WBR1105

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Author PageAuthor::William J Gibson
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Anatomy, MainCategory::Physiology
Sub Category SubCategory::Pulmonology
Prompt [[Prompt::The Kayan tribe is a Burmese tribe known for a tradition in which the women gradually extend the length of their necks with coils. A 35-year-old Kayan woman presents to her local physician for shortness of breath that occurs when she travels to the farm. Before administering a methacholine challenge test, the physician makes some baseline measurements:
  • Tidal volume= 600mL
  • PaCO2= 40 mmHg
  • Pexpired CO2= 28 mmHg
  • Respiratory Rate: 16/min

Which of the following represents the physiologic dead space in this patient?]]

Answer A AnswerA::150 mL
Answer A Explanation AnswerAExp::150mL is approximately the normal physiologic dead space, but the dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180mL.
Answer B AnswerB::175 mL
Answer B Explanation AnswerBExp::Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL.
Answer C AnswerC::180 mL
Answer C Explanation AnswerCExp::Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL.
Answer D AnswerD::190 mL
Answer D Explanation AnswerDExp::Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL.
Answer E AnswerE::225 mL
Answer E Explanation AnswerEExp::Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL.
Right Answer RightAnswer::C
Explanation [[Explanation::The patient in this vignette is being evaluated for asthma after reporting feeling pulmonary symptoms on exposure to a foreign stimulus (the farm). In this case, the patient has a unique anatomy due to the tribe’s tradition of using series of coils to extend their necks. Dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it remains in the conducting airways, or reaches alveoli that are not perfused or poorly perfused. We are asked to calculate the physiologic dead space in this patient.

Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution it is possible to measure anatomical and alveolar dead space, employing the concept of mass balance, as expressed by Bohr equation (below).


In normal individuals, physiologic dead space is approximately 150mL. However, plugging in the values given in the stem to the Bohr equation, we find that the physiologic dead space is increased to 180 mL.
Educational Objective: The physiologic dead space can be calculated with the Bohr equation, which quantifies the using the dilution of alveolar CO2 as it is exhaled.
References: First Aid 2015 page 602
First Aid 2014 page 596]]

Approved Approved::Yes
Keyword WBRKeyword::Lung, WBRKeyword::Lung volume, WBRKeyword::Dead space, WBRKeyword::Pulmonology, WBRKeyword::Physiology, WBRKeyword::Respiratory physiology
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