Amniotic fluid (AF) volume evaluation has become important in the assessment of pregnancy at risk for an adverse pregnancy outcome.1 The two tests of fetal well-being used most commonly in the United States, the modified biophysical profile2 and the biophysical profile,3 include ultrasound estimation of AF volume. Recognition of low AF volumes may lead to additional fetal surveillance testing, induction of labor in the term patient, and initiation of potent glucocorticoids to accelerate fetal lung maturity followed by delivery of the preterm pregnancy.
Using color Doppler to assist in the identification of the umbilical cord in a pocket of fluid being insonated for the assessment of AF volume adequacy is increasing. A MEDLINE search from 1966 to 2000 using the search terms Doppler, color Doppler, AF volume, and oligohydramnios identified only one report concerning the measuring of the amniotic fluid index (AFI) with and without color Doppler. Bianco et al4 found the use of color Doppler resulted in significantly lower AF volume estimation by ultrasound and hypothesized that color Doppler might enhance the detection of oligohydramnios. No investigation to date has evaluated the use of color Doppler with ultrasound measurements and dye-determined actual AF volume to explore if color Doppler does increase the detection of true oligohydramnios.
The purpose of this investigation was to determine if concurrent use of color Doppler positively or negatively impacts ultrasound-based techniques in estimating actual dye-determined AF volume.
MATERIALS AND METHODS
This prospective clinical investigation included pregnant women undergoing amniocenteses to evaluate fetal lung maturity status before a planned labor induction or cesarean delivery. Exclusion criteria included women declining study participation following informed consent or patients for whom an amniocentesis could not be performed. All women signed an informed consent, which was approved by the Institutional Review Board at the University of Mississippi Medical Center, Jackson, Mississippi.
Participants underwent an ultrasound estimation of AF volume by one sonologist using the AFI,5 single-deepest pocket technique6 and subjective assessment (visualization without measurements).7 The AFI technique is used to estimate AF volume by dividing the abdomen into four quadrants. The linea nigra divides the abdomen into right and left halves and the umbilicus divides the abdomen into upper and lower halves. With the woman supine on the examining table and the transducer held at a right angle to the floor, the largest vertical pocket of fluid measured to fetal small parts and/or cord is measured and recorded. The sum of the four quadrants is the AFI. An AFI up to 5 was defined as low fluid, 5–20 as normal, and more than 20 as high fluid volume. The single-deepest pocket measures the largest vertical pocket of AF present. A single-deepest pocket up to 2 is low fluid, 2–8 is labeled as normal, and above 8 is high fluid volume.
Subsequently, a second evaluation of the AFI, single-deepest pocket, and subjective assessment was performed on each patient by a second sonologist using color Doppler. The ultrasound estimates of the first sonologist were unknown to the second sonologist. Measurements of the largest vertical pocket or single-deepest pocket were measured from the bottom of each pocket or to the cord identified by color Doppler. Measurements of low, normal, and high fluid volume were calculated using the same established criteria as used in the group without color. Thereafter, an ultrasound-guided amniocentesis was performed to estimate AF volume by the dye-dilution technique after removal of an aliquot for fetal maturity studies.8,9 Two mL of a 20% aqueous solution of amninohippurate sodium (400 mg, Merck Pharmaceuticals, Inc., West Point, PA) was injected into each amniotic cavity. The needle remained in place over the next 20 minutes with continuous ultrasonic monitoring of needle placement and fetal position. Three mL of the aminohippurate sodium and AF mixture were withdrawn at 20 minutes. Samples were frozen and stored at −20C until assayed for aminohippurate concentration and calculation of AF volume. Resulting values were classified as low (up to 5%), normal (5–95%) or high (above 95%) volumes after comparison with published normal volumes for gestational age.10
Sample size estimation and power analysis were done before the start of this investigation. We assumed the AFI would correctly identify 17% of the patients with oligohydramnios, based on an earlier investigation.7 Sixty subjects would be required to determine if color Doppler would increase the detection of low fluid by 15% (alpha = .05, beta = 80%). The measurement techniques without and with color were not independent, therefore, the paired t-test or McNemar's test was used where appropriate for statistical analysis. A P-value of < .05 was considered significant.
Sixty-seven women at a mean gestational age of 37.1 ± 2.5 weeks were entered into this investigation between June 1999 and March 2000. Mean maternal age at enrollment was 26.1 ± 6.3 years (range 16–41), the majority were African-American (47 of 67, 70.14%), and approximately half were primigravidas or secundigravidas (33 of 67, 49.25%). Dye-determined AF volume was classified as low in 18 patients, high in seven, and the remaining 42 were within normal range.
Both the AFI and the single-deepest pocket techniques, when performed using color Doppler, resulted in significantly lower estimates of the AF volume compared with those techniques used without color. When color Doppler was added, the AFI, which had been estimated without color (11.6 ± 5), declined by 20% ([9.3 ± 4.9], P < .001) after color Doppler was used. Similarly, the single-deepest pocket without color (4.5 ± 1.5) declined by 18% ([3.7 ± 1.5], P < .003) after color Doppler was added.
The detection of low dye-determined amniotic fluid volumes using the AFI technique was not significantly different with the addition of color Doppler. Two of the 18 low volumes were identified without color and five of the 18 were identified with color (P = .083). In addition to these five identified with color as having low fluid volume, nine with normal dye-determined AF volume were erroneously estimated as having low fluid volume. This incorrect estimation resulted in the total number of women with estimated low volume to be greater with color (14 of 67, [21%]) compared with women evaluated without color (two of 67, [3%], P = .002). A greater number of the 42 normal dye-determined volumes were detected without color (41 of 42, 96.7% versus 33 of 42, 78.5%, P = .011). The seven high dye-determined volumes were identified similarly without and with the addition of color Doppler.
The single-deepest pocket technique detected a similar number of the normal dye-determined AF volumes without color (41 of 42, 96.7%) and with color Doppler (40 of 42, 95.2%). The seven high dye-determined volumes were identified similarly without and with the use of color Doppler. None of the 18 low dye-determined volumes were detected without color and only two with color Doppler.
Subjective assessment of the 18 low dye-determined AF volumes was improved from five of 18 (28%) correctly identified without color to nine of 18 (50%, P = .046) correctly identified with color. The 42 normal dye-determined AF volumes were identified better without color (40 of 42, 95.2%) than when color Doppler was added (29 of 42, 69%, P = .002). Moreover, 12 subjects with normal AF volume were erroneously labeled as having low fluid when color Doppler was used with the subjective evaluation of AF volume. The accuracy of using the subjective technique without and with color for the seven patients with high dye-determined volumes could not be determined using McNemar's test because there were no high volume patients with color.
Color Doppler is being used with increased frequency in the ultrasound estimation of AF volume. If color Doppler increases the diagnosis of low AF volume, this could increase the number of labor inductions in otherwise normal pregnancies, based solely on a diagnosis of oligohydramnios. This increasing use of color Doppler, without the validation that its use is related to a reduction of adverse pregnancy outcome, is a concerning trend. Does this technique truly assist in identifying more patients with low fluid or does it lead the sonographer to overdiagnosis of low AF volume with the potential for excessive intervention?
This investigation confirms the earlier work of Bianco4 by revealing that the AFI is decreased by 20% by using color Doppler. This resulted in a significantly greater number of women being categorized as having low fluid volume with the AFI in this investigation. However, when the diagnosis of low fluid with and without color was correlated with dye-determined volumes, the use of color Doppler did not enhance detection of actual oligohydramnios. It is disturbing that with color Doppler nine of 42 women (21%) with normal AF volume were diagnosed as having low fluid volume. The diagnosis of low fluid often leads to additional testing and interventions with increased morbidity and cost—this overdiagnosis is undesirable.
Measurements obtained for the single-deepest pocket were also decreased using color Doppler by 18%. Normal AF volumes were accurately detected without and with color using the single-deepest pocket technique in over 95% of the pregnancies with normal AF volume, but low fluid volume was poorly identified. The overall accuracy of the AFI and single-deepest pocket without color appears to be similar. This finding is consistent with our previous observation that both the AFI and the single-deepest pocket without color are similar, but weak indicators of abnormal AF volume.11
Subjectively, the addition of color Doppler did not increase the detection rate of low dye-determined fluid by correctly detecting 50% of the low volumes, but it did erroneously label 31% of the women with normal fluid as having low fluid. Thus, the subjective evaluation of AF volume with color Doppler appears to be disadvantageous because of the overdiagnosis of low fluid volume in pregnancies with normal fluid volume.
Although the diagnosis of either extreme of AF volume (oligohydramnios, hydramnios) appears not to be impaired by concurrent use of color Doppler, its use in association with the techniques of AFI and subjective assessment in the normal volume patient leads to frequent underestimation of actual AF volume. Overdiagnosis of oligohydramnios could lead the clinician to intervene unnecessarily with potentially adverse maternal-fetal consequences. If color Doppler is to become integrated into routine AFI, single-deepest pocket AF estimation, and subjective estimation, new definitions for normal and abnormal limits must be determined. If color Doppler is used in association with AF volume assessments, then the preferred technique is single-deepest pocket, since it appears least likely to overdiagnose low AF volume and mislead the clinician.
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