A nurse reviews the arterial blood gas results of a patient and notes the following: pH 7.45; PCO2 30 mm Hg; and bicarbonate concentration of 22 mEq/L. The nurse analyzes these results as indicating:
A. Metabolic acidosis, compensated.
B. Metabolic alkalosis, uncompensated.
C. Respiratory alkalosis, compensated.
D. Respiratory acidosis, compensated.
Correct Answer: C. Respiratory alkalosis, compensated.
The normal pH is 7.35 to 7.45. In a respiratory condition, an opposite (see-saw) will be seen between the pH and the PCO2. In this situation, the pH is at the high end of the normal value and the PCO2 is low. In an alkalotic condition, the pH is up. Therefore, the values identified in the question indicate a respiratory alkalosis. Compensation occurs when the pH returns to a normal value. Because the pH is in the normal range at the high end, compensation has occurred.
Option A: The pCO2 determines whether an acidosis is respiratory or metabolic in origin. Metabolic acidosis is due to alterations in bicarbonate, so the pCO2 is less than 40 since it is not the cause of the primary acid-base disturbance. In metabolic acidosis, the distinguishing lab value is a decreased bicarbonate (normal range 21 to 28 mEq/L). Respiratory compensation is the physiologic mechanism to help normalize a metabolic acidosis, however, compensation never completely corrects acidemia.
Option B: HCO3 functions as an alkalotic substance. CO2 functions as an acidic substance. Therefore, increases in HCO3 or decreases in CO2 will make blood more alkalotic. The opposite is also true where decreases in HCO3 or an increase in CO2 will make blood more acidic. CO2 levels are physiologically regulated by the pulmonary system through respiration, whereas the HCO3 levels are regulated through the renal system with reabsorption rates. Therefore, metabolic alkalosis is an increase in serum HCO3.
Option D: Respiratory acidosis typically occurs due to failure of ventilation and accumulation of carbon dioxide. The primary disturbance is an elevated arterial partial pressure of carbon dioxide (pCO2) and a decreased ratio of arterial bicarbonate to arterial pCO2, which results in a decrease in the pH of the blood. To compensate for the disturbance in the balance between carbon dioxide and bicarbonate (HCO3-), the kidneys begin to excrete more acid in the forms of hydrogen and ammonium and reabsorb more base in the form of bicarbonate. This compensation helps to normalize the pH.
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