Swallowing, urine flow, and amniotic fluid volume responses to prolonged hypoxia in the ovine fetus

Thurlow, Ralph W; Brace, Robert A.

doi:10.1067/s0002-9378(03)00494-0

Cited by 26 publications

(42 citation statements)

References 27 publications

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“…The largest previously published rate known to us is about 1 l/day, although this was stated to be somewhat of an underestimate (17). Each of the present seven fetuses was shown to be capable of an amniochorionic absorption rate in excess of 4.5 l/day.…”

Section: Discussionmentioning

confidence: 85%

Function curve of the membranes that regulate amniotic fluid volume in sheep

Faber

Anderson

Hohimer

et al. 2005

American Journal of Physiology-Heart and Circulatory Physiology

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-Seven singleton 120-day fetal lambs were prepared with a shunt from the lung to the gastric end of the esophagus, a bladder catheter, and multiple amniotic fluid and vascular catheters. The urachus was ligated. Beginning 7 days later, amniotic fluid volumes were determined by drainage, followed by replacement with 1 liter of lactated Ringer (LR) solution. Urine flow into the amnion was measured continuously. In 14 of 27 experiments, amniotic fluid volumes were determined again 2 days after the inflow into the amnion had consisted of urine only and in 13 experiments after the inflow of urine had been supplemented by an intraamniotic infusion of LR solution. Intramembranous absorption was calculated from the inflows and the changes in volume between the beginning and end of each experiment. The relations between absorption rate and amniotic fluid volume, the "function curves," were highly individual. Urine production during the infusion of LR solution did not decrease, fetal plasma renin activity decreased (P Ͻ 0.001), and amniotic fluid volume increased by 140% [SE (27%), P Ͻ 0.005], but the increase in the amniochorionic absorption rate of 411% [SE (48%), P Ͻ 0.001] was greater (P Ͻ 0.005) than the increase in volume. Each of the seven fetuses was proven capable of an average intramembranous absorption rate that exceeded 4.5 liters of amniotic fluid per day. During the infusion of LR solution, the increase in the rate of absorption matched the rate of infusion (both in ml/h), with a regression coefficient of 0.75 (P Ͻ 0.001). Thus, even for large amniotic fluid volumes, volume is not limited by the absorptive capacity of the amniochorion, and, at least in these preparations, the position of the function curve and not the natural rate of inflow was the major determinant of resting amniotic fluid volume. fetus; polyhydramnios; amniochorion; intramembranous absorption DURING THE LAST 50 DAYS of the 150-day gestation period of fetal sheep, the production rate of amniotic fluid is on the order of 500 ml/day (6, 19). Under steady-state conditions, the rate of absorption will be the same. Because the volume of amniotic fluid is relatively constant over this period of time, it follows that this volume is almost perfectly controlled.Production of amniotic fluid results from fetal urination and lung fluid secretion (5, 13, 16). There is good evidence that urine production and lung fluid flow do not serve to regulate amniotic fluid volume because neither lung fluid flow nor urine production properly respond to induced changes in volume (9, 11). Thus, although substances secreted in these fluids may affect volume regulation (14), whatever regulatory mechanism exists must modulate the rate of fluid elimination to adapt to changes in fluid production. Fluid elimination consists of swallowing by the fetus and of absorption of fluid by the amniochorion, the membrane that surrounds the amniotic cavity. Amniochorionic absorption of amniotic fluid ["intramembranous absorption" (7, 14)] proceeds by means of an as-yetunresolved mec...

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Section: Discussionmentioning

confidence: 85%

Function curve of the membranes that regulate amniotic fluid volume in sheep

Faber

Anderson

Hohimer

et al. 2005

American Journal of Physiology-Heart and Circulatory Physiology

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“…Water injected into the amniotic fluid will enter the fetus, even in the presence of esophageal ligation (9), as will arginine vasopressin (10). Intramembranous absorption will increase in response to severe hypoxia (14,16) or in response to excess volumes of Ringer solution (6). Increases in intramembranous absorption would serve to limit increases in amniotic fluid volume.…”

Section: Dilution Periodmentioning

confidence: 99%

“…Intramembranous absorption refers to the movement of fluid from the amniotic fluid into the fetal blood perfusing the amnion and chorion (11) and ranges from ϳ250 to 500 ml/day (8). While intramembranous absorption rate will change in response to a variety of fetal perturbations including hypoxia (13,16), esophageal ligation (9), and changes in the osmotic gradient between the fetal plasma and the amniotic fluid (7), the basis for any regulatory control over the intramembranous absorption rate is unknown.…”

mentioning

confidence: 99%

Intramembranous absorption rate is unaffected by changes in amniotic fluid composition

Anderson¹,

Yang²,

Hohimer³

et al. 2005

American Journal of Physiology-Renal Physiology

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Experiments were performed to determine the effect of amniotic fluid dilution on the rate of intramembranous absorption. Seven fetal sheep at 118 days gestation were instrumented with a shunt between the trachea and esophagus and arterial and venous vascular catheters. In addition, the urachus of the fetal bladder was ligated, and a catheter was placed in the bladder. Ligation of the urachus does not interfere with urine flow into the amnion. After 5 days of recovery, fetuses were randomly assigned to one of two protocols; all fetuses completed both protocols. In the fetuses in the control period, continuous urine flow measurement was begun. In the fetuses assigned to the isovolumic dilution protocol, continuous urine flow measurement was also begun and, in addition, amniotic fluid was continually exchanged with lactated Ringer solution on an isovolumic basis. After 3-4 days, fetal blood pressures and amniotic fluid volumes were determined. Amniotic fluid volumes were determined by drainage. Each fetus was then assigned to the remaining protocol. The presence of the tracheal-esophageal shunt and the ligation of the urachus allowed the rate of intramembranous absorption to be calculated. Isovolumic exchange showed no effect on fetal vascular pressures, blood-gas values, or urine production. We could demonstrate no effect of isovolumic dilution of amniotic fluid on its volume. However, we were able to demonstrate an inverse relationship between amniotic fluid volume and intramembranous absorption (P Ͻ 0.02).fetal fluid balance THE RATE OF CHANGE IN VOLUME of amniotic fluid is determined by the difference between the inflow of fluid and the outflow of fluid into the amniotic space. The most significant sources of fluid are fetal urine and lung fluid secretions, whereas the loss of fluid occurs primarily through swallowing and intramembranous absorption (1)(2)(3)8). Intramembranous absorption refers to the movement of fluid from the amniotic fluid into the fetal blood perfusing the amnion and chorion (11) and ranges from ϳ250 to 500 ml/day (8). While intramembranous absorption rate will change in response to a variety of fetal perturbations including hypoxia (13, 16), esophageal ligation (9), and changes in the osmotic gradient between the fetal plasma and the amniotic fluid (7), the basis for any regulatory control over the intramembranous absorption rate is unknown.The present series of experiments was designed to test the hypothesis that the rate of intramembranous absorption is independent of the composition of the amniotic fluid. The contribution of intramembranous absorption was isolated by continually measuring urine flow and eliminating the contributions of lung fluid secretion and fetal swallowing through creation of a fistula between the esophagus and the trachea. Under these conditions, the composition of the amniotic fluid is dependent on the contribution of the urine flow and the contribution of diffusional exchange between the amniotic fluid and the fetal plasma. In an attempt to determine whether the ...

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“…Although lung fluid, urine flow, and swallowing are regulated by the fetus, esophageal ligation does not routinely result in increased amniotic fluid volume (16), and neither lung fluid production nor swallowing is necessary for an adequate homeostatic response to abnormal production of amniotic fluid (7). Furthermore, lung fluid production and urine flow often do not change or change in the opposite direction necessary to restore amniotic fluid volume (6,12,17,19).…”

mentioning

confidence: 99%

Regulatory response to washout of amniotic fluid in sheep

Yang¹,

Davis²,

Hohimer³

et al. 2005

American Journal of Physiology-Heart and Circulatory Physiology

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To test the hypothesis that a substance present in the amniotic fluid could serve as a regulator of amniotic fluid volume, we drained and discarded amniotic fluid while replacing it with lactated Ringer solution that was isotonic to amniotic fluid. Seven ewes with singleton fetuses at 119 +/- 1 days of gestation (mean +/- SE) were instrumented with multiple indwelling catheters in the pedal artery, pedal vein, and amniotic cavity. During the exchange periods, an average of 3,019 +/- 171 ml/day of lactated Ringer solution was infused into the amniotic cavity while an equal amount of amniotic fluid was pumped out and discarded. During the control period, amniotic fluid composition and volume were not altered. Exchange and control periods started with the same amniotic fluid volume, lasted 3 or 4 days, and were randomized with regard to order. Amniotic fluid volume measured by vacuum drainage was 556 +/- 98 ml at the end of the control period and 986 +/- 209 ml (P = 0.03) at the end of the exchange period. Fetal arterial blood gases, hemodynamic parameters and the osmolality gradient between fetal plasma and amniotic fluid were not altered by the exchange process. A linear relationship between the control amniotic fluid volume and the volume at the end of the exchange period (P = 0.003) suggests that the animals with larger control volumes responded to isovolumic dilution with a larger volume increase. We conclude that amniotic fluid may contain a substance that regulates amniotic volume.

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Swallowing, urine flow, and amniotic fluid volume responses to prolonged hypoxia in the ovine fetus

Cited by 26 publications

References 27 publications

Function curve of the membranes that regulate amniotic fluid volume in sheep

Function curve of the membranes that regulate amniotic fluid volume in sheep

Intramembranous absorption rate is unaffected by changes in amniotic fluid composition

Regulatory response to washout of amniotic fluid in sheep

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