Andersons pediatric cardiology 171

There are two main techniques, each with its strengths and weaknesses Color Doppler myocardial imaging157 records velocities measured at two points a distance apart and their change within the cardiac cycle It records a strain rate from which it derives strain in the longitudinal plane.154 This technology also shows the extent of torsion and twist of the heart during the normal cardiac cycle The technique is particularly useful following myocardial infarction in the adult because it enables regional abnormalities to be detected Strain rate has been recorded using this technique in small numbers of children with aortic valvar atresia or hypoplastic left heart syndrome.158,159 The particular technologic difficulties of this approach in the fetus relate to the small myocardial mass interrogated by each pixel and also to the fact that myocytes are aligned longitudinally in the fetal right ventricle but circumferentially in the fetal left ventricle.114 However, Doppler velocity color coding permits the detection of motion of the aortic wall and measurement of the velocity of the pulse wave, and this technique may prove to be a useful noninvasive technique to assess fetal arterial compliance Speckle tracking or vector velocity imaging (VVI) is a technique that tracks the acoustic markers seen following constructive and destructive interactions of the ultrasound beam from frame to frame It is a non-Doppler technique and does not require the close alignment of Doppler techniques, but it is recommended to capture images at high frame rates of 140 Hz or more.155 Image acquisition should be made using high frequency (without harmonics) and good contrast between the cavity and the endocardium The limitations of strain assessment using both these methods include the difficulty in tracking acoustic markers in the fetal heart because of small size, fast heart rate, the variable scanning depth during pregnancy and lack of ECG gating in the recording of the original frame rates Assessment of strain using color Doppler myocardial imaging requires high frame rates of more than 200 frames per second and may be limited in later gestation if harmonic imaging is required to image the fetal heart.154–156 It requires close alignment parallel to the wall motion because it is a Doppler technique, and one of the more difficult aspects of this methodology is to ensure that the Doppler sample remains within the area of interest.150–153 As with all Doppler-based measurements in the fetus, it is prone to artifacts from fetal or maternal movement or maternal aortic pulsations The differences in reports using VVI may be explained by several technical factors specific to the application of this to the fetus.160 Speckle tracking is not suitable on stored clips (unless they are stored in DICOM) because the video capture rate of these is between 25 and 30 Hz If cardiac function is considered to be similar to that of a piston, its time intervals can be measured from the change in static to dynamic work using dynamic adaptive piston pump technology The cardiac state diagram is a novel visualization tool to display the phases of cardiac function and provide measurement of myocardial tissue velocities without a simultaneous ECG signal.142 Comparison of Doppler, M-Mode, and Newer Techniques Application of newer techniques to the fetus are limited by technical issues, particularly in the time domain.148 From paired studies of strain measurement using speckle-tracking VVI comparing high and low frame rate recordings, it is clear that the peak diastolic velocities and peak positive strain rates are reduced when lower frame rate recordings are analyzed.155 Moreover, the detection of biphasic diastolic waveforms requires at least 29 frames per heartbeat and is rarely detected using speckle tracking, whereas they are invariably seen with tissue Doppler techniques (Fig 6.16).154–156 FIG 6.16 Comparison of velocities at the atrioventricular annulus recorded using speckle tracking vector velocity imaging (VVI; A) and tissue Doppler (TDI; B) techniques in the human fetus TDI was recorded between 300 Hz and VVI at 120 Hz The systolic (Sm), early diastolic (Em), and late diastolic (Am) myocardial velocities can be measured by both techniques However, in a paired comparison, measurement of the usual biphasic diastolic inflow pattern using VVI required at least 29 frames per heartbeat As in measurements of long axis function, velocities recorded by speckle tracking technology are lower than those recorded using spectral tissue Doppler because speckle tracking calculates the mean of the X and Y coordinate velocities and does not provide peak velocities.155 Correlation of myocardial displacement measurements made using M-mode and VVI is good In comparative studies, VVI or speckle tracking correlates better with traditional Doppler or M-mode in the spatial parameters such as displacement or strain but more poorly in those requiring temporal parameters such as strain rate and velocities.155 These techniques remain subject to wide variation, likely due to both physiologic variability in the fetus, as well as operator variation