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Old 10-25-2012
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Question The cause of acid base disturbance

An anxious young woman presents to the emergency department with an acute onset of severe abdominal pain. She states that she "partied a little bit last night" and consumed approximately 8 or 9 alcoholic drinks. She also admits to using diuretics to "lose water weight." Her stool is guaiac-negative, but she has periumbilical tenderness to palpation. An arterial blood gas study shows that her pH is 7.55 and her bicarbonate level is 21 mEq/L, with a partial pressure of carbon dioxide of 25 mm Hg. Her serum shows normal sodium chloride levels. Which of the following is the origin of her acid-base disturbance?

A A build-up of unmeasured anions due to hepatic metabolism of alcohol
B Electrolyte imbalance due to diuretic use
C Hyperventilation secondary to pain and anxiety
D Hypoventilation due to the respiratory depression caused by alcohol ingestion
E Vomiting due to alcohol toxicity
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ABGs show respiratory alkalosis. so answer is C : Hyperventilation secodary to pain and anxiety.
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C Hyperventilation secondary to pain and anxiety
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Default my answer :)

C Hyperventilation secondary to pain and anxiety
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Default My answer c

Yep absolutely C
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The correct answer is C. According to her lab data, this young woman has an acute respiratory alkalosis. Respiratory alkalosis is caused by a loss of CO2, which is balanced by an increased excretion of HCO3−. Hence, a low CO2 and low HCO3− level indicate respiratory alkalosis. The key to this question is to recognize that respiratory alkalosis can be caused only by an increase in ventilation, which can be caused by low oxygen (in high altitudes) or by sympathetic stimulation such as anxiety, panic attack, or pain. In this case, the patient is described as anxious and presents with severe abdominal pain, which is most likely the result of acute alcohol-induced pancreatitis. Both the anxiety and the pain could be causing her to hyperventilate.

Answer A is incorrect. An increase in anions would be consistent with anion-gap metabolic acidosis. Metabolic acidosis is indicated by the presence of a low pH with a low plasma HCO3− and a low CO2, and an increased anion gap, measured by ([Na+] - [Cl−] - [HCO3−]), which is normally between 10 and 16 mEq/L.

Answer B is incorrect. Diuretic use can cause metabolic alkalosis by volume contraction. This causes the kidney to compensate by reabsorbing sodium and excreting hydrogen ions. A metabolic alkalosis would present with elevated pH, elevated CO2, and elevated HCO3−.

Answer D is incorrect. Hypoventilation causes a reduction in pH due to CO2 retention. This will lead to a respiratory acidosis with a low pH, a high CO2, and a high HCO3−. The compensatory mechanism for respiratory acidosis is an increase in HCO3− retention by the kidneys to normalize the pH.

Answer E is incorrect. Vomiting causes a metabolic alkalosis secondary to the loss of acid and chloride from the stomach. If this were the cause, this patient's lab results would show a high pH, a high HCO3−, and (with respiratory compensation) a high CO2. The causes of metabolic alkalosis include vomiting, diuretic therapy, and chloride restriction. The compensation for metabolic alkalosis is hypoventilation.
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