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The effect of low amniotic pressure without oligohydramnios on fetal lung development in a rabbit model
The effect of low amniotic pressure without oligohydramnios on fetal lung development in a rabbit model Fatih Kizilcan, MD,” F. Cahit Tanyel, and Akgi.in HiqsGnmez, MD” Ankara, Turkey
MD,” Nur @kar,
MD,b Nebil
Blyiikpamukqu,
MD,”
OBJECTIVE: Our purpose was to examine fetal lung development in reduced intraamniotic pressure without amniotic fluid loss. STUDY DESIGN: A circular portion of uterine wall measuring 1 cm in diameter was excised while the chorionic and amniotic membranes were leti intact at the twenty-third day of gestation in New Zealand White rabbits. The chorionic and amniotic membranes herniated spontaneously through the defect. RESULTS: Amniotic pressure was significantly reduced after herniation. Lung weight/body weight ratios at term were significantly reduced in experimental fetuses compared with controls. Residual amniotic fluid volumes at term did not differ. Histopathologic examination of lung specimens showed that fetal lungs had not matured in the experimental group as fully as in the control group. CONCLUSION: This experimental study demonstrated that low amniotic pressure impaired fetal lung development, even without oligohydramnios. (AM J OBSTET GYNECOL 1995; 173:36-41.)
Key words:
Fetus, oligohydramnios,
lung hypoplasia
Pulmonary hypoplasia is one of the major causes of neonatal mortality. Oligohydramnios is the most common predisposing factor, but its pathogenesis remains to be elucidated. It has been widely speculated that lung hypoplasia in this situation is the result of fetal compression by the uterine wall leading to reduced thoracic cavity available for lung growth. This theory is supported by other sequelae of oligohydramnios such as Potter’s facies and limb deformities. However, the mechanism of the selective effect of compression on the lung, rather than other visceral organs, without thoracic cage deformity is not clarified. Alternatively, several other explanations proposing deficiency of fetal breathing movements,‘. ’ internal stenting effect of lung liquid,3s 4 or a possible humoral or amniotic pulmonary growth facto? also exist. However, none of the explanations are entirely satisfactory. On the other hand, amniotic fluid contains pulmonary growth factors such as epidermal growth factor and fibroblast-pneumocyte factor.‘-* Loss of such contents
From the Departments of Pediatric Surgery” and Morphology,” Hacettepe University Faculty of Medicine. Supported by the Scient$c and Technical Research Council of Turkey (TAG 1014). Received for publication August 8, 1994; revised November 17, 1994; accepted November 18, 1994. Reprint requests: F. Cahit Tanyel, MD, Hacettepe Universitesi Tip Fakultesi, Cocuk Cerrahisi Anabilim Dali, 06100, Sihhiye, Ankara, Turkey. Copyright 0 1995 by Mosby-Year Book, Inc. 0002-9378/95 $3.00 f 0 6/l/62323 36
through leakage or altered fetus-amniotic fluid exchanges may also be responsible in the pathogenesis of pulmonary hypoplasia associated with oligohydramnios. In pregnancies complicated by severe oligohydramnios intraamniotic pressure has been found to be reduced,‘, I0 accordingly, the low amniotic pressure has been proposed as a cause of lung hypoplasia associated with oligohydramnios.” However, no experimental studies supporting this concept have been carried out. Therefore an in utero experimental study was planned to evaluate the effect of reduced intraamniotic pressure on fetal lung development while keeping the amniotic cavity intact with the aim at preserving natural volume and exchange of amniotic fluid.
Material and methods Assuming that herniation of the amniotic sac through the uterine wall would change the intraamniotic pressure, an experimental model was created. The experiment began after institutional care and use committee approval was received, and guidelines for responsible use with the highest standards of humane care were strictly followed during the study. In the experimental group 20 time-mated pregnant New Zealand White rabbits underwent operation on day 23 of pregnancy (normal 31 to 32 day gestation) according to the general principles.‘” After a 12-hour fasting period rabbits were premeditated with ketamine (10 mg/kg) intravenously. Anesthesia was maintained with 2% to 3% halothane in oxygen by mask inhalation.
Volume 173, Number Am J Obstet Gynecol
Kizilcan
1
Fig. 1. Schematic representation
A 0.5 gm dose of sulbactam-ampicillin was given intramuscularly at the beginning of the operation. The abdomen was shaved and prepared with povidoneiodine. While aseptic practices were followed, the uterus was delivered through the midline vertical incision without kinking uterine vessels. The uterine wall covering one of the fetuses in each horn was operated on. A 5-O silk purse-string suture with a diameter of 1.5 cm was placed on the uterine wall and tied without constricting the circle. With the chorionic and amniotic membranes left intact, a circular portion of the uterine wall measuring 1 cm in diameter was excised, resulting in a defect in the uterine wall that was surrounded by the purse-string suture. The chorionic and amniotic membranes herniated spontaneously through the defect in the uterine wall. The herniated sacs moved outward and inward while the uterus contracted and relaxed. Hence the amniotic sac was partially outpouched and devoid of uterine support (Fig. 1). Cesarean sections were performed on the thirty-first day of gestation, and live births with intact membranes were studied. Fetuses were extracted from the uterus with their amniotic sacs. Amniotic membranes were torn, and fluid was then collected and measured. After death the body weight and lung weight of the fetuses were measured and lung weight/body weight ratios were calculated. The lungs were histopathologically examined. Unoperated fetuses served as controls. Statistical analysis was performed with the Mann-Whitney test. Another group of rabbits was included in pressure measurement studies at 23 days of gestation. Intraamniotic pressures of 10 unoperated fetuses in five rabbits were measured extraabdominally under conditions similar to those of the experimental group. Pressure measurements were also carried out in another 10
of herniation
et al.
37
of amniotic membranes.
fetuses of five rabbits after amniotic herniation. Pressures were measured with a 20-gauge needle connected to a silicone strain-gauge transducer (Statham P 23 AC, Gould Instrument Systems, Inc., Valley View, Ohio) by a saline solution-filled catheter. The output of the transducer was displayed on a chart recorder (Grass Polygraph 7 DAE, Grass Instrument Co., Quincy, Mass.), whereby scoring was calibrated with a saline solutionfilled tube manometer. The gain of the recorder was amplified to allow pressure measurements of a minimum of 0.3 cm H,O. Working of the system was checked by inserting the 20-gauge needle into the rubber cuff connected to a tube manometer. Intraamniotic pressures were then measured by placing the needle in the amniotic cavity through the uterine wall and recorded for 60 to 90 seconds after stable readings had been established. The values of intraamniotic pressure were compared between unoperated and herniated pregnancies. Statistical analysis was performed with the Mann-Whitney test. Results Eleven of 20 rabbits aborted (55%). In the remaining nine rabbits 10 of 18 operated fetuses were alive and available for evaluation. In addition to the 10 experimental fetuses 14 control litters were obtained. During cesarean sections outpouched membranes were found to be shrunken. Amniotic fluids of the fetuses were scant, because the mean volumes were measured to be 0.61 ? 0.1 and 0.58 ? 0.1 ml in experimental and control fetuses, respectively. They were not significantly different (p > 0.05). Body weight, lung weight, and lung weight/body weight ratios of live litters were tabulated (Table I). The mean lung weight/body weight ratio of experimental litters was 0.017 ? 0.003, and the range was 0.012 to
38
Kizilcan
et al.
Am J Obstet
Fig. 2. Photomicrograph of control lung. Sections of fully developed eosin stain. Original magnification X ZOO.) Table
I. Results of amniotic
herniation
Controls Amniotic herniation Mann-Whitney U test Significance SW,
procedure
airways. (Hematoxylin
July 1995 Gynecol
and
(mean -+ SD)
37.17 -c 10.66 40.39 2 15.40
0.91 t 0.22
0.025 -c 0.004
0.72
0.017
” 0.34
t
0.003
135 p <7i.05
p cg”o.05
p < 0.05
Body weight; LW, lung weight.
0.020. The mean lung weight/body weight ratio of controls was 0.025 2 0.004, and the range was 0.018 to 0.032. Lung weight/body weight ratios of experimental litters were found to be significantly reduced when compared with those of controls (p < 0.05). However, no significant difference was found in absolute lung weight and body weight determinations Cp > 0.05). On histopathologic examination of the control lungs the airways were lined primarily by cuboidal epithelium. In some regions, however, the epithelium appeared to be flattened. The airways were almost regular in size (Fig. 2). On the other hand, the lungs of experimental fetuses had rather immature structures compared with controls. The size of airways was variable. They were irregular, thick-walled, and highly cellular. Maturation, number, and size of alveoli showed variation in the operated group. In some lungs alveoli were mature (Fig. 3), but in others they were reduced in size and number with thick, hypercellular interalveolar septa (Fig. 4). Pressure studies showed that the mean intraamniotic pressures of unoperated and operated fetuses were 4.94 ? 0.32 cm H,O and 2.17 4 0.73 cm H,O, respectively (Fig. 5). They differed significantly (p < 0.05).
Comment The normal volume of amniotic fluid is primarily determined by the relative rates of fetal intestinal ab-
sorption and fetal urination,13, ” with a substantial contribution from the lung.” Additionally, volume regulation occurs by transmembranous and intramembranous pathways, whereas amniotic fluid is absorbed through amnion-chorion and the fetal surface of placenta, respectively.16 These exchange mechanisms maintain amniotic fluid volume within its normal range. Normal fetal lung development requires a critical amniotic fluid volume, and its paucity results in fetal lung hypoplasia, although the mechanism is not clearly understood. There is evidence that maintenance of normal fluid volume in fetal airways is a factor in lung growth.3 Alterations in the fetus-amniotic fluid exchange may lead to alteration in lung liquid volume. Reduction in the lung liquid volume may also be caused by low amniotic pressure, leading to enhanced fluid in the amniotic cavity.” The pressure of amniotic fluid may, additionally, apply a transpulmonary distending pressure on the developing lung. In our created model amniotic herniation significantly reduced lung weight/body weight ratios in experimental fetuses compared with controls. A lung weight/body weight ratio <0.018, together with morphometric analysis of the lung, has been regarded as a criterion for the diagnosis of lung hypoplasia.” A low lung weight/body weight ratio has indeed been associated with a decreased lung cell population.‘”
Volume 173, Kumber AIII J Obstet Gynecol
1
Kizilcan
Fig. 3. Section from amniotic (Hematoxylin and eosin stain.
Fig. 4. Most immature and number of alveoli Original magnification
histologic dispersed X 200.)
herniation Original
group. Walls magnification
appearance of lung within hypercellular
The experiment was carried out during the last pseudoglandular or early canalicular phases of lung development. “-‘I It is during these periods that airways, particularly the acini with formation of the primary respiratory bronchioles, develop.‘Y Histologically, these phases are characterized by an irregular configuration of the airways separated by relatively thick, sparsely cellular interstitial septa.” Histopathologic findings of a reduced number of alveoli with thicker and hypercellular interalveolar septa in the experimental group suggested that fetal lungs had matured from the pseudoglandular stage but not as fully as had the lungs of the control newborns. Furthermore, in some specimens histologic appearances resemble the lung of rabbit fetus at 23 to 26 days of gestation” (Fig. 4).
between
airways
are thick
and highly
et
al.
39
cellular.
X 100.)
in experimental interstitium.
group (Hematoxylin
showing reduced size and
eosin
stain.
In this experiment intraamniotic pressure was reduced at amniotic herniation, which was consistent with our assumption. Inevitable leakage around the catheter, which should lead to oligohydramnios, prevented us from measuring intraamniotic pressure continuously through the amniotic herniation to the end of pregnancy. Nevertheless, it was suggested that intraamniotic pressure should have been kept reduced until termination of pregnancy, because the amniotic cavity was persistently deprived, to some extent, of uterine tonicity. Preserving the integrity of the fetal membranes while creating a defect in the uterine wall, which should subsequently remain open, allowed the fetus to be able to exchange a normal volume of amniotic fluid including the outpouched fluid also. The residual amniotic
40
Kizilcan
et al. Am J Obstet
Amniotic
July 1995 Gynecol
Pressure
cm I320
Un-operated
50 mm/min
Amniotic herniation
Fig. 5. Mean intraamniotic pressures of unoperated fetuses to 7.2 and 1.9 to 2.5 cm H,O, respectively. Representative
fluid volume at term did not in fact differ between the experimental and control fetuses. This suggested that amniotic herniation did not alter the equilibrium of fluid movements into and out of the amniotic cavity, although the relative rates of contributions from each route may have been changed. Body weight values of operated fetuses did not differ, even a little, from those of controls. Therefore the intrauterine growth of the surviving fetuses was regarded as normal, although herniation of the amniotic membranes caused a high fetal death rate. In contrast to findings of our experiment, low weights of fetuses in the experiments of oligohydramnios with amnionoperitoneal shunting may have been related to the negative balance in the fetusamniotic fluid exchangesz3 On the other hand, the created model resembled the situation in oligohydramnios resulting from ruptured membranes, because some part of the amniotic fluid was outside the uterus. The volume of amniotic fluid remaining inside the uterus was smaller in the operated than in the unoperated group. Therefore in the operated group the fetus was probably subjected to postural changes. Hence this would have led to the lower extremities of the fetus being pushed into the abdomen with consequent abdominothoracic compression.“4, 25 This could possibly explain lung hypoplasia in oligohydramnios within the globular uteri, such as in the human or sheep. However, compression of the fetus in cephalocaudal direction seemed to be unlikely in the tubular rabbit uteri.
with amniotic charts from
herniation each group
ranged from are presented.
4
This study represents a new model of pulmonary hypoplasia concerning the importance of intraamniotic pressure on fetal lung development. Herniation of amniotic membranes through the uterine wall impaired lung development, even without oligohydramnios. REFERENCES
1. Wigglesworth JS, Desai R. Is fetal respiratory function a major determinant of perinatal survival? Lancet 1982;l: 264-7. 2. Fewell JE, Lee CC, Kitterman JA. Effects of phrenic nerve section on the respiratory system of fetal lambs. J Appl Physiol 1981;51:293-7. 3. Fewell JE, Hislop AA, Kitterman JA, Johnson P. Effects of tracheostomy on lung development in fetal lambs. J Appl Physiol 1983;55:1103-8. 4. Harding R, Backing AD, Sigger JN. Influence of upper respiratory tract on lung liquid flow to and from the lungs. J Appl Physiol 1986;61:68-74. 5. Glick PL, Siebert TR, Beniamin DR. Pathophvsiologv < ., -, of congenital diaphragmatic hernia, I: renal enlargement suggests feedback modulation by pulmonary derived renotropins - an unifying hypothesis to explain pulmonary hypoplasia, polyhydramnios, and renal enlargement in the fetus/newborn with congenital diaphragmatic hernia. J Pediatr Surg 1990;25:492-5. Watanabe H. Epidermal growth factor in urine of pregnant women and in amniotic fluid throughout pregnancy. Gynaecol Endocrinol 1990;4:43-50. Post M, Barsomian A, Smith BT. The cellular mechanism of glucocorticoid acceleration of fetal lung maturation: fibroblast-pneumocyte factor stimulates choline-phosphate cytidylyl transferase activity. J Biol Chem 1986;261: 2179-84. Smith BT, Post M. Fibroblast-pneumocyte factor. Am J Physiol 1989;257:L174-8.
Volume 173, Number Am J Obstet Gynecol
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9. Fisk NM, Tannirandorn Y, Nicolini U, Talbert DG, Rodeck CH. Amniotic pressure in disorders of amniotic fluid volume. Obstet Gynecol 1990;76:210-4. 10. Nicolini U, Fisk NM, Talbert DG, et al. Intrauterine manometry: technique and application to fetal pathology. Prenat Diag 1989;9:243-54. 11. Nicolini U, Fisk NM, Rodeck CH, Talbert DG, Wigglesworth JS. Low amniotic pressure-is this the cause of pulmonary hypoplasia? tLM J OBSTET GYNECOL 1989;161: 1098-101. 12. Kizilcan F, Tanyel FC, Buyukpamukcu N, Hicsonmez A. Fetal survival after in-utero experiment in the rabbit. Lab Anim Sci 1994;44:144-7. 13. Seeds AE Jr. Water metabolism of the fetus. AM J OBSTET GYKECOL 1965;92:727-45. 14. Lind T, Kendall A, Hytten FE. The role of the fetus in the formation of the amniotic fluid. J Obstet Gynaecol Br Commonw 1972;79:289-98. 15. Strang LB. Growth and development of the lung-fetal and postnatal. Annu Rev Physiol 1977;39:253-76. 16. Gilbert WM, Bruce RA. Amniotic fluid volume and normal flows to and from the amniotic cavity. Semin Perinatol 1993;17:150-7. 17. Sherer DM, Davis JM, Woods JR Jr. Pulmonary hypoplasia: a review. Obstet Gynecol Surv 1990;45:792-
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41
18. Wigglesworth JS, Desai R. Use of DNA estimation for growth assessment in normal and hypoplastic fetal lungs. Arch Dis Childh 1981;56:601-5. 19. Adzick NS, Harrison MR, Glick PL. Experimental pulmonary hypoplasia and oligohydramnios: relative contributions of lung fluid and fetal breathing movements [Discussion]. J Pediatr Surg 1984;19:658-65. 20. Wigglesworth JS, Desai R. Effects on lung growth of cervical section in the rabbit fetus. Early Hum Dev 1979; 3:51-65. 21. Pringle KC. Human fetal lung development and related animal models. Clin Obstet Gynecol 1986;29:502-13. 22. Ohi R, Suzuki H, Kato T, Kasai M. Development of the lung in fetal rabbits with experimental diaphragmatic hernia. J Pediatr Surg 1976;11:955-9. 23. Nakayama DK, Glick PL, Harrison MR, Villa RL, Noall R. Experimental pulmonary hypoplasia due to oligohydramnios and its reversal by relieving thoracic compression. J Pediatr Surg 1983;18:347-53. 24. Strang LB. Fetal lung liquid: secretion and reabsorption. Physiol Rev 1991;71:991-1016. 25. Harding R, Hooper SB, Dickson KA. A mechanism leading to reduced lung expansion and lung hypoplasia in fetal sheep during oligohydramnios. AM J OBSTET GYNECOL 1990;163:1904-13.
803.
Availability
et al.
back issues
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MD,” Nur @kar,
MD,b Nebil
Blyiikpamukqu,
MD,”
OBJECTIVE: Our purpose was to examine fetal lung development in reduced intraamniotic pressure without amniotic fluid loss. STUDY DESIGN: A circular portion of uterine wall measuring 1 cm in diameter was excised while the chorionic and amniotic membranes were leti intact at the twenty-third day of gestation in New Zealand White rabbits. The chorionic and amniotic membranes herniated spontaneously through the defect. RESULTS: Amniotic pressure was significantly reduced after herniation. Lung weight/body weight ratios at term were significantly reduced in experimental fetuses compared with controls. Residual amniotic fluid volumes at term did not differ. Histopathologic examination of lung specimens showed that fetal lungs had not matured in the experimental group as fully as in the control group. CONCLUSION: This experimental study demonstrated that low amniotic pressure impaired fetal lung development, even without oligohydramnios. (AM J OBSTET GYNECOL 1995; 173:36-41.)
Key words:
Fetus, oligohydramnios,
lung hypoplasia
Pulmonary hypoplasia is one of the major causes of neonatal mortality. Oligohydramnios is the most common predisposing factor, but its pathogenesis remains to be elucidated. It has been widely speculated that lung hypoplasia in this situation is the result of fetal compression by the uterine wall leading to reduced thoracic cavity available for lung growth. This theory is supported by other sequelae of oligohydramnios such as Potter’s facies and limb deformities. However, the mechanism of the selective effect of compression on the lung, rather than other visceral organs, without thoracic cage deformity is not clarified. Alternatively, several other explanations proposing deficiency of fetal breathing movements,‘. ’ internal stenting effect of lung liquid,3s 4 or a possible humoral or amniotic pulmonary growth facto? also exist. However, none of the explanations are entirely satisfactory. On the other hand, amniotic fluid contains pulmonary growth factors such as epidermal growth factor and fibroblast-pneumocyte factor.‘-* Loss of such contents
From the Departments of Pediatric Surgery” and Morphology,” Hacettepe University Faculty of Medicine. Supported by the Scient$c and Technical Research Council of Turkey (TAG 1014). Received for publication August 8, 1994; revised November 17, 1994; accepted November 18, 1994. Reprint requests: F. Cahit Tanyel, MD, Hacettepe Universitesi Tip Fakultesi, Cocuk Cerrahisi Anabilim Dali, 06100, Sihhiye, Ankara, Turkey. Copyright 0 1995 by Mosby-Year Book, Inc. 0002-9378/95 $3.00 f 0 6/l/62323 36
through leakage or altered fetus-amniotic fluid exchanges may also be responsible in the pathogenesis of pulmonary hypoplasia associated with oligohydramnios. In pregnancies complicated by severe oligohydramnios intraamniotic pressure has been found to be reduced,‘, I0 accordingly, the low amniotic pressure has been proposed as a cause of lung hypoplasia associated with oligohydramnios.” However, no experimental studies supporting this concept have been carried out. Therefore an in utero experimental study was planned to evaluate the effect of reduced intraamniotic pressure on fetal lung development while keeping the amniotic cavity intact with the aim at preserving natural volume and exchange of amniotic fluid.
Material and methods Assuming that herniation of the amniotic sac through the uterine wall would change the intraamniotic pressure, an experimental model was created. The experiment began after institutional care and use committee approval was received, and guidelines for responsible use with the highest standards of humane care were strictly followed during the study. In the experimental group 20 time-mated pregnant New Zealand White rabbits underwent operation on day 23 of pregnancy (normal 31 to 32 day gestation) according to the general principles.‘” After a 12-hour fasting period rabbits were premeditated with ketamine (10 mg/kg) intravenously. Anesthesia was maintained with 2% to 3% halothane in oxygen by mask inhalation.
Volume 173, Number Am J Obstet Gynecol
Kizilcan
1
Fig. 1. Schematic representation
A 0.5 gm dose of sulbactam-ampicillin was given intramuscularly at the beginning of the operation. The abdomen was shaved and prepared with povidoneiodine. While aseptic practices were followed, the uterus was delivered through the midline vertical incision without kinking uterine vessels. The uterine wall covering one of the fetuses in each horn was operated on. A 5-O silk purse-string suture with a diameter of 1.5 cm was placed on the uterine wall and tied without constricting the circle. With the chorionic and amniotic membranes left intact, a circular portion of the uterine wall measuring 1 cm in diameter was excised, resulting in a defect in the uterine wall that was surrounded by the purse-string suture. The chorionic and amniotic membranes herniated spontaneously through the defect in the uterine wall. The herniated sacs moved outward and inward while the uterus contracted and relaxed. Hence the amniotic sac was partially outpouched and devoid of uterine support (Fig. 1). Cesarean sections were performed on the thirty-first day of gestation, and live births with intact membranes were studied. Fetuses were extracted from the uterus with their amniotic sacs. Amniotic membranes were torn, and fluid was then collected and measured. After death the body weight and lung weight of the fetuses were measured and lung weight/body weight ratios were calculated. The lungs were histopathologically examined. Unoperated fetuses served as controls. Statistical analysis was performed with the Mann-Whitney test. Another group of rabbits was included in pressure measurement studies at 23 days of gestation. Intraamniotic pressures of 10 unoperated fetuses in five rabbits were measured extraabdominally under conditions similar to those of the experimental group. Pressure measurements were also carried out in another 10
of herniation
et al.
37
of amniotic membranes.
fetuses of five rabbits after amniotic herniation. Pressures were measured with a 20-gauge needle connected to a silicone strain-gauge transducer (Statham P 23 AC, Gould Instrument Systems, Inc., Valley View, Ohio) by a saline solution-filled catheter. The output of the transducer was displayed on a chart recorder (Grass Polygraph 7 DAE, Grass Instrument Co., Quincy, Mass.), whereby scoring was calibrated with a saline solutionfilled tube manometer. The gain of the recorder was amplified to allow pressure measurements of a minimum of 0.3 cm H,O. Working of the system was checked by inserting the 20-gauge needle into the rubber cuff connected to a tube manometer. Intraamniotic pressures were then measured by placing the needle in the amniotic cavity through the uterine wall and recorded for 60 to 90 seconds after stable readings had been established. The values of intraamniotic pressure were compared between unoperated and herniated pregnancies. Statistical analysis was performed with the Mann-Whitney test. Results Eleven of 20 rabbits aborted (55%). In the remaining nine rabbits 10 of 18 operated fetuses were alive and available for evaluation. In addition to the 10 experimental fetuses 14 control litters were obtained. During cesarean sections outpouched membranes were found to be shrunken. Amniotic fluids of the fetuses were scant, because the mean volumes were measured to be 0.61 ? 0.1 and 0.58 ? 0.1 ml in experimental and control fetuses, respectively. They were not significantly different (p > 0.05). Body weight, lung weight, and lung weight/body weight ratios of live litters were tabulated (Table I). The mean lung weight/body weight ratio of experimental litters was 0.017 ? 0.003, and the range was 0.012 to
38
Kizilcan
et al.
Am J Obstet
Fig. 2. Photomicrograph of control lung. Sections of fully developed eosin stain. Original magnification X ZOO.) Table
I. Results of amniotic
herniation
Controls Amniotic herniation Mann-Whitney U test Significance SW,
procedure
airways. (Hematoxylin
July 1995 Gynecol
and
(mean -+ SD)
37.17 -c 10.66 40.39 2 15.40
0.91 t 0.22
0.025 -c 0.004
0.72
0.017
” 0.34
t
0.003
135 p <7i.05
p cg”o.05
p < 0.05
Body weight; LW, lung weight.
0.020. The mean lung weight/body weight ratio of controls was 0.025 2 0.004, and the range was 0.018 to 0.032. Lung weight/body weight ratios of experimental litters were found to be significantly reduced when compared with those of controls (p < 0.05). However, no significant difference was found in absolute lung weight and body weight determinations Cp > 0.05). On histopathologic examination of the control lungs the airways were lined primarily by cuboidal epithelium. In some regions, however, the epithelium appeared to be flattened. The airways were almost regular in size (Fig. 2). On the other hand, the lungs of experimental fetuses had rather immature structures compared with controls. The size of airways was variable. They were irregular, thick-walled, and highly cellular. Maturation, number, and size of alveoli showed variation in the operated group. In some lungs alveoli were mature (Fig. 3), but in others they were reduced in size and number with thick, hypercellular interalveolar septa (Fig. 4). Pressure studies showed that the mean intraamniotic pressures of unoperated and operated fetuses were 4.94 ? 0.32 cm H,O and 2.17 4 0.73 cm H,O, respectively (Fig. 5). They differed significantly (p < 0.05).
Comment The normal volume of amniotic fluid is primarily determined by the relative rates of fetal intestinal ab-
sorption and fetal urination,13, ” with a substantial contribution from the lung.” Additionally, volume regulation occurs by transmembranous and intramembranous pathways, whereas amniotic fluid is absorbed through amnion-chorion and the fetal surface of placenta, respectively.16 These exchange mechanisms maintain amniotic fluid volume within its normal range. Normal fetal lung development requires a critical amniotic fluid volume, and its paucity results in fetal lung hypoplasia, although the mechanism is not clearly understood. There is evidence that maintenance of normal fluid volume in fetal airways is a factor in lung growth.3 Alterations in the fetus-amniotic fluid exchange may lead to alteration in lung liquid volume. Reduction in the lung liquid volume may also be caused by low amniotic pressure, leading to enhanced fluid in the amniotic cavity.” The pressure of amniotic fluid may, additionally, apply a transpulmonary distending pressure on the developing lung. In our created model amniotic herniation significantly reduced lung weight/body weight ratios in experimental fetuses compared with controls. A lung weight/body weight ratio <0.018, together with morphometric analysis of the lung, has been regarded as a criterion for the diagnosis of lung hypoplasia.” A low lung weight/body weight ratio has indeed been associated with a decreased lung cell population.‘”
Volume 173, Kumber AIII J Obstet Gynecol
1
Kizilcan
Fig. 3. Section from amniotic (Hematoxylin and eosin stain.
Fig. 4. Most immature and number of alveoli Original magnification
histologic dispersed X 200.)
herniation Original
group. Walls magnification
appearance of lung within hypercellular
The experiment was carried out during the last pseudoglandular or early canalicular phases of lung development. “-‘I It is during these periods that airways, particularly the acini with formation of the primary respiratory bronchioles, develop.‘Y Histologically, these phases are characterized by an irregular configuration of the airways separated by relatively thick, sparsely cellular interstitial septa.” Histopathologic findings of a reduced number of alveoli with thicker and hypercellular interalveolar septa in the experimental group suggested that fetal lungs had matured from the pseudoglandular stage but not as fully as had the lungs of the control newborns. Furthermore, in some specimens histologic appearances resemble the lung of rabbit fetus at 23 to 26 days of gestation” (Fig. 4).
between
airways
are thick
and highly
et
al.
39
cellular.
X 100.)
in experimental interstitium.
group (Hematoxylin
showing reduced size and
eosin
stain.
In this experiment intraamniotic pressure was reduced at amniotic herniation, which was consistent with our assumption. Inevitable leakage around the catheter, which should lead to oligohydramnios, prevented us from measuring intraamniotic pressure continuously through the amniotic herniation to the end of pregnancy. Nevertheless, it was suggested that intraamniotic pressure should have been kept reduced until termination of pregnancy, because the amniotic cavity was persistently deprived, to some extent, of uterine tonicity. Preserving the integrity of the fetal membranes while creating a defect in the uterine wall, which should subsequently remain open, allowed the fetus to be able to exchange a normal volume of amniotic fluid including the outpouched fluid also. The residual amniotic
40
Kizilcan
et al. Am J Obstet
Amniotic
July 1995 Gynecol
Pressure
cm I320
Un-operated
50 mm/min
Amniotic herniation
Fig. 5. Mean intraamniotic pressures of unoperated fetuses to 7.2 and 1.9 to 2.5 cm H,O, respectively. Representative
fluid volume at term did not in fact differ between the experimental and control fetuses. This suggested that amniotic herniation did not alter the equilibrium of fluid movements into and out of the amniotic cavity, although the relative rates of contributions from each route may have been changed. Body weight values of operated fetuses did not differ, even a little, from those of controls. Therefore the intrauterine growth of the surviving fetuses was regarded as normal, although herniation of the amniotic membranes caused a high fetal death rate. In contrast to findings of our experiment, low weights of fetuses in the experiments of oligohydramnios with amnionoperitoneal shunting may have been related to the negative balance in the fetusamniotic fluid exchangesz3 On the other hand, the created model resembled the situation in oligohydramnios resulting from ruptured membranes, because some part of the amniotic fluid was outside the uterus. The volume of amniotic fluid remaining inside the uterus was smaller in the operated than in the unoperated group. Therefore in the operated group the fetus was probably subjected to postural changes. Hence this would have led to the lower extremities of the fetus being pushed into the abdomen with consequent abdominothoracic compression.“4, 25 This could possibly explain lung hypoplasia in oligohydramnios within the globular uteri, such as in the human or sheep. However, compression of the fetus in cephalocaudal direction seemed to be unlikely in the tubular rabbit uteri.
with amniotic charts from
herniation each group
ranged from are presented.
4
This study represents a new model of pulmonary hypoplasia concerning the importance of intraamniotic pressure on fetal lung development. Herniation of amniotic membranes through the uterine wall impaired lung development, even without oligohydramnios. REFERENCES
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9. Fisk NM, Tannirandorn Y, Nicolini U, Talbert DG, Rodeck CH. Amniotic pressure in disorders of amniotic fluid volume. Obstet Gynecol 1990;76:210-4. 10. Nicolini U, Fisk NM, Talbert DG, et al. Intrauterine manometry: technique and application to fetal pathology. Prenat Diag 1989;9:243-54. 11. Nicolini U, Fisk NM, Rodeck CH, Talbert DG, Wigglesworth JS. Low amniotic pressure-is this the cause of pulmonary hypoplasia? tLM J OBSTET GYNECOL 1989;161: 1098-101. 12. Kizilcan F, Tanyel FC, Buyukpamukcu N, Hicsonmez A. Fetal survival after in-utero experiment in the rabbit. Lab Anim Sci 1994;44:144-7. 13. Seeds AE Jr. Water metabolism of the fetus. AM J OBSTET GYKECOL 1965;92:727-45. 14. Lind T, Kendall A, Hytten FE. The role of the fetus in the formation of the amniotic fluid. J Obstet Gynaecol Br Commonw 1972;79:289-98. 15. Strang LB. Growth and development of the lung-fetal and postnatal. Annu Rev Physiol 1977;39:253-76. 16. Gilbert WM, Bruce RA. Amniotic fluid volume and normal flows to and from the amniotic cavity. Semin Perinatol 1993;17:150-7. 17. Sherer DM, Davis JM, Woods JR Jr. Pulmonary hypoplasia: a review. Obstet Gynecol Surv 1990;45:792-
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