Velocities can be estimated from Doppler effect. Dimensions can be measured by 2-D imaging.
Volumes and flows can be derived from this.
Pressure gradient across a restrictive orifice can be estimated using simple Bernoulli equation(P=4V2).
The rate of change of pressure gradient - pressure half time - can be used to estimate the orifice size in MS and AR. (orifice area = 220/PHT)
PISA & JETS:
Fluid speeds up as it approaches narrow orifices. This results in characteristic appearances proximally and distally.
Proximally more than one PISA - Proximal Isovelocity Surface Area - may be visible in colour flow doppler. The dimensions of PISA can be used to estimate valve orifice.
Distally a jet is formed. This jet entrains mass and its volume increases. The area of the jet seen with colour flow is assumed to be proportional to the volume. But jet area may vary from plane to plane as it is only 2-D.
Also jets may be free or confined.
Free jet is unaffected by the boundaries of the chamber.
If the jet is close to the walls of the chamber, its volume is confined due to Coanda effect. Reliance on jet size will lead to underestimation of lesion severity.
Jets in the vicinity are affected by each other. Jets in the same direction result in accentuation and those in the opposite direction attenuate.
The intensity of the flow signal obtained with CW doppler depends on:
1. Gain setting
2. Number of blood cells from which echo beam has reflected.
VELOCITY, FLOW & AREA:
If we assume laminar flow in a cylinder of constant diameter, then velocity
V = s/t
where
s - distance covered
t - time taken to cover the distance
Volume is cross section area x length
ie CSA x s
Flow = Rate of change of volume
= CSA x s/t
= CSA x V
So Q = CSA x V
V can be estimated from doppler.
Obtaining CSA is more difficult. Planimetry can be difficult. So from 2-D dimensions CSA is calculated approximately applying standard geometric formulae.
But the fact is:
1. Flow is not laminar.
2. Flow does not have flat profile.
3. FLow is pulsatile.
Velocity-time integral (VTI) gives the distance travelled by blood over any particular period.
V=s/t
so
s = Vxt.
= Vmean x time.
Mean flow Qmean = CSA x Vmean
Maximum flow Qmax = CSA x Vmax
Stroke volume SV = CSA x VTI.
Cardiac output CO = SV x HR.
The sites of obtaining the velocity spectra are:
1. CW doppler across aortic valve
2. PW doppler positioned in LVOT
3. PW across competent mitral valve.
The CSA can be derieved from the following equations:
1. Aortic valve. Measure intercommisural length "s".
CSA = 0.433xs2.
2. Mitral annulus. Measure radii r1 and r2.
CSA = π r1 x r2
3. LVOT. Measure diameter.
CSA = π x r2
Principle of continuity of flow:
In the absence of shunts, net forward SV in any one part of the circulation must equal net forward SV in any other part.
This can be used to calculate orifice areas of stenotic or regurgitant valves.
Adjacent structures in same phase or non adjacent structures in differnt phase of circualtion can be used.
1. LVOT and AV can be used to calculate AV valve area.
Qav = Qlvot
CSA av x Vmax-av = CSA lvot x Vmax-lvot
CSAav= CSAlvot xVmax-lvot/Vmax-av.
Similarly
CSAav = CSAlvot x VTIlvot/VTIav
Doppler velocity index Vmax-lvot/Vmax-av is also used as a simple index of severity of stenosis.
2.MV doppler in diastole and AV doppler in systole can be used in a similar way.
If one of the structure is normal the area of other can be derieved.
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