The physiologic impact of a stenotic valve on the heart is determined by the _____ and the ______.
-valve position -obstruction severity (orifice size)
Stenotic valves effect on orifice size, flow velocity
-decrease orifice size which per the continuity equation requires increase in flow velocity to achieve physiologic flow rates--generates a clinically important pressure gradient
Pressure gradient due to a stenotic valve leads to a pressure load where?
-subjects the chamber UPSTREAM from the valve to a pressure load
Per Bernoulli, the relationship between the pressure gradient and flow velocity is _____.
How does blood acquire a velocity within the heart?
-transfer of blood across an orifice requires the blood have kinetic energy associated with the velocity it acquires -this is accomplished by transforming hydraulic potential (pressure) to haudric kinetic (velocity) energy -this causes a pressure decrease as the blood acquires velocity, thus generating a pressure gradient across the orifice
What does the continuity of flow state?
-fluid is incompressible -flow RATE in any section in a pipe is the same -flow rate=mean velocity X CSA -flow VELOCITY is inversely related to cross sectional area -if you decrease CSA of a pipe (say within stenosis), the blood must travel at a higher velocity. Per the Bernoulli equation, this means their must be a pressure gradient.
Given the Bernoulli theorem and continuity equation, how can one measure the degree of valve stenosis?
-measure the volume of blood and CSA at time 1, and the velocity outside of orifice. This will allow you to determine the CSA aka stenosis. The amount of velocity needed to cross a valve will determine the necessary pressure drop.
The pressure GRADIENT is related _______ to the flow VELOCITY.
Greatest magnitude of a pressure gradient between the LV and Aorta occurs when during systole?
-when peak flow velocity occurs into the aorta
Is there normally a big pressure gradient between the LV and aorta during peak systolic ejection velocity?
-no, at physiological flow rates, normal orifice sizes yield flow velocities that evoke small physiologically unimportant pressure gradients
Determinants of flow rate, flow velocity, and pressure gradients
1. flow rate (mL/sec) is determined by CO (linear, direct) and time available for flow (linear, inverse, time valves are open) 2. flow velocity: (cm/sec) is determined by flow rate (linear, direct) and valve orifice area (linear, inverse) 3. pressure gradient (mmHg): flow velocity (direct, QUADRATIC)
Gorlin Valve Area Equation
-allows us to measure CSA of orifice from pressure gradient and flow rate
Aortic stenosis can be viewed as a disorder of pure ___________.
-increased left ventricular afterload
-LV must generate a substantially greater pressure gradient to achieve the flow velocity needed to maintain flow rate across the stenotic valve= large aortic valve pressure gradient
Major differences comparing aortic and ventricular pressure waveforms in the setting of aortic stenosis.
-large aortic valve pressure gradient (necessary to achieve flow velocity across stenotic valve)
-aortic P does not rise with the ventricular pressure and it is very uneven--this is because it takes longer for the blood to reach aorta to generate pressure (usually 2/3 of flow occurs durign 1st 1/3 of systole) and it is very turbulent.
-LVEDP is also very elevated
Normal peak aortic valve flow velocity and pressure gradient
4 things seen on echo of aortic stenosis and the LV
-heavily calcified poorly mobile aortic valve
-normal LV cavity size
-normal LV contractile function (recall HFpEF with concentric LVH)
How does Reynold's Law play into stenosis?
-decides velocity at which turbulent flow develops and in stenosis, glow across aortic valve into ascending aorta during systolic ejection is turbulent
-this turbulence= the murmur we hear
-can also see mild aortic regurgitation
Systolic flow velocity in normal vs AS
-normal: 80-100 cm/s (~1 m/sec); mostly uniform at this speed
-AS:~4m/sec and very turbulent and variable in speed distribution; rounded velocity envelope=uniform ejection rate
Mild, severe, and critial levels of valve orifice area in AS
-critical: 0.8 cm2
Describe the effct of decreasing orifice surface area on flow rate and mean systolic P gradient.
-decreases in valve orifica area means that increasing larger pressure gradients are required for flow rates
Mean systolic pressure gradient able to be establish in AS
So, does merely checking a patients pressure gradient and finding it to be low mean they are free of AS?
NO! the quadratic relationship between Pressure gradient, orifice area and flow rate means that at low flow rates, even with a small orifice area, the valve pressure gradient may be deceptively small. Therefore, the valve area must always be calculated using the pressure gradient and CO.
Why might AS limit ability to increase CO in response to demand?
-increasing blood flow across the valve requires a large increase in pressure gradient and thus, increased systolic pressure load on the LV..which will eventually max out
-limits ability to increase CO in response to demand
Consequences of severe AS
-at small valve orifice areas, small changes in orifice area or flow require large changes in the aortic valve pressure gradient
-this means that minor progressions in anatomic severity may cause large changes in LV afterload
-limited ability to increase CO in response to exercise-- must draw mostly on HR increase (long time spent in systole) which drastically raises myocardial metabolic requirements
Is AS acute or chronic?
it is a chronic disease that does not develop acutely
The heart can adapt to AS by developing ___________. What are the limitations of adaptive responses?
-concentric LVH to offset the increase in LV wall stress
limits: effect on LV diastolic compliance, limits of extent of hypertrophy due to coronary circulation and coexisting epicardial coronary disease, degradation of myocardial peformance due to fibrosis; can also have progression of stenosis severity
3 mechanisms of decompensation due to AS
1. angina pectoris: limitation of coronary circulation's response to hypertrophy and increased wall stress
2. effort-related syncope or presyncope: inadequate CO response to exercise, arrhythmias provoked by exercise-induced hypotension or ischemia
3. CHF (systolic and/or diastolic): inadequacy of LVH to normalize wall stress leads to degradation of contractile performance and decrease in diastolic compliance; progression of obstruction severity
Implications of the presence or absence of symptoms in AS
-absence: compensatory mechanisms are working and patient is doing okay. Though not likely asymptomatic--will likely have reduced exercise capacity
-appearance of sxs: compensatory mechanisms are breaking down due to further progression of stenosis or degradation of LV performance=BIG TROUBLE
Natural history of aortic stenosis and sxs timing
-once sxs arise, you need to treat them ASAP!!! They will likely deteriorate SOON after sxs appear