11.3.2 Doppler echocardiography

Spectral display

When the structure being scanned by the ultrasound beam is moving, it sends back an echo with a positive frequency shift if it is moving towards it or a negative one if it is moving away; this is the Doppler effect. The difference between the transmitted frequency and the received frequency is used to calculate the speed at which the target is moving. By measuring the velocity of the blood cells, we can measure the speed of the blood flow. This is displayed on the screen in the form of a spectrum of velocities over the observation time (spectral Doppler) and gives a representation of the blood flow through the observed structure. Two technical modalities are available (see Figure 25.17).

  • Pulsed Doppler: The Doppler analysis takes place at a precise point along the interrogation axis; the position of the sampling window (sampling volume) is set by the observer. This advantage is offset by a disadvantage: the maximum speed that can be recorded is limited (0.6-1.2 m/s depending on depth).
  • Continuous Doppler: The Doppler analysis takes place along the entire interrogation axis and records all velocities encountered along this axis; only the observer knows where the maximum displayed velocity occurs. This disadvantage is offset by the fact that there is no limit to the maximum speed that can be recorded.

 Colour Doppler

Colour Doppler is a variant of pulsed Doppler in which the direction of flow is translated by a colour code and the velocity by the intensity of that colour. Accelerated or vortex flow takes on a multi-coloured mosaic appearance. Colour flow is the simplest way to visualise valvular insufficiency and to demonstrate acceleration through a stenosis. At rest, blood flow in the ventricles is physiologically laminar and therefore quiet. Any vortex indicates pathological acceleration: valvular stenosis, regurgitation from a high-pressure cavity to a low-pressure cavity, simple cardiac etherism or localised turbulence in the vicinity of a sclerosis. The vortices of these eddies cause disturbances that are visible on Doppler echocardiography. The colour flow gives rise to a phenomenon known as spectral overlap or aliasing, which consists of an inversion of the colour code when the speed of the flow exceeds the recording rate of the device. This phenomenon is analogous to the slow reversal that occurs in cinema when the speed of a wheel (number of revolutions per second) exceeds the recording rate of the camera (number of frames per second).

Colour flow imaging allows quick and easy assessment of flow in valvular disease, but has limitations that must be understood to avoid dangerous misinterpretation (Figure 11.25 and Figure 11.26).

Fig11 25 en

 Figure 11.25: Echocardiography in Doppler mode. A: Mitral regurgitation in the long-axis view of the LV; the Doppler beam (dotted) is well aligned with the flow of the MI, represented by the coloured Doppler flow. B: Schematic representation of the Doppler beam through the  aortic stenosis in transgastric view; in order to analyse the flow through the stenosis consistently, the Doppler axis must pass through the prestenotic acceleration zone (PISA), the narrowed orifice with the vena contracta (laminar flow, red arrow) at its exit, and the turbulent zone in the aortic root; the maximum velocity flow is recorded at the level of the vena contracta.

Fig11 26 en

 Figure 11.26: Mitral regurgitation (MI) in a 4-chambers TEE view (0°). A: Concentric, circularly symmetric MI; this type of regurgitation has the same appearance in all planes. As the MI (Vmax 5-6 m/s) recruits blood already in the LA  by the Venturi effect and is directed towards the sensor, the extent of the colour jet tends to overestimate the size of the regurgitant volume. In this case, the MI is mild to moderate because it is thin at its origin and does not have a zone of concentric acceleration (PISA) on the venous side. B: Eccentric MI in posterior leaflet prolapse. This MI slows down when it hits the atrial wall, has little Venturi effect and presents a very different image depending on the plane of analysis; in fact, it spreads over a large part of the LA wall, but the image is always a tomography and shows only a section of this wide jet. As a result, the colour image underestimates the extent of the regurgitation. In this case, the MI is severe, as shown by the width of the vena contracta (yellow arrow) and the presence of a large PISA on the ventricular side.

  •  The colour flow image is a map of velocities, not of actual blood volume. The velocity of a flow is an expression of the instantaneous pressure gradient between the upstream and downstream cavities; for example, Vmax in mitral regurgitation (MI) decreases when left ventricular function is reduced, and Vmax in aortic regurgitation (AR) increases when SAR is high.

  • For the same upstream pressure, flow velocity is inversely proportional to the size of the orifice: it decreases when the orifice is very wide.

  • Two-dimensional imaging produces a tomography and only shows the size of the jet in one plane; whereas central jets tend to be circularly symmetric, eccentric jets hitting a wall have a highly variable geometry that is not visible in the cross-sectional plane. The two-dimensional colour jet gives a reliable representation of the former, but tends to underestimate the importance of the latter because it does not visualise their extent in space.


    Vidéo: Mitral insufficiency type 1

 Video: Type II eccentric mitral insufficiency due to prolapse of the posterior leaflet; the jet is directed towards the interatrial septum.

  • A high velocity central MI jet (Vmax 5-6 m/s) recruits blood volume already in the LA by the Venturi effect, whereas an eccentric MI jet slows down as it hits the wall of the LA and is not amplified by the Venturi effect. Again, the colour jet tends to overestimate the importance of the former and underestimate that of the latter [4].

  • The Doppler effect is maximal when the flow and the interrogation axis are in the same direction. The jet of an MI shows maximum development in mid-oesophageal retrocardiac views because its flow is well aligned with the Doppler axis, whereas the jet of an AI is greatest in transgastric views where it is oriented towards the sensor.

  • Maximum velocity should be measured at the point of greatest stenosis, i.e. between the leaflets of the valve or immediately downstream (vena contracta), not at a distance; this applies to both stenosis and regurgitation.

  • Imaging intuitively assumes that orifices are circular, but they are often oval, slit-shaped or of complex geometry, so it is important to always assess in at least two orthogonal planes.

  • In a murmur, the volume regurgitated depends on the duration of the flow; if the flow is short, the murmur is much smaller than in a pansystolic (MI) or pandiastolic (AI) murmur.

  • The measurement scale (Nyquist limit) must be adapted to flow velocity being measured; if it is too low, vortices will appear throughout the sector, but if it is too high, slow flows will no longer be represented.

  • The gain must be adjusted so that it just eliminates the small coloured spots that appear outside the ventricles and vessels if it is too high (this adjustment is automatic in most machines).

 Vena contracta

In a constriction, the flow tends to flow in the central part because the outer zones are slowed down by friction against the walls. The flow is laminar and remains so at the outlet, only becoming turbulent at some distance. Its smallest diameter is not at the orifice itself, but immediately after, at a point known in hydrodynamics as the vena contracta (Figure 11.27) [14,15]. As the blood flow there is still laminar, the size of this zone faithfully reproduces the shape of the flow in the stenosis, but its cross-sectional area is slightly smaller than that of the anatomical orifice (GOA geometrical orifice area). Measurement of the diameter of the vena contracta is an excellent technique for assessing the effective orifice area (EOA effective orifice area) of a stenosis or regurgitation because it is independent of flow velocity and pressure force. As a result, it remains valuable in acute failure where the lack of downstream cavity dilatation leads to rapid pressure equalisation and ventricular dysfunction reduces  jet velocity  and hence its apparent area. The size of the vena contracta correlates better with haemodynamic effects and prognosis than the anatomical area of the orifice. Failure is severe when the diameter of the vena contracta is > 0.7 cm in MI or IT and > 0.6 cm in IA.

 Fig11 27 en

Figure 11.27: Proximal isovelocity surface area (PISA) and vena contracta in mitral regurgitation in a mid-esophageal retrocardiac view. A: Schematic representation; the PISA is a concentric hemisphere centred on the orifice and located in the chamber upstream of the flow; the vena contracta is the narrowest zone of laminar flow exiting the orifice. B: The cross-sectional area of the vena contracta (1) is slightly smaller than that of the anatomical orifice (2). C: As it gradually accelerates towards the orifice, the Doppler flow crosses the Nyquist limit (aliasing) and changes colour, moving from the yellow end of the colour scale to the blue end (E). The greater the volume of blood passing through the orifice, the greater the PISA. D: Mild to moderate mitral regurgitation: The PISA is tiny and the vena contracta is narrow. E: Image of severe mitral regurgitation; when the colour scale is set to 35-50 cm/s (here 41 cm/s), an aliasing radius of 1 cm (white arrow) corresponds to a regurgitant orifice area of 0.5 cm2 (severe MI). The main difficulty with this measurement is to accurately define the plane of the regurgitant orifice at leaflets level .

PISA
 
When passing from a wide cavity to a narrow orifice, the fluid molecules accelerate in a concentric zone upstream of the constriction, since fluids are incompressible. The greater the acceleration, the larger the zone of hemispherical convergence and therefore the greater the volume of fluid that must pass through the orifice. This zone is called the Proximal Isovelocity Surface Area, or PISA, because the fluid velocity is the same over the entire surface of each successive concentric hemisphere (see Chapter 25, Concentric Acceleration Zone) [6,13]. According to the continuity equation, the product of the velocity (V) at the surface of a hemisphere and the surface area (S) of this hemisphere is equal to the product of the maximum velocity through the orifice and the surface area of the orifice: (S - V)hemisphere = (S - V)orifice. 

Video: zone of concentric acceleration (PISA) on the ventricular side of a central mitral insufficiency; as it accelerates, the flow changes from red to yellow to blue.

 
Video: Zone of concentric acceleration (PISA) on the ventricular surface of a mitral regurgitation orifice (prolapse of the posterior leaflet). 
 
It is easy to measure the surface area of the concentric hemisphere by locating the boundary where flow colour inversion  occurs. This inversion (aliasing) occurs when the Vmax of the flow exceeds that which can be read for the chosen colour scale. For mitral regurgitation:
 
S regurgitation orifice = (Shémisph - Valiasing) / VmaxIM = (2π r2 - Valiasing) / VmaxIM
   
 
In mitral regurgitation, the calculation can be simplified: if the colour scale is set to an aliasing velocity of 50 cm/s, a radius of 1 cm for the hemisphere with 1 aliasing corresponds to a regurgitant orifice ≥ 0.5 cm2 , which corresponds to severe MI [7,11,12].
 
 Cardiac output
 
Stroke volume is the product of the cross-sectional area (S) measured by two-dimensional imaging and the integral of the velocities (ITV) measured by the Doppler effect at the same point, i.e.: VS = S (cm2) - ITV (cm). Multiplying the result by the heart rate (HR) gives the cardiac output:
 
Q (ml/min) = S (cm2) · ITV (cm) · HR (min-1)
 
This calculation can be made at many points, but is most accurate when the local cross section is circular (LVOT) or of simple geometric shape (triangular for the aortic valve) and does not change during ejection [5]. Velocity and diameter must strictly be measured at the same point and in systole. Unfortunately, they cannot be measured simultaneously because they require different angles of view, which introduces a significant bias into the measurements.
 
  • LVOT: Measurement of diameter (mean: 2.0-2.2 cm) in 4-chamber or long-axis retrocardiac mid-oesophagus; measurement of flow by deep transgastric approach (long-axis 0° or 120°) with pulsed Doppler. 3D echo shows that two-dimensional measurements of the LVOT underestimate its surface, which is elliptical rather than circular [10].
  • Aortic valve: measurement of surface area in its triangular mesosystolic orifice (basal retrocardiac view, short axis 40°); measurement of flow by transgastric route (long axis 0° or 120°) with continuous Doppler. The aortic valve has a circular orifice only in the protocole; for more than 2/3 of systole it is triangular (S = 0.433 · L 2).
  • Mitral valve: measure the area of the orifice (S = 0.785 · D2 ) by averaging the diameter of the annulus at 60° and 120° retrocardiac in protodiastole; measure the flow using pulsed Doppler by positioning the window at the mitral annulus (and not at the tips of the leaflets). This is the least reliable method.
  • Right cardiac output is measured via the RVOT (transgastric view 40° or 100°) or via the pulmonary artery (short axis view ascending aorta 0-20°); measure PA diameter in systole and pulsed Doppler flow strictly at the same level.
  • 3D echocardiography measures the volume of the LV in systole and diastole fairly reliably; by subtracting Vtd - Vts we obtain the systolic volume.
 However, TEE is much less efficient than Swan-Ganz for calculating cardiac output; the best results, obtained at  aortic valve level by considering its geometry as triangular, remain outside the 30% approval limit with pulmonary thermodilution [3].
 
 Tissue Doppler
 
Tissue Doppler allows us to observe and quantify the speed of movement of anatomical structures in systole and diastole. The descent of the mitral annulus towards the apex in systole is due to longitudinal contraction of the LV. This is due to the longitudinal myocardial bundles located in the subendocardium, the area most at risk in the event of overload or ischaemia. If it decreases (normal value of S': 10-12 cm/s), it is a finer and earlier sign of ventricular dysfunction than the decrease in ejection fraction. Tissue Doppler can also assess a portion of the myocardium and measure its deformation in systole and diastole (strain) and the rate of this deformation (strain rate) [2]. 
 
Speckle tracking
 
The heterogeneity of myocardial echoes makes it possible to isolate small areas of 20-40 pixels, known as speckles, which have a particular configuration in their ultrasound reflection that does not change during the cardiac cycle. Using shape recognition algorithms, the processor can track the movement of these natural acoustic markers, a process known as speckle tracking (see Figure 25.31). By tracking several of them, the computer can continuously assess the distance between them and the deformation of the observed tissue segment (strain and strain rate) [2]. This type of study makes it possible to quantify subtle changes in LV function, such as the degree of longitudinal shortening, which are not apparent in the ejection fraction [1].
 
 Discrepancies
 
The different echocardiographic modalities do not always give consistent results. For example, in narrow aortic stenosis, inconsistencies between gradient, velocity and area occur in 30% of cases [8]. Numerous artefacts (echoes and shadows from calcifications or prostheses) and technical difficulties (misalignment of Doppler axis and blood flow) often lead to underestimation of valvular lesions. It is therefore important not to base a diagnosis or surgical indication solely on echocardiography, but to correlate it with other investigative tools (multibar CT, MRI) and, above all, with the patient's clinical condition [9]. The ejection fraction is a misleading element for estimating LV function in the context of valvular heart disease because it depends on loading conditions - preload and afterload are abnormal in valvular heart disease - and its calculation depends on the shape and dimensions of the LV, which may be very altered. 
 
 
Colour Doppler 
The extension of the colour jet in valvular regurgitation does not represent the volume regurgitated; it is a mapping of velocities, which is itself a function of the colour scale:
- Colour scale (Nyquist limit) and amplification
- Pressure gradient across the valve
- Orifice size
Colour jet overestimates insufficiency in the case of: central jet, high upstream pressure, small orifice.
Colour jet underestimates insufficiency in: eccentric jet, low engine pressure, very large orifice.
 
Valvulopathy: Doppler analysis recommendations
 
Doppler axis well aligned with the axis of insufficiency or stenosis through the valve orifice.
Good anatomical course of the downstream cavity in the Doppler axis.
Analysis of Vmax at the level of the vena contracta.
Measurement in 2 orthogonal planes.
Consistency of results with 2D measurements (regurgitation orifice) and cavity remodelling.
 
 
 
 

 © CHASSOT PG, BETTEX D, August 2011, last update November 2019

 

References

 

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