SAQ 163 – Respiratory – severe asthma acid-base analysis and ventilation

Question 6. severe asthma acid-base analysis and ventilation.

a. Calculations

• Winter's formula: expected pCO2 = 1.5 x HCO3 + 8, approximately 35 mmHg; actual pCO2 is 80 mmHg.
• Alveolar gas calculation gives PAO2 approximately 256 mmHg and an A-a gradient approximately 10 mmHg, which is normal.

b. Primary abnormality

• Severe respiratory acidosis due to hypercapnic respiratory failure.

c. Secondary abnormality

• Concurrent metabolic acidosis because the bicarbonate is lower than expected for an isolated respiratory acidosis.

d. Unifying explanation

• Type 2 hypercapnic respiratory failure from severe obstructive airways disease with fatigue and hypoventilation.
• Possible acute-on-chronic respiratory acidosis related to obesity hypoventilation or chronic airways disease.
• Concurrent lactic metabolic acidosis from beta-agonists, adrenaline, or impaired perfusion.
• Normal A-a gradient suggests no major shunt, diffusion limitation, or V/Q mismatch.

e. Ventilation aims

• Ensure adequate oxygenation while avoiding excessive peak pressures and dynamic hyperinflation.
• Allow permissive hypercapnia and use a low respiratory rate with prolonged expiratory time.
• Use low tidal volumes based on ideal body weight, adequate sedation, and neuromuscular blockade if required.