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A 19-year-old woman with 5-day history of vomiting and diarrhea...

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asked Apr 5, 2017 in Medicine by Rajesh Bahara

A 19-year-old woman with a severe gastrointestinal infection presents to the emergency department with a 5-day history of vomiting and diarrhea. Serum chemistry tests show:

Na+: 138 mEq/L
K+: 3.0 mEq/L
Cl–: 88 mEq/L HCO3–: 21 mEq/L; BUN: 10 mg/dL Creatinine: 0.8 mg/dL Glucose: 101 mg/dL 

Arterial blood gas analyses shows a pH of 7.38, partial arterial carbon dioxide pressure of 37 mm Hg, partial arterial oxygen pressure of 82 mm Hg, and an oxygen saturation of 96% on room air. Which of the following statements is most accurate regarding this patient’s acid-base status?

(A) She has a metabolic acidosis
(B) She has a mixed metabolic alkalosis and metabolic acidosis
(C) She has a mixed respiratory alkalosis and respiratory acidosis
(D) She has no acid-base disturbances
(E) She has a respiratory alkalosis

1 Answer

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Best answer
(B) She has a mixed metabolic alkalosis and metabolic acidosis
While this patient’s pH, bicarbonate, and carbon dioxide levels are all very close to normal, it is always important to look more closely before concluding that there is no disorder. Vomiting is a common cause of a metabolic alkalosis, while diarrhea is a common cause of non-anion-gap metabolic acidosis. The patient has had gastrointestinal symptoms that have led to acute dehydration, which indicates that these symptoms are prob- ably quite severe. It is also important to look at the serum chemistry. One would expect a hypokalemic hypochloremic metabolic alkalosis from vomiting, but only the electrolyte deficiencies are present. The equalized pH suggests that the patient is losing an equal amount of acid through vomiting as she is base through diarrhea. Therefore, it is more likely that she has a mixed acid-base disorder than no electrolyte imbalances at all.

Answer A is incorrect. Non-anion-gap metabolic acidosis is the presence of a low pH with a low plasma bicarbonate level and without an elevated anion gap. It is characterized by a compensatory retention of the other main body anions, which results in hyperchloremia. The cause is generally diarrhea and renal tubular acidosis.

Answer C is incorrect. While a mixed respiratory disorder could lead to this electrolyte profile, the patient has no respiratory pathology. Therefore, it is more likely that her acid-base status is being determined by a metabolic process.

Answer D is incorrect. Although this patient’s pH, bicarbonate, and carbon dioxide levels are close to normal, the gastrointestinal symptoms (vomiting, diarrhea) suggest that she has a mixed acid-base disorder than no electrolyte imbalances at all.

Answer E is incorrect. Respiratory alkalosis can be caused only by an increase in ventilation leading to excessive loss of carbon dioxide, which is balanced by an increased excretion of bicarbonate. Hence, a high pH and low carbon dioxide and bicarbonate levels indicate respiratory alkalosis.
answered Apr 5, 2017 by Rajesh Bahara
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