Case: A 67 year old female with severe COPD and chronic respiratory failure on home oxygen at 2 L/min (28%) is admitted with symptoms of wheezing, cough, chest congestion and increased shortness of breath. She had increased her home oxygen to 4 L/min (36%) with some improvement. Chest x-ray shows COPD without infiltrates; no evidence of pneumonia. Pulse ox reading (SpO2) on 4 L/min is 92%. ABG on oxygen at 4L/min is pH 7.38 / pCO2 55 / pO2 64. What are the diagnoses and why?
Answer: Acute exacerbation of COPD with acute on chronic (hypoxemic) respiratory failure. The diagnosis of acute on chronic respiratory failure can be challenging. If an ABG is obtained showing acute hypercapnic respiratory failure (pCO2 >50mmHg and pH <7.35) this diagnosis is easily confirmed. In this case, the patient does not have acute hypercapnic respiratory failure because the pH is normal indicating only chronic hypercapnic respiratory failure.
Could she have acute hypoxemic respiratory failure? Yes indeed, she may! The usual indicators of hypoxemic respiratory failure (pO2, SpO2, pO2/FIO2 ratio) must be interpreted with caution when the patient has pre-existing chronic hypoxemia as this patient certainly does. It is essential to keep in mind that home oxygen is titrated to maintain the patient’s pO2 just above 60mmHg (SpO2 >91%).
In this case, she had increased shortness of breath suggesting an acute fall in oxygenation levels probably below her expected baseline. Worsening symptoms alone suggest an acute exacerbation of her chronic baseline state. The fact that she required a substantial increase in home oxygen from 2 L/min (28%) to 4L/min (36%) to improve symptoms and ultimately went home on her 2 L/minute also indicates an acute decompensation of her baseline state.
See further discussion on how to differentiate acute and chronic respiratory failure in the 2014 CDI Pocket Guide.