How do preload and afterload differential shift the cardiac function curve?

How do preload and afterload differential shift the cardiac function curve?

• A. Increased preload moves the Frank-Starling curve to the right, increasing stroke volume for a given afterload.
• B. Increased preload moves the Frank-Starling curve to the left, decreasing stroke volume.
• C. Increased afterload steepens the ESPVR, increasing stroke volume.
• D. Increased afterload has no effect on stroke volume.

Answer: (A)

Preload increases the stretch of cardiac muscle at the start of contraction, which, according to the Frank-Starling mechanism, makes the heart contract more forcefully and pump more blood. When preload is higher, you get a larger stroke volume even if the afterload stays the same. In the way we describe the cardiac function relationship, this means the curve shifts so that for a given afterload you achieve a greater stroke volume. Afterload, the pressure the ventricle must overcome to eject blood, works in the opposite direction: increasing afterload makes it harder to eject blood, so stroke volume tends to decrease at a fixed preload. This is why choices suggesting that afterload would increase stroke volume or that it has no effect don’t fit the basic physiology. ESPVR slope relates to contractility, not to a preload-driven increase in output from higher filling.

Preload increases the stretch of cardiac muscle at the start of contraction, which, according to the Frank-Starling mechanism, makes the heart contract more forcefully and pump more blood. When preload is higher, you get a larger stroke volume even if the afterload stays the same. In the way we describe the cardiac function relationship, this means the curve shifts so that for a given afterload you achieve a greater stroke volume.

Afterload, the pressure the ventricle must overcome to eject blood, works in the opposite direction: increasing afterload makes it harder to eject blood, so stroke volume tends to decrease at a fixed preload. This is why choices suggesting that afterload would increase stroke volume or that it has no effect don’t fit the basic physiology. ESPVR slope relates to contractility, not to a preload-driven increase in output from higher filling.