Respiratory · Year 1 · from Respiratory
Case 2: COPD Exacerbation with Acute-on-Chronic Respiratory Acidosis
Case Presentation
A 70-year-old man with severe COPD (FEV1 28% predicted) on home oxygen presents with worsening dyspnea and increased sputum production for 3 days. On examination, he is in respiratory distress with pursed-lip breathing and use of accessory muscles. Vital signs show heart rate 105 bpm, blood pressure 148/88 mmHg, respiratory rate 28/min, and oxygen saturation 82% on his usual 2 L/min nasal cannula.
Arterial blood gas on 2 L/min nasal cannula shows pH 7.28, PaCO2 72 mmHg, PaO2 52 mmHg, HCO3 33 mEq/L. Previous baseline ABG from 6 months ago showed pH 7.38, PaCO2 58 mmHg, HCO3 34 mEq/L on 2 L/min O2.
Interpretation of this ABG requires understanding his chronic baseline. At baseline, he has chronic respiratory acidosis (PaCO2 58) with full metabolic compensation (HCO3 34, pH near normal). The expected compensation for chronic respiratory acidosis is 3.5 mEq/L rise in HCO3 for each 10 mmHg rise in PaCO2 above 40. His baseline: (58-40)/10 x 3.5 = 6.3 mEq/L increase, predicting HCO3 of 24 + 6 = 30 mEq/L. His actual HCO3 of 34 is appropriate.
Now with acute exacerbation: PaCO2 has risen acutely from 58 to 72 mmHg. For this acute rise of 14 mmHg, only chemical buffering can occur (not enough time for renal compensation), predicting HCO3 rise of about 1 mEq/L per 10 mmHg, or approximately 1.4 mEq/L. Since his chronic compensation was already at 34, we'd expect HCO3 around 35. Actual HCO3 is 33, confirming acute-on-chronic respiratory acidosis without additional metabolic component. The pH of 7.28 reflects the acute, uncompensated portion of the CO2 rise.
The patient is treated with BiPAP (non-invasive ventilation), bronchodilators, systemic corticosteroids, and antibiotics for presumed infectious exacerbation. Oxygen is titrated carefully to maintain SpO2 88-92% to avoid suppressing hypoxic respiratory drive. Within 48 hours, his PaCO2 returns toward baseline and pH normalizes.
Key Learning Points
- Chronic respiratory acidosis shows full metabolic compensation (3.5 mEq/L HCO3 per 10 mmHg PCO2)
- Acute respiratory acidosis shows only chemical buffering (1 mEq/L HCO3 per 10 mmHg PCO2)
- Acute-on-chronic respiratory acidosis shows acidemia despite elevated bicarbonate
- Comparison to baseline ABG is essential for accurate interpretation
- Supplemental oxygen must be titrated carefully in chronic CO2 retainers