WBR0949

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	USMLE Step 1
Main Category	Biochemistry, Physiology
Sub Category	Pulmonology
Prompt	A 22-year-old woman is brought to the emergency department (ED) after several members of her family witnessed her suffering a seizure. On the way to the hospital, she admitted to swallowing 50 aspirin pills in a suicide attempt. She is unconscious but responds to painful stimuli. In the ED, results from arterial blood gas on room air demonstrate the following: pH=7.32, pCO2=18 mmHg, pO2=111 mmHg, Bicarbonate=15 mEq/L. Blood samples are withdrawn, and electrolytes are as follows: Sodium=140 mEq/L, Chloride=100 mEq/L. Which of the following would have a similar effect on the oxygen-hemoglobin dissociation curve as this patient’s condition?
Answer A	Decrease in altitude
Answer A Explanation	A decrease in altitude would left-shift the oxygen-hemoglobin dissociation curve.
Answer B	Decrease in temperature
Answer B Explanation	A decrease in temperature would left-shift the oxygen-hemoglobin dissociation curve.
Answer C	Decrease in CO2
Answer C Explanation	A decrease in CO2 would left-shift the oxygen-hemoglobin dissociation curve.
Answer D	Increase in 2,3-bisphosphoglycerate (2,3-BPG)
Answer D Explanation	An increase in 2,3-BPG would right-shift the oxygen-hemoglobin dissociation curve.
Answer E	Increase in carbon monoxide (CO)
Answer E Explanation	Because carbon monoxide (CO) has a 200x higher affinity for hemoglobin than does oxygen, an increase in CO would cause a non-competitive decrease in the oxygen binding capacity of hemoglobin.
Right Answer	D
Explanation	This patient has a high anion gap metabolic acidosis due to salicylate toxicity. The criteria for high anion gap metabolic acidosis is pH < 7.35 and an anion gap >16 (anion gap limit differs with age). The anion gap is defined by the following equation: Anion Gap = Na - Cl - Bicarbonate In this case, the anion gap equals 140-100-15= 25. The oxygen-hemoglobin dissociation curve is right shifted by a decrease in pH (increasing acidity). The following factors result in a right shift of the oxygen-hemoglobin dissociation curve: CO2 BPG (2,3-BPG) Exercise Acidic pH High altitude High temperature Educational Objective: The following factors result in a right shift of the oxygen-hemoglobin dissociation curve: CO2 BPG (2,3-BPG) Exercise Acidic pH High altitude High temperature References: First Aid 2013 page 548
Approved	Yes
Keyword	Oxygen, Hemoglobin, Curve, Metabolic acidosis, Aspirin, Toxicity, Overdose, BPG, High anion gap
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