The respiratory therapist is assessing a patient with end-stage COPD who was admitted to the medical ward for an exacerbation of COPD due to increasing sputum purulence. The therapist notes the following ABG results in the patient's electronic medical record: pH 7.52, PaCO₂ 51; HCO₂ 40; PaO₂ 46. Which of the following is the best interpretation of these ABG results?

Question

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An end-stage COPD patient would be expected to have chronic ventilatory failure for baseline ABG results. During an exacerbation, he or she may also experience acute periods of hyperventilation. If able, these patients have the mechanical reserve to increase their alveolar ventilation significantly in an attempt to maintain their baseline PaO₂, which has decreased in relation to an acute pulmonary problem, in this case an increase in sputum purulence. When excessive alveolar ventilation occurs, this action causes the patient's PaCO₂ to decrease from its normally "high baseline" level. As the PaCO₂ decreases, the arterial pH increases. As this condition intensifies, the patient's baseline ABG values can quickly change from chronic ventilatory failure to acute alveolar hyperventilation superimposed on chronic ventilatory failure. If the therapist does not know the past history of the patient with acute alveolar hyperventilation superimposed on chronic ventilatory failure, he or she might initially interpret the ABG values as signifying partially compensated metabolic alkalosis with severe hypoxemia. However, the clinical situation that offsets this interpretation is the presence of marked hypoxemia. A low oxygen level is not normally seen in patients with pure metabolic alkalosis. Thus, whenever the ABG values appear to reflect partially compensated metabolic alkalosis but the condition is accompanied by significant hypoxemia, the respiratory therapist should be alert to the possibility of acute alveolar hyperventilation superimposed on chronic ventilatory failure.

The Respiratory Therapist Is Assessing a Patient with End-Stage COPD