Expansion
Aortic pressure, systemic vascular resistance, ventricular radius and wall thickness
Expansion
Afterload is the load the ventricle must overcome to eject. By the law of Laplace, wall stress is proportional to pressure multiplied by radius, divided by twice the wall thickness.
So afterload rises with:
- Arterial pressure and systemic vascular resistance
- Aortic valve obstruction, as in aortic stenosis
- Ventricular dilatation, which increases the radius
- Reduced arterial compliance, as in stiff, aged arteries
And falls with:
- Vasodilators
- Ventricular hypertrophy, which increases wall thickness
The dilatation point explains a vicious cycle in heart failure: a dilated ventricle faces a higher wall stress for the same blood pressure, which further impairs ejection and promotes more dilatation.