Department of Perinatology, Kagawa Medical University, 1750-1 Ikenobe, Miki, Kagawa 761-0793, Japan
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Abstract |
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Key words: appropriate-for-gestational-age fetus/Doppler ultrasound/fetal artery/multiple pregnancy/pulsatility index/singleton pregnancy
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Introduction |
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It has been suggested (Hata et al., 1991; Xu et al., 1995
) that the deposition of soft tissue seen in normal singletons during the third trimester occurs to a much lesser extent in normal twins and triplets. Kuno et al. (1999) gave strong support to these works (Hata et al., 1991
; Xu et al., 1995
). Therefore, further study is needed to clarify whether this decrease in soft tissue deposition in multiple pregnancies represents a true growth abnormality or merely a physiological adaptation to the energy demands associated with the support of growth in multiple fetuses.
With recent advances in Doppler ultrasound, especially in colour Doppler flow imaging, various fetal vessels have been investigated (Arduini and Rizzo, 1990; Manabe et al., 1995
). The use of a sensitive colour flow Doppler apparatus to identify and reliably insonate specific small diameter vessels has greatly facilitated the study of the blood flow within individual fetal organs (Belfort et al., 1993
). However, most fetal Doppler studies were done in singleton pregnancies, and there have been a few reports on fetal Doppler velocimetry in multiple pregnancies (Nimrod et al., 1987
; Giles et al., 1990
; Gaziano et al., 1991
; Degani et al., 1992
). Moreover, to date, there has been no known report on fetal peripheral arterial Doppler velocimetry, such as splenic artery, renal artery, and femoral artery, in multiple pregnancies.
The objective of the present study was to construct reference limits for pulsatility index (PI) values from middle cerebral artery (MCA), descending aorta (DA), splenic artery (SA), renal artery (RA), femoral artery (FA), and umbilical artery (UA), based on a longitudinal study of 35 singleton appropriate-for-gestational-age (S-AGA), 52 twin appropriate-for-gestational-age (Tw-AGA), and 12 triplet appropriate-for-gestational-age (Tri-AGA) infants, and to evaluate the alterations in those PI values among these three groups during gestation.
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Materials and methods |
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Fetal age determination was estimated from the first day of the last menstrual period and confirmed by the first-trimester and early second-trimester ultrasound examinations (crownrump length, biparietal diameter, and femur diaphysis length measurements) (Tsuzaki et al., 1982; Iwamoto, 1983
) or determined from the date of conception (IVF pregnancies) plus 2 weeks (the ultrasound age estimates confirmed these age determinations).
Ultrasound examinations were carried out at 2 week intervals beginning at ~15 weeks of menstrual age continuing until delivery. The number of examinations of individual patients ranged (mean ± SD) from 5 to 13 (9.3 ± 2.1) in the S-AGA group, from 5 to 12 (8.0 ± 2.0) in Tw-AGA group, and from 8 to 10 (9.5 ± 0.9) in the Tri-AGA group. At each examination, each fetus was identified by its position in the uterus, its size, or its sex in multiple pregnancies. Measurements of the biparietal diameter, head circumference, abdominal circumference and femur diaphysis length were obtained at each examination for each fetus using procedures described in a previous publication (Deter et al., 1981). The estimated weight was determined from values for the biparietal diameter, abdominal circumference, and femur diaphysis length as described previously (Shinozuka et al., 1987
).
Results are expressed as mean ± SD. Statistical analysis for comparison of maternal age, parity, and Apgar score among the groups was done using a KruskalWallis one-way analysis of variance. Maternal height, maternal weight, birth age, birth weight, neonatal crownheel length, neonatal head circumference, neonatal abdominal circumference, and neonatal thigh circumference were compared using an analysis of variance (ANOVA) and NeumanKeuls multiple comparison test. P < 0.05 was considered to be significant.
Colour Doppler flow imaging and pulsed Doppler ultrasonography with a 3.5 MHz convex transducer (Aloka SSD-2000, Tokyo, Japan) was used for blood flow velocity measurements in the fetal MCA, DA, SA, RA, FA and UA. In the colour Doppler mode, the flow directed toward the transducer was displayed in shades of red and the flow directed away from the transducer was in shades of blue. In the colour and pulsed Doppler mode, the lowest possible measurable velocity was 1.54 cm/s. Pulse repetition frequencies were 125 kHz, the maximum penetration depth was 24.6 cm, and the gate width was 110 mm. Wall filters (50 Hz) eliminated low-frequency signals occurring from vessel wall motion. The spatial peak temporal average intensity at the maximum amplitude and minimum gate width in simultaneous colour and pulsed Doppler mode was <80 mW/cm2, according to the manufacturer's specification.
The technique of recordings previously described (Manabe et al.,1995) has been reported in detail elsewhere. Briefly, a good real-time image of the plane in which the vessel is situated was first obtained and the colour flow function was then superimposed in an attempt to visualize blood flow through the vessel and to minimize the angle of insonation between Doppler beam and flow direction. The pulsed Doppler sample volume (3 mm) was therefore placed on the point of maximum signal intensity as expressed by the degree of colour brightness and the flow velocity waveforms were then recorded. MCA was insonated at the level of the greater wings of the sphenoid. Velocity waveforms from DA were recorded at the lower thoracic level whereas RA waveforms were measured at the level of branching from the abdominal aorta. SA velocity waveforms were obtained at the level of the splenic hirus, and FA velocity waveforms were gained at the level of the upper third of the thigh. UA velocity waveforms were analysed in a position considered equidistant from abdominal and placental insertion points. Unfortunately, DA, RA, SA, and FA velocity waveform measurements in triplet pregnancy could not be performed, because the vessels could not be identified due to fetal crowding.
Recordings were made in the absence of fetal body or breathing movements. Waveforms were recorded over five cardiac cycles. PI [(peak systolic velocity end-diastolic velocity)/time-averaged mean peak velocity] was calculated. All examinations were performed by one examiner (M.A.). The intra-observer coefficient of variation for the measurement of PI was determined by performing five consecutive examinations on 10 patients within 30 min. The results were 7.5, 8.8, 9.7, 8.5, 7.2 and 9.6% for measurements of PI for MCA, DA, RA, SA, UA and FA respectively.
For each artery in each fetal group, data set regression analysis was carried out, testing the regression of PI value on menstrual age, using polynomials of the first to the third degree (Dunn and Clerk, 1974; Rohatgi, 1976; Bertagnoli et al., 1983
). Different models were tested and independent variable deletion carried out by ANOVA applied to the regression was followed by calculation of the step-down method coefficients (Snedecor and Cochran, 1967). The choice of the optimal model was based on the following criteria: largest R2, all coefficients different from 0, and low standard deviation of regression (SDR) (Bertagnoli et al., 1983
).
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Results |
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Discussion |
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There have been a few reports on fetal Doppler velocimetry in multiple pregnancies (Nimrod et al., 1987; Giles et al., 1990
; Gaziano et al., 1991
; Degani et al., 1992
). However, these Doppler studies were carried out only in MCA and UA. Available data for other fetal peripheral arteries in multiple pregnancies are limited. There is no known report on fetal peripheral arterial Doppler velocimetry, such as SA, RA and FA in multiple pregnancies. Therefore, it was thought that a different standard of fetal Doppler velocimetry in multiple pregnancies was needed for different peripheral arteries. The Doppler ultrasonography-based reference limits constructed in this longitudinal population provide an additional tool for the evaluation of fetal regional vascular resistance in multiple pregnancies. In singleton small-for-gestational-age fetuses, a decrease in the SA-PI (Abuhamad et al., 1995
; Capponi et al., 1997
), increase in the RA-PI (Mari et al., 1995
), and increased FA-PI (Mari et al., 1991) have been reported. Since in this study all fetuses were AGA, one would not expect dramatic differences in comparing these fetuses. However, the results of this study on different fetal peripheral artery PI in multiple pregnancies may have clinical usefulness, especially in the case of discordant fetal growth. Further study is needed to clarify the clinical significance of these measurements in multiple pregnancies.
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Notes |
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References |
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Submitted on April 7, 1999; accepted on June 25, 1999.