Hypertension & Coronary Artery Disease Q 29

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Which of the following terms is used to describe the amount of stretch on the myocardium at the end of diastole?

  

     A. Afterload

     B. Cardiac index

     C. Cardiac output

     D. Preload

    

    

Correct Answer: D. Preload

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Preload is the amount of stretch of the cardiac muscle fibers at the end of diastole. The volume of blood in the ventricle at the end of the diastole determines the preload. Also termed left ventricular end-diastolic pressure (LVEDP), preload is a measure of the degree of the ventricular stretch when the heart is at the end of diastole. Preload, in addition to afterload and contractility, is one of the three main factors that directly influence stroke volume (SV), the amount of blood pumped out of the heart in one cardiac cycle.

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Option A: Afterload is the force against which the ventricle must expel blood. Afterload refers to the resistance normally maintained by the aortic and pulmonic valves, the condition and tone of the aorta, and the resistance offered by the systemic and pulmonary arterioles. The afterload of any contracting muscle is defined as the total force that opposes sarcomere shortening minus the stretching force that existed before contraction. Applying this definition to the heart, afterload can be most easily described as the “load” against which the heart ejects blood.

Option B: Cardiac index is the individualized measurement of cardiac output, based on the client’s body surface area. One measure of heart function is the cardiac index. The cardiac index relies on another important parameter, cardiac output, and turns cardiac output into a normalized value that accounts for the body size of the patient. The function of the cardiac index is to create a normalized value for the cardiac function, which effectively corrects for the patient’s body size.

Option C: Cardiac output is the amount of blood expelled from the heart per minute. Cardiac output (CO) is the amount of blood pumped by the heart minute and is the mechanism whereby blood flows around the body, especially providing blood flow to the brain and other vital organs. The body’s demand for oxygen changes, such as during exercise, and the cardiac output is altered by modulating both heart rate (HR) and stroke volume (SV).

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