Figure 13-28: A. Pulsed-wave Doppler tracing from a patient with aortic regurgitation. The transducer is in the apical position and the sample volume is in the left ventricular outflow tract. A laminar envelope is seen during systole, while aliased flow is present during diastole because of high-velocity flow. B. Continuous-wave Doppler tracing through the left ventricular outflow tract (with transducer in the apical position). The maximal velocity of the aortic regurgitation is now measurable, but all other velocities along the Doppler beam are recorded as well.
The problem of range ambiguity can be overcome by pulsed-wave Doppler. In this mode, short bursts of signal are transmitted from the transducer at a given pulse-repetition frequency (PRF). The instrument then receives the signal for only a brief period-an interval that corresponds to the time required for sound energy to travel and return from a specific site along the beam path. In practice, the operator selects the location at which flow is to be examined by positioning a sample volume, and the instrument determines the period during which to receive the incoming reflected frequencies. With pulsed-wave Doppler, only a single piezoelectric crystal is needed and flow can be recorded in one small area within the heart or vasculature.54,55 Unfortunately, pulsed Doppler techniques employ intermittent sampling and are therefore susceptible to a problem of range ambiguity referred to as aliasingfM By definition, aliasing is the erroneous representation of flow in the direction opposite to that in which it is actually occurring. To correctly record the velocity of blood flow by pulsed Doppler, the PRF must be at least double the Doppler shift frequency, a value known as the Nyquist limit. If the blood flow examined is of very high velocity or far from the transducer (requiring a long transit time), it may necessitate an unobtainably high PRF. In such cases, aliasing will occur as Doppler signals that depict flow at high velocity in ambiguous or opposite directions compared to actual flow (Fig. 13-28). An intermediate mode between pulsed and CW methods, high-PRF Doppler, is also available.57,58 This mode enables higher-velocity recordings to be obtained at a compromise of depicting two to four sample sites simultaneously.
The major limitation of pulsed and CW Doppler (sometimes referred to as spectral Doppler) is that no spatial information regarding the size, shape, and 2D direction of flow is provided. An extension of pulsed-wave Doppler techniques, color-flowDoppler (CFD), provides real-time M-mode or 2D imaging of blood flow by presenting the velocity and direction of RBC movement as shades of color superimposed upon gray-level 2D tissue structure. Standard pulsed Doppler yields flow signals from a single site along a single scan line. In CFD, rapid pulsed-wave interrogations are performed at multiple sites for multiple scan lines to create a spatially correct and dynamic display of moving blood within the heart and vasculature59 61 (Q+iH; Fig. 13-29). Doppler signals are presented as colors assigned to individual sites Fig. 13-30, Plate
54). Blood flow moving toward the transducer is displayed in red, flow away from the transducer is displayed in blue, and increasing velocity is depicted in brighter shades of each color. The variance within each signal is calculated as a statistical marker of turbulence and is presented by adding green to the image Fig. 13-31, Plate 55). Therefore, turbulent flow jets appear as a mosaic mix of colors. CFD also can be superimposed onto M-mode tracings Fig. 13-32, Plate 56), often termed M/Q imaging, and is helpful in clarifying the timing of flow phenomena. Given the time constraints imposed by collecting the large volume of data required by CFD, velocity estimates are performed by autocorrelation techniques that are less accurate than fast Fourier transform analysis.62 Nevertheless, CFD technology is a major advance that has improved the rapid detection of cardiac pathology, especially valvular regurgitation and intracardiac shunts.
The clinical application of Doppler recordings is based on the fundamental differences between normal and disturbed blood flow. Normal flow is laminar, with all RBCs exhibiting the same velocity and direction of flow. Although some abnormalities, such as atrial septal defects, involve laminar flow, most pathologic conditions involve disturbed or turbulent flow and share a common hydrodynamic basis for the resultant flow dynamics. Specifically, nearly all circulatory disturbances (stenosis, regurgitation, shunt) involve blood flow from a high-pressure chamber to a lower-pressure chamber through a restricted orifice.53 Aortic valve disease is a perfect example. Aortic stenosis is a forward flow disturbance in which turbulent blood travels from a high-pressure LV to a lower-pressure aorta through a restricted aortic orifice in systole. Aortic regurgitation is a retrograde flow disturbance in which turbulent blood regurgitates from a high-pressure aorta to a lower-pressure left ventricle through a small regurgitant orifice in diastole. In each case, the pressure gradient results in a high-velocity jet coursing through a restricted orifice, reaching its maximal velocity at a site just distal to the orifice, designated the vena contracta, at which time shear forces produce vortices resulting in flow of varying direction and velocity (Fig. 13-33). In each case, the velocity of the jet is related to the pressure gradient across the orifice. Thus, the hallmark of disturbed flow is a very high velocity jet with adjacent vortices of varying direction and velocity of flow. On pulsed Doppler recordings, these hemodynamic abnormalities cause broadening of the spectral signal and aliasing. On CW recordings, high velocity represents the primary abnormality. By color-flow imaging, the disturbance is manifest by the increased variance and higher velocities in the signal. With any of these techniques, of course, inappropriate timing of flow serves to highlight the abnormality (e.g., high-velocity LA flow during systole in mitral regurgitation).
The Standard Doppler Examination
A clinical Doppler examination must be performed with full consideration of the three different Doppler modalities available, the types of information each can provide, the multiple sites for flow interrogation, and the spectrum of pathologic lesions that produces flow disturbances. In light of these considerations, it is understandable that the Doppler examination may not be as standardized as the format for 2D cardiac imaging; however, a number of usual practices have emerged. A vast majority of echocardiographic examinations include screening for flow disturbances by CFD. Since Doppler signals are best recorded with the ultrasound beam parallel to flow, screening is typically performed in long-axis or apical views. Any flow disturbances visualized are subsequently examined by CW spectral recordings and, in most laboratories, by pulsed-wave Doppler. Although CW examination is typically reserved for flow disturbances, pulsed-wave Doppler also may be of value in quantifying flow dynamics in the setting of laminar flow. In this regard, pulsed Doppler recordings obtained at the mitral, tricuspid, and aortic valve orifices, pulmonary artery, and pulmonary veins constitute part of a standard echocardiogram in many laboratories (&+-B- Figs. 13-26, 13-34, 13-35, 13-36 and 13-37).
The normal Doppler examination is characterized by uniformity of flow velocity and the absence of highvelocity turbulent flow. CFD recordings demonstrate laminar flow through the atrioventricular valves in diastole and the semilunar valves in systole. Since the Doppler examination is usually performed with a long-axis or apical transducer orientation, diastolic filling is characteristically encoded in red and ejection in blue Fig. 13-30, Plate 54). Color aliasing is often observed at the levels of the mitral annulus and
LV outflow tract as an abrupt change from bright red to bright blue or vice versa, usually in the center of the flow stream. Pulsed Doppler recordings of transmitral flow velocities are often recorded at the level of both the leaflet tips and annulus. Velocities are higher at the tips, while recordings at the annulus offer the ability to calculate flow through a cross-sectional area that is relatively uniform throughout the cardiac cycle. A sample volume positioned in the right upper pulmonary vein reveals systolic and diastolic emptying flow of nearly equal magnitude followed by a short, low-velocity reversal of flow into the pulmonary veins following atrial contraction (Fig. 13-36). Flow in the LV outflow tract and aortic annulus area is characterized by a progressive increase of velocity peaking in early systole, followed by a more gradual deceleration of flow Fig. 13-35). Minimal if any flow velocities are detected in the mitral valve orifice and LV outflow tract in systole and diastole, respectively, in normal examinations. Examinations of the tricuspid and pulmonic valves give qualitatively similar results to those of the mitral and aortic valves Figs. 13-26 and 13-37). Normal values for forward flow velocity are given in Table 13-4. As can be seen, velocity in normal individuals is highest in the aorta and is less than 2 m/s.63 Other commonly made measurements include the acceleration time (from the beginning of flow to peak velocity of flow in the ascendingaorta or pulmonary artery); and the deceleration time, from LV inflow peak E-wave velocity extrapolated to baseline zero velocity.
Table 13-4: Normal Intracardiac Doppler Velocities
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