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James M. Norton
Advan Physiol Educ 25:53-61, 2001.
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P E R S O N A L
V I E W
TOWARD CONSISTENT DEFINITIONS
FOR PRELOAD AND AFTERLOAD
James M. Norton
Department of Physiology and Pharmacology; University of New England College of Osteopathic Medicine,
Biddeford, Maine 04005
S
ADV PHYSIOL EDUC 25: 53– 61, 2001.
Key words: Law of LaPlace; wall tension; wall stress; cardiac remodeling; hypertrophy
Recent changes in the basic science curriculum at the
University of New England College of Osteopathic
Medicine have included a reduction in the number of
classroom contact hours devoted to the traditional
lecture format. These reductions have prompted (i.e.,
required) faculty to be more creative and productive
in their use of their formal classroom time with students, with much more attention being paid to outlining major concepts, establishing linkages among
topics, utilizing classroom case presentations and
breakout groups–a variety of techniques intended to
foster student understanding. One outcome of this
approach is that students, in turn, are more clearly
required to master much of the basic knowledge
(facts, definitions, etc.) on their own, with guidance
from the faculty in the form of recommended and
required readings in texts, review articles, and current literature.
This seemingly rational use of published written materials by students to obtain the factual underpinnings
for the concepts and relationships developed in class
has one consequence that is very troublesome: published sources often have very different and sometimes conflicting definitions for important physiological terms. Because medical students generally dislike
ambiguity and because faculty generally strive for
accuracy, such discrepancies are annoying to both
groups. But if conflicts among definitions of important terms and concepts remain unresolved, students
may carry into their clinical training incomplete or
inaccurate working definitions of these terms that
1043 - 4046 / 01 – $5.00 – COPYRIGHT © 2001 THE AMERICAN PHYSIOLOGICAL SOCIETY
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ignificant differences exist among textbook definitions for the terms preload
and afterload, leading to confusion and frustration among students and faculty
alike. Many faculty also chose to use in their teaching simple terms such as
“end-diastolic volume” or “aortic pressure” as common-usage approximations of
preload and afterload, respectively, but these are only partial representations of these
important concepts. Straightforward definitions both of preload and afterload that
are concise yet still comprehensive can be developed using the Law of LaPlace to
describe the relationships among chamber pressure, chamber radius, and wall thickness. Within this context, the term “preload” can be defined as all of the factors that
contribute to passive ventricular wall stress (or tension) at the end of diastole, and
the term “afterload” can be defined as all of the factors that contribute to total
myocardial wall stress (or tension) during systolic ejection. The inclusion of “wall
stress” in both definitions helps the student appreciate both the complexities of
cardiac pathophysiology and the rationale for therapeutic intervention.
P E R S O N A L
may be adequate most of the time but may fail the
students in novel or complicated clinical situations.
V I E W
ical definition and what might serve as an approximation of preload clinically according to common usage
(9, 16).
A similar degree of variability was observed among
the textbook definitions of afterload. This term was
defined as the load against which a muscle exerts
force (9), the pressure in the arteries leading from the
ventricles (1, 4, 8, 9, 14, 16, 17, 21, 22, 24, 25,
27–29), aortic and ventricular pressures (assumed to
be identical) (15, 16, 31), myocardial wall tension or
stress (15, 16, 31), peripheral resistance (6, 11, 12,
16), force needed to overcome opposition to ejection
(18), output impedance (19, 20), and diastolic aortic
pressure (26). As was the case for preload, some texts
gave multiple definitions (16, 19) and some gave specific definitions but allowed for afterload to be approximated as aortic pressure according to common
clinical usage (16, 18, 20, 27). This is where I first
encountered the discrepancy between my own physiological approach to defining afterload and the
choice by some clinical faculty during their lectures
to use simpler terms such as “arterial pressure” or
“peripheral resistance” to define afterload, terms that
are only indirect and incomplete representations of
the real concept.
Compilations of textbook definitions of preload
and afterload. To assess the extent of variability in
the definitions of preload and afterload, I decided
simply to compile a list of definitions for these two
terms from all of the comprehensive physiology texts,
cardiovascular physiology monographs, and physiology review books in a representative faculty collection (namely, the books sitting on my office shelves)
as well as from two selected websites. Most, but not
all, of the 29 texts I surveyed were the current editions, but for such basic concepts as preload and
afterload, I did not feel that an edition or two would
make much of a difference. With the use of the index,
I searched in each text for the clearest statement that
defined either term and generated a table summarizing my findings (Table 1). In the quotes provided in
this table, words in italics or bold print appeared as
such in the original text.
Definitions of preload and afterload. The basis for
the definitions of both preload and afterload is the
Law of LaPlace (also known as the surface tension law
or the Law of Young-LaPlace), stated as follows for a
thin-walled spherical structure: T ⫽ PR/2, where T is
wall tension, P is chamber pressure, and R is chamber
radius. For a thick-walled structure such as the left
ventricle, a more appropriate form of the equation
would be ␴ ⫽ PR/2w, where wall stress (␴) is related
to T and wall thickness (w) as follows: T ⫽ ␴w.
With the use of the format of LaPlace’s equation,
preload for the left ventricle can be best described as
the left ventricular ␴ or T at the end of diastolic filling,
as follows: preloadLV ⫽ (EDPLV)(EDRLV)/2wLV, where
EDPLV is left ventricular end-diastolic filling pressure,
EDRLV is left ventricular end-diastolic radius, and wLV
is left ventricular w. The preload for the right ventricle would be described mathematically in an analogous fashion. Defined in words, therefore, preload
represents all the factors that contribute to passive ventricular wall stress (or tension) at the
The variability among the textbook definitions was
surprising. For example, preload is variously defined
as: muscle fiber tension (7, 9, 20, 22), muscle fiber
stretch (1, 6, 25), muscle fiber length (3, 18), enddiastolic volume (2, 5, 12, 19, 24, 27–29), end-diastolic filling pressure (7, 9, 12, 14, 17, 21, 26, 28, 29,
31), force (3), or, as part of a description of basic
muscle mechanics, weight (15). A number of texts
describe preload as either volume or pressure (24,
29), and some distinguish between a strict physiolog-
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Such is the case with the concepts of preload and
afterload, major determinants of cardiac function
along with heart rate and myocardial contractility.
Each year, after the completion of the pathophysiology segment of our second-year Cardiovascular System course, students begin turning up at my office
door claiming that some of the clinical faculty defined
preload and afterload much differently, and more simply, than I had. As I routinely do when students bring
to my attention evidence of differing opinions or
contrary definitions provided by faculty, I suggested
that they check the textbooks and the biomedical
literature to resolve the differences. This year, because of an inordinate number of complaints and
confusion about the definition of afterload, in particular, I decided to follow my own advice.
P E R S O N A L
V I E W
TABLE 1
Summary of published definitions of preload and afterload
Reference
Afterload Definition
Textbook of Medical
Physiology (9)
“. . . the degree of tension on the muscle when it
begins to contract . . . is the preload . . .”
(p. 115)
“For cardiac contraction, the preload is usually
considered to be the end-diastolic pressure
when the ventricle has become filled.”
(p. 115)
“. . . the load against which the muscle exerts its
contractile force . . . is called the afterload.”
(p. 115)
“The afterload of the ventricle is the pressure in
the artery leading from the ventricle.” (p. 115)
Physiology (2)
“. . . the resting muscle is stretched by a preload,
which in the intact heart represents the end of
filling of the left ventricle during diastole (in
other words, it represents the end diastolic
volume).” (p. 366)
“During ejection, the afterload is represented by
aortic and intraventricular pressures, which
are virtually equal to each other.” (p. 366)
Best and Taylor’s
Physiological Basis of
Medical Practice (31)
“The end-diastolic pressure in the ventricle can
also be equated to the preload.” (p. 220)
“In the whole heart the preload should
constitute the tension in the wall at the end of
diastole (which determines the resting fiber
length), but for practical purposes the
ventricular end-diastolic volume of the
ventricular end-diastolic pressure is used to
indicate the preload.” (p. 227)
“Afterload ⫽ wall tension (stress). Diagram
showing how the systolic wall tension or wall
stress (which represents the afterload in the
myocardial fibers during left ventricular
ejection) is affected by the geometry of the
left ventricle (LV).” (p. 220, legend of Figure
2.111)
Review of Medical Physiology
(6)
“In vivo, the preload is the degree to which the
myocardium is stretched before it
contracts . . .” (p. 546)
“. . . and the afterload is the resistance against
which blood is expelled.”
(p. 546)
Essential Medical Physiology
(14)
“The filling pressure is often termed the preload
because this is the load on the muscle fibers
before contraction.” (p. 192)
“The afterload for the contraction is the aortic
pressure . . .” (p. 192)
Medical Physiology (23)
“At the end of diastole the intraventricular
pressure is analogous to the ‘preload’ in a
simple muscle strip preparation (i.e., the
weight that is suspended from such a strip to
stretch it to the desired initial length).”
(p. 994)
“During ejection the aortic pressure is related to
the ‘afterload,’ or weight that the muscle
[strip] is required to lift.”
(p. 994)
Human Physiology (26)
“The preload is given by the end-diastolic
pressure . . .” (p. 387)
“. . . and the afterload by the diastolic arterial
pressure.” (p. 387)
Human Physiology:
Foundations and Frontiers
(21)
“In the case of the heart, the preload is the
right atrial pressure . . .” (p. 376)
“In the case of the heart, . . . the afterload is
the aortic pressure.” (p. 376)
Textbook of Physiology:
Circulation, Respiration,
Body Fluids, Metabolism,
and Endocrinology (24)
“The end-diastolic filling pressure or maximal
diastolic volume (preload) is the most
important determinant of stroke volume.”
(p. 973)
“The second intrinsic factor that determines the
stroke volume is the aortic pressure or
afterload.” (p. 974)
Physiology (3)
“The force required to stretch the muscle . . . is
called the preload. The term preload is also
used to indicate the length of the sarcomere or
muscle before contraction.” (p. 43)
“The afterload represents an impediment to the
shortening of muscle fibers or to ejection in
the heart. The afterload for the ventricles is
the arterial pressure . . .” (p. 163)
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Preload Definition
P E R S O N A L
V I E W
TABLE 1—Continued
Summary of published definitions of preload and afterload
Reference
Afterload Definition
Circulatory Physiology (28)
“. . . during diastole a greater influx of blood into
the ventricle will cause the ensuing
contraction to be more forceful. This may be
thought of as a ‘preload’ stimulus since it was
applied before contraction began.” (p. 76)
“. . . he [Starling] controlled the right atrial
pressure (and thereby the right ventricular
diastolic pressure or preload) . . .” (p. 76)
“He [Starling] also controlled the aortic pressure
(or afterload) by means of an artificial aortic
resistance.” (p. 76)
Cardiovascular Physiology
(22)
“During diastolic ventricular filling, for example,
the progressive increases in ventricular
pressure and volume combine to increase
muscle tension (T ⫽ P 䡠 r) . . .” (p. 55)
“End-diastolic pressure is referred to as
ventricular preload because it sets the resting
tension of the cardiac muscle fibers at the end
of diastole.” (p. 55)
“Systemic arterial pressure is often referred to as
the ventricular afterload because it
determines the tension which must be
developed by cardiac muscle fibers before
they can shorten.” (p. 56)
Cardiovascular Physiology
(1)
“. . . the preload refers to the stretch of the left
ventricle just before the onset of contraction
(the so-called end-diastolic volume) . . .”
(p. 65)
“. . . and the afterload refers to the aortic
pressure during the period when the aortic
valve is open.” (p. 65)
Physiology of the Heart and
Circulation (18)
“It is customary to refer to this length [just
before contraction] in terms of the force or
preload required to stretch the muscle to its
precontraction length.” (p. 77)
“In summary, cardiac afterload . . . is the left
ventricular myocardial force necessary to
overcome opposition to ventricular ejection.
In the clinical setting, as a rough index, it is
frequently related to aortic pressure.” (p. 177)
An Introduction to
Cardiovascular Physiology
(15)
“To study the effect of stretch, the relaxed
muscle is stretched to a known length by
means of a small weight or preload . . .”
(p. 78)
“The afterload is the stress, S [force per unit
cross-sectional area of wall], during systole,
and from the statement S ⫽ Pr/2w we see
that it depends not only on the arterial
pressure but also on chamber radius and wall
thickness.” (p. 89)
Modern Cardiovascular
Physiology (11)
“This volume [end-diastolic volume] is often
termed preload because it is a load applied to
the muscle fibers before they contract.” (p. 9)
“This resistance [to outflow] is termed the
afterload, since it is applied after contraction
is initiated.” (p. 11)
Cardiovascular Physiology
(20)
“. . . the upper end of the muscle is anchored,
and a weight (preload) is suspended from the
lower end. The resting force is equal to the
weight attached . . .” (p. 94)
“Preload, the force just prior to contraction
(Chapter 3), is related in the ventricle to enddiastolic pressure.” (p. 118)
“The afterload on an intact ventricle is
consequently not a simple quantity, and
authorities do not agree on how it should be
measured or expressed.”
(p. 118)
“The input impedance of the systemic or
pulmonary arteries is the most appropriate
measure of ventricular afterload, but it is
complicated to analyze and takes the form of
a frequency-dependent spectrum (Chapter 6).
The choice is thus between a simple variable
like mean aortic pressure, which is an
indirect, partial representation of the real
afterload, and the more complete but
complicated analysis involving in computing
impedance.” (p. 118)
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Preload Definition
P E R S O N A L
V I E W
TABLE 1—Continued
Summary of published definitions of preload and afterload
Reference
Preload Definition
Afterload Definition
n/a
“The Frank-Starling Law was established in
animal studies in which a constant aortic
pressure (afterload) and a constant
contractility were maintained.” (p. 23)
Fundamental Cardiovascular
and Pulmonary Physiology
(7)
“The ventricle begins to contract . . . at a
measurable end-diastolic pressure that
represents the initial load of the ventricle or
preload.” (p. 73)
“Ejection begins . . . and the ventricular pressure
at this point (equal to aortic pressure)
represents ventricular afterload.” (p. 73)
Pathophysiology of Heart
Disease (16)
“. . . the preload can be thought of as the amount
of myocardial stretch at the end of diastole,
just prior to contraction.” (p. 195)
“preload: the ventricular wall tension at the end
of diastole. In clinical terms, it is the stretch
on the ventricular fibers just prior to
contraction, often approximated by the enddiastolic volume or end-diastolic pressure”
(p. 196, Table 9.1)
“afterload: the ventricular wall tension during
contraction; the resistance that must be over
come in order for the ventricle to eject its
contents. It is often approximated by the
systolic ventricular (or arterial) pressure.” (p.
196, Table 9.1)
“It [afterload] is more formally defined as the
ventricular wall stress that develops during
systolic ejection.” (p. 196)
Pathophysiology of Disease:
An Introduction to Clinical
Medicine (19)
“ ‘Preload’ is the amount of filling of the
ventricle at end-diastole.” (p. 227–229)
“The impedance against which the heart must
work is termed ‘afterload;’ increased
afterload (aortic pressure for the left ventricle)
will cause a decrease in stroke volume.” (p.
227)
Cardiopulmonary System
(25)
“In terms of muscle performance, the preload is
the stretch on a muscle fiber prior to
contraction.”
(p. 39)
“Aortic pressure (PAo, the afterload) . . .” (p. 39)
Harrison’s Online (10)
“In the heart-lung preparation the stroke volume
within limits correlates directly with the
diastolic fiber length (preload) . . .”
(Chap. 232)
“. . . the stroke volume of the intact ventricle [is]
determined by three influences: (1) the length
of the muscle at the onset of contraction, i.e.,
the preload; . . .” (Chap. 232)
“In the intact heart the afterload may be
defined as the tension or stress developed in
the ventricular wall during ejection.
Therefore, the afterload is determined by the
aortic pressure as well as the volume and
thickness of the ventricular cavity.”
(Chap. 232)
“ . . . the tension that the muscle is called upon
to develop during contraction, i.e., the
afterload.” (Chap. 232)
Integrated Medical
Curriculum Online (13)
“Preload is end diastolic volume (EDV).”
“Afterload . . . can be defined as ‘the force the
heart has to overcome to eject blood.’ ”
Physiological Medicine: a
clinical approach to basic
medical physiology (17)
“Preload is the venous pressure that results in
filling of the heart in diastole.” (p. 322)
“Afterload is the pressure against which the
heart must work to pump blood.” (p. 322)
Human Physiology: From
Cells to Systems (27)
“The extent of filling is referred to as the
preload, because it is the workload imposed
on the heart before contraction begins.”
(p. 292)
“The arterial pressure is referred to as the
afterload because it is the workload imposed
on the heart after the contraction has begun.”
(p. 294)
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Clinical Cardiology (4)
P E R S O N A L
V I E W
TABLE 1—Continued
Summary of published definitions of preload and afterload
Reference
Preload Definition
Afterload Definition
“The preload is the ventricular end-diastolic
volume or pressure.” (p. 83)
“The third factor that determines stroke volume
is the afterload, or pressure load, i.e., the total
peripheral resistance. This is the load which
the heart must pump against in order to eject
blood, and its magnitude is best represented
by the [arterial] diastolic pressure.”
(pp. 84–85)
Physiology (5)
“The preload for the left ventricle is left
ventricular end-diastolic volume, or enddiastolic fiber length; that is, the resting length
from which the muscle contracts.” (p. 127)
“The afterload for the left ventricle is aortic
pressure.” (p. 127)
Physiology: An Illustrated
Review with Questions and
Answers (29)
“Preload, the ventricular end-diastolic volume
(or pressure), reflects how much the heart is
stretched before contraction.” (p. 45)
“Afterload, the pressure against which the
heart must pump to eject blood, is a function
of the total peripheral resistance.” (p. 45)
Blond’s Medical Guides:
Physiology (8)
EDV (or preload) is directly affected by the
amount of blood that enters the ventricle
while the heart is in diastole.” (p. 156)
“While afterload relates mainly to the blood
pressures in the arteries . . .”
(p. 157)
Taber’s Cyclopedic Medical
Dictionary (30)
preload. In cardiac physiology, the end-diastolic
stretch of the muscle fiber. In the intact
ventricle, this is approximately equal to the
end-diastolic volume or pressure. (p. 1585)
afterload. In cardiac physiology, the stress or
tension that develops in the ventricular wall
during systole. (p. 51)
Definitions of preload and afterload drawn from 31 different sources including textbooks, monographs, and websites.
end of diastole. From this expression, one can see
that end-diastolic filling pressure or end-diastolic volume (manifested in the equation above as radius)
contribute to preload, but should not be equated
with preload. A summary flow chart of factors contributing to preload is provided in Fig. 1.
here.) From the expression above, it is clear that
anything that increases left ventricular output impedance and therefore requires a greater ventricular pressure during systole (aortic stenosis, hypertension, increased total peripheral resistance, hypertrophic
cardiomyopathy, etc.) will cause an increase in afterload. Also, if the chamber radius is increased as the
result of increased filling during diastole or ventricular
remodeling in response to chronic increases in filling
pressures, afterload will be increased even if arterial
pressure is normal. Arterial pressure and total peripheral resistance contribute to afterload but should not
be equated with afterload. A summary flow chart of
factors contributing to afterload is provided in Fig. 2.
Similarly, with the use of LaPlace’s equation again, left
ventricular afterload can be best described as the left
ventricular ␴ or T during systolic ejection: afterloadLV ⫽ (SPLV)(SRLV)/2wLV, where SPLV is left ventricular systolic pressure and SRLV is left ventricular
systolic radius. The afterload for the right ventricle
would be described mathematically in an analogous
fashion. Defined in words, therefore, afterload represents all the factors that contribute to total
myocardial wall stress (or tension) during systolic ejection. (In vivo, both systolic pressure and
systolic volume are changing constantly during the
ejection phase of the cardiac cycle, and, therefore, so
is afterload; but this variability during systole doesn’t
significantly affect the basic arguments presented
These definitions for preload and afterload fit the
psychological need for conciseness and brevity, yet
by their mention of wall stress, these definitions force
a consideration of the Law of LaPlace and of the
complex relationships among pressure, volume, and
wall tension in the beating heart. The importance of
focusing on wall stress in the definitions of preload
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Physiology: A Review with
Questions and
Explanations (12)
P E R S O N A L
V I E W
and afterload (afterload, in particular) relates to the
metabolic costs associated with the development of
myocardial wall tension and, therefore, chamber pressure. The greater the tension requirement during systole, the greater the demand for oxygen and metabolic substrate by the myocardium. In the presence of
cardiac disease, both physiological compensatory
mechanisms and therapeutic regimens have as their
goals the reduction of myocardial wall tension (and,
therefore, myocardial oxygen consumption) and the
restoration of a balance between oxygen supply and
demand, especially important in patients with impaired coronary blood flow. A comprehensive definition of afterload, such as that provided here, would
help students appreciate this therapeutic rationale.
preload and increased afterload. If filling pressures,
output pressures, and stretch (factors in the numerators of the equations described above) are loads imposed on the heart by conditions within the circulatory system, then a change in myocardial wall thickness
(in the denominator) can be considered as a major
myocardial response to these externally imposed perturbations. For increased preload, the additional wall stress
caused by a larger chamber radius is normalized by
increasing the wall thickness enough to restore the ratio
EDR LV/wLV in the equation above for preload. Likewise,
for an increased afterload generated by a greater output
impedance requiring higher ventricular pressures during systole, the systolic wall stress is normalized by
hypertrophy that restores the ratio SPLV/wLV.
The relationships among pressure, radius, and wall
thickness described above provide a clear physiological explanation for the different patterns of hypertrophy and remodeling seen in response to increased
In conclusion, the tendency clearly exists in texts, in
conversation, and even in formal lectures to use short,
simple definitions of preload and afterload. Preload is
defined variously as “filling pressure” or end-diastolic
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FIG. 1.
Factors determining preload: a flow diagram illustrating the various
factors within the cardiovascular system that determine preload and
therefore end-diastolic myocardial passive wall stress based on the parameters in the Law of LaPlace: chamber radius, chamber pressure, and
wall thickness.
P E R S O N A L
V I E W
volume; afterload is often simplified as “total peripheral resistance” or arterial pressure. According to the
above analysis, these are only components of preload
and afterload and don’t tell the whole story. If, in the
mind of a student, afterload is defined only as aortic
pressure, then that student will not be able to appreciate fully the increases in afterload (left ventricular
wall stress) and, therefore, oxygen consumption that
would accompany aortic stenosis, obstructive cardiomyopathy, or ventricular remodeling associated with
increased chamber radius.
pathophysiology and the therapeutic approaches to
heart disease.
It is my contention that preload and afterload should
be consistently defined in terms of myocardial wall
stress (or tension) and that the definitions should
always include the major factors affecting wall tension for each, namely, chamber pressure, chamber
radius, and wall thickness. If you keep wall stress or
“wall tension” built into your definitions of preload
and afterload, you will be better able, in my opinion,
to help your students understand cardiovascular
Received 22 June 2000; accepted in final form 30 October 2000
The author acknowledges the students in the College of Osteopathic Medicine and in Physician Assistant and Nurse Anesthesia
programs of the University of New England for many constructive
criticisms and comments regarding what really works in the classroom.
Address for reprint requests and other correspondence: J. M. Norton, Dept. of Physiology and Pharmacology, Univ. of New England
College of Osteopathic Medicine, 11 Hill’s Beach Rd., Biddeford,
ME 04005 (E-mail: [email protected]).
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FIG. 2.
Factors determining afterload: a flow diagram illustrating the various factors within the cardiovascular system that determine afterload and therefore myocardial wall tension during systole based on the parameters of the
Law of LaPlace: chamber radius, chamber pressure, and wall thickness.
P E R S O N A L
V I E W
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