WBR0990

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Pulmonology
Prompt	[[Prompt::An attending in the cardiac intensive care unit at major academic medical center is teaching a resident about the pulmonary vasculature. A patient with a history of hypertrophic cardiomyopathy is recovering from an anterior myocardial infarction and the following values are measured. Pulmonary artery pressure (mean): 15 Pulmonary capillary wedge pressure (mean): 10 Aortic systolic pressure: 140 Aortic diastolic pressure: 90 Heart rate: 90 per minute Stroke Volume: 40 mL Which of the following corresponds to the pulmonary vascular resistance in this patient?]]
Answer A	AnswerA::0.72 mmHg·min/L
Answer A Explanation	AnswerAExp::Incorrect: See calculation in the explanation
Answer B	AnswerB::1.39 mmHg·min/L
Answer B Explanation	AnswerBExp::Correct: See calculation in the explanation
Answer C	AnswerC::11.25 mmHg·min/L
Answer C Explanation	AnswerCExp::Incorrect: See calculation in the explanation
Answer D	AnswerD::13.9 mmHg·min/L
Answer D Explanation	AnswerDExp::Incorrect: This result can be achieved by dividing the difference in systolic and diastolic aortic pressures and dividing by the cardiac output (3.6L/min)
Answer E	AnswerE::20.83 mmHg·min/L
Answer E Explanation	AnswerEExp::Incorrect: See calculation in the explanation
Right Answer	RightAnswer::B
Explanation	[[Explanation::In this question we are asked to calculate the pulmonary vascular resistance for this patient. Recall that the resistance of a vessel can be calculated by the change in pressure across the segment divided by the flow (R=P/Q). Generally, the pulmonary capillary wedge pressure is a good estimate of the pressure in the left atrium. Therefore, pulmonary vascular resistance is given by the formula: <math>PVR = (P_{pulmonary artery} - P_{capillary wedge})/Cardiac Output</math> α Plugging in values from the above case we get the following: (15 mmHg - 10 mmHg)/(90 beats/minute*(.040 L/min))=5/3.60=1.39 mmHg·min/L. Normal values for pulmonary vascular resistance range from 0.25–1.6 mmHg·min/L. Educational objective: References: Educational Objective: Pulmonary vascular resistance is given by the change in pressure across the pulmonary vasculature divided by the cardiac output. References: First Aid 2014 page 599]]
Approved	Approved::Yes
Keyword	WBRKeyword::Lung, WBRKeyword::Vasculature, WBRKeyword::Pulmonary hypertension, WBRKeyword::Hemodynamics, WBRKeyword::Hemodynamic
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::