Definition of Afterload

Afterload is the tension or stress developed in the wall of the left ventricle during ejection. In other words, it is the 'load' against which the heart MUST contract to eject blood. The following are components of the after load: aortic pressure and/or the pressure the ventricle must over come to eject blood. The greater the aortic/pulmonary pressure, the greater the after load on the left/right ventricle, respectively. Following Laplace's law, the tension upon the muscle fibers in the heart wall is the product of the pressure within the ventricle, multiplied by the volume within the ventricle, divided by the wall thickness. Therefore, when comparing a normal heart to a heart with a dilated left ventricle, if the aortic pressure is the same in both hearts, the dilated heart must create a greater tension to overcome the same aortic pressure to eject blood because it has a larger internal radius and volume. Thus, the dilated heart has a greater total load (tension) on the myocytes, i.e., has a higher afterload. Conversely, a hypertrophied left ventricle has a lower afterload. When contractility becomes impaired and the ventricle dilates, the afterload rises and limits output. This may start a vicious circle, in which cardiac output is reduced as oxygen requirements are increased.