₃⁻ is regulated by kidneys. This difference in regulation speed is key to compensation.

Acid-Base Disorders - pH Rollercoaster

• Four primary types:
  • Respiratory Acidosis: ↑PaCO₂, ↓pH (e.g., hypoventilation)
  • Respiratory Alkalosis: ↓PaCO₂, ↑pH (e.g., hyperventilation)
  • Metabolic Acidosis: ↓HCO₃⁻, ↓pH. Check Anion Gap (AG): $AG = Na^+ - (Cl^- + HCO_3^-)$. Normal: 8-12 mEq/L. (e.g., DKA)
  • Metabolic Alkalosis: ↑HCO₃⁻, ↑pH (e.g., vomiting)
• 📌 ROME: Respiratory Opposite (pH & PaCO₂); Metabolic Equal (pH & HCO₃⁻).
• Compensation: Body's response to normalize pH.
  • Renal (slow) for respiratory; Respiratory (fast) for metabolic.

⭐ High Anion Gap (>12 mEq/L) in Metabolic Acidosis suggests MUDPILES causes.

ABG Interpretation Steps - Decode the Code

Systematic 5-step ABG analysis:

• 1. pH Status:
  • Acidemia: pH < 7.35
  • Alkalemia: pH > 7.45
• 2. PaCO₂ (Respiratory): Normal 35-45 mmHg.
  • Altered PaCO₂ suggests respiratory component.
• 3. HCO₃⁻ (Metabolic): Normal 22-26 mEq/L.
  • Altered HCO₃⁻ suggests metabolic component.
• 4. Primary Disorder:
  • 📌 ROME: Respiratory Opposite (pH & PaCO₂), Metabolic Equal (pH & HCO₃⁻).
• 5. Compensation:
  • Non-primary system attempts to correct pH.
  • Metabolic Acidosis: Check Anion Gap ($Na^+ - (Cl^- + HCO_3^-)$). Normal AG: 8-12 mEq/L.

⭐ If pH is normal (7.35-7.45) with abnormal PaCO₂ and HCO₃⁻, suspect a fully compensated disorder or a mixed acid-base disorder.

Compensation & Gaps - Body's Balancing Act

• Compensation: Aims to normalize pH.
  • Metabolic Acidosis (Winter's): Expected $PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm \mathbf{2}$.
  • Metabolic Alkalosis: Expected $PaCO_2 \approx (0.7 \times HCO_3^-) + \mathbf{20} \pm \mathbf{5}$.
  • Resp. Acidosis: Acute $\Delta HCO_3^- \uparrow \mathbf{1}$ for $\Delta PaCO_2 \uparrow \mathbf{10}$; Chronic $\Delta HCO_3^- \uparrow \mathbf{3.5}$ for $\Delta PaCO_2 \uparrow \mathbf{10}$.
  • Resp. Alkalosis: Acute $\Delta HCO_3^- \downarrow \mathbf{2}$ for $\Delta PaCO_2 \downarrow \mathbf{10}$; Chronic $\Delta HCO_3^- \downarrow \mathbf{4-5}$ for $\Delta PaCO_2 \downarrow \mathbf{10}$.
• Anion Gap (AG): $Na^+ - (Cl^- + HCO_3^-)$. Normal: 8-12 mEq/L.
  • High AG (HAGMA): 📌 MUDPILES/GOLDMARK.
  • Normal AG (NAGMA): 📌 HARDUPS.
• Delta-Delta Ratio ($\Delta AG/\Delta HCO_3^-$):
  • 1-2: Pure HAGMA.
  • <1: HAGMA + NAGMA.
  • >2: HAGMA + Met. Alkalosis.
• Osmolal Gap: Measured Osm - Calculated Osm. Normal: <10-15 mOsm/kg.
  • Calculated Osm: $(2 \times Na^+) + (Glucose/18) + (BUN/2.8)$.

⭐ In mixed acid-base disorders, the change in $PCO_2$ and $HCO_3^-$ may not fit simple compensatory formulas, requiring careful assessment of AG and delta ratio.

High‑Yield Points - ⚡ Biggest Takeaways

• pH defines acidosis (<7.35) or alkalosis (>7.45).
• PaCO₂ is respiratory; opposite pH in primary respiratory disorders.
• HCO₃⁻ is metabolic; same direction as pH in primary metabolic disorders.
• Anion Gap (Na⁺ - [Cl⁻ + HCO₃⁻]) for metabolic acidosis causes; normal 8-12 mEq/L.
• Compensation: Respiratory is fast; metabolic is slow.
• Winter's formula (PCO₂ = 1.5 × HCO₃⁻ + 8 ± 2) for PCO₂ in metabolic acidosis.
• Suspect mixed disorders with inappropriate compensation.