WBR0868: Difference between revisions

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|MainCategory=Pathophysiology
|MainCategory=Pathophysiology
|SubCategory=Pulmonology
|SubCategory=Pulmonology
|Prompt=A 42 year old man is brought to the emergency department with altered mental status. He was reportedly found in the garage by his son where he had spent the afternoon working on fixing up the motor of his hot rod. EMS convey the suspicion of toxic fume inhalation. On admission, the patient is tachycardic and tachypneic, and is markedly confused. If the patient's oxygen dissociation curve was to be plotted, which of the following curves would closely fit his current presentation?
|Prompt=A 59 year old woman presents to the emergency department for 2 days of severe vomiting after consuming some raw meat at a county fair. On admission, the patient is afebrile, her heart rate is 103 bpm, and her blood pressure is 92/64 mmHg. She appears frail and very uncomfortable. She is promptly rehydrated and blood is withdrawn for appropriate work-up. If the patient's oxygen dissociation curve is to be plotted on admission, which of the following curves would closely fit her current presentation?
 


[[Image:Oxygen_dissociation.png|500px]]
[[Image:Oxygen_dissociation.png|500px]]
|Explanation=Carbon monoxide (CO) is an odorless toxic gas released in the process of incomplete combustion. Hemoglobin binding sites have 250 times better affinity to CO than to oxygen. This would typically cause rapid binding of CO as well as cause conformational changes in the hemoglobin molecule leading to increased affinity to oxygen. While CO occupies oxygen binding sites, it decreases the oxygen saturation of hemoglobin. In addition, as it increases affinity of hemoglobin to oxygen it causes a left shift leading to a decrease in oxygen dissociation at the peripheral tissues. Both of those phenomena combined would lead to the red curve seen in the graph above.  
|Explanation=Metabolic alkalosis is a classical complication of excessive vomiting. As excess acid is lost, the arterial pH increases gradually. Acutely, before renal compensation begins excreting excess bicarbonate, the increasing pH causes a left shift in the oxygen dissociation curve, increasing oxygen affinity and binding at the level of the lungs but also causing less oxygen dissociation at the level of the peripheral tissues. Other factors causing a left shift include hypothermia, CO poisoning (although with a decrease in Hb saturation - Curve E), and low CO2 concentrations. A left shift in the oxygen dissociation curve signifies a lower P50, thus hemoglobin would reach 50% saturation at a lower partial pressure oxygen.




Educational Objective: Carbon monoxide poisoning causes 2 distinct effects on the oxygen dissociation curve: a left shift and a decrease in hemoglobin oxygen saturation.
Educational Objective: Metabolic alkalosis causes a left shift in the oxygen dissociation curve leading to increased affinity of Hb to oxygen.




Reference:
Reference:


Ernst A, Zibrak JD. Carbon monoxide poisoning. N Engl J Med. 1998;339(22):1603-8.
Astrup P, Engel K, Severinghaus JW, Munson E. The influence of temperature and pH on the dissociation curve of oxyhemoglobin of human blood. Scand J Clin Lab Invest. 1965;17(6):515-23.  
|AnswerA=A
|AnswerA=A
|AnswerAExp=This curve refers to the oxygen dissociation trend of myoglobin, typically exponential not sigmoid in shape due to the lack of allosteric interactions.
|AnswerAExp=This curve refers to the oxygen dissociation trend of myoglobin, typically exponential not sigmoid in shape due to the lack of allosteric interactions.
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|AnswerE=E
|AnswerE=E
|AnswerEExp=This curve depicts both a left shift and a decrease in hemoglobin saturation usually seen in patients with CO poisoning.
|AnswerEExp=This curve depicts both a left shift and a decrease in hemoglobin saturation usually seen in patients with CO poisoning.
|RightAnswer=E
|RightAnswer=B
|WBRKeyword=CO poisoning, Oxygen dissociation curve
|WBRKeyword=Metabolic alkalosis, oxygen dissociation curve
|Approved=No
|Approved=No
}}
}}

Revision as of 08:46, 7 November 2013
Author [[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Pathophysiology
Sub Category SubCategory::Pulmonology
Prompt [[Prompt::A 59 year old woman presents to the emergency department for 2 days of severe vomiting after consuming some raw meat at a county fair. On admission, the patient is afebrile, her heart rate is 103 bpm, and her blood pressure is 92/64 mmHg. She appears frail and very uncomfortable. She is promptly rehydrated and blood is withdrawn for appropriate work-up. If the patient's oxygen dissociation curve is to be plotted on admission, which of the following curves would closely fit her current presentation?

]]

Answer A AnswerA::A
Answer A Explanation AnswerAExp::This curve refers to the oxygen dissociation trend of myoglobin, typically exponential not sigmoid in shape due to the lack of allosteric interactions.
Answer B AnswerB::B
Answer B Explanation AnswerBExp::This curve typically depicts a left shift of the normal oxygen dissociation curve seen in situations of hypothermia, alkalosis, and low CO2 concentrations.
Answer C AnswerC::C
Answer C Explanation AnswerCExp::This curve represents the normal oxygen dissociation curve with typical P50 (50% of Hemoglobin saturated) at approximately 26 mmHg of PO2.
Answer D AnswerD::D
Answer D Explanation AnswerDExp::This curve typically depicts a right shift of the normal oxygen dissociation curve seen in situations of hyperthermia, acidosis, exercise and high CO2 concentrations.
Answer E AnswerE::E
Answer E Explanation AnswerEExp::This curve depicts both a left shift and a decrease in hemoglobin saturation usually seen in patients with CO poisoning.
Right Answer RightAnswer::B
Explanation [[Explanation::Metabolic alkalosis is a classical complication of excessive vomiting. As excess acid is lost, the arterial pH increases gradually. Acutely, before renal compensation begins excreting excess bicarbonate, the increasing pH causes a left shift in the oxygen dissociation curve, increasing oxygen affinity and binding at the level of the lungs but also causing less oxygen dissociation at the level of the peripheral tissues. Other factors causing a left shift include hypothermia, CO poisoning (although with a decrease in Hb saturation - Curve E), and low CO2 concentrations. A left shift in the oxygen dissociation curve signifies a lower P50, thus hemoglobin would reach 50% saturation at a lower partial pressure oxygen.


Educational Objective: Metabolic alkalosis causes a left shift in the oxygen dissociation curve leading to increased affinity of Hb to oxygen.


Reference:

Astrup P, Engel K, Severinghaus JW, Munson E. The influence of temperature and pH on the dissociation curve of oxyhemoglobin of human blood. Scand J Clin Lab Invest. 1965;17(6):515-23.
Educational Objective:
References: ]]

Approved Approved::No
Keyword WBRKeyword::Metabolic alkalosis, WBRKeyword::oxygen dissociation curve
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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