Ảnh hưởng của hậu gánh trên Frank-Starling Curves Sự gia tăng trong hậu gánh thay đổi đường cong FrankStarling xuống và bên phải (Hình 4-15). Vì vậy, tại một tải trước (LVEDP trong hình 4-15), tăng trong hậu gánh giảm khối lượng đột quỵ | CARDIAC FUNCTION 79 of as an adaptive mechanism by which the ventricle is able to offset the increase in wall stress that accompanies increased aortic pressure aortic valve stenosis or ventricular dilation. Effects of Afterload on Frank-Starling Curves An increase in afterload shifts the Frank-Starling curve down and to the right Fig. 4-15 . Therefore at a given preload LVEDP in Figure 4-15 an increase in afterload decreases stroke volume. Conversely decreasing afterload shifts the curves up and to the left thereby increasing the stroke volume at any given preload. For reasons discussed later changes in afterload result in a subsequent change in preload so that as the Frank-Starling curve shifts the operating point for the curve shifts diagonally as shown in Figure 4-15. In the normal heart changes in afterload do not have pronounced effects on stroke volume however the failing ventricle is very sensitive to changes in afterload. Effects of Afterload on the Velocity of Fiber Shortening Force-Velocity Relationship The decrease in stroke volume that accompanies an increase in afterload is caused by im- FIGURE 4-15 Effects of changes in afterload on Frank-Starling curves. An increase in afterload shifts the operating point of the Frank-Starling curve from A to B whereas a decrease in afterload shifts the operating point from A to C. Therefore increased afterload decreases stroke volume and increases end-diastolic pressure preload . The converse also is true. paired emptying of the ventricle. The basis for this is found in the force-velocity relationship of cardiac myocytes. The forcevelocity relationship shows how afterload affects the velocity of shortening when muscle fibers contract isotonically. To illustrate this a papillary muscle is placed in an in vitro bath and a load is attached to one end Fig. 4-16 left panel . When the muscle contracts the fiber first generates tension isometrically right panel a to b until the developed tension exceeds the load imposed on
