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Old 06-08-2011
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Question Hypoxemia Case

The medical intern on-call is summoned, at 11:30 pm, to the bedside of a 66-year-old man with O2 saturation by pulse oximetry of 60% on 2 L/min O2 via nasal cannula. The patient was admitted at 8:00 am today after presentation to the emergency room with a 2-day history of fever, dyspnea, and productive cough. Chest x-ray (CXR) showed a left lower lobe infiltrate and sputum cytology showed gram-positive cocci in pairs. Past medical history is significant for hypertension and coronary artery disease (CAD) specifically acute myocardial infarction (AMI) 2 years ago. Medications are erythromycin 1 g intravenous (IV) every 6 hours and cefuroxime 750 mg IV every 8 hours. On physical examination, the patient is cyanotic and difficult to arouse. Temperature is 102°F; blood pressure (BP) is 150/90 mm HG; heart rate (HR) is 95 beats/min and irregular; respiration rate (RR) is 8 breaths/min.
What is most likely present in this patient?

Answer Choices
A. Alkaline urine pH
B. Decreased serum lactate
C. Increased plasma ion gap
D. Increased serum bicarbonate
E. Respiratory alkalosis
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Old 06-08-2011
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C. Septicemia, tissue hypoxia, high lactate resulted in high anion gap
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C. Increased plasma ion gap
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For my fellows, useful to have a quick look!

Normal range: 8 to 16 meq/L

Anion gap = [Na+] - [Cl-] - [HCO3-]
or alternative formula:
AG = [Na+] + [K+] - [Cl-] - [HCO3-].

The anion gap can be used to help identify the cause of metabolic acidosis.
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Option C (Increased plasma ion gap) is correct. Severe hypoxemia inevitably results in a lactic acidosis as a result of increased cellular anaerobic metabolism. Increased non-HCl plasma acid concentration results in an increase in the plasma ion gap.

Option A (Alkaline urine pH) is incorrect. Severe hypoxemia inevitably results in a lactic acidosis as a result of increased cellular anaerobic metabolism. The normal renal response would be to generate bicarbonate and secrete hydrogen ions, resulting in an acidic urine pH.

Option B (Decreased serum lactate) is incorrect. Severe hypoxemia inevitably results in a lactic acidosis as a result of increased cellular anaerobic metabolism.

Option D (Increased serum bicarbonate) is incorrect. Lactic acidosis should be present in this patient; serum bicarbonate levels are decreased with a metabolic acidosis.

Option E (Respiratory alkalosis) is incorrect. The patient will have a lactic acidosis because of severe hypoxemia. A compensatory respiratory alkalosis is a normal physiologic response, but the patient is in respiratory failure and can not ventilate at a sufficient minute volume to generate a compensatory hypocapnea.
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Ok then to make it more interesting!

Is it Type A or Type B Lactic Acidosis?
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Cardiology-, Internal-Medicine-, Pulmonology-, Step-2-Questions

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