Renal response of the lamb fetus to partial occlusion of the umbilical cord

Amniotic fluid composition in the fetal lamb with intrauterine growth restriction

OBJECTIVE

Our aim was to examine changes from normal in the composition of amniotic fluid in fetal lambs with mild and severe hypoxemia and intrauterine growth restriction.

STUDY DESIGN

Pregnant sheep underwent maternal catheterization at 88 to 93 days' gestation and fetal catheterization at 105-112 days' gestation. Twelve pregnancies (group 1) provided control data (fetal PaO 2 18-22 mm Hg), in 12 fetuses (group 2) mild hypoxemia (PaO 2 16-19 mm Hg) was induced by prevention of the normal expansion of maternal blood volume, and in 7 fetuses (group 3) chronic hypoxemia (PaO 2 12-18 mm Hg) developed spontaneously.

RESULTS

In group 2 amniotic fluid osmolality and sodium concentrations were lower (approximately 30 mOsm/kg and 10 mEq/L, P <.05) and urea nitrogen level was higher (10 mg/dL, P <.05) than in group 1. In group 3 osmolality and sodium concentrations at approximately 120 days' gestation were similar to those in group 1. Whereas these values decreased with gestation in groups 1 and 2 (P <.05), they remained unchanged or increased in all fetuses in group 3. Mortality rates in groups 1, 2, and 3 were 1 of 12, 4 of 12 (difference not significant), and 5 of 7 (P <.05), respectively.

CONCLUSION

Absence of normal decrease in amniotic fluid osmolality with gestation, in association with a high perinatal mortality rate, was found in severely but not in mildly hypoxemic fetuses with intrauterine growth restriction.

Alterations in fetal urine production during prolonged hypoxaemia induced by reduced uterine blood flow in sheep: mechanisms

1. Our aim was to identify mechanisms whereby prolonged fetal hypoxaemia alters renal function and urine production in fetal sheep. 2. Fetal hypoxaemia was induced for 24 h by reducing uterine blood flow at 129.0 +/- 2.1 days of gestation (term 145-147 days), causing a reduction in fetal arterial O2 saturation (SaO2) from 52.5 +/- 2.3 to 22.0 +/- 1.3% (P < 0.05). This hypoxaemia was initially associated with a mild acidaemia (pH 7.23 +/- 0.03). 3. The glomerular filtration rate (GFR) increased from a control value of 1.8 +/- 0.3 mL/min per kg to a maximal value of 2.8 +/- 0.6 mL/min per kg (P < 0.05) at 4-5 h of hypoxaemia, returning to control levels by 6-9 h of hypoxaemia. After 4 h of hypoxaemia renal blood flow was no different to control values (144 +/- 8 mL/min per 100 g kidney weight) but after 24 h of hypoxaemia it had increased to 190 +/- 8 mL/min per 100 g kidney weight (P < 0.05). Fractional reabsorption of Na+ in the proximal tubules decreased from a control value of 81.5 +/- 2.2 to 65.2 +/- 3.9% at 2-3 h of hypoxaemia (P < 0.05) and remained reduced (68.5 +/- 3.1%) at the end of hypoxaemia (P < 0.05). Fetal mean arterial pressure transiently increased (P < 0.05) but returned to control values by 4-5 h of hypoxaemia. Fetal renal vascular resistance was not significantly altered during hypoxaemia. Fetal urine production increased from a control value of 12.3 +/- 2.1 mL/h per kg to a maximal value of 19.1 +/- 4.2 mL/h per kg at 4-5 h of hypoxaemia (P < 0.05) and returned to control by 24 h of hypoxaemia. 4. Our results indicated that prolonged fetal hypoxaemia leads to the inhibition of Na+ reabsorption in the proximal portion of the renal tubules. Changes in GFR induced by hypoxaemia were similar to those in fetal urine production and were not associated with changes in renal blood flow. We conclude that prolonged fetal hypoxaemia affects renal haemodynamics and the reabsorptive capacity of the renal tubules, resulting in a diuresis.

In utero defecation of the nondistressed fetus: a roentgen study in the goat

An experimental study was performed to investigate gastrointestinal motility and meconium passage, with simultaneous blood gas measurements, in the fetuses of eight pregnant goats at 110 to 114 days' gestation (full term, 147 to 155 days). With the goats under halothane anesthesia, a nasogastric tube and a heparinized central venous catheter were inserted into the fetuses. 24 hours after surgery, 10 mL of gastric juice from the fetus was replaced with a nonhydrosoluble contrast medium, and serial roentgenograms and blood samples (for pH, PO2, and PCO2 measurement) were taken every 4 hours. All fetuses began to pass the contrast medium into the amniotic cavity within 16 to 22 hours, and central venous blood gas values were normal. The results of this study suggest that the fetus, which urinates routinely, also defecates routinely into the amniotic cavity, even in the absence of distress. The increased incidence of meconium staining in fetal distress conditions may be indicative of impaired clearance of amniotic fluid.

Fetal cerebral and renal Doppler in small for gestational age fetuses in hypertensive pregnancies

The aim of the present work is to evaluate the hemodynamic disorders induced in several fetal vascular areas by maternal hypertension and to check the sensitivity and the specificity of the various Doppler parameters in the detection of small for gestational age (SGA) infants. The population consisted of 90 pregnant mothers aged 26.3 +/- 5.8 years with pregnancy-induced hypertension. Seventeen of them delivered moderate small for age babies (between the 10th and 5th centiles), without any neonatal complication. The opposition to flow in the fetal brain and kidney and in the placenta was evaluated using the Resistance Index (RI). The ratio of cerebral RI and umbilical RI, called the cerebroplacental ratio (CPR), was calculated and used as an indicator of fetal flow redistribution. The ability of CPR and renal RI to predict SGA at birth was evaluated; the sensitivity, specificity, positive predictive value, and negative predictive value for the CPR were 88.2%, 98.6%, 93.8% and 97.3%, respectively. The corresponding figures for the renal RI were 58.8%, 94.5%, 71.4% and 91%, respectively. Furthermore, in the SGA group, the abnormal renal RI values were both above and below the normal range, whereas the CPR values demonstrated consistent changes (always < 1).

CONCLUSION

this investigation demonstrates that in pregnancy-induced hypertension (even with moderate growth retardation, and no neonatal complication), the diagnostic efficacy of CPR for predicting SGA at birth is very high and that of renal RI correlates very poorly with fetal growth.

The effects of twenty-four hours of reduced uterine blood flow on fetal fluid balance in sheep

OBJECTIVE

Our aim was to determine the effects of a sustained reduction in uteroplacental perfusion, leading to fetal hypoxia, on determinants of amniotic fluid volume in sheep.

STUDY DESIGN

Surgery was performed on five pregnant ewes 110 to 116 days after mating. At 127.3 +/- 2.2 days uterine blood flow was reduced for 24 hours, which reduced fetal SaO2 from 61.9% +/- 1.2% to 24.9% +/- 0.8%.

RESULTS

Fetal urine production was increased from a control value of 193.0 +/- 24.0 ml/kg per 24 hours to 279.3 +/- 30.0 ml/kg per 24 hours during periods of reduced uterine blood flow and remained above control values for up to 48 hours after the reduced uterine blood flow period. A substantial loss of fetal water and electrolytes occurred through urine, which was associated with changes in the composition of fetal plasma and fetal tracheal, fetal swallowed, and amniotic fluids. Fetal swallowing was reduced throughout the reduced uterine blood flow period from a control value of 200.8 +/- 56.0 ml/kg per 24 hours to 32.7 +/- 8.4 ml/kg per 24 hours and returned to control levels after the cessation of the reduced uterine blood flow.

CONCLUSION

We conclude that 24 hours of reduced uterine blood flow causes major changes in fetal renal function and fetal swallowing that, in spite of an expected reduction in lung liquid production, would increase the flow of fluid and electrolytes from the fetus into the amniotic sac.

Oligohydramnios: antepartum fetal urine production and intrapartum fetal distress

Animal and human studies suggest that fetal oliguria is a normal physiologic response to hypoxemia. To assess the clinical significance of this observation, we studied (before their admission) 51 fetuses of women whose pregnancies were complicated by oligohydramnios at greater than or equal to 38 weeks' gestation. We found that the mean hourly fetal urine production decreased significantly in relation to the severity of subsequent intrapartum fetal compromise. The mean antepartum rate was 95 ml/hr in the 21 fetuses with a normal intrapartum heart rate pattern; this fell to 59 ml/hr in the 18 fetuses who had an abnormal intrapartum heart rate pattern but who responded to intrauterine resuscitation. The rate was 33 ml/hr in the 12 fetuses who were delivered by cesarean section as a result of fetal distress. These findings suggest that oligohydramnios associated with fetal oliguria may be used to identify those fetuses who have less intrinsic or uteroplacental reserve than do those of women with oligohydramnios who have a higher rate of fetal urine production.

Fetal recirculation of amniotic fluid arginine vasopressin

Amniotic fluid volume reflects a balance between fetal lung fluid and fetal urine production and fluid reabsorption via fetal swallowing. Arginine vasopressin (AVP) infusion decreases both fetal lung fluid and urine production and increases amniotic fluid osmolality and AVP concentration. In the present study we assessed the effect of amniotic fluid AVP injection on plasma AVP (n = 6) and renal function (n = 4) in chronically catheterized fetal lambs (X gestation = 130 days). Thirty minutes after addition of 25 micrograms of synthetic AVP into the amniotic cavity, mean +/- SE fetal plasma AVP increased from a base line of 2.7 +/- 0.2 to 14.6 +/- 3.4 pg/ml (P less than 0.01). One hundred and twenty minutes after injection, plasma AVP had increased to 26.9 +/- 5.7 pg/ml. Fetal urine volume did not change (0.78 +/- 0.01 ml/min) but significant increases in urine osmolality (169 +/- 19 to 315 +/- 25 mosm) and urine sodium (64 +/- 11 to 125 +/- 11 mueq/ml) were observed 120 min after AVP administration. In conclusion, amniotic fluid AVP levels can affect fetal plasma AVP concentration, and AVP absorbed from the amniotic fluid by the fetus remains biologically active.

The renal response to acute asphyxia in spontaneously breathing newborn lambs

The effects of acute asphyxia on neonatal renal function were examined in spontaneously breathing newborn lambs. 25 min of asphyxia were induced by addition of a respiratory dead space to reduce PaO2 to 41 +/- 3 mm Hg, pH to 7.03 +/- 0.05 and increase PaCO2 to 68 +/- 5 mm Hg (mean +/- S.E.M.). Glomerular filtration rates did not change significantly during or after asphyxia. Immediately following asphyxia significant (P less than 0.05) increases over the baseline were found in urinary flow rates, fractional sodium excretion, absolute sodium excretion and osmolar clearances. These changes were in part secondary to significant (P less than 0.05) increases in plasma glucose concentrations associated with increases (P less than 0.05) in circulating arterial catecholamine concentrations. The percentage of tubular reabsorption of glucose decreased significantly (P less than 0.05) and urinary glucose excretion increased significantly. Renal blood flow was unchanged. Therefore, asphyxia induced significant hyperglycemia which contributed to the concomitant natriuresis and osmotic diuresis in these newborn lambs.

Ratio of the concentration of hypoxanthine to creatinine in urine from newborn infants: a possible indicator for the metabolic damage due to hypoxia

The ratio of the urinary concentrations of the ATP metabolite, hypoxanthine, to that of creatinine was determined in normal newborn infants. An increase in this ratio reflects high hypoxanthine excretion and thus ATP breakdown. The ratio can be determined on random urine samples, thus simplifying sampling. Urinary changes are persistent; abnormalities are detectable on the second day of life after intrapartum hypoxia. Preliminary results suggest that this ratio on a sample during the second day of life could 'diagnose' intrapartum hypoxia and might therefore quantitatively assess those obstetric 'risk factors' believed to operate through hypoxia.

The preejection period of the fetal cardiac cycle. II. Maternal common internal iliac artery occlusions

The preejection period (PEP) of the fetal cardiac cycle was studied in the chronically instrumented fetal lamb. An inflatable cuff occluder was positioned around the maternal common internal iliac artery, the pregnant ewe's main uterine blood-supplying vessel. Occlusion of the maternal common internal iliac artery produced a fall in pO2 and pH and a rise in pCO2. Fetal arterial blood pressure increased and PEP shortened, especially in the longer lasting occlusions. Fetal heart rate exhibited a bradycardia also during this type of occlusion. The fetal lamb's cardiovascular adjustments to biochemical changes caused by occlusion of the maternal uterine blood supply appear to be determined during the initial phase of occlusion by a chemoreceptor reflex-induced peripheral vasoconstriction mediated by the adrenergic nervous system. The condition is comparable to the one observed during the second part of the umbilical cord occlusions. The interaction between a supersensitive fetal myocardium and the adrenal release of catecholamines may play a dominant role during the second part of occlusion in the longer lasting experiments. Increase in peripheral resistance in the course of redistribution of blood flow during hypoxemia induces a prolongation of the PEP in the early phase of occlusion; overriding of this effect by an increase in myocardial contractility due to rising levels of adrenal catecholamines abbreviates the PEP in the later part of the longer lasting occlusions.

Developmental aspects of the renal response to hypoxemia in the lamb fetus

The effects of fetal hypoxemia on renal hemodynamics and renal function were studied in two groups of chronically catheterized young (< 120 days of gestation) and near-term lamb fetuses (> 130 days). Fetal hypoxemia produced, in both groups, a significantly decrease in renal blood flow (RBF) and a significant increase in the filtration fraction. However, the glomerular filtration rate (GFR) did not change significantly suggesting that the renal vasoconstriction associated with fetal hypoxemia was more important at the efferent than at the afferent arteriolar level. In the group of near-term fetuses, the decrease in RBF correlated closely with changes in plasma renin activity (PRA) (r = 0.77). No changes in PRA were observed during hypoxemia in the group of young fetuses. After hypoxemia, reactive hyperemia associated with a significant increase in urinary prostaglandin excretion (PGE and PGF2 alpha) was observed in near-term fetuses but not in young fetuses. It also was demonstrated that fetal hypoxemia produced a significant increase in fetal plasma concentrations of vasopressin associated with an antidiuresis in all but one near-term fetus and in 50% of the young fetuses, suggesting that the ability of the fetal kidney to reabsorb free water is more developed in near-term fetuses. Finally, fetal hypoxemia had no effect on mean arterial pressure and heart rate in young fetuses; however, in near-term fetuses, a significant increase in blood pressure and a decrease in heart rate were observed. In summary, it appears that the response of the fetal kidney to hypoxemia depends on the degree of fetal maturation.

Renal response of fetal lamb to complete occlusion of umbilical cord

The renal response of the fetal lamb to repeated complete occlusion of the umbilical cord was studied in nine chronically instrumented animals. Five episodes of occlusion of the umbilical cord, each lasting for two minutes, produced a twofold rise in fetal urine osmolality and sodium, chloride, and potassium concentrations. Output of urine and glomerular filtration rate remained essentially unchanged while free water clearance decreased from a control of +0.10 to -0.02 ml. per kilogram per minute at the end of the fifth episode. Electrolyte concentrations in urine remained elevated for at least two hours following the occlusions. In addition to changes in urine composition, there was a 50- to 200-fold increase in the fetal plasma concentration of vasopressin. These studies indicate that complete interruption of the umbilical circulation, even though of short duration, produces disturbances in fetal renal function that can lead to loss of electrolytes in the urine. They provide an explanation for the low sodium levels reported in asphyxiated newborn infants in renal failure.