Solution to Blood Gas #5

History: 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

5-step-approach-to-blood-gas-analysis

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”.

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