Arterial System Flashcards
What is hydraulic filtering?
The compressibility of air trapped converts intermittent inflow of water to a steady outflow of water
What does the windkessel effect explain?
hypertension
it separates the conduit and cushioning function of the arterial tree
an increase in TPR + decrease in arteral compliance = hypertenison
What occurs in the windkessel effect when only resistance increases?
MAP rises to an equal increment in systolic and diastolic pressures
What occurs in the windkessel effect when resistance increases and compliance decreases?
MAP increases to the same extent as with only increased resistance
but pressure oscillations are increased resulting in disproportionate increase in systolic BP and little changes to diastolic
What occurs during systole with compliant arteries?
a substantial fraction of SV is stored in the arteries, the arterial walls are stretched
*reservoir effect
What occurs during diastole with compliant arteries?
previously stretches arteries recoil, the volume of blood displaced by recoil facilitates continuous capillary flow in diastole (secondary pump to maintain diastolic pressure)
What occurs during systole with rigid arteries?
virtually none of the SV can be stored in the arteries (increased force to push SV, increased speed of flow)
What occurs during diastole with rigid arteries?
arteries cannot recoil
increased *MVO2
What is the duration of cardiac SV discharge during systole?
about 1/3 of a cardiac cycle
When is most of the SV pumped?
during rapid ejection (about 1/2 of systole)
How is part of the energy of cardiac contraction dissipated?
as forward capillary flow during systole
remaining energy in distensible arteries is stored as potential energy
What does the elastic recoil of arterial walls convert potential energy into?
into capillary blood flow
What occurs to capillary flow during diastole if arterial walls are rigid?
capillary flow would have ceased
What effect does hydraulic filtering have on cardiac workload?
decreases MVO2
more work is required to pump a given flow intermittently than steadily
*The steadier the flow, the less the excess work
At any given stroke volume, what is the relationship of MVO2 with rigid arteries?
A rigid artery will increase MVO2
What is the equation for aortic compliance?
Dv/DP
change in volume/change in pressure
What is the effect of age on aortic compliance?
For any pressure about 80 mmHg, the aortic compliance will decrease with age
What causes the decrease in aortic compliance with age?
progressive changes in collagen and elastin contents of arterial walls (decrease)
decrease in diameter of the aorta during each cardiac contraction
*offset with exercise
How does heart failure affect the relationship between age and aortic compliance?
reduction in aortic compliance with age is normal
heart failure (disease) + age = magnified decrease in compliance
What are the two major limitations of the windkessel model?
- the arterial tree doesn’t separate conduit and cushioning functions
- it makes the assumption that PWV is of infinite value
Explain the limitations of the windkessel model: the arterial tree doesn’t separate conduit and cushioning functions
both are features of the aorta and its major branches which are distensible tubes
there is a progressive loss of cushioning function, from the ascending aorta to more muscular and increasing conduit function of large arteries from heart to periphery
Explain the limitations of the windkessel model: it makes the assumption that PWV is of infinite value
can’t be the case because of heterogeneity of PWV along the arterial tree
- determines by cushioning and conduit functions in adjacent arterial segments
peripheral arteries are stiffer than central, leading to an increase in amplitude of pressure wave in vessels from heart to periphery
the stiffness of medium-sized peripheral arteries is modulated by vasomotor tone (how constricted) either depending on the endothelial function or sympathetic NS or renin-angiotensin system
What is the gold standard for measuring arterial stiffness?
PWV
Why is a propagative model (like PWV) widely used?
assumes each pulse wave has a finite value
it clearly illustrates the facts that the propagation of the pulse wave is inversely related to compliance of the artery
What will a faster PWV do to the compliance of a vessel?
decreases the compliance
What is the equation for PWV?
PWV = Length/change in time
How is a tonometer used to measure PWV?
used at different sites
difference in the onset of the wave forms with one heart contraction
What does less time between the onset of wave forms with a tonometer indicate?
a faster PWV
Is a faster or slower PWV better?
slower
with compliance energy will be absorbed by walls
without compliance energy is forced forward = increased velocity
* PWV is an indication of compliance
What is an arterial pressure waveform made up of?
forward pressure wave exerted by ventricular contraction + reflected wave
Where are waves mainly reflected from in the periphery?
branch points or sites of impedance mismatch (sudden narrowing)
What is the effect of reflected waves in elastic vessels?
because PWV is low they tend to arrive back at aortic root during diastole
What is the effect of reflected waves in stiff vessels?
PWV rises and the reflected wave arrives back at the central arteries earlier (during systole), adding to the forward wave and augmenting the systolic pressure
What is the augmentation index (Alx)?
difference between the second and first systolic peaks
expressed as a percentage change of pulse pressure
= peak of reflected wave / peak of ejection wave
What physical factors can play an important role in wave reflection?
geometry
number of arterioles
architecture of microvascular network
What does arterial and arteriolar constriction result in regarding reflected waves?
reflection points closer to the heart, leading to earlier aortic wave reflections
*increased augmentation
What does increased aortic stiffness cause regarding reflection waves?
waves to travel more rapidly and arrive earlier in systole
superimposing on the forward wave and boosting systolic pressure
What is the primary source of stiffness?
aorta
What physiological factors affect arterial blood pressure?
CO (HR x SV)
peripheral resistance
What physical factors affect arterial blood pressure?
arterial blood volume
arterial compliance
What is pulse pressure?
systolic - diastolic
a function of SV and arterial compliance
During rapid ejection what occurs to blood volume?
*compliance is held constant
the volume of blood introduced into the arterial system exceeds the volume that exits
- arterial pressure and volume will increase
When is maximum arterial blood volume reached?
at the end of rapid ejection, corresponds to peak pressure (systolic pressure)
What does pulse pressure correspond to?
some arterial volume increment
What does the volume increment equal?
volume of blood discharged by LV during rapid ejection minus volumes that that run off to the periphery during same phase of cardiac cycle
*~80% of SV
raises arterial volume rapidly
What occurs to the pressures and volume increment during diastole?
volume and pressure will fall
* peripheral runoff will exceed cardiac ejection
If SV is suddenly doubled, what occurs to mean arterial pressure?
*compliance is held constant
it will double
arterial pressure will now oscillate with each heart beat about this new value of mean arterial pressure
How would a normal heart eject the new greater SV?
*compliance is held constant
during a fraction about equal to what prevailed at lower SV
How would the greater volume increment be reflected in the pulse pressure?
*compliance is held constant
pulse pressure would be twice as great (driven by volume)
What occurs when both mean and pulse pressure increase?
*compliance is held constant
the increase in systolic pressure exceeds the increase in diastolic pressure
What is the effect of heart failure on pulse pressure and SV?
very small pulse pressure because SV is abnormally small
What is the effect of a large SV on pulse pressure?
likely to have large pulse pressure
What is a well-trained athlete’s pulse pressure?
have low resting HR
prolonged ventricular filling times induce the ventricles to pump a large quantity of blood per beat
thus a large pulse pressure
What is the effect of compliance on pulse pressure?
with the same volume increment, there will be greater pulse pressure in less compliant arteries
* workload on LV will be greater even if SV, TPR, and arterial pressures are equal
What does pulse pressure vary as a function of?
compliance
