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Results: AWHONN Lifelines
Copyright (C) 1999 by the Association of Women's Health, Obstetric and
Neonatal Nurses
Volume 3(6), December/January 1999/2000, pp 20-24
Focus on Fluids: Examining Maternal Hydration and Amniotic Fluid Volume
[Features]
Calhoun, Simone RNC, BSN
Simone Calhoun, RNC, BSN, is a perinatal nurse clinician, at St. Francis
Hospital and Medical Center in Manchester, CT.
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Outline
Factors Affecting Amniotic Fluid Volume
Conditions Associated with Oligohydramnios
Mechanics of Amniotic Fluid Balance
Evaluating Amniotic Fluid Volume
Nursing Implications
References
Graphics
Figure 1
Figure 2
Figure 3
Case Study Rebecca is a 33-year-old gravida 3, para 1011, health care
professional at 31.5 weeks' gestation currently being treated with 2.5 mg
terbutaline and being checked every 4 hours for signs of preterm labor. She
has had an upper respiratory infection with severe nasal congestion for
which she had taken Chlor-trimetron (4 mg) twice overnight. A nonstress test
(NST) was nonreactive. Fetal heart rate baseline was 150 beats/minute, with
an average LTV and no decelerations. One contraction was noted in 40
minutes. Biophysical profile score was 6/10 (-2 NST, -2 amniotic fluid
volume [AFV]), with no cord-free pocket of >2 x 2 cm in two dimensions.
Amniotic fluid index (AFI) was 2.4 cm. Placenta was posterior and fundal
(grade 1). A sonogram 1 week earlier showed fetal growth in the 50th
percentile and an AFI of 14.6 cm.
After discussion with maternal-fetal medicine personnel, who suggested a
possible need for delivery because of severe oligohydramnios, the AFI was
reevaluated and 6 hours later was found to be 7.8 cm. The patient stated
that she had been drinking copious amounts of fluid since the initial AFI.
The patient continued pregnancy with a twice-weekly NST and amniotic fluid
evaluation. The AFI remained in the 8- to 9-cm range. Terbutaline was
discontinued at 37 weeks. The patient spontaneously delivered a 6-pound,
9-ounce healthy boy with Apgar scores of 8 and 9 at 37 weeks, 3 days. Mother
and infant were discharged after 48 hours.
Factors Affecting Amniotic Fluid Volume
Historically, oligohydramnios in the absence of rupture of membranes has
been considered to be a sign of chronic suboptimal placental function that
leads to decreased fetal urine output. Acute decreases in AFV because of
inadequate maternal hydration or medication effects aren't widely reported.
This begs two
questions:
* Assuming no technical errors, was the rapidly improved amniotic fluid
observed in this case patient due to the effect of oral hydration?
* Could lack of maternal hydration adversely affect AFV?
Studies support the notion that maternal fluid volume may play an important
role in maintaining AFV. Sherer et al. (1990) reported a case of severe
oligohydramnios in a woman who presented with severe dehydration because of
gastroenteritis. After the patient was hydrated with 6,500 ml intravenous
crystalloid fluid and was no longer hypovolemic, a rapid reaccumulation of
AFV to normal status was observed. Also, Kilpatrick and Safford (1993) found
that fluid restriction decreased the AFI by 8 percent. Kilpatrick and
Safford also showed that maternal oral hydration with 2 liters of water over
a 2-hour period increased the AFI in pregnancies with normal AFV by 16
percent. Previously, Kilpatrick et al. (1991) had shown that maternal oral
hydration with 2 liters of fluids over 2 hours increased the AFI by 31
percent in women with decreased AFV.
In contrast, Flack et al. (1995) and Kerr (1996) found that oral hydration
increased the AFI in women with oligohydramnios, but did not significantly
increase AFI in women with normal amounts of amniotic fluid. Flack et al.
(1995) found that the increased AFI was not attributable to increased fetal
urine production but instead was probably attributable to improved
uteroplacental perfusion caused by maternal plasma volume expansion. Flack
et al.'s research also suggests that there may be fluid passage from the
maternal intravascular compartment into the amniotic fluid compartment.
Although demonstrated in animal studies, this hasn't been evaluated in
humans. Animal studies have found significant flows of amniotic fluid
through the transmembranous pathway between the fetal vessels on the
chorionic plate and the amniotic cavity (Flack et al., 1995). Flack et al.
also suggested that AFV volume may be increased by intramembranous net water
transfer between mother and fetus across the chorionic plate, fetal skin,
and the surface of the umbilical cord. Kilpatrick and Safford
(1993) noted a significant increase in umbilical artery mean velocity after
maternal hydration and theorized that hydration may work to increase the AFI
by improving placental blood flow or by bulk transfer of water across the
placenta.
Conditions Associated with Oligohydramnios
Oligohydramnios, when identified in patients with intact membranes in a
singleton pregnancy, is defined in several ways in the literature:
* AFI of less than 5 cm
* AFI below the fifth percentile for gestational age (normograms have been
established for gestational ages)
* No cord-free fluid pocket that measures >2 cm in two diameters
* No cord-free fluid pocket with at least one 1 cm deep vertical pocket
(Moore,
1997)
FIGURE 1
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Figure 1. Evaluating AFI
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Oligohydramnios complicates approximately 3 to 5 percent of all pregnancies
(Flack et al., 1995). Intrauterine growth restriction, postmaturity
syndrome, fetal anomalies, stillbirth, fetal distress in labor, and
operative delivery are all associated with oligohydramnios (Kilpatrick,
1997). Prolonged oligohydramnios restricts fetal movement, which may also
lead to compression orthopedic abnormalities, and also may interfere with
normal fetal lung development leading to life-threatening compromise caused
by to pulmonary hypoplasia (Nimrod et al., 1984). Another concern with
inadequate AFV is risk for umbilical cord compression, which may lead to
inadequate umbilical circulation.
Decreasing AFV in the absence of congenital anomalies is generally thought
to be the result of chronically impaired placental function leading to
decreased renal blood flow, which decreases fetal urine production (Flack et
al., 1995). Often when the fluid falls below a certain critical point,
delivery is recommended (Phelan et al., 1987). If the patient is at or near
term, this is not a difficult medical decision. However, when
oligohydramnios is identified in the preterm patient, a decision must be
made as to the safety of continuing the pregnancy. In the meanwhile,
modified bed rest in lateral recumbent position is often recommended.
Prenatal diagnosis should be offered to the family to rule out aneuploidy
(having an abnormal number of chromosomes). A targeted sonogram should be
done to screen for anomalies. The use of corticosteroids if the gestation
has been less than 34 weeks also is recommended because of the risk for
preterm delivery. Women with an AFI below the 5th percentile should be
followed-up with a twice-weekly NST and AFI to ensure continued fetal
well-being (Moore, 1997).
Mechanics of Amniotic Fluid Balance
Although the mechanics of amniotic fluid production are not completely
understood, it is known that, in most cases, the AFV is maintained by a
balance between production and absorption. Fetal urine is thought to be the
major source of amniotic fluid once the fetal kidney begins production at 10
to 12 weeks (Moore, 1997). Moore reports on studies showing that at term the
human fetus may produce as much as 1,200 ml of urine daily. Another source
of amniotic fluid is fetal lung fluid, as evidenced by the presence of
pulmonary surfactant in amniotic fluid. Also, it may be movement of amniotic
fluid through transmembranous and intramembranous routes, which have been
well documented in animal studies (Moore, 1995).
Reduction of amniotic fluid from the amniotic cavity is primarily
accomplished by fetal swallowing, at a rate of approximately 500 ml/day.
Because this process accounts only for about 50 percent of the fluid
produced, other mechanisms must be operative. It's thought that amniotic
fluid also may be removed by a transmembranous movement of fluid across the
membranes into the maternal circulation (Moore, 1997). It is also possible
that amniotic fluid is removed through an intramembranous route into the
fetal circulation by way of the blood vessels on the fetal placental surface
(Moore, 1997).
Evaluating Amniotic Fluid Volume
Because of the increased incidence of poor fetal outcomes associated with
oligohydramnios, evaluating AFV has become an important component of
antepartal fetal testing. Several sonographic techniques are used to do so.
Amniotic fluid index appears to be the most widely used technique because of
its reproducibility, which allows for comparison among different examiners.
This technique divides the uterus into four quadrants, and in each quadrant
measures the largest vertical pocket free of umbilical cord or fetal small
parts. These dimensions are then totaled to calculate the AFI. Normal AFI
ranges between 8 and 18 cm (Moore, 1997). An AFI of 5.1 to 8.0 cm is
considered to be borderline, and less than 5 cm is defined as
oligohydramnios (Kilpatrick et al., 1991; Moore, 1997). Normograms have been
developed that establish the expected AFI ranges for each gestational week,
because AFI fluctuates throughout gestation. Magann et al.
(1997) reported that AFI increased until term, but most research supports
that AFV rises progressively throughout pregnancy until about 32 weeks
(Flack et al., 1995; Moore, 1997). From 32 weeks until term, AFV is
relatively stable, usually at about 700 to 800 ml. Post-term AFV decreases
at about 8 to 12 percent per week to about 400 ml at 42 weeks (Moore, 1997).
The maximum vertical pocket technique (MVP) measures the largest fluid
pocket that is free of umbilical cord or fetal small parts. The use of the
MVP technique defines oligohydramnios as the absence of any fluid pocket of
at least 1 cm in depth (Moore, 1997). A criticism of this technique is that
this criterion was developed from high-risk patients and may not apply to
healthy populations (Moore, 1997). Because of this, the MVP technique isn't
widely used today. Another similar technique in use is the two-diameter
pocket, which measures the largest vertical and horizontal pockets. The
two-diameter technique defines oligohydramnios as no cord-free pocket
measuring greater than 2 x 2 cm (Magann et al., 1992). FIGURE 2
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Figure 2. FOCUS ON FLUIDS Are you getting enough for You, and Baby,
Too?8 to 12-That's how many cups of Fluid you need every day if you're
pregnant or breast-feeding. Here's a guide to help you make sure you get
them.
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Another method is the subjective assessment of AFV by the ultrasonographer.
This is done by comparing the relative amount of echo-free fluid with the
space taken up by the fetus and the placenta. When performed by expert
sonographers, this technique can identify oligohydramnios as efficiently as
the MVP technique (Moore, 1997). However, this technique is rarely used
today, because of the lack of objective measurement, which makes it
difficult to follow trends.
Nursing Implications
Nurses are in an excellent position to educate women about the importance of
adequate daily fluid intake, which, among other benefits, seems to be
important in the maintenance of adequate AFV. This may be critical in:
* The latter stages of pregnancy, when amniotic fluid amounts normally
decline
* In patients at increased risk for oligohydramnios
No published studies determine the actual amount of daily fluid intake
needed in pregnancy. Suggested fluid amounts are based on anecdotal
information only. Kitzinger (1996) recommends 4 to 5 glasses of water per
day, whereas Cherry
(1992) recommends that sedentary pregnant women should increase fluids to
satisfy thirst. Cherry also states that active pregnant women need 8 to 12
extra glasses of water per day during exercise. Eagle Family Foods Corp. has
a patient teaching tool, Focus on Fluids, that recommends 8 to 12 glasses of
fluid daily.
All women should be made aware of the importance of adequate fluid intake
during pregnancy, not just women at risk for uteroplacental insufficiency.
However, it would be of even more importance for women with pregnancies at
risk, because inadequate fluid intake may cause further fetal compromise.
Studies also point to the importance of maintaining adequate hydration
during exercise, thermal exposure, or gastrointestinal losses in pregnancy.
The use of antihistamines as described in the case report have played a role
in the decreased amniotic fluid. Women should be urged to increase their
fluid intake if using these types of over-the-counter medications.
In educating clients regarding adequate oral fluids intake, it is very
important to explain the function of amniotic fluid in protecting the fetus
from injury and preventing cord compression. Without this information, some
women may limit their fluid intake because of increasing edema or the
inconvenience of increased urination late in pregnancy. A fluid tracking
chart is available that can be distributed to patients (see "Focus on
Fluids").
In addition to adequate fluid intake, patients with oligohydramnios and
intrauterine fetal growth retardation should be encouraged to rest as much
as possible in the lateral recumbent position, because this should optimize
uteroplacental perfusion, which also may help to increase AFV as well as
optimize fetal growth. Beginning at the 28th week, all gestating women
should be instructed to report decreased fetal movement. Some institutions
use a modification of the Piacquadio protocol, which quantifies fetal
movement in low-risk patients (Moore & Piacquadio, 1989). Patients are given
instructions to observe fetal movement 1 hour per day. If the baby moves
fewer than 10 times in that hour, they are to eat something nutritious and
repeat the observation. If the baby doesn't move 10 times during the second
hour, they are instructed to notify their health care provider immediately.
It is important to explain that this does not mean that something is wrong
with the baby, but simply that they will need to have fetal evaluation,
which should include an AFI to rule out oligohydramnios, which can restrict
fetal movement.
Because of the many unanswered questions about AFV, more studies are needed
to determine adequate fluid intake during pregnancy as well as the degree of
fluid restriction that results in oligohydramnios.
References
Cherry, S. H. (1992). Understanding pregnancy and childbirth (3rd ed.). New
York: Collier Books: Macmillan Publishing Co.
Flack, N., Sepulveda, W., Bower, S., & Fisk, N., (1995). Acute maternal
hydration in third trimester oligohydramnios: effects on uteroplacental
perfusion, and fetal urine output. American Journal of Obstetrics and
Gynecology, 173(4), 1186-1191.
Focus on fluids. (1999). Patient Education Pamphlet. Columbus, Ohio: Eagle
Family Foods.
Kerr, J., et al. (1996). Maternal hydration and its effect on the amniotic
fluid status. (Abstract No. 85). American Journal of Obstetrics and
Gynecology, 174(1), 416.
Kilpatrick, S. J. (1997). Therapeutic interventions for oligohydydramnios:
Amnioinfusion and maternal hydration. Clinical Obstetrics and Gynecology,
40(2), 328-336.
Kilpatrick, S. J., Safford, K. L., Pomeroy, T., Hoedt, L., Scheerer, L.,
Laros, R. K. (1991). Maternal hydration increases amniotic fluid index.
Obstetrics and Gynecology, 78(6), 1098-1102.
Kilpatrick, S. J., & Safford, K. L. (1993). Maternal hydration increases
amniotic fluid index in women with normal amniotic fluid. Obstetrics
Gynecology, 81(1), 49-52. Bibliographic Links
Kitzinger, S., (1996). The Complete Book of Pregnancy and Childbirth. New
York: Alfred A. Knopf, 99.
Magann, E. F., Nolan, T. E., Hess, L. W., Martin, R. W., Whitworth, N. S.,
Morrison, J. C. (1992). Measurement of amniotic fluid: accuracy of
ultrasound techniques. American Journal of Obstetrics and Gynecology, 167,
1533-1537.
Magann, E. F., Bass, J. D., Chauhan, S. P., Young, R. A., Whitworth, N. S.,
Morrison, J. C. (1997). Amniotic fluid volume in normal singleton
pregnancies. Obstetrics & Gynecology 90(4), 524-528. Ovid Full Text
Bibliographic Links
Moore, T. R. (1997). Clinical assessment of amniotic fluid. Clinical
Obstetrics & Gynecology, 40(2), 303-313. Ovid Full Text Bibliographic Links
Moore, T. R. (1995). Assessment of amniotic fluid in at-risk pregnancies.
Clinical Obstetrics and Gynecology, 38(1), 78-90.
Moore, T. R., Piacquadio, K. (1989). A prospective evaluation of fetal
movement screening to reduce the incidence of fetal death. American Journal
Obstetrics & Gynecology, 160, 1075-1080.
Nimrod, C., Varela-Gettings, F., Machin G., Campbell, D., & Wesenberg, R.
(1984). The effect of very prolonged rupture of membranes on fetal
development. American Journal of Obstetrics and Gynecology, 148, 40-43.
Phelan, J. P., Ahn, M. O., & Smith, C. V. (1987). Amniotic fluid index
measurements during pregnancy. Journal of Reproductive Medicine 32, 601-604.
Bibliographic Links
Sherer, D. M. (1990.) Transient oligohydramnios in a severely hypovolemic
gravid women 35 weeks gestation with fluid reaccumulating immediately after
intravenous maternal hydration. American Journal Obstetrics and Gynecology,
162(3), 770-771.
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Figure 3. No caption available.
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