History: A 67-year-old man presents with respiratory distress. Below is his arterial blood gas on arrival:
| pH 7.31 | Na 142 mmol/L |
| pCO2 107 mmHg | K 4 mmol/L |
| pO2 47 mmHg (FiO2 0.21) | Cl 89 mmol/L |
| HCO3 52 mmol/L | Glucose 10 mmol/L |
What’s the pH?
7.31 = acidaemia
What’s the primary process?
pCO2 = 107
∴ this is a primary respiratory acidosis
Is there any compensation?
Acute respiratory acidosis: 1 for 10 rule
Expected HCO3 = 24 + (measured pCO2 – 40)/10
= 24 + (107 – 40)/10
= 30.7
Chronic respiratory acidosis: 4 for 10 rule
Expected HCO3 = 24 + 4((measured pCO2 – 40)/10)
= 24 + 4((107 – 40)/10)
= 50.8
Actual HCO3 is 52, which is consistent with a chronic, maximally-compensated acute-on-chronic respiratory acidosis.
Are there any other clues?
Electrolyte clues:
Chloride is low, glucose mildly elevated, sodium and potassium within normal limits.
Expected PAO2 = 150 – (pCO2 x 1.25)
= 150 – (107 x 1.25)
= 16.25
A-a gradient = PAO2 – PaO2
= 16.25 – 47
= -30.75
Therefore there is no A-a gradient.
What’s the differential diagnosis?
Description: This arterial blood gas shows a maximally-compensated acute-on-chronic respiratory acidosis. The chloride is low, contributing to the metabolic compensation, and glucose is mildly elevated.
Interpretation: In this clinical context, this arterial blood gas is consistent with an acute exacerbation of chronic obstructive pulmonary disease, with chronic, severe carbon dioxide retention. Management may include intravenous antibiotics, inhaled/nebulised bronchodilators, non-invasive positive pressure ventilation (bilevel positive airway pressure/BPAP), and a titrated fractional inspired oxygen concentration targeted to a peripheral capillary oxygen saturation of 88 – 92%.
Additional information: Infective exacerbation of COPD. “Normal” pCO2 is approximately 60 mmHg when “well”.
