Conceito de pré carga e pós carga em cardiologia
9 pág.

Conceito de pré carga e pós carga em cardiologia


DisciplinaFisioterapia8.381 materiais24.220 seguidores
Pré-visualização5 páginas
Summary of published definitions of preload and afterload
Reference Preload Definition Afterload Definition
Clinical Cardiology (4) n/a \u201cThe Frank-Starling Law was established in
animal studies in which a constant aortic
pressure (afterload) and a constant
contractility were maintained.\u201d (p. 23)
Fundamental Cardiovascular
and Pulmonary Physiology
(7)
\u201cThe ventricle begins to contract . . . at a
measurable end-diastolic pressure that
represents the initial load of the ventricle or
preload.\u201d (p. 73)
\u201cEjection begins . . . and the ventricular pressure
at this point (equal to aortic pressure)
represents ventricular afterload.\u201d (p. 73)
Pathophysiology of Heart
Disease (16)
\u201c. . . the preload can be thought of as the amount
of myocardial stretch at the end of diastole,
just prior to contraction.\u201d (p. 195)
\u201cpreload: 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 end-
diastolic volume or end-diastolic pressure\u201d
(p. 196, Table 9.1)
\u201cafterload: 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.\u201d (p.
196, Table 9.1)
\u201cIt [afterload] is more formally defined as the
ventricular wall stress that develops during
systolic ejection.\u201d (p. 196)
Pathophysiology of Disease:
An Introduction to Clinical
Medicine (19)
\u201c \u2018Preload\u2019 is the amount of filling of the
ventricle at end-diastole.\u201d (p. 227\u2013229)
\u201cThe impedance against which the heart must
work is termed \u2018afterload;\u2019 increased
afterload (aortic pressure for the left ventricle)
will cause a decrease in stroke volume.\u201d (p.
227)
Cardiopulmonary System
(25)
\u201cIn terms of muscle performance, the preload is
the stretch on a muscle fiber prior to
contraction.\u201d
(p. 39)
\u201cAortic pressure (PAo, the afterload) . . .\u201d (p. 39)
Harrison\u2019s Online (10) \u201cIn the heart-lung preparation the stroke volume
within limits correlates directly with the
diastolic fiber length (preload) . . .\u201d
(Chap. 232)
\u201c. . . 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; . . .\u201d (Chap. 232)
\u201cIn 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.\u201d
(Chap. 232)
\u201c . . . the tension that the muscle is called upon
to develop during contraction, i.e., the
afterload.\u201d (Chap. 232)
Integrated Medical
Curriculum Online (13)
\u201cPreload is end diastolic volume (EDV).\u201d \u201cAfterload . . . can be defined as \u2018the force the
heart has to overcome to eject blood.\u2019 \u201d
Physiological Medicine: a
clinical approach to basic
medical physiology (17)
\u201cPreload is the venous pressure that results in
filling of the heart in diastole.\u201d (p. 322)
\u201cAfterload is the pressure against which the
heart must work to pump blood.\u201d (p. 322)
Human Physiology: From
Cells to Systems (27)
\u201cThe extent of filling is referred to as the
preload, because it is the workload imposed
on the heart before contraction begins.\u201d
(p. 292)
\u201cThe arterial pressure is referred to as the
afterload because it is the workload imposed
on the heart after the contraction has begun.\u201d
(p. 294)
P E R S O N A L V I E W
VOLUME 25 : NUMBER 1 \u2013 ADVANCES IN PHYSIOLOGY EDUCATION \u2013 MARCH 2001
57
end of diastole. From this expression, one can see
that end-diastolic filling pressure or end-diastolic vol-
ume (manifested in the equation above as radius)
contribute to preload, but should not be equated
with preload. A summary flow chart of factors con-
tributing to preload is provided in Fig. 1.
Similarly, with the use of LaPlace\u2019s equation again, left
ventricular afterload can be best described as the left
ventricular s or T during systolic ejection: after-
loadLV 5 (SPLV)(SRLV)/2wLV, where SPLV is left ven-
tricular 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 rep-
resents all the factors that contribute to total
myocardial wall stress (or tension) during sys-
tolic 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\u2019t
significantly affect the basic arguments presented
here.) From the expression above, it is clear that
anything that increases left ventricular output imped-
ance and therefore requires a greater ventricular pres-
sure during systole (aortic stenosis, hypertension, in-
creased total peripheral resistance, hypertrophic
cardiomyopathy, etc.) will cause an increase in after-
load. 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 periph-
eral 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.
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
TABLE 1\u2014Continued
Summary of published definitions of preload and afterload
Reference Preload Definition Afterload Definition
Physiology: A Review with
Questions and
Explanations (12)
\u201cThe preload is the ventricular end-diastolic
volume or pressure.\u201d (p. 83)
\u201cThe 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.\u201d
(pp. 84\u201385)
Physiology (5) \u201cThe preload for the left ventricle is left
ventricular end-diastolic volume, or end-
diastolic fiber length; that is, the resting length
from which the muscle contracts.\u201d (p. 127)
\u201cThe afterload for the left ventricle is aortic
pressure.\u201d (p. 127)
Physiology: An Illustrated
Review with Questions and
Answers (29)
\u201cPreload, the ventricular end-diastolic volume
(or pressure), reflects how much the heart is
stretched before contraction.\u201d (p. 45)
\u201cAfterload, the pressure against which the
heart must pump to eject blood, is a function
of the total peripheral resistance.\u201d (p. 45)
Blond\u2019s 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.\u201d (p. 156)
\u201cWhile afterload relates mainly to the blood
pressures in the arteries . . .\u201d
(p. 157)
Taber\u2019s 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.
P E R S O N A L V I E W
VOLUME 25 : NUMBER 1 \u2013 ADVANCES IN PHYSIOLOGY EDUCATION \u2013 MARCH 2001
58
and afterload