Quantification of Cardiac Function
Probably the most widely accepted measure of LV function is the ejection fraction (EF). This volume represents the percentage of blood in the ventricle that is ejected per beat. A reduced EF indicates a lower survival rate in medically treated patients with CAD, valvular heart disease and primary cardiomyopathy, and patients undergoing CABG, aneurysm or valve replacement.
Ejection fraction (%) = [ED (net) – ES (net)] ÷ ED x 100
Counts detected in the LV are proportional to volume, and the change from end-systolic counts (ESC) to end-diastolic counts (EDC) of the LV can be used for the calculation of left ventricular ejection fraction.
The normal range for the left ventricle is 50-75%, the right side is about 45%.
Cardiac output (CO) is the volume of blood pumped out to the body by the LV. It is a measurement of unit of blood per unit of time, usually expressed in liters per minute.
Because body mass can cause this value to vary significantly, clinicians sometimes use the Cardiac Index (CI) to measure function. The cardiac index is defined as the cardiac output divided by the body surface area measured in square meters.
In the normal adult male, the average CO is 5.6 liters/minute and the average body surface area is 1.7 square meters; thus the average CI is about 3 liters/min per square meter.
In normal resting subjects, 25 percent of the total cardiac output goes to the splanchnic bed (liver and gut) and 20 per cent goes to the renal circulation; the brain receives 12%, while the coronary flow accounts for about 4%. During exercise, vasodilation in muscle beds permits flow to the involved muscles to increase from about 20% to 50%, while flow to the splanchnic and renal bed drops proportionally.
There is a significant difference in ventricular volume responses to the exercise of patients with and without CAD. In normal patients, the increased stroke volume and cardiac output that accompany exercise are met by a reduction in the end-systolic volume without much change in end-diastolic volume. As a result, EF increases.
Exercise induced ischemia increases End Diastolic and End Systolic Volumes, maintaining the stroke volume. STROKE VOLUME is the difference between the end diastolic volume (EDV) and end systolic volume (ESV).
By exercise radionuclide techniques, patients with CAD fail to increase EF by at least 5% and/or the presence of one or more exercise-induced wall motion abnormalities. Patients with an ischemic response will demonstrate either no change or a decrease in EF value due to impaired contractility.