What is the interpretation of a blood gas that shows a pH of 7.44, a PaCO₂ of 50 mm Hg, a PaCO₂ of 48 mm Hg, and an [HCO₃^-] of 33 mEq/L in a patient with advanced COPD?

Question

Quizplus · Accepted Answer

A patient with advanced but stable COPD and compensated respiratory acidosis may exhibit the following arterial blood gas values: pH = 7.38 PaCO₂ = 58 mm Hg HCO₃^- = 33 mEq/L Note that the HCO₃- level is high due to renal compensatory activity. If the person now experiences an acute exacerbation of his disease and becomes severely hypoxic, this may stimulate the peripheral chemoreceptors enough to increase alveolar ventilation and acutely lower the PaCO? despite the patient's disturbed lung mechanics. In this case, let us assume that the patient was not capable of hyperventilating enough to lower the PaCO₂ all the way down into the normal range. Even though the PaCO₂ is still above normal, the acute lowering of the PaCO₂coupled with the preexisting high compensatory level of HCO₃^- causes the pH to swing from the acidic to the alkalotic side of normal: pH = 7.44 PaCO₂ = 50 mm Hg HCO₃^- = 33 mEq/L If one were to pay attention only to the blood gas data and apply the strict criteria for interpretation, this would lead to the completely incorrect interpretation of compensated metabolic alkalosis. This interpretation would lead the novice to conclude that the patient is hypoventilating to compensate for a preexisting primary metabolic alkalosis; in light of the patient's medical history, this would be an absurd conclusion. This example illustrates the fact that blood gas data alone are not a sufficient basis for rational acid-base assessment. The patient's medical history, the physical examination, and the nature of the current problem are vital in accurately evaluating blood gas abnormalities. The blood gases here are correctly described as acute hyperventilation superimposed on compensated respiratory acidosis.

What Is the Interpretation of a Blood Gas That Shows