Editing Afterload

{{Short description|Pressure in the wall of the left ventricle during ejection}}
[[File:Heart systole.svg|thumb|Ventricular systole. Red arrow is path from left ventricle to aorta. Afterload is largely dependent upon aortic pressure.]]
'''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.<ref name=":0">{{Cite book|title=Cardiovascular Physiology, 9e|author=Mohrman, David E.|date=2018|publisher=McGraw-Hill Education LLC|isbn=9781260026115|oclc=1055827575}}</ref> 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 [[Circulatory system|cardiovascular system]].<ref name=":0" /> 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]].<ref name=":0" />  Cardiac output is the product of [[stroke volume]] and [[heart rate]].<ref name="King Lowery 2019 p. ">{{citation | last=King | first=J | last2=Lowery | first2=DR | title=Physiology, Cardiac Output | chapter=article-18897 | publisher=StatPearls Publishing | location=Treasure Island (FL) | year=2019 | pmid=29262215 | url=http://www.ncbi.nlm.nih.gov/books/NBK470455/ | access-date=2019-12-20 }}</ref>  Afterload is a determinant of stroke volume (in addition to [[Preload (cardiology)|preload]], and [[Contractility|strength of myocardial contraction]]).<ref name=":0" /> 

Following [[Young–Laplace equation|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.<ref name="Harrison's">{{cite book|author1=Kasper, Dennis L |author2=Braunwald, Eugene |author3=Fauci, Anthony |title=Harrison's Principles of Internal Medicine |url=https://archive.org/details/harrisonsprincip00kasp |url-access=limited |edition=16th |publisher=McGraw-Hill|location=New York|year=2005|pages= [https://archive.org/details/harrisonsprincip00kasp/page/n1374 1346]|isbn=0-07-139140-1|display-authors=etal}}</ref>

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 [[aortic pressure]] (for the left ventricle) and [[pulmonic pressure]] or [[pulmonary artery pressure]] (for the right ventricle). The pressure in the ventricles must be greater than the systemic and pulmonary pressure to open the [[aortic valve|aortic]] and [[pulmonic valve]]s, respectively. As afterload increases, [[cardiac output]] decreases. [[Cardiac imaging]] is a somewhat limited modality in defining afterload because it depends on the interpretation of volumetric data.{{Citation needed|date=August 2011}}

== Calculating afterload ==
Quantitatively, afterload can be calculated by determining the wall stress of the left ventricle, using the [[Young–Laplace equation]]:{{cn|date=March 2021}}

<math display="inline">\left ( \frac{EDP \cdot EDR}{2h} \right )</math> where

EDP is [[end-diastolic pressure]] in the left ventricle, which is typically approximated by taking [[pulmonary artery wedge pressure]],

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 [[pathology]] that include indicators such as an increasing [[left ventricle|left ventricular]] afterload include elevated [[blood pressure]] and [[aortic valve]] disease.<ref>{{cite web |last1= LaCombe | first1= P |last2= Tariq |first2= M |last3= Tariq |first3= S |date= | url= https://www.ncbi.nlm.nih.gov/books/NBK493174/ | title= Physiology, Afterload Reduction
| website= National Center for Biotechnology Information, U.S. National Library of Medicine |access-date= 30 June 2021}}</ref>

[[Systolic hypertension]] (HTN) (elevated blood pressure) increases the left ventricular (LV) afterload because the LV must [[Work (physics)|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.<ref name='Homoud2008'>{{cite web|url=http://ocw.tufts.edu/data/50/636804.pdf |title=Introduction to Cardiovascular Pathophysiology |access-date=2010-05-04 |last=Homoud |first=MK |date=Spring 2008 |work=Tufts Open Courseware |publisher=Tufts University|page=10 }}</ref>

[[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]].<ref>{{cite web |last1= LaCombe | first1= P |last2= Tariq |first2= M |last3= Tariq |first3= S |date= | url= https://www.ncbi.nlm.nih.gov/books/NBK493174/ | title= Physiology, Afterload Reduction
| website= National Center for Biotechnology Information, U.S. National Library of Medicine |access-date= 30 June 2021}}</ref>

In the natural aging process, [[aortic stenosis]] often increases afterload because the left ventricle must overcome the pressure gradient caused by the calcified and stenotic aortic valve, in addition to the blood pressure required to eject blood into the [[aorta]]. For instance, if the blood pressure is 120/80, and the aortic valve stenosis creates a trans-valvular gradient of 30 [[mmHg]], the left ventricle has to generate a pressure of 110 mmHg to open the aortic valve and eject blood into the aorta.<ref>{{cite web |last1= LaCombe | first1= P |last2= Tariq |first2= M |last3= Tariq |first3= S |date= | url= https://www.ncbi.nlm.nih.gov/books/NBK493174/ | title= Physiology, Afterload Reduction
| website= National Center for Biotechnology Information, U.S. National Library of Medicine |access-date= 30 June 2021}}</ref>

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.<ref>{{cite web |last1= LaCombe | first1= P |last2= Tariq |first2= M |last3= Tariq |first3= S |date= | url= https://www.ncbi.nlm.nih.gov/books/NBK493174/ | title= Physiology, Afterload Reduction
| website= National Center for Biotechnology Information, U.S. National Library of Medicine |access-date= 30 June 2021}}</ref>

[[Aortic insufficiency|Aortic insufficiency (Aortic Regurgitation)]] increases afterload, because a percentage of the blood that ejects forward regurgitates back through the diseased aortic valve.  This leads to elevated [[Systole (medicine)|systolic]] blood pressure. The diastolic blood pressure in the aorta falls, due to regurgitation. This increases pulse pressure.<ref>{{cite web |url= https://www.lecturio.com/concepts/aortic-regurgitation/ | title= Aortic Regurgitation | website= The Lecturio Medical Concept Library |access-date= 30 June 2021}}</ref>

[[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.<ref name='KlabundeMR2007'>{{cite web|url=http://www.cvphysiology.com/Heart%20Disease/HD009c.htm |title=Mitral Regurgitation |access-date=2010-01-01 |author=Klabunde RE |date=2007-04-05 |work=Cardiovascular Physiology Concepts |publisher=Richard E. Klabunde | archive-url= https://web.archive.org/web/20100103195916/http://cvphysiology.com/Heart%20Disease/HD009c.htm| archive-date= 3 January 2010 | url-status= live}}</ref>  Afterload is largely dependent upon aortic pressure.

== See also ==
* [[Cardiac output]]
* [[Hemodynamics]]
* [[Preload (cardiology)|Preload]]

==References==
{{Reflist}}

==Further reading==
* {{cite journal |doi=10.1016/0033-0620(76)90021-9 |pmid=128034 |title=Afterload mismatch and preload reserve: A conceptual framework for the analysis of ventricular function |journal=Progress in Cardiovascular Diseases |volume=18 |issue=4 |pages=255–264 |year=1976 |last1=Ross |first1=John }}
* {{cite journal |doi=10.1136/hrt.77.4.346 |pmid=9155614 |pmc=484729 |title=Prognostic significance of right ventricular afterload stress detected by echocardiography in patients with clinically suspected pulmonary embolism |journal=Heart |volume=77 |issue=4 |pages=346–349 |year=1997 |last1=Kasper |first1=W. |last2=Konstantinides |first2=S. |last3=Geibel |first3=A. |last4=Tiede |first4=N. |last5=Krause |first5=T. |last6=Just |first6=H. }}
* {{cite journal |doi=10.1016/0002-9149(75)90048-X |pmid=1124716 |title=Effects of changes in preload, afterload and inotropic state on ejection and isovolumic phase measures of contractility in the conscious dog |journal=The American Journal of Cardiology |volume=35 |issue=5 |pages=626–634 |year=1975 |last1=Mahler |first1=Felix |last2=Ross |first2=John |last3=O'Rourke |first3=Robert A. |last4=Covell |first4=James W. }}
* {{cite journal |doi=10.1161/CIRCULATIONAHA.106.668681 |pmid=17533183 |title=Paradoxical Low-Flow, Low-Gradient Severe Aortic Stenosis Despite Preserved Ejection Fraction is Associated with Higher Afterload and Reduced Survival |journal=Circulation |volume=115 |issue=22 |pages=2856–2864 |year=2007 |last1=Hachicha |first1=Z. |last2=Dumesnil |first2=J. G. |last3=Bogaty |first3=P. |last4=Pibarot |first4=P. |doi-access=free }}
* {{cite journal |pmid=2107077 |year=1990 |last1=Kelly |first1=R. P. |title=Nitroglycerin has more favourable effects on left ventricular afterload than apparent from measurement of pressure in a peripheral artery |journal=European Heart Journal |volume=11 |issue=2 |pages=138–144 |last2=Gibbs |first2=H. H. |last3=O'Rourke |first3=M. F. |last4=Daley |first4=J. E. |last5=Mang |first5=K |last6=Morgan |first6=J. J. |last7=Avolio |first7=A. P. | doi = 10.1093/oxfordjournals.eurheartj.a059669 }}

==External links==
* [http://www.cvphysiology.com/Cardiac%20Function/CF008.htm Overview at cvphysiology.com]

{{Cardiovascular physiology}}

[[Category:Cardiovascular physiology]]