Interpreting Arterial Blood Gases (ABGs)

Key Parameters to Know

• pH
  • Normal: 7.35 \text{ – } 7.45
  • Unit-less; expresses overall acidity/alkalinity of the extracellular fluid bathing every cell.
• PaCO₂ (arterial carbon-dioxide tension)
  • Normal: 35 \text{ – } 45 mm Hg
  • Primary respiratory component; rises stimulate the brainstem to trigger breathing.
• HCO₃⁻ (Bicarbonate)
  • Normal: 22 \text{ – } 26 mEq L⁻¹
  • Metabolic/renal component; produced by the kidneys to buffer the constant acid by-products of cellular metabolism.

Physiology & Significance

• Every cell generates acidic waste (metabolic acids).
  • If not buffered → pH drifts, cellular efficiency falls (organs still survive but work sub-optimally).
• Kidneys create bicarbonate to “package” acids, return them to the kidney, and excrete them – thus stabilising pH.
• Lungs excrete CO₂, the volatile acid; rising PaCO₂ is the dominant stimulus to breathe.

Fundamental Relationship Rules (MUST Memorise)

• Direct relationships with pH:
  • \uparrow \text{HCO}_3^- \Rightarrow \uparrow \text{pH}
  • \downarrow \text{HCO}_3^- \Rightarrow \downarrow \text{pH}
• Inverse relationships with pH:
  • \uparrow \text{PaCO}_2 \Rightarrow \downarrow \text{pH}
  • \downarrow \text{PaCO}_2 \Rightarrow \uparrow \text{pH}

The Three-Question Method for ABG Interpretation

1. What’s wrong? (Check pH)
   • \text{pH} < 7.35 → Acidosis
   • \text{pH} > 7.45 → Alkalosis
2. What caused it? (Compare size/direction of change in PaCO₂ vs HCO₃⁻)
   • If the larger deviation is in HCO₃⁻ → Metabolic origin.
   • If the larger deviation is in PaCO₂ → Respiratory origin.
3. What’s the body doing about it? (Look at the variable NOT responsible for the primary change)
   • If the non-causative parameter shifts in the pH-correcting direction → Compensation is occurring.
   • If compensation normalises pH → Fully compensated.
   • If pH still abnormal → Partially (or uncompensated).

Worked Examples (from lecture)

Example 1 – Uncompensated Metabolic Acidosis

• ABG: \text{pH}=7.21, \text{PaCO}2=32, \text{HCO}3^-=14
  • pH ↓ (acidosis).
  • ΔPaCO₂ = 35 – 32 = 2 units ↓.
  • ΔHCO₃⁻ = 22 – 14 = 8 units ↓ (largest change).
  • Cause = low HCO₃⁻ → Metabolic acidosis.
  • PaCO₂ fell (↓ CO₂ → ↑ pH) but pH still acidotic → Uncompensated.

Example 2 – Uncompensated Respiratory Acidosis

• ABG: \text{pH}=7.18, \text{PaCO}2=66, \text{HCO}3^-=29
  • pH ↓ (acidosis).
  • ΔPaCO₂ = 66 – 45 = 21 units ↑ (largest change).
  • Cause = high PaCO₂ → Respiratory acidosis.
  • HCO₃⁻ mildly ↑ (29) trying to raise pH, but still acidotic → Uncompensated.

Example 3 – Compensated Respiratory Acidosis

• ABG: \text{pH}=7.36 (low-normal), \text{PaCO}2=54, \text{HCO}3^-=32
  • pH officially normal but at low end → look for hidden problem.
  • ΔPaCO₂ = 54 – 45 = 9 units ↑ (largest).
  • Cause = elevated PaCO₂ → Respiratory acidosis.
  • HCO₃⁻ ↑ by 6 units, raising pH back into range → Compensated.

Step-by-Step Checklist for Any ABG

1. Write normals beside actual values for quick reference.
2. For each parameter, note: “↑ or ↓” & “how many units?”.
3. Identify the largest deviation (PaCO₂ vs HCO₃⁻) → primary disorder.
4. Classify:
   • Metabolic acidosis / alkalosis (primary HCO₃⁻ shift)
   • Respiratory acidosis / alkalosis (primary PaCO₂ shift)
5. Examine the non-primary component for compensatory movement.
6. Label final status:
   • “Uncompensated”, “Partially compensated”, or “Fully compensated”.

Practical Tips & Insights

• Memorise a single slide with normals & the four arrow rules; interpretation becomes pattern recognition.
• The body always prioritises returning pH to normal to optimise organ efficiency; however, self-correction can fail ("uncompensated").
• You cannot voluntarily hold your breath to unconsciousness; rising PaCO₂ overrides voluntary control.
• An impression that ABGs are “mystical” is unfounded—systematic use of the three questions rapidly yields the answer.

Recap of Terminology

• Acidosis / Alkalosis – describes the direction of pH change.
• Metabolic – driven by renal (HCO₃⁻) disturbance.
• Respiratory – driven by ventilatory (CO₂) disturbance.
• Compensation – the unaffected system’s attempt to restore pH (kidneys for respiratory issues, lungs for metabolic issues).

Remember

• \text{Normal pH}=7.35\text{–}7.45
• \text{Normal PaCO}_2=35\text{–}45
• \text{Normal HCO}_3^-=22\text{–}26
• Four arrow rules are non-negotiable knowledge.

"Whichever of PaCO₂ or HCO₃⁻ moves farthest from its normal is responsible; the other tries to fix it—always."