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Chapter 28. Acid–Base Disturbances

Student Questions

The clinical scenario and laboratory findings suggest which of the following?

A 68-year-old woman has a history of very heavy tobacco abuse with severe chronic obstructive pulmonary disease (COPD). She presented to the ER with respiratory extremis, diffuse wheezing, and decreased oxygen saturations. Laboratory data drawn immediately before intubation are as follows: pH 7.24, PaCO2 60 mm Hg (8.0 kPa), HCO3 26 mEq/L (mmol/L).

A. Erroneous specimen processing as these lab values are impossible

B. An acute exacerbation of COPD with CO2 retention

C. An acute salicylate poisoning

D. Bacterial pneumonia

Answer: B

Option A: Incorrect. These values do not suggest an error in processing as the pH, PaCO2, and HCO3 are valid and internally consistent.

Option B: Correct. The patient's history of tobacco abuse and COPD coupled with the physical finding of diffuse wheezing make an acute COPD exacerbation the most likely diagnosis. This is confirmed by the arterial blood gas showing an increase in the PaCO2—but not much change in the serum HCO3 (see Question 2)—consistent with acute respiratory acidosis.

Option C: Incorrect. Acute salicylate poisoning would cause hyperventilation with respiratory alkalosis.

Option D: Incorrect. Bacterial pneumonia would cause hypoxia, but the resulting hyperventilation should lead to respiratory alkalosis.

Is the observed degree of compensation appropriate?

A 68-year-old woman has a history of very heavy tobacco abuse with severe chronic obstructive pulmonary disease (COPD). She presented to the ER with respiratory extremis, diffuse wheezing, and decreased oxygen saturations. Laboratory data drawn immediately before intubation are as follows: pH 7.24, PaCO2 60 mm Hg (8.0 kPa), HCO3 26 mEq/L (mmol/L).

A. No: the HCO3 has increased by 2 mEq/L (mmol/L)

B. Yes: the HCO3 has increased by 2 mEq/L (mmol/L)

C. Yes: the PaCO2 has increased by 20 mm Hg (2.7 kPa)

D. No: the PaCO2 has increased by 20 mm Hg (2.7 kPa)

Answer: B

Option A: Incorrect. The 2 mEq/L (mmol/L) change in the serum HCO3 is appropriate.

Option B: Correct. Using the compensation formula for acute respiratory acidosis from Table 27-1 we see that the expected change in HCO3 is 0.1 x the observed change in the PaCO2. Our measured PaCO2 of ...

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