WBR1105

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= 28mL Respiratory Rate: 16/min Which of the following represents the physiologic dead space in this patient?]]
Answer A	AnswerA::
Answer A Explanation	AnswerAExp::
Answer B	AnswerB::
Answer B Explanation	AnswerBExp::
Answer C	AnswerC::
Answer C Explanation	AnswerCExp::
Answer D	AnswerD::
Answer D Explanation	AnswerDExp::
Answer E	AnswerE::
Answer E Explanation	AnswerEExp::
Right Answer	RightAnswer::
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. An equation and example are provided below: <math> V_\mathrm{D} = V_\mathrm{T} \,\frac{P_\mathrm{a\,CO_2}-P_\mathrm{E\,CO_2 Educational Objective: References: ]]
Approved	Approved::
Keyword
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::

{P_\mathrm{a\,CO_2}} </math>

<math> V_\mathrm{D} = 0.5\,\frac{0.056-0.040}{0.056} = 0.143 \ \mathrm{L} </math>

• V_D = dead space
• V_T = tidal volume
• P_aCO2 = partial pressure of carbon dioxide in arteries
• P_ECO2 = partial pressure of carbon dioxide in exhaled air

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

Educational Objective: The physiologic dead space can be calculated by the dilution of alveolar CO2 as it is exhaled, using the Bohr equation.

References: First Aid 2014 page 596. |AnswerA=150 mL |AnswerAExp=Incorrect: 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 190mL. |AnswerB=175 mL

|AnswerBExp=Incorrect: Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL. |AnswerC=180 mL

|AnswerCExp=Correct: Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL. |AnswerD=190 mL

|AnswerDExp=Incorrect: Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL.

|AnswerE=225 mL |AnswerEExp=Incorrect: Inserting the values given above into the Bohr equation shows that the physiologic dead space in this patient is 180 mL. |RightAnswer=C |WBRKeyword=Lung, Lung volume, Dead space, Pulmonology |Approved=Yes }}