Blood Gas Interpretation (ABG Basics)
Arterial blood gas (ABG) analysis helps diagnose respiratory, metabolic, and mixed acid-base imbalances.
🔹 Normal ABG Values
| Parameter | Normal Range | Interpretation |
|---|---|---|
| pH | 7.35 – 7.45 | Acid-Base Balance |
| PaCO₂ | 35 – 45 mmHg | Respiratory Component |
| HCO₃⁻ | 22 – 26 mEq/L | Metabolic Component |
| PaO₂ | 75 – 100 mmHg | Oxygenation |
| Base Excess (BE) | -2 to +2 | Marker of Metabolic Status |
🔹 How to Interpret ABG in 3 Steps
- Assess pH → Acidosis (<7.35) or Alkalosis (>7.45)?
- Determine Primary Cause → Respiratory (PaCO₂ change) or Metabolic (HCO₃⁻ change)?
- Check Compensation → If lungs/kidneys compensate for imbalance.
🔴 ACID-BASE DISORDERS & SAMPLE CASES
🔵 CASE 1: METABOLIC ACIDOSIS (Diabetic Ketoacidosis – DKA)
📌 Case:
A 25-year-old male with Type 1 Diabetes presents with vomiting, deep rapid breathing (Kussmaul respiration), and altered mental status.
🔹 ABG Results:
| Parameter | Value | Normal Range |
|---|---|---|
| pH | 7.21 | 7.35 – 7.45 (Acidic) |
| PaCO₂ | 28 mmHg | 35 – 45 mmHg (Compensation) |
| HCO₃⁻ | 14 mEq/L | 22 – 26 mEq/L (Low) |
| Anion Gap | 18 (High) | 8 – 12 (Normal) |
🔹 Interpretation:
- pH ↓ → Acidosis
- HCO₃⁻ ↓ → Metabolic Acidosis
- Anion Gap >12 → High Anion Gap Metabolic Acidosis (HAGMA)
🔹 Diagnosis: Diabetic Ketoacidosis (DKA)
🔹 Management: IV fluids, insulin, potassium correction.
🔴 CASE 2: RESPIRATORY ACIDOSIS (COPD Exacerbation)
📌 Case:
A 65-year-old male smoker with COPD presents with worsening breathlessness, confusion, and cyanosis.
🔹 ABG Results:
| Parameter | Value | Normal Range |
|---|---|---|
| pH | 7.28 | 7.35 – 7.45 (Acidic) |
| PaCO₂ | 60 mmHg | 35 – 45 mmHg (High) |
| HCO₃⁻ | 28 mEq/L | 22 – 26 mEq/L (Compensating) |
🔹 Interpretation:
- pH ↓ → Acidosis
- PaCO₂ ↑ → Respiratory Acidosis
- HCO₃⁻ compensating → Chronic condition (COPD-related)
🔹 Diagnosis: Chronic Respiratory Acidosis in COPD
🔹 Management: Oxygen therapy, bronchodilators, steroids, BiPAP.
🔵 CASE 3: METABOLIC ALKALOSIS (Severe Vomiting)
📌 Case:
A 30-year-old female with persistent vomiting for 3 days presents with lethargy and muscle cramps.
🔹 ABG Results:
| Parameter | Value | Normal Range |
|---|---|---|
| pH | 7.52 | 7.35 – 7.45 (Alkalotic) |
| PaCO₂ | 48 mmHg | 35 – 45 mmHg (Compensation) |
| HCO₃⁻ | 32 mEq/L | 22 – 26 mEq/L (High) |
🔹 Interpretation:
- pH ↑ → Alkalosis
- HCO₃⁻ ↑ → Metabolic Alkalosis
- PaCO₂ compensation (respiratory retention of CO₂)
🔹 Diagnosis: Hypochloremic Metabolic Alkalosis from Vomiting
🔹 Management: IV fluids with potassium chloride.
🔴 CASE 4: RESPIRATORY ALKALOSIS (Anxiety-Induced Hyperventilation)
📌 Case:
A 22-year-old female presents to the ER with chest tightness, dizziness, and tingling in hands after a stressful event.
🔹 ABG Results:
| Parameter | Value | Normal Range |
|---|---|---|
| pH | 7.48 | 7.35 – 7.45 (Alkalotic) |
| PaCO₂ | 30 mmHg | 35 – 45 mmHg (Low) |
| HCO₃⁻ | 22 mEq/L | 22 – 26 mEq/L (Normal) |
🔹 Interpretation:
- pH ↑ → Alkalosis
- PaCO₂ ↓ → Respiratory Alkalosis (Hyperventilation)
- HCO₃⁻ normal (acute condition, no compensation)
🔹 Diagnosis: Anxiety-Induced Hyperventilation
🔹 Management: Reassurance, breathing into a paper bag.
🔵 CASE 5: MIXED ACID-BASE DISORDER (Septic Shock)
📌 Case:
A 75-year-old male with sepsis presents with hypotension, tachypnea, and altered mental status.
🔹 ABG Results:
| Parameter | Value | Normal Range |
|---|---|---|
| pH | 7.30 | 7.35 – 7.45 (Acidic) |
| PaCO₂ | 30 mmHg | 35 – 45 mmHg (Low) |
| HCO₃⁻ | 16 mEq/L | 22 – 26 mEq/L (Low) |
| Lactate | 5 mmol/L | <2 mmol/L (High) |
🔹 Interpretation:
- Metabolic Acidosis (Lactic Acidosis from Sepsis)
- Respiratory Alkalosis (Hyperventilation in Shock)
- Mixed disorder → Need to treat underlying infection
🔹 Diagnosis: Septic Shock with Mixed Acidosis
🔹 Management: IV fluids, vasopressors, antibiotics.
🔹 SUMMARY TABLE OF ACID-BASE DISORDERS
| Case | Disorder | Key Findings |
|---|---|---|
| Case 1 | Metabolic Acidosis (DKA) | Low pH, Low HCO₃⁻, High Anion Gap |
| Case 2 | Respiratory Acidosis (COPD) | Low pH, High PaCO₂, Compensated HCO₃⁻ |
| Case 3 | Metabolic Alkalosis (Vomiting) | High pH, High HCO₃⁻, High PaCO₂ |
| Case 4 | Respiratory Alkalosis (Anxiety) | High pH, Low PaCO₂, Normal HCO₃⁻ |
| Case 5 | Mixed Acidosis (Sepsis) | Low pH, Low HCO₃⁻, Low PaCO₂, High Lactate |
🔹 CLINICAL Spots
✅ Always check compensation mechanisms
✅ Anion Gap helps differentiate metabolic acidosis types
✅ Respiratory alkalosis common in hyperventilation & sepsis
✅ Mixed disorders require treating the primary cause