med 610 clinical respiratory diseases & critcare med

Arterial Blood Gas

Case 5 Answers

A 68 year-old man with a history of very severe COPD (FEV1 ~ 1.0L, <25% predicted) and chronic carbon dioxide retention (Baseline PCO2 58) presents to the emergency room complaining of worsening dyspnea and an increase in the frequency and purulence of his sputum production over the past 2 days. His oxygen saturation is 78% on room air. Before he is place on supplemental oxygen, a room air arterial blood gas is drawn and reveals: pH 7.25, PCO2 68, PO2 48, HCO3- 31

Acid-base status:

  • The patient has a low pH (acidemia)
  • The patient has a high PCO2 (respiratory acidosis) and a high bicarbonate (metabolic alkalosis). The combination of the low pH and the high PCO2 tells us that the respiratory acidosis is the primary process.
  • The metabolic alkalosis is the compensatory process. The pH is still low despite this metabolic compensation
  • Summary: Primary respiratory acidosis with compensatory metabolic alkalosis.

Alveolar-arterial oxygen difference:

The alveolar-arterial oxygen difference is 17 mmHg. This value is elevated, suggesting that the hypoxemia is due to either shunt or areas of low V/Q (the more likely explanation in a patient with COPD) and cannot be explained by hypoventilation alone.

Explanation for the clinical picture:

The patient has very severe COPD and chronic carbon dioxide retention. As a result, you expect that at baseline, they will have a chronic respiratory acidosis (his baseline PCO2 was 58) with a compensatory metabolic alkalosis. In this case, the clinical history suggests the patient is in an exacerbation. When the patient presents to the ER, his PCO2 is elevated above his baseline. Because this is an acute change, the bicarbonate has not had time to adjust and the pH falls. This case is, therefore, an example of an acute on chronic respiratory acidosis.

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