WBR1105
| Author | [[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Physiology |
| Sub Category | SubCategory::Pulmonology |
| Prompt | [[Prompt::A 35-year-old female immigrant presents to the physician's for shortness of breath that occurs when she travels to the farm. Upon further questioning, the patient informs the physician that she belongs to the Kayan tribe of Burma that is known for a tradition in which the women gradually extend the length of their necks with coils. Prior to performing a methacholine challenge test, the physician obtains baseline measurements for the following:
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 180 mL. |
| Answer B | AnswerB::175 mL |
| Answer B Explanation | AnswerBExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL. |
| Answer C | AnswerC::180 mL |
| Answer C Explanation | [[AnswerCExp::Although the approximately normal physiologic dead space is 150 mL, the patient has an unusual anatomy that may increase the volume of her physiologic dead space. The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL.]] |
| Answer D | AnswerD::190 mL |
| Answer D Explanation | AnswerDExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 180 mL. |
| Answer E | AnswerE::225 mL |
| Answer E Explanation | AnswerEExp::The dead space in this patient is increased as the volume can be determined by the Bohr equation to be 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).
References: First Aid 2015 page 602 |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Lung, WBRKeyword::Lung volume, WBRKeyword::Dead space, WBRKeyword::Pulmonology, WBRKeyword::Physiology, WBRKeyword::Respiratory physiology |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
