Download Preload

Physiology of Ventricular
Function
Dr. Chris Glover
Interventional Cardiology
Director of Education
University of Ottawa Heart Institute
January 12, 2015
Objectives
 Define preload, afterload, and
contractility
 Define cardiac output, stroke volume
 How does load and contractility interplay
with pump function
 Fick equation (not)
 Everyday examples
Terms to Define
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PRELOAD
AFTERLOAD
CONTRACTILITY
COMPLIANCE
STROKE VOLUME
CARDIAC OUTPUT
CARDIAC OUTPUT
 Volume of blood ejected per minute
Factors Affecting Stroke Volume
 Stroke Volume is amount of blood ejected
during each cardiac cycle
 Determinants are
– Preload : +ve
– Afterload : -ve
– Contractility : +ve
PRELOAD
 The load on the ventricle wall set just prior
to systole (the end of diastolic volume)
Greater distension = Greater contraction
 End-diastolic volume (EDV) and/or Enddiastolic pressure (EDP) correlate with
myocardial stretch
Starling’s Law of the Heart
 the extent to which cardiac muscle
contracts is determined by the initial
fiber length prior to initiation of
contraction". The preload (end diastolic
volume) determines the initial fiber length
and hence the degree of overlap of the
actin-myosin filaments in each sarcomere.
The force generated is thus proportional to
the end diastolic volume.
STARLING LAW OF THE
HEART
 The more blood that enters the heart, the
more blood that is pumped out of the heart
Preload - Muscle Fiber
 Relation between muscle length and tension
is curvilinear
 Isometric contraction – tension generated
proportional to length of muscle at time of
contraction
 Stretching optimizes myosin-actin
interaction and increases myofilament
sensitivity to calcium
Preload – Muscle fiber
 Relationship between fiber length and
tension in intact heart is fundamental to
understanding function
 Larger diastolic ventricular volume = More
stretch on muscle fibers = greater
contraction
 Frank-Starling mechanism
PRELOAD
Preload
 ↑ Venous Pressure - ↑ Preload
 ↑ Atrial Contractility - ↑ Preload
 ↑ length of diastole - ↑ Preload
 ↓ ventricular compliance - ↓ Preload
CARDIAC OUTPUT
 Volume of blood ejected per minute
Factors Affecting Stroke Volume
 Stroke Volume is amount of blood ejected
during each cardiac cycle
 Determinants are
– Preload : +ve
– Afterload : -ve
– Contractility : +ve
Pressure-Volume loops
Preload
Afterload and contractility are constant
PRELOAD
Preload
 ↑ Venous Pressure - ↑ Preload
 ↑ Atrial Contractility - ↑ Preload
 ↑ length of diastole - ↑ Preload
 ↓ ventricular compliance - ↓ Preload
Afterload
 Pressure generated by ventricle and
size of chamber at end of contraction
depends upon load against which the
ventricle contracts
Afterload
 Muscle fibers contract against fixed load
(isotonic contraction)
 Length of muscle at end of contraction
directly related to magnitude of load - ↑ load
→ ↓ shortening ( ↓ stroke volume)
 Final length → independent of length prior to
stimulation
AFTERLOAD
 the "load" to be lifted by contraction i.e. the aortic
pressure
 Equals arterial systolic pressure in absence of
aortic obstruction
Afterload
Constant preload/contractility
Contractility
 Changes in the force of contraction
independent of the initial fiber length and
afterload
 Relation b/w initial fiber length and force
developed during contraction shifted upward
 Achieve shorter final length for fixed
afterload
 Changes due to chemical hormonal
interaction
CONTRACTILITY
 When contractility is increased, a greater
force is generated by the contraction,
starting from a given preload. When
contractility is decreased, a smaller force is
generated by the contraction, starting from a
given preload.
Contractility
Constant preload/afterload
Left ventricular performance
Compliance
 Pressure-volume relationship of chamber
during filling
 Reflects ease or difficulty with which
chamber can be filled
 ΔP/ΔV
 Decreased with ischemia, hypertrophy,
infiltration
 ?increased in athletes
Compliance
Clinical - ↓ Preload
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Low Preload – hypovolemia, bleeding
↓ stroke volume
↓ cardiac output
Treatment – replace losses (fluid, blood)
Clinical - ↑ preload
 High Preload – CHF, Mitral/Aortic
insufficiency
 ↑ preload → ↑ EDP → Pulmonary
congestion
 Treatment – optimize contractility (Digoxin);
reduce preload (diet, diuretics); fix valve
(MR/AR)
Clinical - ↑ afterload
 High afterload – Hypertension, aortic obstruction
(aortic stenosis, hypertrophic cardiomyopathy)
 Cardiac muscle hypertrophy reduces tension
 ↑ hypertrophy → ↓ compliance → ↑ EDP → may
lead to Heart Failure
 Treatment – antihypertensives for hypertension;
relieve obstruction for aortic obstruction
Clinical - Contractility
 ↓ Contractility → Myocardial Infarction/
Cardiomyopathy → ↓ SV → ↓ CO
 Treatment - ↑ contractility (digoxin,
catecholamines); optimize preload/afterloead
 Treat cause
Clinical - Compliance
 ↓ compliance ; hypertrophy, ischemia, scar,
infiltration
 ↓ compliance → ↑ EDP for given EDV
 Pulmonary congestion occurs at lower
preload
Thank you
Questions?