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M G Ross

Publications and source records attributed to M G Ross.

At least 55 records · Page 3Linked to original sources
PubMed1999

Nitric oxide modulates spontaneous swallowing behavior in near-term ovine fetus.

Human and ovine fetuses demonstrate an enhanced rate of swallowing, an activity critical for amniotic fluid regulation. Fetal swallowing may be modulated by both systemic and central factors. Nitric oxide (NO) is a central neuromodulator that has been localized to brain regions regulating thirst and swallowing. We sought to determine if NO contributes to the regulation of spontaneous ovine fetal swallowing. Six time-dated pregnant ewes with singleton fetuses (129 +/- 1 day) were chronically prepared with fetal vascular and lateral ventricle catheters and electrocorticogram (ECoG) and esophageal electromyogram electrodes. After a 2-h control period, fetuses were given lateral ventricle injection of NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) and monitored for 2 h. NO precursor L-arginine was then injected into the lateral ventricle, and fetuses were monitored for a final 2 h. All fetuses received an additional control study of fetal swallowing before and after lateral ventricle injection of artificial cerebrospinal fluid (aCSF). Data were analyzed with repeated-measures ANOVA and paired t-test (P < 0.05). Suppression of a central NO with central L-NAME significantly reduced mean (+/-SE) spontaneous fetal swallowing (1.2 +/- 0.1-0.6 +/- 0.1 swallows/min low-voltage ECoG; P < 0.01). Restoration of central NO by L-arginine significantly increased fetal swallowing to pre-L-NAME levels (1.2 +/- 0.1 swallows/min low voltage). There were no changes in fetal swallowing during the control study of aCSF. Fetal ECoG activity and blood pressure did not change during the study or control aCSF injection. We conclude that NO is an important neuromodulator of fetal swallowing activity. Central NO synthase activity may contribute to the heightened level of spontaneous fetal swallowing and thus amniotic fluid regulation.

Animals↗
PubMed1999

Fetal diuretic responses to maternal hyponatremia: contribution of placental sodium gradient.

Maternal hyponatremia induces fetal hyponatremia and increased fetal urine flow. We sought to examine the relative contributions of the placental Na(+) gradient vs. the absolute decrease in fetal plasma Na(+) in the fetal diuretic response to hyponatremia. Seven ewes with singleton fetuses (130 +/- 2 days) were prepared. Ewes received intravenous 1-desamino-8-D-arginine vasopressin (20 microg) and warm tap water (2 liters). Maternal plasma Na(+) was decreased to achieve two levels of maternal hyponatremia. Maternal and fetal blood volume were measured with radiolabeled red blood cells. In response to the first decrease in maternal plasma Na(+), fetal plasma Na(+) did not change initially. Subsequently, fetal plasma Na(+) decreased, normalizing the gradient. The second decrease in maternal plasma Na(+) similarly induced a reduced and normalized placental gradient at lower fetal plasma Na(+) values. Fetal urine flow increased in direct proportion to the degree of fetal hyponatremia (13, 38, 63, 100%, respectively). Maternal, although not fetal, blood volume significantly increased in response to hyponatremia. These results suggest that chronic fetal hyponatremia will result in a persistent diuresis, despite placental equilibration.

Animals↗
PubMed1998

Regulation of hypothalamic arginine vasopressin messenger ribonucleic acid and pituitary arginine vasopressin content in fetal sheep: effects of acute tonicity alterations and fetal maturation.

OBJECTIVE: Fetal arginine vasopressin contributes to fetal and amniotic fluid homeostasis by increasing water resorption in the kidney and, at higher plasma levels, circulatory homeostasis by vasopressor effects. In utero and neonatal exposure of rat pups to prolonged alterations in plasma osmolality may permanently alter (imprint) pituitary arginine vasopressin content and adult responses to osmotic challenges. Our objective was to investigate fetal developmental changes and the impact of maternal dehydration and maternal hyponatremia on fetal pituitary arginine vasopressin content and hypothalamic arginine vasopressin messenger ribonucleic acid expression. STUDY DESIGN: Ten pregnant ewes with singleton fetuses (135 +/- 1 day) were chronically prepared with maternal vascular catheters. Ewes were assigned to receive water deprivation (n = 4) [desamino, D-Arg8]-arginine vasopressin-induced plasma hyponatremia (n = 3), or 4 days of observation (n = 3). Three additional pregnant ewes with preterm (110 +/- 1 day) singleton fetuses were also included for a study of maturational effects. Daily maternal blood samples were analyzed for determination of plasma arginine vasopressin, electrolytes, and osmolality. After the study protocol, fetuses were operatively delivered, umbilical blood samples obtained, and fetuses put to death for pituitary and hypothalamic tissues. Pituitary arginine vasopressin content was determined by radioimmunoassay, and hypothalamus arginine vasopressin messenger ribonucleic acid expression was detected by Northern blotting. RESULTS: Dehydration significantly (P < .05) increased, and hyponatremia significantly decreased maternal plasma sodium concentration compared with controls. Fetal plasma sodium concentration significantly changed in parallel with maternal values (dehydration: 139 +/- 1 to 150 +/- 1 mEq/L; hyponatremia: 138 +/- 1 to 128 +/- 5 mEq/L). Fetal hypothalamic arginine vasopressin messenger ribonucleic acid expression and pituitary content did not change in relation to these relatively acute alterations in plasma tonicity. However, among all animals, arginine vasopressin messenger ribonucleic acid expression was significantly negatively correlated with pituitary arginine vasopressin content (r2 = 0.563; P = .02). Arginine vasopressin messenger ribonucleic acid expression was significantly lower in both preterm and near-term fetuses (P < .05) than that in the maternal ewe, although pituitary arginine vasopressin content (in micrograms per milligram of protein) was significantly greater in preterm fetuses (P < .01, vs maternal; P < .05, vs near term). CONCLUSIONS: The significant inverse relation between arginine vasopressin content and arginine vasopressin messenger ribonucleic acid suggests a dynamic arginine vasopressin synthesis-content feedback relationship is functional in the near-term fetus. Although relatively acute periods of maternal hypertonicity or hypotonicity do not alter fetal pituitary arginine vasopressin content or hypothalamic arginine vasopressin messenger ribonucleic acid expression, longer-term plasma tonicity alterations may potentially have an impact on the fetal arginine vasopressin hypothalamic-pituitary axis.

Animals↗
PubMed1998

Objective definition of shoulder dystocia: a prospective evaluation.

OBJECTIVE: The current study was undertaken to validate the objective definition of shoulder dystocia in a prospectively evaluated group of patients. STUDY DESIGN: Selected vaginal deliveries from January 1995 to December 1996 (N = 722) were evaluated for head-to-body delivery time and use of ancillary obstetric maneuvers. Charts were reviewed for perinatal and outcome data. RESULTS: Ninety-nine deliveries were complicated by shoulder dystocia and 623 deliveries had no shoulder dystocia. The objective definition described infants with lower 1-minute Apgar scores and increased birth weight. All fetal injuries were in the shoulder dystocia group. Duration of the second stage was significantly associated with a diagnosis of shoulder dystocia. The risk of shoulder dystocia was increased with maternal diabetes, but it was not correlated with birth weight in diabetics. CONCLUSIONS: The objective definition of shoulder dystocia identified a group of patients with an increased birth weight and risk of fetal injuries. The use of an objective definition will assist the evaluation of prophylactic and treatment proposals for shoulder dystocia.

Adult↗
PubMed1998

Human amniotic fluid mathematical model: determination and effect of intramembranous sodium flux.

OBJECTIVE: A recently described mathematical model of human amniotic fluid dynamics used known and estimated rates of fetal fluid production (lung liquid and urine) and composition (osmolality) to enable calculation of previously unmeasured routes of amniotic fluid resorption, including fetal swallowing and intramembranous (across the amnion) water flow. This "osmolar" model assumed that only free water resorption occurred across the intramembranous route. We hypothesized that intramembranous flow also may include solutes and electrolytes because significant concentration gradients exist between amniotic fluid and fetal plasma. We used mass balance analysis to determine the direction and magnitude of intramembranous sodium flux and to assess the ability of a newly described "sodium" model to predict changes in amniotic fluid volume in response to changes in intramembranous electrolyte flow. Mathematical modeling was used to predict changes in amniotic fluid volume in response to changes in intramembranous electrolyte flow. STUDY DESIGN: Model predictions were calculated using published values for human amniotic fluid and fetal urine composition and volume. Ovine studies were used to derive lung fluid volumes and composition. Fetal swallowing and intramembranous flow were independently determined using net amniotic fluid osmolar (osmolality model) and sodium (sodium model) balance. Differences between osmolality and sodium model predictions were normalized to calculate the net intramembranous sodium flux, assuming a net balance of intramembranous osmotic solute flow. RESULTS: Both sodium and osmolality models predicted swallowed volume to be greater than intramembranous flow until 28 to 32 weeks' gestation, after which the relationship reversed. However, the sodium model predicted greater intramembranous flow and lower swallowing rates compared with the osmolality model at all gestational ages. Osmolar mass balance required daily intramembranous sodium flux into the amniotic fluid, which increased with gestational age. Furthermore, assuming stable swallowing and intramembranous water flow, the model predicts that 5% increases or decreases in amniotic fluid solute concentrations caused by intramembranous flux result in polyhydramnios or oligohydramnios, respectively. CONCLUSION: Sodium and osmolality models demonstrate similarities in determinations of amniotic fluid dynamics. However, mass balance equations demonstrate a net intramembranous flow of sodium into the amniotic fluid under normal conditions. Mathematical modeling suggests that small alterations in daily intramembranous sodium flux may evoke large changes in amniotic fluid volume.

Amniotic Fluid↗
PubMed1998

Fos response of fetal sheep anterior circumventricular organs to osmotic challenge in late gestation.

We hypothesized that the anterior circumventricular organs (ACVO) and the supraoptic (SON) and hypothalamic paraventricular nuclei (PVN), among other structures that play a role in sensing extracellular body fluid volume and composition in postnatal animals (as demonstrated by Fos protein production by the immediate-early gene c-fos), would show similar activation in fetal sheep during an osmotic challenge. The brains of 10 fetal sheep [6 treated, 4 controls; 129-131 days of gestational age (dGA) = 0.87 gestation] were immunostained for Fos. Seventy-five minutes before tissue collection the dams were given intravenous 20% mannitol (1 ml . min-1 . kg-1 for 10 min). Subsequently, the ACVO, SON, and PVN were scored for the amount of neuronal Fos immunostaining. The subfornical organ (SFO; 24.5 +/- 9.0 vs. 1.7 +/- 1.2), the organum vasculosum of the lamina terminalis (OVLT; 26.8 +/- 5.6 vs. 7.0 +/- 2.0), the SON (39.8 +/- 3.0 vs. 0.15 +/- 0.1), and the PVN (59.8 +/- 7.9 vs. 0.7 +/- 0.7) had increases (P < 0.05) in the average number of Fos-positive cells per field compared with controls, whereas the median preoptic nucleus did not. Double immunostaining for Fos and arginine vasopressin (AVP) or oxytocin (OT) indicated that AVP- but not OT-immunopositive neurons in SON and PVN respond to osmotic challenge. These results demonstrate that the SFO, OVLT, SON, and PVN are activated by osmotic challenge in fetal sheep at 130 dGA.

Animals↗
PubMed1998

Development of ingestive behavior.

Swallowing represents a primary physiological function that provides for the ingestion of food and fluid. In precocial species, swallowing activity likely develops in utero to provide for a functional system during the neonatal period. The chronically instrumented ovine fetal preparation has provided the opportunity for recent advances in understanding the regulation of in utero swallowing activity. The near-term ovine fetus swallows fluid volumes (100-300 ml/kg) that are markedly greater, per body weight, than that of the adult (40-60 ml/kg). Spontaneous in utero swallowing and ingestive behavior contribute importantly to the regulation of amniotic fluid volume and composition, the acquisition and potential recirculation of solutes from the fetal environment, and the maturation of the fetal gastrointestinal tract. Fetal swallowing activity is influenced by fetal maturation, neurobehavioral state alterations, and the volume of amniotic fluid. Furthermore, intact dipsogenic mechanisms (osmolality, angiotensin II) have been demonstrated in the near-term ovine fetus. It remains unknown to what degree, if any, fetal swallowing may be influenced by nutrient appetite, salt appetite, or taste. Nevertheless, the development of dipsogenic and additional regulatory mechanisms for ingestive behavior occurs during fetal life and may be susceptible to changes in the pregnancy environment. This review describes what is currently known regarding the in utero development of ingestive behavior and the importance of this activity for fetal and perhaps ultimately adult fluid homeostasis.

Aging↗
PubMed1997

Anticholinergic suppression of ovine fetal swallowing activity.

OBJECTIVE: Fetal swallowing contributes importantly to amniotic fluid volume regulation as the primary route of fluid resorption, reaching 500 to 1000 ml/day near term. Near-term ovine fetal swallowing activity occurs predominantly during low-voltage electrocortical activity. In view of the potential to pharmacologically alter electrocortical activity, we hypothesized that fetal administration of a centrally acting cholinergic antagonist may be used to modulate fetal swallowing activity. To explore cholinergic modulation of swallowing activity, we examined fetal swallowing and electrocortical activity in response to central and peripheral cholinergic suppression by atropine sulfate. STUDY DESIGN: Singleton ovine fetuses (n = 6) were chronically prepared with vascular catheters and thyrohyoid, nuchal, and thoracic esophageal electromyogram and biparietal electrocortical electrodes. Swallowing and electrocortical activity were monitored for 2 hours before and after intravenous injection (1 ml of 0.15 mol/L sodium chloride) of atropine sulfate (1 mg/kg). On a subsequent day an identical study was performed with use off atropine methyl nitrate (3 mg/kg), an atropine analog that does not cross the blood-brain barrier. RESULTS: Atropine sulfate decreased low-voltage electrocortical activity (56% +/- 5% to 14% +/- 4%), increased high-voltage electrocortical activity (40% +/- 5% to 81% +/- 5%), and did not change intermediate electrocortical activity (4% +/- 1% to 5% +/- 1%). Fetal swallowing activity decreased from 46 +/- 12 to 12 +/- 2 swallows per hour after atropine sulfate administration. Atropine methyl nitrate had no discernible effect on either fetal electrocortical or swallowing activity. Fetal arterial pressure, plasma osmolality, pH, PCO2, and PO2 did not change. CONCLUSIONS: Central cholinergic antagonism suppresses low-voltage fetal electrocortical and swallowing activity in the ovine fetus. Studies exploring spontaneous or induced fetal swallowing should consider the behavioral state of the fetus when conclusions are drawn about changes in the swallowing activity.

Animals↗
PubMed1997

Three-dimensional method for determination of amniotic fluid volume in intrauterine pockets.

BACKGROUND: Although current ultrasound techniques provide a linear (amniotic fluid index; AFI) or two-dimensional area index of amniotic fluid (AF), these indices have limited correlation with actual AF volume. We sought to quantify the three-dimensional volume of ultrasound-identified AF pockets, as assessed by the AFI and two-dimensional area methods. The BVI 2500 (Bladder Volume Instrument 2500; Diagnostic Ultrasound Corp., Redmond, WA) has been used to quantify the volume of residual urine in the bladder. INSTRUMENT AND METHOD: The BVI 2500 (Diagnostic Ultrasound Corp.) ultrasound uses a rotating 2-MHz transducer, computer-defined fluid interface, and computer integration of 12 cross-sectional images to calculate three-dimensional fluid volume. After providing written informed consent, 14 term pregnant patients (36-42 weeks) were evaluated using the BVI 2500 and an Ultramark 8 sector scan (Advanced Technology Laboratory, Bothell, WA). The largest vertical fluid pocket in each quadrant of the abdomen was identified with the sector scan, and vertical and horizontal measurements for AFI and two-dimensional area were recorded. Simultaneous AF volume measurements of each pocket were performed three times with the bladder volume instrument, and maximum values were used. Three-dimensional volume, two-dimensional area, and AFI values were compared by correlation analysis, with P < or = .05 considered statistically significant. EXPERIENCE: Among all patients, the average (+/- standard deviation) AFI was 7.6 +/- 4.1 (range 1.5-16.4) cm, and the average two-dimensional area was 30.9 +/- 21.1 (range 4.3-81.3) cm2. This corresponded to an average three-dimensional volume of 215 +/- 134 (range 23-497) cm3. Three-dimensional volume correlated highly with both AFI (r = 0.9; P < .001) and two-dimensional area (r = 0.86; P < .001). One AFI centimeter was equivalent to a volume of 30 cm3. CONCLUSION: There are highly significant linear correlations of three-dimensional amniotic fluid volumes with AFI and two-dimensional area. The four pockets used in AFI determination account for only 50% of total AF volume. Three-dimensional determinations may aid in clinical assessments of AF volume.

Adult↗
PubMed1997

Ovine fetal laryngeal chemoreflex thresholds and respiratory effects.

In newborn infants, laryngeal contact with solutions of low chloride concentration or pH evokes swallowing, laryngeal adduction, and respiratory inhibition (laryngeal chemoreflex). To determine whether the laryngeal chemoreflex is present during fetal life and its effect on fetal respiratory activity, eight time-bred ewes (128 +/- 2 days) were prepared with fetal electrocortical diaphragm and esophageal electrodes and a nasopharyngeal catheter. After a 60-minute control period, increasing volumes (0.1 to 1.0 ml/kg) of 0.15 mol/L NaCl or distilled water (0.05 to 1.0 ml/kg) and decreasing concentrations of NaCl (0.15 to 0.02 mol/L) at a fixed volume (0.3 ml/kg) were sequentially administered through the nasopharyngeal catheter (38 degrees C). The minimum water volume that stimulated swallowing was significantly less than the minimum 0.15 mol/L NaCl volume (0.10 +/- 0.02 vs. 0.70 +/- 0.05 ml/kg). The maximum NaCl concentration that stimulated swallowing was 0.04 +/- 0.01 mol/L During the control period, respiratory activity averaged 14.6 +/- 0.7 breaths/minute and did not change during absent swallow responses or isotonic saline-induced swallows. However, respiratory activity significantly decreased during water (4.7 +/- 0.6 breaths/minute) and hypotonic saline-induced swallow responses (3.7 +/- 0.7 breaths/minute). Fetal electrocortical activity did not change during absent or stimulated swallows. We conclude that laryngeal water or hypotonic saline solution may stimulate fetal swallowing and suppress fetal respiratory activity, similar to the newborn laryngeal chemoreflex. We speculate that an exaggeration of the laryngeal chemoreflex apnea response in the newborn may predispose to sudden infant death syndrome.

Animals↗
PubMed1997

Fetal swallowing: relation to amniotic fluid regulation.

In summary, fetal swallowing activity contributes importantly to fetal and amniotic fluid homeostasis, and fetal somatic and gastrointestinal development. Human and ovine fetal swallowing increases throughout gestational with fetal swallowed volumes markedly greater (relative to body weight) than adults. Although the regulation of swallowing activity in early gestation is unknown, intact central and systemic dipsogenic mechanisms have been shown during the last third of ovine gestation. Recent studies suggest that swallowing behavior may be modulated in accordance with neurobehavioral state changes and influenced by hypoxia, hypotension and plasma osmolality changes. Whether fetal swallowing also is regulated by the development of "hunger" sensation, salt appetite, or the development of taste is uncertain. Nevertheless, it is likely that, for species in which swallowing behavior develops in utero, there are potentially dramatic influences of the maternal-fetal pregnancy environment on the imprinting of regulatory mechanisms controlling ingestive behavior. Ultimately, the regulation of fetal swallowing may aid in the prevention and/or therapy of human amniotic fluid disorders.

Adult↗
PubMed1996

Intraamniotic deamino(D-Arg8)-vasopressin: prolonged effects on ovine fetal urine flow and swallowing.

OBJECTIVE: Fetal urine is the main source of amniotic fluid, and alterations in urine production have an impact on amniotic fluid dynamics. Intraamniotic arginine vasopressin is absorbed into fetal plasma, but fetal antidiuretic responses may be obscured by vasoconstrictor (V1 receptor) actions of arginine vasopressin. We determined the effects of intraamniotic administration of a specific V2 receptor agonist, deamino(D-Arg8)-vasopressin, on fetal plasma arginine vasopressin immunoreactivity, fetal urine output, and swallowing. STUDY DESIGN: Six chronically prepared pregnant ewes (129 +/- 1 days' gestation) received a single deamino(D-Arg8) (50 micrograms injection into the amniotic fluid with subsequent measurement of fetal plasma arginine vasopressin immunoreactivity, fetal urine flow and osmolality, and fetal swallowing. RESULTS: Within 30 minutes of deamino(D-Arg8)-vasopressin administration, fetal plasma arginine vasopressin immunoreactivity (4.5 +/- 0.5 to 76.4 +/- 21.9 pg/ml) and urine osmolality (135 +/- 13 to 285 +/- 43 mOsm/kg H2O) significantly increased and urine flow decreased (0.18 +/- 0.01 to 0.05 +/- 0.01 ml/kg per minute). After 48 hours urine flow remained reduced (0.08 +/- 0.03 mg/kg per minute) and osmolality remained elevated (399 +/- 18 mOsm/kg H2O). There were no changes in fetal swallowing, systolic (64 +/- 3 mm Hg) or diastolic (42 +/- 3 mm Hg) blood pressure, or heart rate (169 +/- 6 beats/min). CONCLUSIONS: Intraamniotic deamino(D-Arg8)-vasopressin administration evokes persistent fetal antidiuresis in the absence of arginine vasopressin-induced fetal cardiovascular effects or changes in fetal swallowed volume, indicating its suitability as a potential therapeutic agent in amniotic fluid volume disorders such as polyhydramnios.

Amniotic Fluid↗
PubMed1996

Treatment of oligohydramnios with maternal 1-deamino-[8-D-arginine] vasopressin-induced plasma hypoosmolality.

OBJECTIVE: Maternal 1-deamino-[8-D-arginine] vasopressin (a selective antidiuretic agonist) and oral water loading decrease maternal and fetal plasma osmolality and markedly increase fetal urine flow in sheep. We hypothesized that a titrated reduction in maternal plasma osmolality would increase human amniotic fluid volume. STUDY DESIGN: Pregnant women (n = 5) with oligohydramnios at term were administered oral water loading (20 ml/kg) and intravenous 1-deamino-[8-D-arginine] vasopressin (2 micrograms) to induce antidiuresis. Maternal plasma and urine osmolality and urine production were measured hourly, and water replacement was titrated for 8 hours to reduce plasma osmolality by 15 to 20 mOsm/kg. The amniotic fluid index determined by ultrasonography was measured at baseline, 8 hours, and 24 hours. A control group of pregnant women (n = 5) with oligohydramnios at term was observed for 8 hours with maintenance intravenous hydration. RESULTS: In 1-deamino-[8-D-arginine] vasopressin-treated women, maternal urine flow increased with oral water loading, decreased with 1-deamino-[8-D-arginine] vasopressin administration, and remained reduced for 8 hours. Maternal plasma osmolality significantly decreased (285 +/- 4 to 265 +/- 4 mOsm/kg) and the amniotic fluid index significantly increased (4.1 +/- 0.6 to 8.2 +/- 1.5 cm) at 8 hours. Although maternal urine osmolality returned to basal values at 24 hours, plasma osmolality was reduced and the amniotic fluid index remained significantly increased (8.2 +/- 1.3 cm). There was no change in the amniotic fluid index (4.3 +/- 0.4 to 4.7 +/- 0.7 cm) in control patients observed with maintenance intravenous hydration. CONCLUSIONS: Maternal 1-deamino-[8-D-arginine] vasopressin and oral water administration can reduce and stabilize plasma osmolality and increase amniotic fluid volume. 1-Deamino-[8-D-arginine] vasopressin therapy has potential for the prevention and treatment of oligohydramnios.

Adult↗
PubMed1996

1-Deamino-[8-D-arginine] vasopressin-induced maternal plasma hypoosmolality increases ovine amniotic fluid volume.

OBJECTIVE: Maternal 1-deamino-[8-D-arginine] vasopressin (a selective antidiuretic agonist) and oral water loading decrease maternal and fetal plasma osmolality and markedly increase fetal urine flow in sheep over 24 hours. We hypothesized that a sustained reduction in plasma osmolality would increase the amniotic fluid volume. STUDY DESIGN: Pregnant sheep (130 +/- 1 days, n = 6) were given oral water loading (2000 ml) and intravenous 1-deamino-[8-D-arginine] vasopressin (20 microgram injection followed by 4 micrograms/hr) to induce maternal urinary antidiuresis and to maintain a maternal plasma osmolality decrease of approximately 20 mOsm/kg for 48 hours. Fetal plasma osmolality, electrolytes, atrial natriuretic factor, and arginine vasopressin levels, fetal swallowing activity, and fetal urine flow and composition were measured each study day, and amniotic fluid volume was measured by tracer dilution before and at 48 hours of plasma hypoosmolality. RESULTS: In response to 1-deamino-[8-D-arginine] vasopressin and water loading, maternal plasma osmolality decreased (298 +/- 1 to 275 +/- 3 mOsm/kg), mirrored by fetal plasma hypoosmolality (293 +/-1 to 271 +/- 3 mOsm/kg). Fetal urine osmolality (155 +/- 16 to 117 +/- 10 mOsm/kg) and amniotic fluid osmolality significantly decreased (281 +/- 3 to 269 +/- 5 mOsm/kg). Fetal urine flow rate significantly increased (0.33 +/- 0.06 to 0.51 +/- 0.04 ml/min), whereas fetal swallowing activity decreased (56 +/- 7 to 33 +/-5 swallows per hour). In response to alterations in fetal fluid dynamics, amniotic fluid volume significantly increased from 493 +/- 80 to 839 +/- 160 ml. CONCLUSION: Chronic maternal 1-deamino-[8-D-arginine vasopressin-induced plasma hypoosmolality induces a sustained reduction in fetal swallowing activity and an increase in fetal urine production, which results in a marked increase in amniotic fluid volume. Use of maternal 1-deamino-[8-D-arginine] vasopressin and oral fluid loading has the potential for manipulation of amniotic fluid volume homeostasis and may prove useful in the treatment of oligohydramnios.

Amniotic Fluid↗
PubMed1996

Mathematic modeling of human amniotic fluid dynamics.

OBJECTIVE: We sought to develop a model quantifying the relative contributions of fetal swallowing and intramembranous flow to amniotic fluid dynamics during human gestation. We then used the model to simulate the impact of absent swallowing on amniotic fluid volume. STUDY DESIGN: The model was developed with published data for normal human amniotic fluid volume and composition, human fetal urine flow rate and composition (11 to 42 weeks), and extrapolated data from ovine lung fluid production. Fetal swallowing and intramembranous flow were calculated with assumptions that (1) swallowed fluid is isotonic to amniotic fluid, (2) intramembranous flow is free water diffusion, and (3) 50% of lung fluid is swallowed. The model was then applied to simulate absent fetal swallowing and variable (0%, 50%) proportions of swallowed lung fluid were used as a representation of esophageal atresia-tracheal fistula variations. RESULTS: Fetal swallowed volume and intramembranous flow linearly increase until 28 to 30 weeks. Daily swallowed volume then exponentially increases to a maximum of 1006 ml/day at term, whereas intramembranous flow continues on a linear trend to reach 393 ml/day at term. With absent swallowing and variable amounts of lung fluid swallowed (0%, 50%), predicted amniotic fluid volume is similar to normal values through 20 weeks, exceeds the 95% confidence interval for normal amniotic fluid volume at 29 to 30 weeks' gestation (approximately 2000 ml), and then exponentially increases. Predicted amniotic fluid osmolality (280 to 257 mOsm/kg) is slightly lower than actual values although within the clinically normal range. CONCLUSIONS: This model indicates that the normal reduction in amniotic fluid volume beginning at 34 weeks results from the marked increase in swallowed volume during the third trimester. Additionally, this model correlates well with the timing of the initial clinical presentation of polyhydramnios observed in some fetuses with conditions that result in absent or reduced swallowing or gastrointestinal atresia. Modeling of amniotic fluid dynamics can predict normal changes in fetal fluid exchange and may aid in understanding of amniotic fluid imbalances.

Amniotic Fluid↗
PubMed1996

Amniotic fluid index predicts the relief of variable decelerations after amnioinfusion bolus.

OBJECTIVE: Our purpose was to determine whether intrapartum amniotic fluid index before amnioinfusion can be used to predict response to therapeutic amnioinfusion. STUDY DESIGN: Intrapartum patients (n = 85) with repetitive variable decelerations in fetal heart rate that necessitated amnioinfusion (10 ml/min for 60 minutes) underwent determination of amniotic fluid index before and after bolus amnioinfusion. The fetal heart tracing was scored (scorer blinded to amniotic fluid index values) for number and characteristics of variable decelerations before and 1 hour after initiation of amnioinfusion. The amnioinfusion was considered successful if it resulted in a decrease of > or = 50% in total number of variable decelerations or a decrease of > or = 50% in the rate of atypical or severe variable decelerations after administration of the bolus. Spontaneous vaginal births before completion of administration of the bolus (n = 18) were excluded from analysis. The probability of success of amnioinfusion in relation to amniotic fluid index was analyzed with the chi(2) test for progressive sequence. RESULTS: The mean amniotic fluid index before amnioinfusion was 6.2 +/- 3.3 cm. An amniotic fluid index of < or = 5 cm was present in 40% of patients (27/67), and an amniotic fluid index of < or = 8 cm was present in 72% of patients (48/67). The probability of success of amnioinfusion decreased with increasing amniotic fluid index before amnioinfusion (76% [16/21] when initial amniotic fluid index was 0 to 4 cm, 63% [17/27] when initial amniotic fluid index was 4 to 8 cm, 44% [7/16] when initial amniotic fluid index was 8 to 12 cm, and 33% [1/3] when initial amniotic fluid index was > 12 cm, p = 0.03). The incidence of nuchal cords or true umbilical cord knots increased in relation to amniotic fluid index before amnioinfusion. CONCLUSIONS: Amniotic fluid index before amnioinfusion can be used to predict the success of amnioinfusion for relief of variable decelerations in fetal heart rate. Failure of amnioinfusion at a high amniotic fluid index before amnioinfusion may be explained by the increased prevalence of nuchal cords or true knots in the umbilical cord.

Adult↗