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Obstetric ultrasonography has become an important part of prenatal care. In Norway, one routine fetal examination in the second trimester is offered to all pregnant women; thus this is the only opportunity to detect the majority of the developmental disorders in the population. The scan is received by 98% of the pregnant population and is mainly performed by ultrasound-trained sonographer/midwives1.
Studies evaluating the prenatal detection of fetal developmental disorders show a variety of detection rates2–6. Common to most of the studies is the relatively poor detection rate of congenital heart defects (CHDs). Worldwide, the fetal heart examination has been reported as difficult, in particular when applied to non-selected populations2, 7–10. Various ways of improving the prenatal detection of CHDs have been suggested: additional views through the fetal heart, such as the four-chamber view11, 12, the great arteries8, 13 and the three-vessel view14; training programs15, 16; and the use of advanced technology17.
The need to provide training programs specifically for the fetal heart to improve the skills of ultrasound personnel may indicate that there is insufficient basic training in examining the fetal heart. Several ultrasound specialists have expressed the importance of teaching and training to detect CHDs18, 19, but the impact of teaching and training has not yet been clarified scientifically. As the fetal heart seems to be the most difficult organ to examine, it makes it a suitable one to assess the operator's ultrasound competence.
The present study was designed to see if the educational aspect was a factor that would influence the detection rate of CHDs, and thus should demand as much or more attention than incorporating new views and new technology. Sonographer/midwives with different levels of ultrasound experience were assessed regarding their ability to detect CHDs during routine fetal examinations of a large, non-selected population. The intention was not to determine the detection rate as such, but rather to determine the relative detection rate between the examiners. On the basis of these empirical data, it was anticipated that the current ultrasound training program could be evaluated and possible suggestions for improvements made.
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All sonographer/midwives assessed during the study period were specially trained in obstetric ultrasonography. The basic training included a minimum of 200 routine examinations supervised by experienced sonographer/midwives in addition to a Level I ultrasound course. For the present study, sonographer/midwives who had performed between 200 and 2000 routine examinations subsequent to the basic training were categorized as less experienced and those who had performed more than 2000 routine examinations as experienced. The sonographer/midwives' experience with ultrasonography following basic training varied from 3 months to 17 years.
The fetal population was collected from a well-defined area consisting of the city of Trondheim and eight surrounding communities. More than 97% of the pregnant women residing in the defined area receive a prenatal ultrasound examination and deliver at Trondheim University Hospital, thus the population is non-selected. Women who had a routine scan in the second trimester performed by a sonographer/midwife during the study period from February 1991 to December 2001 were included in the study. The routine scan was scheduled at 18 completed weeks based on the last menstrual period or early clinical assessment. Thirty minutes were allocated for the complete consultation. The gestational age was based on the fetal biparietal diameter between 16 and 22 completed weeks. Those who did not receive a routine ultrasound scan between 16 and 22 completed weeks, or those who had a routine scan performed by a physician, were excluded from this study.
The routine scan comprised regular fetal biometry and a detailed anatomical survey including the four-chamber view and the great arteries of the heart (Table 1). General data from the ultrasound examinations were prospectively registered. During the first 5 years of the study period, heart structures obtained from every fetus were registered in detail on a separate form: four chambers; intact ventricular septum; tricuspid valve closer to the apex than mitral valve; ascending aorta; main pulmonary artery; aortic arch; ductal arch. These registrations made it possible to follow each sonographer/midwife's progression over time.
Table 1. Details of the fetal heart obtained at the second-trimester fetal examination
|Transverse||Occupies 1/3 of thoracic area|
| thorax||Located in the middle with apex to the left|
|Four-chamber||Two atria of equal size (foramen ovale flap)|
| view||Two ventricles of equal size|
| || ‘Moderator band’ in right ventricle|
| || Left ventricle forms the apex|
| ||Two sets of atrioventricular valves|
| || Tricuspid valve closer to the apex than|
| || mitral valve|
| ||Intact ventricular septum|
|Great arteries||Ascending aorta from the left to the right|
| || Anterior wall continues with ventricular septum|
| ||Pulmonary artery from the right to the left|
From January 1995 the pregnant women were offered a new scan if a proper four-chamber view had not been obtained at the routine scan. The second scan was performed by a more experienced colleague at approximately 20 gestational weeks and included the four-chamber view, the great arteries and the venous return to the fetal heart.
The classification of heart defects was done retrospectively after a final diagnosis was made either by postnatal echocardiography or by autopsy. A perinatal pathologist performed the postmortem examinations20. The defects were classified as major or minor. A heart defect was classified as major when surgical repair was likely to be required because of gross structural complexity of functional significance21, for example transposition of the great arteries, hypoplastic left heart syndrome, atrioventricular septal defect, coarctation of the aorta, and large ventricular septal defect. Minor heart defects, arrhythmias with a structurally normal heart and absent inferior vena cava with azygos continuation were excluded from this study. Rhabdomyomas were categorized as tumors and thus excluded as a heart defect. Major heart defects detected prior to the routine scan in the second trimester were not included in this study. Major heart defects not detected at the routine scan, but detected later in the pregnancy, were considered not detected for the purpose of this study, in which only the performance of the routine scan in the second trimester was evaluated.
The major heart defects were grouped as ‘isolated’ for fetuses with a normal karyotype and absence of extra-cardiac malformations or as ‘associated malformations including abnormal karyotype’ if extracardiac malformations and/or abnormal karyotype were present. The detection rates of isolated CHDs and of CHDs with associated malformations, including abnormal karyotype, were compared between experienced and less experienced sonographer/midwives.
A CHD that affected the appearance of the four-chamber view was considered a heart defect with an ‘expected abnormal four-chamber view’ (atrioventricular septal defect; hypoplastic left heart syndrome; tricuspid atresia)15. A CHD that did not affect the four-chamber view was considered an ‘expected normal four-chamber view’ (transposition of the great arteries (TGA); moderate coarctation of aorta; tetralogy of Fallot), unless additional defects from the heart changed the expected appearance of this view from normal to abnormal (TGA with serious pulmonary artery stenosis; moderate coarctation of aorta with mitral stenosis). The numbers of CHDs with expected abnormal four-chamber views recognized and missed by experienced sonographer/midwives were compared with the numbers recognized and missed by less experienced sonographer/midwives.
The ultrasound machines used were a Hitachi EUB-415 with a 5-MHz and a 3.5-MHz curvilinear transducer (Hitachi, Tokyo, Japan), a VingMed CFM 800 with a 5-MHz mechanical sector and a VingMed System Five with a 3.5-MHz curvilinear transducer (Vingmed, Horton, Norway). Pre- and postnatal data were prospectively maintained in a computer database. Groups were compared in 2 × 2 contingency tables using Pearson's Chi-Square or Fisher's exact test. The level of significance was set at P < 0.05.
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During the study period 29 460 fetuses were examined by ultrasonography at Trondheim University Hospital. Of those, 29 035 (99%) fetuses had a routine examination in the second trimester performed by a sonographer/midwife. The median gestational age at the time of the routine scan was 18.3 (range, 16.1–22.5) weeks.
Figure 1 shows the experienced and the less experienced sonographer/midwives' ability to obtain the four-chamber view, the ascending aorta and the main pulmonary artery at the routine scan in approximately 1000 fetuses from each of the years 1991, 1993 and 1995. A significant difference was found in their ability to obtain both the four-chamber view and the great arteries (P < 0.001). The ability to obtain the four-chamber view and the great arteries over time is shown in Figure 2 for the five experienced sonographer/midwives and the four less experienced sonographer/midwives who were new to this work in 1991, 1993 and 1995. During the period from 1991 to 1995 there was a significant increase in the ability to obtain the four-chamber view and the great arteries among the new, less experienced sonographer/midwives (P < 0.0001).
Figure 1. The experienced and less experienced sonographer/midwives' ability to obtain the four-chamber view, the ascending aorta and the main pulmonary artery in 2994 fetuses. , experienced; , less experienced. *P < 0.001.
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Of the 82 major CHDs in the fetal population, 35 (43%) were detected prenatally. The median gestational age at the time of the diagnosis was 18.0 (range, 16.1–22.0) weeks. Three (6%) of the 47 major CHDs not detected were suspected by a sonographer/midwife at the routine scan, but after further referral were dismissed as a defect by the physician who was consulted.
Table 2 shows the ability of the experienced and less experienced sonographer/midwives to detect major CHDs altogether, and to detect isolated CHDs and CHDs with associated malformations, including abnormal karyotype.
Table 2. The experienced and less experienced sonographer/midwives' ability to detect major congenital heart defects (CHDs) altogether, isolated CHDs, and CHDs with associated malformations including abnormal karyotype, at the routine examination in the second trimester
|Experience of operator||CHDs||Isolated CHDs||CHDs with extracardiac malformations|
|Total, n||Detected, n (%)||Total, n||Detected, n (%)||Total, n||Detected, n (%)|
|Experienced||42||22 (52)*||18||8 (44)†||24||14 (58)‡|
|Less experienced||40||13 (32.5)*||22||6 (27)†||18||7 (39)‡|
|Total||82||35 (43)||40||14 (35)§||42||21 (50)§|
Of the 82 major CHDs, 58 (71%) would be expected to have an abnormal four-chamber view. Of these, 32 fetuses were examined by experienced sonographer/midwives at the routine scan and 28 (87.5%) of the abnormal views were recognized. The corresponding number for the less experienced sonographer/midwives was 13/26 (50%) (P = 0.002).
Sixteen (62%) of the 26 isolated major heart defects not detected at the routine scan would be expected to have an abnormal four-chamber view at that gestation (Table 3). Of those, the experienced sonographer/midwives missed 5/10 (50%) CHDs and the less experienced sonographer/midwives 11/16 (69%) CHDs.
Table 3. Isolated major congenital heart defects not detected at the second-trimester routine scan with expected four-chamber view presentation
|Congenital heart defect|| ||Four-chamber view|
|n||Expected normal, n||Expected abnormal, n|
|TGA, simple||3||3|| |
|TGA, complex||1|| ||1|
|TGA, corrected||2||2|| |
|HLHS, simple||4|| ||4|
|Coarctation of aorta, simple||3||3|| |
|Coarctation of aorta, complex||1|| ||1|
|Pulmonary valve stenosis||3|| ||3|
|Ventricular septal defect, critical||3|| ||3|
|Tetralogy of Fallot||2||1||1|
|Pulmonary valve atresia||1|| ||1|
|Tricuspid atresia||1|| ||1|
|Aortic valve stenosis||1|| ||1|
|Total||26||10 (38%)||16 (62%)|
Of the 19 237 fetuses scanned between 1995 and 2001, a total of 392 fetuses were re-examined owing to difficulties in obtaining a proper four-chamber view at the routine scan. The percentage of fetuses referred to such a scan per year decreased from 2.4% in 1995 to 1% in 2001 (Figure 3). The reasons for not obtaining a proper four-chamber view were: unfavorable fetal position in 160 (41%) fetuses, poor imaging in 156 (40%), maternal obesity in 47 (12%), other in two (0.5%). For 27 (7%) fetuses no reason was given.
Figure 3. Proportion of four-chamber views not obtained at the second-trimester routine examination of 19 237 fetuses.
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The present study shows that experience in working with ultrasonography plays a significant role in the sonographer/midwife's ability to obtain fetal heart structures leading to the detection of major heart defects; thus, teaching and experience are major factors that need to be addressed to optimize the fetal heart examination.
As early as the mid 1980s it had become clear that the fetal heart needed special attention during the fetal examination. The four-chamber view was introduced with the expectation of detecting 50% of major heart defects11, 12. Access to this view is relatively easy and the success rate in obtaining this view has been reported as more than 95%; however, the detection rate of major CHDs has not been as high as expected16, 22–24. The present study also showed a high rate of success in obtaining the four-chamber view both for the experienced and the less experienced sonographer/midwives; in spite of this, of the 26 isolated CHDs that were missed, 62% could be expected to have been associated with an abnormal four-chamber view at the time of the routine scan. Those who reported the four-chamber view as satisfactorily obtained had clearly not included a thorough evaluation of all the details from the fetal heart shown in Table 125, 26. Obviously, future training has to focus on those structures in detail to make sure that they are correctly obtained and interpreted19.
There is still reason to believe that demonstration of the four-chamber view remains the most important way of detecting CHDs. However, 20 years after the introduction of this view, we have to accept that the potential of the four-chamber view has not yet been fully realized26. In the present study, the experienced sonographer/midwives showed a greater ability to evaluate the four-chamber view, and thus detected more CHDs: the experienced sonographer/midwives recognized significantly more of the expected abnormal four-chamber views than did the less experienced sonographer/midwives. However, the fact that experienced sonographer/midwives also missed CHDs in hearts that must have had an abnormal four-chamber view emphasizes the need for targeted training to obtain this view.
To improve the results, the great arteries have been added to the fetal heart examination, increasing the detection rate to some extent, but not as much as anticipated16, 27, 28. The experienced sonographer/midwives did significantly better in obtaining the great arteries, but the results showed room for improvement. Their detection rate after obtaining both the four-chamber view and the great arteries did not exceed the detection rate expected from the four-chamber view alone. The poor detection rates of CHDs from non-selected populations may indicate that neither the four-chamber view nor the great arteries' view have been properly learned2, 9, 10, 23, 29. Obviously, future teaching also has to focus on all the details necessary to evaluate the great arteries. Our data suggest that including the great arteries might not give the expected results as long as the information from the basic four-chamber view is not fully obtained and utilized. The importance of a stepwise introduction to the fetal heart examination remains fundamental to the success in fetal heart scanning, underlining the importance of mastering the first step—the four-chamber view—before the next step is considered19, 25.
Although the difference between experienced and less experienced sonographer/midwives in obtaining the great arteries was statistically significant, the difference between the two groups regarding the detection rate of CHDs was not statistically significantly different. Because heart defects are low in number, a background population of three times the size of our population, approximately 100 000 fetuses, would be needed to reach a level of significance given the same detection rates. However, the experienced sonographer/midwives did significantly better in obtaining both the four-chamber view and the great arteries—details necessary for the detection of CHDs. In addition, the experienced sonographer/midwives' ability to recognize an abnormal four-chamber view was significantly better than that of the less experienced sonographer/midwives. The significant improvement in the ability to obtain fetal heart structures over time also emphasizes the difference between the sonographer/midwives. It is important to note that even though not all the differences were statistically significant, the consistent trend between experienced and less experienced sonographer/midwives that was demonstrated in the present study does suggest a true difference between them.
New technology, such as color Doppler, has been introduced to ease the recognition of the great arteries17. New modalities may well contribute to the detection of CHDs and are without doubt important for the diagnostic process. But this continuous search for, and use of, more sophisticated technology may not improve the detection rate of CHDs as long as the basic views are not fully mastered. When properly practiced, obtaining the four-chamber view and that of the great arteries may be sufficient to detect the majority of major CHDs30.
In the retrospective study by Wong et al.7, which compared ultrasound personnel at tertiary and non-tertiary institutions, sonographers' experience was found to be one of the main factors influencing the detection rate of CHDs. Despite the same basic ultrasound training, the difference in detection rates between sonographers working in tertiary and non-tertiary institutions was significant (61% versus 21%, P < 0.001). The same differences between tertiary and non-tertiary institutions regarding detection of fetal abnormalities in general have also been reported by other investigators2, 31, emphasizing the fact that extensive experience is necessary to perform a high-quality ultrasound examination.
Another measure to evaluate the quality of the fetal heart examination is the ability to detect isolated CHDs, as in these cases the heart is the only affected organ. In our study, the experienced sonographer/midwives also did better in detecting isolated CHDs. On the other hand, isolated CHDs were less frequently detected than CHDs with associated malformations including abnormal karyotype; this suggests that extracardiac malformations triggered the detection of the CHDs. This has also been found by other investigators32, 33. Obviously, during an apparently normal scan without the presence of extra-cardiac malformations, the fetal heart has not been given enough attention, emphasizing the fact that mastering the basic views of the fetal heart has not been adequate.
The superiority of the experienced sonographer/midwives to those less experienced underlines the importance of dedicated personnel performing obstetric ultrasound examinations. It has been speculated that specialists34, such as pediatric cardiologists, could contribute to an improved prenatal detection rate of CHDs. This would imply that specialists would have to scan a population with a low incidence of pathology. It is not obvious, given such a situation, that they would do better than sonographers as the pediatric cardiologist would also need intensive training to master the fetal examination35, 36. In addition, as opposed to sonographer/midwives, pediatric cardiologists are not used to a screening situation where pathology is found only occasionally among a large number of normal cases. Sonographer/midwives are professionals dedicated to performing routine scans in non-selected populations with a low incidence of pathology, with the aim of achieving a high detection rate of pathology when present, and not only of CHDs. From an organizational point of view, one extra scan just to evaluate the fetal heart would most probably not be cost effective. Our data show that dedicated personnel develop a specific sensitivity for deviations, as some true heart defects in the present study were suspected by the sonographer/midwife, but later evaluated as normal by the physician who was consulted. Sonographer/midwives provided with proper training and easy access to referral centers sensitive to the suspected findings may be a better alternative to specialist scanning18, 19, 26, 37. Although this initially may increase the false positive rate, Carvalho et al.37 showed that, over time, the number of referrals decreased and the detection rate of CHDs increased with such a system.
Within the organizational structure of a department, experienced personnel are often responsible for the administrative work, leaving the larger number of examinations to those who are less experienced. The 42% detection rate of major CHDs at the routine scan in the second trimester demonstrated in the present study, was a joint result from both experienced and less experienced sonographer/midwives. Our data indicate that a higher detection rate would have been achieved if only experienced sonographer/midwives had performed the routine scans. However, this seems unrealistic, since all departments need to recruit new personnel. The focus should rather be directed towards ways of improving their basic education.
The present study has shown that experienced sonographer/midwives have a higher detection rate of CHDs than less experienced sonographer/midwives. We have also shown that experienced sonographer/midwives sometimes erroneously evaluated four-chamber views as normal when they were not, but to a significantly lesser degree than the less experienced sonographer/midwives. Thus, mastering the four-chamber view still seems to be a problem that needs to be addressed. Following basic training, our sonographer/midwives' learning curve so far has been dependent on a self-learning process through a large number of ultrasound examinations with limited time allocated for the total scan. This is not an ideal way of learning34. As a consequence of this, we suggest that educational institutions increase their focus primarily on the mastering of the four-chamber view during basic ultrasound education19, 26, 38–40. We have to accept that the fetal heart is the most difficult organ to examine and allow time for the necessary maturation of ultrasound skills to occur. After completion of the basic ultrasound education, the great arteries should be added to the heart examination; this can be done through providing fetal heart courses at the educational sites, maybe as an obligatory extension of the basic education. A prerequisite for proper teaching is that the teachers are sonographers and/or physicians who have considerable experience with ultrasonography.
The discussion of the low detection rates of CHDs has gone on for a long time, during which additional scanning planes and technology have been introduced. We believe our data emphasize that educational institutions need to critically evaluate and improve their fundamental teaching of the four-chamber view and the great arteries to give their students a more solid basis and thus shorten the learning curve.