Afterload
Afterload is the pressure that the heart must work against to eject blood during systole (ventricular contraction). Afterload is proportional to the average arterial pressure.[1] As aortic and pulmonary pressures increase, the afterload increases on the left and right ventricles respectively. Afterload changes to adapt to the continually changing demands on an animal's cardiovascular system.[1] Afterload is proportional to mean systolic blood pressure and is measured in millimeters of mercury (mm Hg).
Hemodynamics
Afterload is a determinant of cardiac output.[1] Cardiac output is the product of stroke volume and heart rate.[2] Afterload is a determinant of stroke volume (in addition to preload, and strength of myocardial contraction).[1]
Following Laplace's law, the tension upon the muscle fibers in the heart wall is the pressure within the ventricle multiplied by the volume within the ventricle divided by the wall thickness (this ratio is the other factor in setting the afterload). 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. This is also true in the eccentric hypertrophy consequent to high-intensity aerobic training. Conversely, a concentrically hypertrophied left ventricle may have a lower afterload for a given aortic pressure. 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.[3]
Afterload can also be described as the pressure that the chambers of the heart must generate to eject blood from the heart, and this is a consequence of
Calculating afterload
Quantitatively, afterload can be calculated by determining the wall stress of the left ventricle, using the Young–Laplace equation:[citation needed]
where
EDP is
EDR is end-diastolic radius at the midpoint of the left ventricle, and
h is the mean thickness of the left ventricle wall. Both radius and mean thickness of the left ventricle may be measured by echocardiography.
Factors affecting afterload
Disease processes
Systolic hypertension (HTN) (elevated blood pressure) increases the left ventricular (LV) afterload because the LV must work harder to eject blood into the aorta. This is because the aortic valve won't open until the pressure generated in the left ventricle is higher than the elevated blood pressure in the aorta.[5]
Pulmonary hypertension (PH) is increased blood pressure within the right heart leading to the lungs. PH indicates a regionally applied increase in afterload dedicated to the right side of the heart, divided and isolated from the left heart by the interventricular septum.[6]
In the natural aging process,
Due to the increased afterload, the ventricle has to work harder to accomplish its goal of ejecting blood into the aorta. Thus, in the long-term, increased afterload (due to the stenosis) results in hypertrophy of the left ventricle to account for the increased work required and also to decrease wall stress since wall thickness and wall stress are inversely proportional.[8]
Mitral regurgitation (MR) decreases afterload. In ventricular systole under MR, regurgitant blood flows backwards/retrograde back and forth through a diseased and leaking mitral valve. The remaining blood loaded into the LV is then optimally ejected out through the aortic valve. With an extra pathway for blood flow through the mitral valve, the left ventricle does not have to work as hard to eject its blood, i.e. there is a decreased afterload.[10] Afterload is largely dependent upon aortic pressure.
See also
References
- ^ OCLC 1055827575.
- PMID 29262215, retrieved 2019-12-20
- ISBN 0-07-139140-1.
- ^ LaCombe, P; Tariq, M; Tariq, S. "Physiology, Afterload Reduction". National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved 30 June 2021.
- ^ Homoud, MK (Spring 2008). "Introduction to Cardiovascular Pathophysiology" (PDF). Tufts Open Courseware. Tufts University. p. 10. Retrieved 2010-05-04.
- ^ LaCombe, P; Tariq, M; Tariq, S. "Physiology, Afterload Reduction". National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved 30 June 2021.
- ^ LaCombe, P; Tariq, M; Tariq, S. "Physiology, Afterload Reduction". National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved 30 June 2021.
- ^ LaCombe, P; Tariq, M; Tariq, S. "Physiology, Afterload Reduction". National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved 30 June 2021.
- ^ "Aortic Regurgitation". The Lecturio Medical Concept Library. Retrieved 30 June 2021.
- ^ Klabunde RE (2007-04-05). "Mitral Regurgitation". Cardiovascular Physiology Concepts. Richard E. Klabunde. Archived from the original on 3 January 2010. Retrieved 2010-01-01.
Further reading
- Ross, John (1976). "Afterload mismatch and preload reserve: A conceptual framework for the analysis of ventricular function". Progress in Cardiovascular Diseases. 18 (4): 255–264. PMID 128034.
- Kasper, W.; Konstantinides, S.; Geibel, A.; Tiede, N.; Krause, T.; Just, H. (1997). "Prognostic significance of right ventricular afterload stress detected by echocardiography in patients with clinically suspected pulmonary embolism". Heart. 77 (4): 346–349. PMID 9155614.
- Mahler, Felix; Ross, John; O'Rourke, Robert A.; Covell, James W. (1975). "Effects of changes in preload, afterload and inotropic state on ejection and isovolumic phase measures of contractility in the conscious dog". The American Journal of Cardiology. 35 (5): 626–634. PMID 1124716.
- Hachicha, Z.; Dumesnil, J. G.; Bogaty, P.; Pibarot, P. (2007). "Paradoxical Low-Flow, Low-Gradient Severe Aortic Stenosis Despite Preserved Ejection Fraction is Associated with Higher Afterload and Reduced Survival". Circulation. 115 (22): 2856–2864. PMID 17533183.
- Kelly, R. P.; Gibbs, H. H.; O'Rourke, M. F.; Daley, J. E.; Mang, K; Morgan, J. J.; Avolio, A. P. (1990). "Nitroglycerin has more favourable effects on left ventricular afterload than apparent from measurement of pressure in a peripheral artery". European Heart Journal. 11 (2): 138–144. PMID 2107077.