Effective use of ultrasound for fetal heart evaluation

Congenital heart defects have a multifactorial etiology and therefore their prenatal detection cannot be achieved solely by screening the high-risk population defined by medical history. Screening ultrasound is usually performed in the second trimester and is still the best means of detecting cardiac defects. Some technical aspects may contribute to unsatisfactory visualization of the heart, even at specialized centers, and include: gestational age, transducer frequency and some fetal and maternal factors.  Maternal body habitus, previous abdominal surgery and early gestational age are the other major factors for suboptimal fetal heart scanning. An additional reason for inadequate examination is ‘lack of time’. In some units the whole fetal examination usually has to be achieved within a short time period due to mass screeening. Under these circumstances there may be no time to wait for the fetus to change its position or to change the different presets of the machine or use other transducers.
Two major problems are commonly encountered during fetal heart examination: firstly, the examiner may not be able to obtain an adequate four-chamber view with different fetal positions and, secondly, the image is not optimized for the analysis of the heart. It is of utmost importance to optimize imaging prior to fetal cardiac scanning.
Selection of the appropriate probe is the most important step in evaluating fetal heart. A transabdominal probe with a frequency range of 4-8 MHz is optimal for the first and second trimesters till 24 weeks of pregnancy. Due to the ossification of ribs after 24 weeks, the frequency range of the transabdominal probe should be lowered to 2-5 MHz to achieve better penetration. On the other hand, for patients with high body mass index,one must also use lower frequency probes. For transvaginal fetal heart examination, a 5-9 MHz probe would be the most appropriate one. However, in recent years high frequency linear transabdominal probes providing 6-12 MHz frquency ranges have become availbale.
After selection of the appropriate probe, time has come to achieve proper 2D image optimization. It is important to start by setting optimal 2D and color paramaters. For heart scanning, we need high contrast images. Therefore, tissue harmonic imaging, higher levels of speckle reduction and lower levels of compunding yield better cardiac images.
The next step is one of the most important steps of fetal heart evaluation which is selection of a suitable acoustic window. In order to avoid shadowing from the fetal spine and ribs, it would be ideal to examine the fetus in a position where the ultrasound beam will insonate the fetus from the thorax anteriorly. An optimal  insonation angle should also be assured in order to visualize the chambers, outflow tracts and interventricular septum of the fetal heart. This is best done by assuring an angle between the beam and interventricular septum of approximately 45 degrees.
Since the fetal heart is unique when compared to other fetal organs in terms of motion, one should select a narrow window of examination to achive higher frame rates. Higher frame rates yield better images on the beating heart. The image should be zoomed to an appropriate level in order to visualize the structures. Optimal analysis of the heart may be achieved by magnification of the image, using the zoom function, so that the heart fills a third to half of the screen, and by the use of the cine-loop to assess different phases of the cardiac cycle.
The next step is the application of color Doppler. If color Doppler is available, it should be used routinely during the screening examination. For the first trimester, scale for color should be kept at levels of 30-40 cm/s, for the second trimester between 50-60  cm/s, and for the third trimester between 70-80 cm/s. Wall motion filtering should be kept at intermediate frequencies.When color mapping covers the walls of cardiac chambers and outflow tracts, color Doppler gain has to be reduced.

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