Urine production rate is related to behavioural states in the near term human fetus

Fetal urine production in late pregnancy

Objective. Hourly fetal urine production rate (HFUPR) was studied in relation to both gestational age and the onset of spontaneous labor in normal term human pregnancies. Methods. Serial volume measurements were obtained from longitudinal ultrasound images of the fetal bladder at 1-5-minute intervals, and HFUPR was subsequently calculated. A total of 178 adequate bladder-filling cycles were recorded in 112 women, and the amniotic fluid index (AFI) was assessed. Results. HFUPR did not change significantly between 37 and 42 weeks' gestation. However, HFUPR decreased during the last 14 days prior to the onset of spontaneous labor (P < 0.005). No significant correlation was found between HFUPR and AFI, neither when measured at the same time nor when HFUPR and AFI were measured at various intervals in time. Conclusion. HFUPR falls before and in relation to the time of onset of labor rather than in relation to gestational age.

Effects of the maternal position and rest on the fetal urine production rate: a prospective study conducted by 3-dimensional sonography using the rotational technique (virtual organ computer-aided analysis)

OBJECTIVES

The purpose of this study was to determine the effect of the maternal left lateral decubitus position and rest on the fetal urine production rate.

METHODS

Fifty-four pregnant women with a normal amniotic fluid volume who had a singleton pregnancy between 26 and 40 weeks' gestation were included in this prospective study. Exclusion criteria included rupture of membranes, intake of any food or liquid within the previous 4 hours, and maternal or fetal complications. The amniotic fluid index was determined by averaging 2 or 3 repetitive measurements of the 4 amniotic fluid quadrants at the time of fetal urinary bladder volume measurements. The fetal urine production rate was measured using virtual organ computer-aided analysis. The amniotic fluid index and fetal urine production rate before and after the left lateral position rest period were compared by a paired Student t test. The Pearson correlation was used to study the relationships among the maternal age, gestational age, test time, amniotic fluid index, and fetal urine production changes.

RESULTS

The mean amniotic fluid indices ± SD before and after the rest period were 151.0 ± 45.0 and 172.5 ± 46.7 mm, respectively, indicating a significant increase in the amniotic fluid index (P < .05). The mean fetal urine production rates before and after the rest period were 73.7 ± 66.8 and 151.8 ± 119.9 mL/h, respectively, indicating a significant increase in fetal urine production (P < .05).

CONCLUSIONS

The fetal urine production rate and amniotic fluid index are markedly increased by maternal rest in the left lateral decubitus position.

Ovine hourly fetal urine production: relation to fetal electrocortical activity

OBJECTIVE

Ultrasound studies of hourly urine production rate in human fetuses have suggested that a fall in urine production occurs in state 2F (fetal quiet sleep) secondary to a state-dependent decrease in renal blood flow. We sought to ascertain the relationship between fetal hourly urine production rate and behavioral state in the near-term ovine fetus, a model in which urine production and fetal brain activity can be directly measured.

METHODS

Six ewes with singleton pregnancies were prepared with vascular and amniotic fluid catheters. Fetuses were prepared with hindlimb vascular catheters, a bladder catheter, and biparietal ECoG electrodes. After at least 5 days of recovery (ga 130 +/- 2 days; term = 145-150 days), each animal was monitored for a 6-h period. Urine production was measured by draining the bladder catheter through a drop counter and fetal ECoG was continuously recorded (sampling rate of 50 Hz). ECoG activity was analyzed using power spectral analysis and periods of active and quiet sleep identified using both signal amplitude and corresponding 85% spectral edge frequency.

RESULTS

Basal fetal arterial pH (7.36 +/- 0.01), pO2 (22.0 +/- 1.2 mmHg) and pCO2 (47.0 +/- 1.6 mmHg) and plasma (295 +/- 2 mOsm/kg) and urine (179 +/- 3 mOsm/kg) osmolalities were within normal ranges. Active and quiet sleep comprised 50 +/- 2 and 43 +/- 1% time, respectively. There was no difference in hourly urine production rate in active sleep (21.4 +/- 9.7 ml/h) and quiet sleep (18.8 +/- 7.7 ml/h).

CONCLUSIONS

1) Hourly fetal urine production rate is independent of ECoG activity state in the near-term ovine fetus. 2) Assuming only minor species differences, ultrasound measurement of human fetal hourly urine production rate can be performed without concern for fetal neurobehavioral state changes.

Acute maternal rehydration increases the urine production rate in the near-term human fetus

OBJECTIVE

We sought to investigate the effect of a decrease of maternal plasma osmolality produced by hypotonic rehydration on the fetal urine production rate in normal near-term human fetuses.

STUDY DESIGN

Twenty-one healthy pregnant women attending the clinic for antenatal care were studied between 37 and 40 weeks' gestation. The fetal urine production rate was assessed by serial measurements of 3 diameters of the fetal bladder. The hourly fetal urine production rate was determined by linear regression analysis of the calculated bladder volumes versus time and was initially determined after a period of 4 hours of fluid deprivation. Thereafter, the women were asked to drink 1 L of water, and the hourly fetal urine production rate was assessed again. The hourly fetal urine production rate was only studied during behavioral state 1F because it is dependent on the behavioral state. The fetal behavioral state was determined by assessment of fetal heart rate, fetal eye movements, and fetal body movements.

RESULTS

Successful recordings were obtained in 10 of the 21 women. The hourly fetal urine production rate increased significantly after hypotonic rehydration (P <.02). Compared with the initial hourly fetal urine production rate after 4 hours of fluid deprivation, the hourly fetal urine production rate showed an increase of 63.2% after hypotonic rehydration, from 38.2 +/- 16.3 mL/h to 62.4 +/- 34.6 mL/h (mean +/- SD). After rehydration, the baseline fetal heart rate fell significantly, from 141 +/- 6 to 132 +/- 8 beats/min (mean +/- SD; P =.005).

CONCLUSION

The fetal urine production rate is augmented after acute maternal oral hypotonic rehydration after 4 hours of fluid deprivation. The current findings demonstrate that the near-term human fetus can handle such acute changes in fluid osmolality by increasing the urine production rate to maintain its fluid homeostasis. This mechanism implies that changes in maternal plasma osmolality and volume probably play an important role in determining amniotic fluid volume. Therefore the application of maternal hydration for the treatment of oligohydramnios should be further investigated.

Renal responses to prolonged (48 h) hypoxemia without acidemia in the late-gestation ovine fetus

The effect of sustained moderate hypoxia on renal blood flow and renal function was studied in the ovine fetus (123-129 days). The experiments consisted of 48 h of isocapnic hypoxia, not resulting in acidemia, but sufficient to produce redistribution of blood flow in favor of the brain at the expense of the carcass. Hypoxemia was induced by maternal nitrogen inhalation. Fetal arterial O(2) saturation and arterial O(2) pressure (Pa(O(2))) decreased from, respectively, 50.6 +/- 3.0% and 17.2 +/- 0.9 mmHg during control to 36.4 +/- 2.7% and 13.4 +/- 0.7 mmHg on the first and to 32.2 +/- 2. 2% and 12.4 +/- 0.7 mmHg on the second day of hypoxemia. Fetal renal blood flow and urine production rate were continuously measured using ultrasonic flow transducers. Fetal renal blood flow increased during hypoxemia from 11.8 +/- 1.6 to 15.6 +/- 1.8 ml/min and remained elevated throughout the 48-h hypoxemia period (P < 0.01). Renal blood flow was inversely correlated with fetal Pa(O(2)) (r is -0.69, P < 0.0001). Fetal urine production rate, glomerular filtration rate, filtration factor, osmotic clearance, and free water clearance did not significantly change from control values during hypoxemia or recovery. We conclude that hypoxemia without acidemia results in an immediate and considerable increase in fetal renal blood flow, which remains elevated for the entire hypoxemic period.

Hourly fetal urine production rate in the near-term fetus: is it really increased during fetal quiet sleep?

Fetal bladder volume and hourly fetal urine production (HFUPR) is calculated on the assumption that the fetal bladder is ellipsoid in shape. A recent validation study demonstrated a progressive overestimation at increasing bladder volumes. This may be due to changes in shape of the fetal bladder at increasing volumes. Two independent papers have shown increased HFUPR during fetal behavioural state 1F (S1F) when compared with S2F. The aim of the present study was to assess whether this increase of HFUPR during S1F, previously observed by others, could be the result of an error introduced by the method of volume calculation. A retrospective evaluation was performed in a series of 208 HFUPR measurements in 123 normal near term pregnant women attending a low-risk atenatal clinic. Adequate bladder filling in both states was identified in 43 recordings. Maximum fetal bladder volumes were greater (> 10 ml) during S1F in comparison to S2F in 56% of these recordings and HFUPR was significantly greater during S1F only in these cases. Bladder volumes are usually lower during S2F as a result of fetal voiding, which occurred in association with 22 of 36 transitions from S1F to S2F, and only 1 of 13 transitions from S2F to S1F (P < 0.001). When disregarding calculated bladder volumes in excess of 20 ml for the purpose of calculating HFUPR, eleven recordings remained. HFUPR calculated in this way was significantly lower in comparison to measurements where larger bladder volumes were included and no difference was observed between states. This implies that the differences observed are the result of the greater error in calculating bladder volumes and HFUPR during S1F, where volumes are usually greater and that calculation of fetal bladder volume should not be performed on the assumption that the bladder is ellipsoid in shape. Alternative techniques include limiting measurements to a maximum volume of approximately 20 ml, when the bladder is usually ellipsoid in shape or basing volume calculation on the surface area of a series of sagittal views as suggested by Hedriana and Moore [Hedriana HL, Moore TR. Ultrasonographic evaluation of human fetal urinary flow rate: accuracy of bladder volume estimations. Am J Obstet Gynecol 1994;170:1250-1254; Hedriana HL, Moore TR. Accuracy limits of ultrasonographic estimation of fetal urinary flow rate.

Urine production rate and renal blood flow in the near-term ovine fetus are not related to high and low voltage electrocortical activity

Studies in both the human and ovine near-term fetus have identified the clustering of physiologic and behavioral parameters into states. In a recent study in the human fetus a considerable decrease was found in fetal urine production during non-rapid eye movement (non-REM) compared with REM sleep. Whether this decrease was caused by decreased renal blood flow or changes in urine concentration is not known. This prompted us to investigate the relation between fetal urine production rate and electrocortical activity in the near-term ovine fetus. We hypothesized that in the ovine fetus urine production and renal blood flow during REM [comparable to low voltage electrocortical activity (LV ECoG)] would be lower than during non-REM [(high voltage (HV) ECoG)]. In eight fetal sheep between 123 and 127 d of gestation (term 147 d), ECoG, renal blood flow, urine flow, and urine osmolality were measured continuously for 6 h on 2 consecutive days. Data were analyzed into HV ECoG and LV ECoG whereafter urine flow, urine osmolality, and renal blood flow data were averaged per state. We found no significant differences in urine flow, urine osmolality, or renal blood flow between the two behavioral states in the ovine fetus. Because these data are in sharp contrast to those found in the human fetus, we conclude that the observed dissimilarities in renal responses between the human and sheep fetus add to the already known differences in behavioral states between the two species.

Ultrasound measurement of fetal urine flow

In summary, the measurement of fetal urinary flow remains controversial because the actual volumes of the human fetal bladder are unknown. The volume formula for an ovoid overpredicts fetal bladder volume and has a wide margin of certainty. The best ultrasonographic prediction of bladder volumes were calculated by applying a regression formula to the area of a sagittal sonographic image of the bladder (0.46323 + 1.39394.sagittal area). Calculations of the HFUFR based on the ovoid volume formula may be overestimated by 40%-70%. However, HFUFR estimated by regression of three to six longitudinal bladder area measurements is better with a narrow margin of uncertainty (+/- 35%). This technique affords improved accuracy and is easier and more convenient.

Acute maternal hydration in third-trimester oligohydramnios: effects on amniotic fluid volume, uteroplacental perfusion, and fetal blood flow and urine output

OBJECTIVE

Our purpose was to determine whether acute maternal hydration in pregnancies with third-trimester oligohydramnios (1) increases amniotic fluid index and hourly fetal urine production rate and (2) alters uteroplacental perfusion and fetal blood flow.

STUDY DESIGN

Ten women with third-trimester oligohydramnios (amniotic fluid index < or = 5 cm) and 10 controls with normal amniotic fluid volume (amniotic fluid index > 7 cm) were prospectively recruited for this study. Maternal plasma and urine osmolality, amniotic fluid index, hourly fetal urine production rate, and Doppler flow velocimetry of maternal uterine artery and fetal umbilical, descending aorta, middle cerebral, and renal arteries were determined before and after oral hydration by having the patient drink 2 L of water over 2 hours.

RESULTS

There was a significant reduction in maternal plasma (p < 0.05) and urine osmolality (p < 0.0001) in both groups after short-term oral hydration. Hydration increased amniotic fluid volume in women with oligohydramnios (mean change in amniotic fluid index 3.2 cm, 95% confidence intervals 1.1 to 5.3; p < 0.02) but not in those with normal amniotic fluid volume (mean change in amniotic fluid index -2.0, 95% confidence intervals -4.1 to +0.2). The hourly fetal urine production rate, however, did not increase in either group (mean change in hourly fetal urine production rate 3.5 ml/hr, 95% confidence intervals -11.7 to +18.7 and -6.8 ml/hr, 95% confidence intervals -2.9 to -10.7, respectively). Hydration was associated with an increase in uterine artery mean velocity in the oligohydramnios group (mean change in mean velocity 16.7 cm/sec, 95% confidence intervals 8.0 to 25.3; p < 0.006) but not in controls (mean change in mean velocity 1.2 cm/sec, 95% confidence intervals -19.7 to +22.1). There was no change in pulsatility index or in velocity in any of the fetal vessels studied in either group.

CONCLUSIONS

Short-term maternal oral hydration increases the amniotic fluid index in women with third-trimester oligohydramnios. Although the mechanism for this effect remains unclear, it could not be accounted for by fetal urination in this study but instead was associated with improved uteroplacental perfusion.

Behavioural state dependency of renal artery and descending aorta velocimetry and micturition in the normal term fetus

OBJECTIVE

To examine the behavioural state dependency of renal artery and descending aorta pulsatility index and hourly fetal urinary production rate (HFUPR) in the normal term fetus.

DESIGN

An observational study.

SUBJECTS

Twenty-three healthy women with a normal singleton pregnancy between 36 and 40 weeks gestation.

MAIN OUTCOME MEASURES

Renal artery and descending aorta pulsatility indices were established using pulsed Doppler ultrasound. HFUPR (ml) was estimated using real-time ultrasound. Fetal behavioural states 1F (quiet sleep) and 2F (active sleep) were determined from combined recordings of fetal heart rate, eye movements and body movements.

RESULTS

An inverse correlation between pulsatility index and fetal heart pattern was established for both the renal artery (r = 0.60; SD = 0.22; P = 0.0001) and the descending aorta (r = 0.73; SD = 0.16; P = 0.0001). The difference in mean pulsatility index between states 1F and 2F adjusted for fetal heart pattern was 0.09 (SD 0.23) for the renal artery (P = 0.02) and 0.26 (SD 0.19) for the descending aorta (P = 0.0001). Mean HFUPR was significantly higher (44.8%, P = 0.01) in state 1F (96 ml) compared with state 2F (54 ml). During the urinary filling phase, a poor correlation existed between renal artery pulsatility index and HFUPR (r = 0.04) and between descending aorta pulsatility index and HFUPR (r = 0.01).

CONCLUSIONS

In the normal term fetus renal artery and descending aorta pulsatility indices are reduced during fetal behavioural state 2F, suggesting reduced downstream impedance at the fetal trunk and in particular at renal level. This behavioural state is, however, also associated with reduced micturition. No correlation exists between renal artery and descending aorta pulsatility indices and fetal urinary output.

Hourly fetal urinary production rate and blood velocity waveforms in the fetal renal artery relative to fetal heart rate pattern and fetal eye movements in normal pregnancies at 30-32 weeks of gestation

In 14 normal pregnancies at 30-32 weeks of gestation, bloodflow velocity waveforms in the fetal renal artery and fetal micturition were studied in relation to fetal heart rate pattern (FHRP) and fetal eye movements. PI in the fetal renal artery was significantly lower in the presence of FHRP-B and eye movements separately and in combination compared with FHRP-A and absent eye movements separately and in combination. Fetal urinary output was higher in the absence of fetal eye movements. There was an inverse relationship between renal artery PI and fetal heart rate. However, since mean fetal heart rate in the various state variables was not essentially different, no standardization for heart rate was necessary. No correlation existed between renal artery PI and fetal urinary production rate. It can be concluded that in the normal fetus at 30-32 weeks of gestation, renal artery PI demonstrates a significant reduction in the presence of FHRP-B and fetal eye movements separately and in combination, suggesting reduced downstream impedance at renal level. However, the decrease in fetal micturition in the presence of fetal eye movements, suggests that reduced urinary output is determined by changes in fetal central nervous system activity rather than by changes in fetal renal perfusion.

Behavioural states and cardiovascular dynamics in the human fetus; an overview

Behavioural state dependent changes occur in the circulation of the normally developing human fetus at term. During fetal behavioural state 2F (active sleep), there seems to be preferential blood flow through the ductus venosus and foramen ovale into the left heart. Increased left ventricular output of well-oxygenated blood during fetal behavioural state 2F will be necessary to meet the raised energy demands both at cerebral and trunk level as demonstrated by reduced pulsatility index values in the descending aorta, renal artery and intracranial arteries during this behavioural state. The limited studies in IUGR suggest behavioural state independency in the descending aorta and internal carotid artery which may be attributed to the fetus being incapable of adjusting its circulation to the existing behavioural state in the presence of circulatory centralization.