20/10/2025
ABG (Arterial Blood Gas) in Clinical Practice
Definition
ABG means Arterial Blood Gas.
It is a vital test that shows how well the lungs and kidneys are maintaining the acid–base balance, oxygenation, and ventilation of the body.
It helps detect problems like respiratory failure, metabolic acidosis, and oxygen imbalance.
🟦 Main Parameters and Normal Values
🔹 pH: 7.35 – 7.45 → shows acid–base balance
🔹 PaCO₂: 35 – 45 mmHg → represents respiratory component (ventilation)
🔹 HCO₃⁻: 22 – 26 mEq/L → represents metabolic component (renal)
🔹 PaO₂: 80 – 100 mmHg → represents oxygenation
🔹 SaO₂: 95 – 100% → percentage of hemoglobin saturated with oxygen
🔹 Base Excess (BE): −2 to +2 mEq/L → indicates metabolic compensation
🔹 Anion Gap (AG): 8 – 12 mEq/L → helps detect type of metabolic acidosis
Step-by-Step Interpretation
🟢 Step 1 — Check the pH
If pH < 7.35 →Acidemia
If pH > 7.45 → Alkalemia
If pH is 7.35–7.45 → Normal (may be compensated)
🔵 Step 2 — Identify the Primary Disturbance
If pH is low and PaCO₂ is high → Respiratory Acidosis
If pH is high and PaCO₂ is low → Respiratory Alkalosis
If pH is low and HCO₃⁻ is low → Metabolic Acidosis
If pH is high and HCO₃⁻ is high → Metabolic Alkalosis
🟣 Step 3 — Check for Compensation
The body tries to restore normal pH.
Respiratory Acidosis → Kidneys increase HCO₃⁻ to compensate
Respiratory Alkalosis → Kidneys excrete more HCO₃⁻
Metabolic Acidosis → Lungs blow off CO₂ (hyperventilation)
Metabolic Alkalosis → Lungs retain CO₂ (hypoventilation)
👉 If compensation is absent or inadequate → there is a mixed disorder.
🟫 Step 4 — Evaluate Oxygenation
PaO₂ between 80–100 mmHg →Normal
PaO₂ between 60–80 mmHg → Mild hypoxemia
PaO₂ between 40–60 mmHg → Moderate hypoxemia
PaO₂ below 40 mmHg →Severe hypoxemia
➡️ PaO₂/FiO₂ ratio
More than 300 → Normal
Less than 300 → Impaired gas exchange
Less than 200 → ARDS (Acute Respiratory Distress Syndrome)
⬛ Step 5 — Calculate the Anion Gap
Formula: AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal value = 8 – 12 mEq/L
If AG > 12 →High Anion Gap Metabolic Acidosis (DKA, lactic acidosis, renal failure, toxins)
If AG normal → Normal Anion Gap (Hyperchloremic) Metabolic Acidosis (diarrhea, renal tubular acidosis)
Common Clinical Patterns
Respiratory Acidosis → Seen in COPD, asthma, hypoventilation, or airway obstruction.
ABG: pH ↓, PaCO₂ ↑, HCO₃⁻ ↑ (if chronic).
Respiratory Alkalosis → Seen in anxiety, pain, fever, sepsis, hypoxia.
ABG: pH ↑, PaCO₂ ↓, HCO₃⁻ ↓ (if chronic).
Metabolic Acidosis → Seen in DKA, renal failure, shock, diarrhea.
ABG: pH ↓, HCO₃⁻ ↓, PaCO₂ ↓ (compensation).
Metabolic Alkalosis → Seen in vomiting, diuretics, hypokalemia.
ABG: pH ↑, HCO₃⁻ ↑, PaCO₂ ↑ (compensation).
🟪 Mnemonic: ROME Rule
ROME = Respiratory Opposite, Metabolic Equal
If pH and PaCO₂ move in opposite directions →Respiratory
If pH and HCO₃⁻ move in the same direction →Metabolic
Examples:
🟥 Respiratory Acidosis → pH ↓, PaCO₂ ↑
🟩 Respiratory Alkalosis → pH ↑, PaCO₂ ↓
🟧 Metabolic Acidosis → pH ↓, HCO₃⁻ ↓
🟦 Metabolic Alkalosis → pH ↑, HCO₃⁻ ↑
🟤 ABG in Critical Care
COPD Exacerbation → Respiratory Acidosis (chronic + metabolic compensation)
Sepsis or Shock → Metabolic Acidosis (lactic acidosis)
Anxiety or Pain → Respiratory Alkalosis
Vomiting → Metabolic Alkalosis
Salicylate Toxicity → Mixed Metabolic Acidosis + Respiratory Alkalosis
DKA → High AG Metabolic Acidosis
Cardiac Arrest → Severe Metabolic Acidosis + Hypoxemia
🩶 Normal ABG Values Summary (for memory)
pH → 7.35 – 7.45
PaCO₂ → 35 – 45 mmHg
HCO₃⁻ → 22 – 26 mEq/L
PaO₂ → 80 – 100 mmHg
SaO₂ → 95 – 100%
Base Excess → −2 to +2 mEq/L
Anion Gap → 8 – 12 mEq/L
🩷 Quick Clinical Tips
Low pH + High CO₂ → Respiratory Acidosis
High pH + Low CO₂ → Respiratory Alkalosis
Low pH + Low HCO₃⁻ → Metabolic Acidosis
High pH + High HCO₃⁻ → Metabolic Alkalosis
Always check O₂ level and compensation
Always calculate Anion Gap in every metabolic acidosis
Thanks
Dr. Akoon Bol Akoon