Arterial Blood Gas (ABG) Interpretation

• Oxygen levels in the blood (PaO₂)
• Carbon dioxide levels (PaCO₂)
• Plasma acid/base status (pH, HCO₃, base excess)

Step 1: Is the Patient Hypoxemic?

Step 2: Is the Patient Hypercarbic?

• PaCO₂ > 45 mmHg = Hypercarbia*

When evaluating CO₂, also look at pH:

• Elevated PaCO₂ with normal pH → Compensatory process (e. g., chronic CO₂ retainer)
• Elevated PaCO₂ with low pH → Clinically significant respiratory acidosis — diagnose and treat the cause

Step 3: Does the Patient Have Acidemia?

• pH < 7.35 = Acidemia*

• Low pH → more O₂ released at tissue level (potentially beneficial)
• But low pH → global peripheral vasodilation → diverts perfusion away from vital organs (harmful)
• The "win" of better O₂ delivery is severely diminished by the "loss" of directed blood flow

Categories of Acidemia

1. Purely Respiratory — elevated PaCO₂ driving low pH
2. Purely Metabolic — low HCO₃ driving low pH (anion gap vs. non-anion gap)
3. Mixed — Combined respiratory + metabolic

Step 4: Does the Patient Have Alkalemia?

• pH > 7.45 = Alkalemia*

Clinical note: Alkalemia affects respiratory drive — the brain responds by slowing breathing to retain CO₂ (lower pH back toward normal). The degree of pH rise needed to cause clinically significant respiratory suppression is typically higher than levels seen in the ICU.

CO₂ levels directly affect cerebral vascular tone and can affect Cerebral Perfusion Pressure (CPP).

In patients with severe traumatic brain injury, tight CO₂ range control may be mandated despite pH measurements.

• Hypercarbia → cerebral vasodilation → ↑ ICP
• Hypocarbia → cerebral vasoconstriction → ↓ CPP (also harmful)

• P/F Ratio
• Anion Gap

• Adapted from SurgCritCare. org. Verify against current clinical standards and attending guidance.*