Herz
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Factors affecting the contraction of isolated cardiac muscle, preload, afterload and contractility, in addition to the heart rate, are also the primary determinants of the cardiac output in the intact ventricle. In the intact heart, ventricular end-diastolic wall stress is analogous to the preload and within physiologic limits ultimately determines the resting length of the sarcomeres in the ventricular wall. The relationship between the end-diastolic pressure and the stroke volume can be used to describe ventricular function and the relationship between stroke volume and end-diastolic volume represents the ejection fraction. ⋯ Construction of a line connecting points of end-systolic pressure-volume values corresponds with that of the force-velocity relationship, the slope of which may accurately reflect the ventricular contractility. None of these indexes, however, completely represents the force-velocity-length relationship of the intact heart. At present, the best measurements of contractility combine use of various parameters as well as data obtained from a series of contractions.