cardiac perf Flashcards
Cardiac Output
volume of blood ejected by the heart per unit time
= heart rate (HR) x stroke volume (SV)
stroke volume
volume of blood pumped per contraction
SV = EDV- ESV
end-diastolic volume
volume of blood in ventricle before contraction
end-systolic volume
volume of blood in ventricle after contraction
preload
- volume of blood (load) inside the heart chamber before contraction
- fiber length or tension when the heart contracts
what influences preload
- venous return, EDV
- ventricular filling time
- ventricular compliance
- atrial systole
what is Frank-Starling’s Law of the Heart
- stretching cardiac muscle fibers increases strength of contraction
- stretch dependent on the extent of ventricular filling or preload
cardiac muscle length-tension relationship
unlike skeletal muscle, cardiac muscle does not normally function at the peak of the length-tension relationship; stretching cardiac muscle fibers increases strength of contraction
*neither summation nor recruitment occurs
what causes an increase in tension in cardiac muscles? List steps.
- overlap of thick and thin filaments
- increases Ca2+ sensitivity of myofilaments
- enhanced CA2+-induced CA2+ release (stretch activated Ca2+ channels)
Why is the length tension relationship different between skeletal and cardiac muscle?
- cardiac muscle is
- stiffer than skeletal muscle
- high resting tension
- small changes in length produce large changes in tension
how are fiber length and stroke related
- increase in LV-end diastolic fiber length = greater force of contraction = greater SV
what does preload dictate
ventricular filling pressure
how does ventricular filling time impact end diastolic volume and stroke volume
- as heart rate increases
- filling of ventricles will decrease
- EDV and SV will decrease
what is ventricle filling pressure
the pressure that builds up in the ventricle as the ventricle is being filled with blood
- typically equivalent to the mean atrial pressure
- more blood = more pressure
what is ventricular compliance
the heart’s ability to distend under pressure
C= change in volume/change in pressure
what happens to ventricular compliance during heart disease
- compliance decreases as ventricles can “stiffen”
what happens to SV during heart disease
- decreased compliance causes
- decrease in end-diastolic fiber length
- decreased SV
why is atrial systole more important at elevated heart rates (i.e. exercise)
- atrial systole accounts for 10-20% of ventricular filling at rest
- accounts for up to 40% of SV at elevated heart rates
what is afterload
- resistance against which the ventricle contracts
OR
- the pressure the ventricle must generate to eject blood
Name some factors that affect afterload
oppose ventricular ejection
- peripheral resistance
- blood viscosity
- valvular dysfunction
afterload is equated to what
aortic pressure
give equation for relationship between mean arterial pressure, diastolic pressure and systolic pressure
MAP = diastolic pressure + 1/3 (systolic pressure - diastolic pressure)
pressure and volume loops provides information about what
ventricular performance
what 4 phases are represented in the pressure and volume loop
what is occuring during isovolumetric contraction
- contraction against a closed chamber and fixed volume of blood
- mital valve and aortic valve closed
- ventricular volume is constant
- LVP increases dramatically
what is occuring during ventricular ejection (top of PV loop)
- LVP becomes greater than aortic pressure and aortic valve opens
- blood is ejected
what occurs to LV pressure and volume during ventricular ejection
- LVP remains high because of contracting ventricle
- LV volume decreases dramatically
What is occuring during isovolumetric relaxation (left side of PV loop)
- systole ends, ventricle relaxes
- LVP drops below aortic pressure => aortic valve closes
- ventricular blood volume remains constant
- closed valves = fixed volume
what initiates ventricular filling (bottom part of PV loop)
- LV pressure falls below atrial pressure
- mitral valve opens
what happens to volume of ventricle during ventricular filling (bottom part of PV loop)
- ventricle fills with blood; volume increases back to its EDV
how does the pressure volume loop change with increased preload
- increase in venous return
- greater filling of LV
- greater end diastolic fiber length
- stroke volume is increased
what changes are made to the pressure volume loop with increased afterload
- increased aortic pressure
- LV must eject blood against a greater pressure
- LV pressure must increase to a level higher than aortic pressure
- aortic valve opens later and closes sooner
- ESV is increased => SV is reduced
What changes are made to the P/V loop with increased contractility (sympathetic stimulation)
- LV develops greater tension and pressure
- LV ejects a greater volume of blood during systole
- less blood remains in ventricle
- SV is increased as expense of a decreased ESV
define work as related to the myocardium
- work = force (pressure) x distance (stroke volume)
- work is stroke work = work the heart performs on each beat
what is the equation for stroke work
stroke volume x MAP (afterload)
what is cardiac work
left ventricular work per unit time
what is the equation for cardiac work
cardiac work = cardiac output x MAP
cardiac work = stroke work x heart rate
look at last lecture slide
- increased filling
- decreased afterload
- increased preload
- increase ventricular pressure
- increased stroke volume
- increased contractility
