Using the Fick Principle, where Cardiac Output (CO) = oxygen consumption (VO2) / arteriovenous oxygen difference (CaO2-CvO2), the cardiac output is determined to be 4 L/min. Plugging into and rearranging the equation CO = SV x HR gives a value for cardiac stroke volume in this patient of 40 mL.
In theory, the greatest level of accuracy in using the Fick Principle to calculate CO is achieved through the measurement of O2 content in both the pulmonary arteries and veins. In practice however, peripheral arterial blood is often used as a surrogate for pulmonary venous blood. The true Fick method of calculating cardiac output or stroke volume is rarely used due to the difficulty of obtaining the requisite variables for gas concentrations and oxygen consumption. As a result, an assumed value for oxygen consumption of 125 ml O2/min/m^2 body surface area is commonly used.
Chavey et al. discuss the evaluation and management of systolic dysfunction heart failure. Assessment begins with history and physical examination, allowing for placement of the patient into a New York Heart Association classification tier based on their symptomatology. Additional measures in the evaluation of heart failure include EKG, echocardiography with estimation of ejection fraction, and, at times, exercise stress testing.
Matthews et al. discuss strategies for monitoring cardiac output. They conclude that traditional measurement strategies included invasive pulmonary artery catheterization and utilization of the direct Fick method in the calculation of stroke volume. However, less invasive techniques now available include impedance cardiography, esophageal doppler monitoring, the derivative Fick method measuring partial carbon dioxide breathing, transpulmonary thermodilution, and pulse contour analysis among others.
Illustration A summarizes the calculations used to reach the above answer. Illustration B shows a pictorial representation of the Fick Principle, highlighting its potential utility in the calculation of cardiac output.
Answers 1,3,4,5: The stroke volume calculated using the Fick Principle, CO=SVxHR relationship, and above values is 40 mL; see Illustration A for further detail.
Chavey WE 2nd, Blaum CS, Bleske BE, Harrison RV, Kesterson S, Nicklas JM. Guideline for the management of heart failure caused by systolic dysfunction: Part I. Guideline development, etiology and diagnosis. Am Fam Physician. 2001 Sep 1;64(5):769-74.
PMID:11563568 (Link to Abstract)
Mathews L, Singh RK. Cardiac output monitoring. Ann Card Anaesth. 2008 Jan-Jun;11(1):56-68.
PMID:18182765 (Link to Abstract)
USMLE World Step 1 QBank Question #1529. Copyright © USMLEWorld, LLC 2012