Clinical scenario: Chronic respiratory issue | Clubbing | Bronchiectasis | ABGs
A 10-year-old boy is evaluated in the pediatric respiratory clinic for progressive exercise intolerance. He has had recurrent episodes of productive cough requiring antibiotics since early childhood and has poor weight gain despite adequate appetite. Examination reveals digital clubbing, increased work of breathing, bilateral coarse crackles, and wheezing. High-resolution CT chest shows extensive bronchiectasis. Arterial blood gas analysis on room air shows:
- pH: 7.35
- PaCO₂: 55 mmHg
- HCO₃⁻: 30 mmol/L
- PaO₂: 58 mmHg
Which of the following best describes his acid–base status?
A. Acute respiratory acidosis without compensation
B. Chronic respiratory acidosis with renal compensation
C. Acute respiratory alkalosis with metabolic compensation
D. Metabolic alkalosis with respiratory compensation
E. Mixed respiratory acidosis and metabolic acidosis
Correct answer & Explanation:
Correct Answer
B. Chronic respiratory acidosis with renal compensation
Detailed Explanation: Cystic fibrosis | Acid base imbalance
This child has features suggestive of chronic suppurative lung disease due to cystic fibrosis:
- Recurrent respiratory infections
- Poor growth
- Digital clubbing
- Bronchiectasis
- Obstructive lung disease
However, the question is testing interpretation of the arterial blood gas.
Step 1: Identify the primary abnormality
pH = 7.35
Normal pH:
7.35–7.45
The pH is at the lower limit of normal, suggesting compensation.
PaCO₂ = 55 mmHg
Normal PaCO₂:
35–45 mmHg
PaCO₂ is elevated.
Therefore:
Primary respiratory acidosis
Step 2: Determine if compensation is appropriate
The key distinction is acute versus chronic respiratory acidosis.
Acute respiratory acidosis
For every 10 mmHg increase in PaCO₂:
↑ HCO₃⁻ by approximately 1 mmol/L
Chronic respiratory acidosis
For every 10 mmHg increase in PaCO₂:
↑ HCO₃⁻ by approximately 3.5–4 mmol/L
PaCO₂ rise:
55 − 40 = 15 mmHg
Expected HCO₃⁻ increase in chronic respiratory acidosis:
15/10 × 4 = 6 mmol/L
Normal HCO₃⁻ ≈ 24 mmol/L
Expected:
24 + 6 = 30 mmol/L
Measured HCO₃⁻:
30 mmol/L
Therefore:
✅ Appropriate renal compensation
Why this occurs
In chronic cystic fibrosis lung disease:
Airway obstruction and bronchiectasis cause:
↓ Effective alveolar ventilation
↓
CO₂ retention
↓
Respiratory acidosis
Over time, kidneys compensate by:
- Increasing hydrogen ion excretion
- Increasing bicarbonate reabsorption
Result:
↑ HCO₃⁻ and partial correction of pH
Why the other options are incorrect
A. Acute respiratory acidosis without compensation
Incorrect.
In acute respiratory acidosis, bicarbonate would not rise enough.
Expected HCO₃⁻:
Approximately 25–26 mmol/L, not 30.
B. Chronic respiratory acidosis with renal compensation
Correct.
The elevated bicarbonate corresponds exactly with expected chronic compensation.
C. Acute respiratory alkalosis with metabolic compensation
Incorrect.
Respiratory alkalosis causes:
↓ PaCO₂
This patient has elevated PaCO₂.
D. Metabolic alkalosis with respiratory compensation
Incorrect.
The primary abnormality is increased PaCO₂, not increased bicarbonate.
E. Mixed respiratory acidosis and metabolic acidosis
Incorrect.
In mixed respiratory and metabolic acidosis, bicarbonate would be lower than expected.
Examples:
- Sepsis
- Renal failure
- Severe shock
CPSP Pearls ⭐
Chronic respiratory acidosis is seen in:
- Cystic fibrosis with advanced lung disease
- Bronchopulmonary dysplasia
- Neuromuscular weakness
- Severe chronic airway disease
Compensation rules:
Respiratory acidosis
Acute:
- HCO₃⁻ rises ~1 mmol/L per 10 mmHg ↑ PaCO₂
Chronic:
- HCO₃⁻ rises ~3.5–4 mmol/L per 10 mmHg ↑ PaCO₂
Important exam point
A near-normal pH does not mean the ABG is normal.
Always identify:
- Primary disorder
- Expected compensation
- Evidence of mixed disorder
