Ventricular Function Flashcards
resting tension
the amount of tension that develops passively by stretching muscle
active tension
the amount of tension that is developed by the muscle by an active process (contraction) at a particular length
preload
force placed on muscle before contraction beings (pre)
muscle is passive as preload is applied
causes muscle to passively lengthen
intact heart, preload is the volume that fills the chamber during the diastolic phase
isometric contraction
contraction without change in length
isotonic contraction
contraction with shortening and constant muscle tone
effects of increasing preload on isometric contraction
increasing preload increases amount of tension developed
time to peak tension unchanged
increases dT/dt
optimal length
the sarcomere length that produces the maximal tension development - due to better crossbridge alignment
length dependent calcium sensitivity
as length of muscle increases, so does the sensitivity to calcium
contractility
a change in force development, independent of change in muscle length due to load
changing contractility alters the amount of tension developed and the time to peak tension (isometric)
increasing preload at constant afterload and contractility
- Decreases the time to the onset of shortening. (Note the dotted vertical lines in A)
- Increases the amount of shortening. (A, B, C)
- Increases shortening velocity. (Note differences in dL/dt in A). Shortening velocity increases with increased preload.
- Does not affect the maximum tension reached (This is determined by the afterload!).
increasing afterload at constant preload and contractility
- Increasing afterload decreases the amount of shortening
- Increasing afterload decreases the velocity of shortening
- Increasing afterload increases the total tension developed in the muscle.
What happens to Vmax when contractility is increased?
increased
ejection fraction equation
EF= { EDV-ESV } / EDV * 100
preload in the heart
stretches cardiac muscle passively toward its optimal length
passively positions contractile elements for interaction
alters calcium sensitivity
equivalent to EDV
afterload in the heart
opposes shortening (ejection)
analogous to arterial pressure
in some diseases, also due to obstrictive structures such as aortic valve stenosis or hypertrophied muscle
contractility of the heart
force development independent of length changes
in intact heart, contractility changes due to changes in sympathetic nerve activity
increased sympathetic activity leads to greater calcium release and greater ejection
What is the adult normal value for ejection fraction?
about 60%
How do you maximize shortening in isolated cardiac muscle or stroke volume in the heart?
increase preload
decrease afterload
increase contractility
heart failure
a condition in which the heart fails to provide a cardiac output sufficient to meet the needs of the body
reasons for decreased contractility in the heart
decrease calcium availability
ischemia
myocardial depressants
chronic volume overload
reasons for decreased compliance
scar tissue
elevated pericardial fluid
pericardial constriction
mechanisms of heart failure
impaired contractility
impaired diastolic filling
increased afterload
causes of impaired diastolic filling
left ventricular hypertrophy
restrictive cardiomyopathy
myocardial fibrosis
transient myocardial ischemia
pericardial constriction or tamponade
reason for increased afterload (chronic and severe)
advanced aortic stenosis
uncontrolled severe hypertension
factors affecting stroke volume
force of contraction
afterload
ventricular size
hypertrophy
determinants of preload
pressure gradient
time for filling
compliance
atrial function
determinants of contractility
neurotransmitters
drugs
disease
determinants of afterload
aortic pressure
ventricular outflow tract resistance
absolute heart rate
heart rate affects the time for ventricular filling
factors influencing “heart rate” (ventricular rate)
pacemaker function
AV nodal conduction
ventricular conduction
autonomic nervous system
external work
external work = stroke volume x mean aortic pressure
