Pinson and Tang CDI Pocket Guide
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Acute on Chronic Respiratory Failure

July 3, 2021

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. An ABG is most helpful in making a diagnosis.

Chronic respiratory failure is caused by abnormalities of oxygenation and carbon dioxide elimination due to chronic lung disease. It may be classified as hypoxemic or hypercapnic.

  • Chronic hypoxemic: SpO2 < 91% on room air or pO2 < 60 on room air, and/or
  • Chronic hypercapnic: Elevated pCO2 > 50 (chronic hypercapnea/chronic respiratory acidosis) with normal pH (7.35 - 7.45)

The use of home oxygen is a reliable indicator of chronic hypoxemic respiratory failure since SpO2 ≤ 88% is required to meet medical necessity criteria for home O2.

Acute-on-chronic hypoxemic respiratory failure is an acute exacerbation or decompensation of chronic respiratory failure recognized by worsening dyspnea and the following:

  • Increase in chronic supplemental oxygen, or
  • pO2 < 60 or SpO2 < 91% on usual home O2 (rather than room air), or
  • Decrease in baseline pO2 by > 10 mmHg on ABG

If an ABG were obtained showing acute hypercapnic respiratory failure (pCO2 >50 mmHg and pH <7.35) this diagnosis is easily confirmed. In this case, however, the patient does not have acute hypercapnic respiratory failure because the pH is normal indicating only chronic hypercapnic respiratory failure.

Could she have chronic hypoxemic respiratory failure? Yes indeed, she does because she requires home O2. How about an acute decompensation of chronic hypoxemic respiratory failure? 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 60 mmHg (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 confirming acute-on-chronic hypoxemic respiratory failure.

See further discussion on how to differentiate acute and chronic respiratory failure in the CDI Pocket Guide.

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This topic is sourced directly from the original 
CDI Pocket Guide®. 

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