A 54-year-old man is being evaluated for exertional dyspnea that started 6 months ago. The patient has no associated cough or wheezing. There is no known history of heart disease in his family. ECG, chest x-ray, exercise stress test, and echocardiogram are obtained to evaluate his symptoms. He subsequently undergoes right and left cardiac catheterization, and the results are as follows: What additional information is necessary to calculate cardiac output in this patient?

Question

A 54-year-old man is being evaluated for exertional dyspnea that started 6 months ago.  The patient has no associated cough or wheezing.  There is no known history of heart disease in his family.  ECG, chest x-ray, exercise stress test, and echocardiogram are obtained to evaluate his symptoms.  He subsequently undergoes right and left cardiac catheterization, and the results are as follows:  What additional information is necessary to calculate cardiac output in this patient?

Quizplus · Accepted Answer

Cardiac output, expressed in liters per minute, is commonly calculated using the following formula: Cardiac output = stroke volume x heart rate Cardiac output can also be determined with a pulmonary artery (Swan-Ganz) catheter by applying the Fick principle, which uses the rate of oxygen consumption and the arteriovenous oxygen difference: Cardiac output = rate of O₂ consumption / arteriovenous O₂ content difference Arterial oxygen content is measured directly from arterial blood, and mixed venous oxygen content is measured from pulmonary arterial blood. The rate of oxygen consumption can be determined with an oxygen meter by measuring the rate of disappearance of oxygen in exhaled air. (Choice A) The arteriovenous difference in oxygen content can be calculated using the information provided in the question stem (eg, arterial blood O₂ content - venous blood O₂ content). (Choices B and C) The oxygen-binding capacity of hemoglobin and percentage of hemoglobin saturation can be used to calculate blood oxygen content using the following equation: Blood O₂ content = (O₂-binding capacity x % saturation) + dissolved O₂ Dissolved oxygen is the amount of oxygen unbound to hemoglobin and typically makes up a small fraction of the total blood oxygen content. (Choice E) The respiratory quotient is the ratio of CO₂ to O₂ across the alveolar membrane, and is used to estimate the metabolic rate. The normal value is typically 0.8. Educational objective: The Fick principle can be applied to calculate cardiac output using the rate of oxygen consumption and the arteriovenous oxygen content difference: Cardiac output = rate of O₂ consumption / arteriovenous O₂ content difference __________ References: Cardiac output monitoring: Basic science and clinical application. (http://www.ncbi.nlm.nih.gov/pubmed/18211449)

A 54-Year-Old Man Is Being Evaluated for Exertional Dyspnea That