Doppler Hemodynamics

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Doppler Echocardiography

  • doppler echo measures blood flow velocities in the heart and great vessels
  • Doppler effect described by Austrian physicist Christian Doppler in 1842
  • stationary target will scatter ultrasound in all directions with frequency being same in all directions
  • If the target moves, the frequency observed when the target is moving towards the observer becomes higher
  • Frequency when target is moving away from observer is lower
  • Ambulance, trains, etc.

Doppler equation

Fd= 2fo/C v cos θ
v= peak velocity
c= speed of sound in blood (1560 m/s)
θ= intercept angle between u/s beam and direction of flow

  • When an ultrasound beam with known frequency transmitted to heart, it is reflected by RBCs with another frequency
  • the difference between these frequencies is the doppler shift
  • The doppler shift is described in this equation and depends on a few other variables
  • Cos0 is 1- as this angle increases, the value falls below 1 and will underestimate the doppler shift and hence, peak velocity
  • This underscores the importance of having the beam as parallel to blood flow as possible
  • Frequency Shift --> Doppler Equation --> Velocity Data
 Bottom Line:
 Frequency shift is described by the doppler equation, from which you get velocity data

Types of Doppler Echo

  • PW, CW, color, tissue
  • Color doppler is always done with 2D echo
  • PW uses a single crystal that sends and receives sound beam signals
    • The crystal emits a short burst of u/s at a certain frequency (pulse repetition frequency)
    • After a certain interval determined by the depth of interest (place your sample volume at a point of interest), it samples the backscatter signals
    • The maximal frequency shift that can be determined is ½ the PRF or Nyquist limit
    • If the velocity exceeds the Nyquist limit, you get aliasing- doppler spectrum gets cut off at the nyquist frequency and the rest is recorded on the opposite side
  • CW uses two u/s crystals- one continuously transmits and another continuously receives
    • Useful for high velocities, not limited by the PRF or nyquist phenomenon
    • Can use the pedhoff probe (nonimaging transducer)

Hemodynamic data obtained by 2-D Doppler echo

  • Volumetric measurements
    • SV and CO
    • Regurgitant volume and fraction
    • Qp/Qs
  • Pressure gradients
    • Maximal instantaneous gradient
    • Mean gradient
  • Valve area
    • Stenotic valve area
    • Regurgitant orifice area
  • Intracardiac pressures
    • PA pressures, LAP, LVEDP

Volumetric Measurments

Stroke Volume and Cardiac Output

  • Flow velocity varies during ejection in a pulsatile system so flow velocity is summed as VTI or velocity-time integral
VTI = area enclosed by baseline and doppler spectrum
  • Flow = area x velocity
  • SV = CSA x VTI
  • CSA = π r2
  • CSA = D2 x 0.785
  • CO = SV x HR

Example: If the LVOT is 2 cm and the VTI is 10 cm

= πr2 x VTI
= π (1)2 x 10 = 31 cc

If the HR is 100 bpm then:

= 31 X 100 == 310 cc/min
 LVOT diameter is best measured in the parasternal long axis
 VTI of LVOT is measured with PW from apical-4 with closing click of AV to ensure you are at the annulus

Regurgitant volume and fraction

Q total = Regurgitant volume + Qs

Regurgitant volume = Q total – Qs

Mitral regurgitant volume = mitral inflow – LVOT flow

  • Regurgitant fraction can be estimated by the volumetric method and PISA
  • You can use the same equation (volume = area x velocity) to calculate regurgitant volume and fraction
  • Diastolic forward flow is equal to the systolic flow across LVOT and back into LA via MR (flail post valve)
  • Measure diameter across mitral valve to get mitral annulus area and mitral flow VTI (Qtotal)
  • Use same method as above to get stroke volume (Qs)

Example: Mitral Regurgitant Volume by Proximal Isovelocity Surface Area

As regurgitant flow enters the mitral annulus during systole, the flow converges and then narrows into an area called the vena contracta (narrowest flow). It then expands into the Atrium into the area of turbulence (what we currently call the size of the jet and can have further down stream effects such as pulmonary vein flow reversal in systole. As blood flow converges toward the regurgitant orifice, blood flow velocity increases with multiple shells of isovelocity in hemispheric shape. As velocity increases, it exceeds the nyquist limit and color reverses. Since we can’t see the orifice so we look at the isovelocity shell instead. By conservation of flow, the flow rate at this surface should equal the flow rate across the regurgitant orifice. So if you find a velocity shell you can move the scale factor to help you identify it.

For example, when blood flows away from the transducer towards the regurgitant orifice and is blue until it reaches the negative aliasing velocity of the color-flow map and then turns orange-red. At that point, you have your aliasing velocity. The area is the surface area of a hemisphere = 2πr2 formed by the radius of the sphere at aliasing velocity. PISA will get bigger with more flow (bigger jets). It also will get bigger as you shift the scale factor down. The scale just helps you identify it.

Regurgitant volume by PISA

FlowPISA = FlowROA

Flow ratepisa = 2πr2 x aliasing velocity

Flow rateROA= ROA x regurgitant velocity

ROA = flow rateROA / peak MR velocity

Regurgitant volume = ROA x MR VTI

= flow rateROA/peak MR velocity x MR VTI

= (2πr2 x aliasing velocity x MR VTI)/ peak MR velocity

Calculating shunt Fraction:

Flow = area x velocity

Example: ASD with folling Doppler Echo measurements.

  • RVOT Velocity 1.8 m/s
  • VTIRVOT 32 cm
  • Diameter 2.6 cm
  • LVOT Velocity 1.1 m/s
  • VTILVOT 16 cm
  • Diameter 2.4 cm

Qp = CSARVOT x VTIRVOT = π(1.3)2 x 32 = 170 cm3

Qs = CSALVOT x VTILVOT = π(1.2)2 x 16 = 72 cm3

Qp/Qs = 170/72 = 2.4

Pressure Gradients

Transvalvular gradients

  • doppler echo measures blood-flow velocities which can be converted to pressures gradients by the bernoulli equation
  • in most clinical situations, flow acceleration and viscous friction components of the equation can be ignored
  • Also, flow velocity proximal to a fixed orifice (v1) is much lower than the peak so this is can be ignored as well
  • Pressure gradient across a fixed orifice can then be calculated with the simplified bernoulli equation

Simplified Bernoulli equation:

ΔP = 4v2

Gradients by echo vs cath

Maximal instantaneous gradient ≠ peak to peak gradient

  • Blood flow velocity by Doppler is instantaneous
  • Therefore, when you are measuring peak gradients, you are measuring peak instantaneous gradients
  • Max instantaneous will always be higher than peak to peak
  • Peak pressures do not occur simultaneously, so peak to peak is a nonphysiologic measurement
  • mean gradients do correlate however

Doppler Area Calculations

Valve Area

Continuity equation

A1V1 = A2V2

A2 = A1V1/V2

Pressure half-time (PHT)
  • PHT will be longer with more severe MS and shorter with more severe AI

  • PHT = time from peak pressure to ½ peak pressure or Vmax / 21/2
  • PHT = 0.29 x DT
  • Severe AI < 250 msec
PISA for Pinheads (MR)
  • You can simplify the PISA equation if you make some assumptions

FlowPISA = FlowROA

2πr2 x aliasing velocity = ROA x regurgitant velocity

ROA = 2πr2 x aliasing velocity / regurgitant velocity

Assuming peak MR velocity = 5 m/s

Normal LV-LA gradient = 100 mmHg

Set aliasing velocity to 40 cm/s

Then ROA Can be Simplified to:

ROA = r2/2

Aortic Stenosis
  • Aortic jet velocity 4.2 m/s
  • VTIAV 68 cm
  • Mean gradient 45 mmHg
  • LVOT diameter 2.1 cm
  • LVOT velocity 0.9 m/s
  • VTILVOT 14 cm

What is the max pressure gradient, AVA, dimensionless index?

  • Max pressure gradient:
= 4 (vmax)2 = 4 (4.2 m/s)2 = 72 mmHg
  • AVA
= CSALVOT x VTILVOT / VTIAV = π(1.05)2 x 14 / 68
= 0.74 cm2
  • Dimensionless index
= vLVOT/vAV = 0.9 m/s / 4.2 m/s = 0.21
Mitral Stenosis
  • Velocity and pressure gradients depend on volume of flow so MVA is more reliable estimate of severity
  • Step 1: Measure maximum velocity using CW Doppler from apical
  • Step 2: Trace mitral flow velocity and determine mean pressure and VTI
  • Step 3: Determine MVA by PHT, continuity equation, PISA method, and planimetry
  • The best window to look at mitral inflow is the apical four chamber
  • The typical CW spectral recording of mitral stenosis demonstrates spectral broadening in diastole, with peak flow in early diastole and a progressive but slowed diastole descent
  • excellent agreement between CW Doppler estimates of the mitral valve pressure gradient, using the simplified Bernoulli equation, and simultaneous estimates derived from cardiac catheterization data.
  • MVA = 220/PHT

What is the MVA? (assuming no AI)

PHT = 0.29 x DT = 203 ms
MVA = 220/203 = 1.08 cm2

Mitral Regurgitation

Examine the CW gradients of the regurgitant jet between the LV and the LA:

  • Small amounts of MR will produce very high pressure differences between the left ventricle and the left atrium
  • Minor degrees of regurgitation will, therefore, result in very high velocity jets as a consequence of these great pressure differences.
  • Severe mitral regurgitation results in relatively smaller differences between the two chambers.
  • Smaller pressure gradients result in relatively lower velocity jets
  • There is more equalization of pressures b/w LA and LV, therefore velocity jets are lower. With greater degrees of regurgitation, pressures will rise in the chamber receiving the Regurgitant volume leading to a general decrease in the velocity of the resultant Doppler spectral recording.
  • The severity of valvular regurgitation is not reflected in an increase in the velocity of the regurgitant jet as detected by Doppler echocardiography

Aortic Insufficiency
  • The best window for the evaluation of aortic insufficiency with CW Doppler is the apical window
  • Aortic insufficiency appears as a holodiastolic, high frequency turbulent jet with spectral broadening and flow toward the transducer
Differentiating Various Systolic Doppler Flows
  • Carefully examine peak velocities, flow duration or ejection time, accompanying diastolic flow signals, location of doppler window, doppler flow configuration
  • AS vs MR
duration of MR longer than AS b/c no AS during isovolumic relaxation and contraction. MR occurs during these periods
MR jet velocity also >4-5 m/s unless LAP elevated
Peak MR velocity is always greater than AS if occur in same patient
May also see AI jet with AS
  • TR has respiratory variation
  • HCM Dagger configuartion of upstoke
  • AS vs PS
difficult to distinguish but obtained from different windows

Intracardiac pressures

Calculating Right Ventricular Systolic Pressure and Pulmonary Artery Systolic Prssure

  • TR velocity:
  • Reflects systolic pressure difference between RV and RA
  • 4 peak TR velocity2 = RVSP – RA
  • RVSP = 4 peak TR velocity2 + RA
  • In the absence of RVOT obstruction, RVSP = PASP
  • Doppler measurement by CW closely approximates pressures measured by simultaneous RA and RV pressure tracings

Mitral Regurgitation velocity: Left Atrial Pressure

  • Systolic pressure difference between LV and LA
  • 4 x MR velocity2 = LV pressure - LAP
  • In absence of LVOT obstruction, LV systolic pressure = systolic bp
  • 4 x MR velocity2 = sbp - LAP
  • LAP = systolic bp – 4 x MR velocity2

Calculating Other Intracardiac Pressures

Velocity of a regurgitant valve directly related to pressure drop across that valve

  • Peak PR Velocity --> Mean RA Pressure
  • End Diastolic PR Velocity --> PA End Diastolic Pressure
  • PFO Velocity --> LA Pressure
  • End Diastolic LA Velocity --> LVEDP
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