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USMLE® Step 1 Question of the Day: Altered Mental State

Osmosis Team
Published on May 25, 2022. Updated on May 23, 2022.

This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case involves a 78-year-old man with altered mental status. The patient has a Swan-Ganz catheter placed in the right heart. Can you figure out the most likely cause of this patient's clinical presentation?

A 78-year-old man presents to the emergency department with altered mental status and fatigue. He is found to be critically ill and is admitted to the intensive care unit (ICU) for further evaluation and monitoring. While in the ICU, the patient has a Swan-Ganz catheter placed in the right heart for continuous hemodynamic measurements. An initial set of readings is demonstrated below:

Hemodynamic Parameter 

 Result

Pulmonary capillary wedge pressure (PCWP)

 20 mmHg (normal 4-12 mmHg)

Systemic vascular resistance (SVR)

 2000 dyn·s/cm5 (normal 700–1600 dyn·s/cm5)

Cardiac Index (CI)

 1.2 L/min/m(normal 2.6–4.2 L/min/m2)

Mixed venous oxygen saturation (SvO2)

 50% (normal 65-70%)

Which of the following is the most likely cause of this patient's clinical presentation?

A. Bacteremia due to a Gram-negative rod

B. Impaired sympathetic outflow

C. Thrombus in the pulmonary artery

D. Hemorrhage due to bleeding peptic ulcer

E. Heart failure

Scroll down to find the answer!


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The correct answer to today's USMLE® Step 1 Question is...

E. Heart failure

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

A. Bacteremia due to a Gram-negative rod

Incorrect: Septic shock is a subtype of distributive shock that presents with decreased systemic vascular resistance, decreased pulmonary capillary wedge pressure, increased cardiac output, and increased mixed venous oxygen saturation. This patient’s findings are inconsistent with those of septic shock.

B. Impaired sympathetic outflow

Incorrect: Neurogenic shock is another subtype of distributive shock that presents with decreased systemic vascular resistance, decreased pulmonary capillary wedge pressure, decreased cardiac output, and an increased mixed venous oxygen saturation. It is most commonly caused by spinal cord injury that disrupts sympathetic outflow. In contrast, this patient has an elevated systemic vascular resistance.

C. Thrombus in the pulmonary artery

Incorrect: Obstructive shock is a subtype of “cold shock” and can be caused by pulmonary embolism. It can present with increased systemic vascular resistance, decreased cardiac output, and decreased mixed venous oxygen saturation. In causes of obstructive shock secondary to pulmonary embolism, pulmonary capillary wedge pressure (surrogate of left atrial pressure) is reduced since less blood is reaching the left heart.

D. Hemorrhage due to bleeding peptic ulcer

Incorrect: Hemorrhagic shock presents with increased systemic vascular resistance, decreased pulmonary capillary wedge pressure, decreased cardiac output, and decreased mixed venous oxygen saturation. It is most commonly due to ongoing hemorrhage from trauma or gastrointestinal bleeding.

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Main Explanation

This critically ill patient had a Swan-Ganz catheter placed and hemodynamic measurements taken. The findings include a decreased cardiac output (of which the cardiac index is a surrogate measure taking into account BMI), increased systemic vascular resistance, increased pulmonary capillary wedge pressure (PCWP), and decreased mixed venous oxygen saturation (SvO2). These parameters are consistent with cardiogenic shock, of which heart failure is a common etiology. 


A Swan-Ganz-catheter is a flexible tube passed into the right side of the heart and the pulmonary artery. It is used to take measurements of cardiac output (heart x stroke volume), systemic vascular resistance (80x (Mean Arterial Pressure - Mean Venous Pressure or CVP) / Cardiac Output), PCWP (surrogate of left atrial pressure), and SvO2 (measure of total body oxygen consumption). While seldom used in modern ICUs, it was historically used to provide useful information in determining the etiology of a patient’s shock.

Osmosis table illustrating different types of shocks.

Major Takeaway

The etiology of shock can be elucidated by placement of a Swan-Ganz catheter and obtaining measurements of SVR, PCWP, CO, and SvO2.

References

Ginosar, Y., Sprung, C.L. (1996) The Swan-Ganz catheter. Twenty-five years of monitoring. Critical Care Clinics. 12(4), 771-776. Doi: 10.1016/s0749-0704(05)70278-5.

Vincent, J.L., De Backer, D. (2013) Circulatory shock. The New England Journal of Medicine. 370(6), 583. Doi: 10.1056/NEJMra1208943.

Walls, R., Hockberger, R., Gausche-Hill, M. (2017) Rosen's Emergency Medicine: Concepts and Clinical Practice. Philadelphia, PA: Elsevier. ISBN: 978-0323354790.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.