Fetal habituation to vibroacoustic stimulation in uncomplicated postterm pregnancies.

OBJECTIVES: Testing of fetal habituation to repeated vibroacoustic stimulation might give additional information concerning the fetal condition and central nervous system (CNS) functioning in postterm pregnancies. This study is designed to investigate whether healthy postterm fetuses are able to habituate and if so, if the habituation pattern of postterm fetuses differs from that of term fetuses. STUDY DESIGN: Twenty women with an uncomplicated pregnancy beyond 41 weeks gestational age (GA) participated, and 37 women with a GA between 37 and 40 weeks served as controls. The vibroacoustic stimulus was repeatedly applied to the maternal abdomen above the fetal legs for a period of 1s every 30s. A fetal trunk movement within 1s of stimulus application was considered a positive response. Lack of response to four consecutive stimuli indicated habituation. The habituation rate is defined as the number of stimuli applied before the fetus stopped responding. Data were compared using Mann-Whitney U test. RESULTS: Of the 18 postterm fetuses in which the presence or absence of habituation could be established, 14 habituated and four persisted in responding. Twenty-six of the 32 term fetuses, in which the presence or absence of habituation could be established, habituated and six persisted in responding. The habituation rate varied widely in postterm (median of eight stimuli, quartile ranges of 6 and 18) as well as in term fetuses (median of 9.5 stimuli, quartile ranges of 6 and 15). There was no difference in median habituation rate between postterm and term fetuses. CONCLUSION: The ongoing maturation of the CNS in the last weeks of pregnancy appears not to be reflected in the fetal habituation pattern. Furthermore, the interfetal variability in habituation of healthy postterm fetuses is such that testing of habituation appears not to be suitable for the identification of the fetus at risk for an adverse neonatal outcome.

Hypoxia in fetal lambs: a study with (1)H-MNR spectroscopy of cerebrospinal fluid.

In fetal lambs, severe hypoxia (SH) will lead to brain damage. Mild hypoxia (MH) is thought to be relatively safe for the fetal brain because compensating mechanisms are activated. We questioned whether MH, leading to mild acidosis, induces changes in cerebral metabolism. Metabolites in cerebrospinal fluid (CSF) samples, as analyzed by proton magnetic resonance spectroscopy, were studied in two groups of seven anesthetized near-term fetal lambs. In group I, SH leading to acidosis with an arterial pH <7.1 was achieved. In group II, MH with an intended pH of 7.23--7.27 was reached [start of MH (SMH)], and maintained during 2 h [end of MH (EMH)]. During SH, choline levels in CSF, a possible indicator of cell membrane damage, were increased. Both during SH and at EMH, CSF levels of lactic acid, alanine, phenylalanine, tyrosine, lysine, branched chain amino acids, and hypoxanthine were increased compared with control values and with SMH, respectively. At EMH, the hypoxanthine CSF-to-blood ratio was increased as compared with SMH. These results indicate that prolonged MH leads to energy degradation in the fetal lamb brain and may not be as safe as assumed.

The responses to repeated vibroacoustic stimulation in a fetus with trisomy 18.

Observation of the fetal movement and fetal heart rate responses to repeated vibroacoustic stimulation might be useful as a measure to assess fetal well-being. Studies of abnormal fetuses are needed to gain insight in the spectrum of possible responses to stimulation. We present a case of a fetus with trisomy 18 that failed to respond to repeated stimulation.

Fetal habituation to vibroacoustic stimulation in relation to fetal states and fetal heart rate parameters.

OBJECTIVES: Fetal habituation to repeated stimulation has the potential to become a tool in the assessment of fetal condition and of the function of the fetal central nervous system (CNS). However, the influence of fetal quiescence and activity on habituation remains to be clarified. We studied habituation and the influence of fetal state and fetal heart rate (FHR) parameters on habituation in healthy term fetuses. SUBJECTS AND METHOD: We studied habituation in 37 healthy fetuses in two tests with an interval of 10 min. The vibroacoustic stimuli were applied to the maternal abdomen above the fetal legs for a period of 1 s every 30 s. A fetal trunk movement within 1 s after stimulation was defined as a positive response. Habituation rate is defined as the number of stimuli applied before an observed non-response to four consecutive stimuli. The FHR patterns (FHRP) of the 10 min observation period before and after the tests were visually classified. Fetal states were defined according to the FHRP. Baseline FHR, FHR variability and the number of accelerations were calculated in a subgroup of 25 fetuses. RESULTS: Of the 32 fetuses that responded normally during the first test, 26 habituated and six had persistent responses. The median habituation rate decreased significantly in the second test (P=0.001). There was no difference in habituation rate between fetuses that where initially in a quiet state and those in an active state. The FHR parameters before the first test and the difference between these FHR parameters before and after the test did not correlate with the habituation rate. CONCLUSIONS: Although the majority of healthy fetuses was able to habituate, the interfetal variability in habituation performance is such that testing of habituation seems not to be a sensitive tool for the assessment of the fetal CNS. This variability is neither the result of differences in fetal state nor of the various FHR parameters before testing, nor of the difference in change of FHR parameters arising from stimulation.

Effect of low-sodium diet on uteroplacental circulation.

OBJECTIVE: To study the influence of chronic dietary sodium restriction on uteroplacental circulation. METHODS: In a randomized trial, Doppler flow velocity waveforms of the uterine and umbilical artery were studied at monthly intervals during pregnancy in 59 women on a low-sodium diet and in 68 controls. RESULTS: Pulsatility index (PI), resistance index (RI), and A/B ratio of the uterine artery were significantly lower during sodium restriction, whereas PI, RI, and A/B ratio of the umbilical artery were significantly higher. CONCLUSIONS: The lower resistance indices of the uterine artery during sodium restriction might reflect an increase in pulse pressure/impedance ratio as a result of activation of the renin-angiotensin system. The increase in umbilical artery resistance indices supports the hypothesis that fetal circulation might be altered by chronic dietary sodium restriction.

Urinary prostaglandin excretion in pregnancy: the effect of dietary sodium restriction.

INTRODUCTION: Dietary sodium restriction results in activation of the renin-angiotensin-aldosterone-system. In the non-pregnant situation renin release in response to a low sodium diet is mediated by prostaglandins. We studied the effect of dietary sodium restriction on urinary prostaglandin metabolism in pregnancy. PATIENTS AND METHODS: In a randomized, longitudinal study the excretion of urinary metabolites of prostacyclin (6-keto-PGF(1 alpha)and 2,3-dinor-6-keto-PGF(1 alpha)) and thromboxane A(2)(TxB(2)and 2,3-dinor-TxB(2)) was determined throughout pregnancy and post partum in 12 women on a low sodium diet and in 12 controls. RESULTS: In pregnancy the excretion of all urinary prostaglandins is increased. The 6-keto-PGF(1 alpha)/ TxB(2)-ratio as well as the 2, 3-dinor-6-keto-PGF(1 alpha)/ 2,3-dinor-TxB(2)-ratio did not significantly change in pregnancy. CONCLUISION Prostacyclin and thromboxane do not seem to play an important role in sodium balance during pregnancy.

Responses to vibroacoustic stimulation in a fetus with an encephalocele compared to responses of normal fetuses.

BACKGROUND: Observation of fetal movement and fetal heart rate (FHR) responses to repeated vibroacoustic stimulation (VAS) might be useful as a measure to assess fetal well-being and to assess the integrity of the fetal central nervous system (CNS). We observed the movement and FHR responses to repeated VAS of a term fetus with a serious brain anomaly as compared to responses of normal term fetuses. SUBJECTS, METHODS, RESULTS: In 37 normal term fetuses and in a term fetus with an encephalocele we studied movement and FHR response to repeated VAS. All normal fetuses responded within 1 s after stimulation with general body movement and FHR acceleration. At 36 gestational weeks, no movement or FHR responses were seen in the fetus with an encephalocele. Repetition of the test in this fetus after one week still showed no response to repeated VAS. CONCLUSION: Normal fetuses showed movement and FHR responses to external stimulation. The fetus with an encephalocele did not respond to repeated VAS with a movement or FHR acceleration. Case studies in fetuses with structural anomalies of the CNS are needed to gain insight into the spectrum of possible responses to VAS.

Fetal learning and memory.

We used fetal habituation to repeated vibroacoustic stimulation to assess fetal memory. After the initial stimulus, we assessed the fetuses 10 min later and again after 24 h. 16 of 19 fetuses habituated rapidly to the stimuli at 10 min (p=0.004) and 24 h (p=0.042) after the initial test. We conclude, therefore, that fetuses are able to learn: they have a short-term memory of at least 10 min, and a long-term memory of at least 24 h.

Hypoxia, the subsequent systemic metabolic acidosis, and their relationship with cerebral metabolite concentrations: An in vivo study in fetal lambs with proton magnetic resonance spectroscopy.

OBJECTIVE: The relationship among decreased fetal arterial oxygen saturation, the subsequent systemic metabolic acidosis, and changes in cerebral metabolite concentrations in the fetal lamb brain was investigated by means of quantitative proton magnetic resonance spectroscopy. STUDY DESIGN: Fetal hypoxia was induced in 6 fetal lambs by gradual reduction of the oxygen supply to the anesthetized pregnant ewe. In vivo proton magnetic resonance spectroscopy was performed on the fetal lamb brain simultaneously with repeated measurements of fetal arterial oxygen saturation and acid-base balance. RESULTS: Proton magnetic resonance spectra showed metabolites such as inositol, choline compounds, creatine, and N-acetylaspartate. A signal for cerebral lactate was below the detection level under normoxic conditions and increased during hypoxia to indicate concentrations varying from 2.8 to 11.1 mmol/kg wet weight brain tissue. N -Acetylaspartate signals decreased during hypoxia, whereas signals of inositol, choline compounds, and creatine remained constant. CONCLUSION: These results support the view that fetal cerebral anaerobic metabolism in fetal lambs does not start under hypoxic conditions if the arterial blood pH is >7.28 or the base excess is >-8 mmol/L.

State transitions in uncomplicated pregnancies after term.

A behavioural state transition is the time interval between two different behavioural states. In low-risk fetuses, the fetal heart rate pattern (FHRP) is the first variable to change in transitions from 1F to 2F ('non-REM-sleep' to 'REM-sleep') and the last variable to change in transitions from 2F to 1F. This is not the case in IUGR (intra-uterine growth retardation), and absence of a specific order in which behavioural variables are changing might be an indication for a (mild) disturbance of the fetal central nervous system (CNS). We investigated whether state transitions in twelve low risk term fetuses (39-41 weeks post menstrual age, PMA; control group) differ from those in twelve uncomplicated pregnancies > 41 weeks PMA (study group). All subjects underwent one behavioural study in which fetal heart rate pattern, eye and body movements were recorded simultaneously. We recorded 23 transitions from 1F to 2F and 20 from 2F to 1F. Median (range) duration for transitions from 1F to 2F was 85 (10-180) s in the study group, and 60 (10-180) s in the control group. Transitions from 2F to 1F lasted 80 (10-140) and 60 (30-100) s, respectively. In both groups, the FHRP was the first variable to change in transitions from 1F to 2F, however, in transitions from 2F to 1F, no specific order in change of variables could be demonstrated. We conclude that the study of transitions does not distinguish between the term and after term fetuses under optimal conditions. Whether or not the analysis of state transitions can be used to distinguish 'normal' from 'abnormal' fetuses and detect the fetus at risk after term awaits further study.

Proton magnetic resonance spectroscopy of fetal lamb brain during hypoxia.

Proton magnetic resonance spectroscopy of fetal lamb brain was performed simultaneously with repeated measurements of fetal arterial oxygen saturation during decrease of oxygen supply. Magnetic resonance spectra displayed the same metabolite peaks as detected in the human fetal brain. Cerebral lactate signals could be detected during fetal hypoxia.

Accuracy of fetal pulse oximetry and pitfalls in measurements.

Pulse oximetry is a technique for estimating arterial oxygen saturation continuously and non-invasively. Reflectance pulse oximetry might become useful for monitoring the fetus during labour but it is much more susceptible to all kinds of physiological variations than the well-established transmission pulse oximetry for neonatal or adult monitoring. This review focuses on the accuracy of reflectance pulse oximetry. Results of human, animal, in vitro and theoretical models indicate that factors such as; blood volume fraction differences, haematocrit, and blood flow differences are major sources for inaccurate pulse oximetry readings in the fetal arterial oxygen saturation range of 10-80%. These factors cannot be overcome by systems using two wavelengths sensors with the 660/890 or 940 nm combination. Reported precision values (S.D. of difference between pulse oximeter and blood sample saturation) range between 2.5 and 12.9% for various 660 nm sensors. Most sensors were tested only once with a limited number of animals. A new 735/890 nm sensor (Nellcor Puritan Bennett) demonstrates a promising accuracy (precision around 5%) in two studies. Various other sensors have also been developed, but are not or scarcely evaluated. Without thorough establishment of the reliability of this technique, clinical fetal oxygen saturation data are still of limited value.

The efficacy of intrapartum fetal surveillance when fetal pulse oximetry is added to cardiotocography.

OBJECTIVE: To determine if oxygen saturation measurement with pulse oximetry (SpO2) in combination with cardiotocograghy (CTG), improves the assessment of the intrapartum fetal condition. STUDY DESIGN: Four expert obstetricians individually evaluated 119 cases that were monitored during labor: during the first session the CTG data were available, and in the second session CTG and SpO2 data were evaluated. They were instructed to indicate the need for intervention and to estimate the umbilical artery pH. RESULTS: In the non-acidotic group (umbilical artery pH > or = 7.15, n = 112) the average(+/-S.D.) number of interventions decreased from 27(+/-17) to 16(+/-9) when SpO2 was available. This reduction in number of interventions resulted in an significantly increased specificity for two referees. In the acidotic group (n = 7) the average number of interventions also decreased, from 6(+/-2) to 4(+/-2), and as a consequence the sensitivity decreased. The pH estimate based on CTG + SpO2 was higher in both acidotic and non-acidotic fetuses than the estimated pH based on CTG alone. CONCLUSION: In this study all referees intervened less frequently when SpO2 was used as an adjunct to CTG. This resulted in fewer unnecessary operative interventions, but may also lead to unidentified fetal acidosis. The number of acidotic newborns (n = 7) was too small, however, to draw definite conclusions. Larger studies should address the efficacy of SpO2 in detecting fetal compromise before clinical use can be advocated.

Validation of reflectance pulse oximetry: an evaluation of a new sensor in piglets.

OBJECTIVE: A new reflectance pulse oximetry sensor, developed for intrapartum estimation of arterial oxygen saturation (SaO2), was calibrated and evaluated. The sensor contains two light emitting diodes of 735 and 890 nm, and a photodetector at a distance of 14 mm from both light emitting diodes. METHODS: In seven Yorkshire/Hampshire piglets, the reflectance sensor (Nellcor Puritan Bennett Inc.) was calibrated using blood sample SaO2 values. The resulting calibration line was evaluated in four Dutch piglets, by comparing pulse oximetry saturation readings (SpO2) with blood sample and intravascular fiberoptic oximetry SaO2 values. Several reflectance sensors were fixed on each animal. Desaturation levels were obtained by changing the gas mixture of oxygen/ nitrous oxide via a tracheal catheter. RESULTS: In the Yorkshire/ Hampshire piglets, the standard deviation of difference (SpO2-SaO2) was 4.7% (n = 364), over an SaO2 range of 17% to 100%. In the Dutch piglets, the mean difference (SpO2-SaO2) was -1.6% and the standard deviation of difference was 5.4%, over the same SaO2 range (n = 254). Comparisons of continuous recordings of reflectance SpO2 and fiberoptic SaO2 revealed variation in individual regression lines. CONCLUSIONS: This new 735/890 nm reflectance sensor demonstrates acceptable accuracy in piglets. A further evaluation during labor should assess its feasibility for fetal surveillance.

Neonatal complications in newborns with an umbilical artery pH < 7.00.

OBJECTIVE: Our purpose was to determine the significance of an umbilical artery pH < 7.00 in relation to neonatal morbidity and mortality. STUDY DESIGN: Between 1986 and 1993 acid-base assessment of the umbilical artery was performed routinely in 10,699 deliveries. In a retrospective cohort study 84 nonanomalous neonates with an umbilical artery pH < 7.00 were individually matched with 84 neonates with an umbilical artery pH > 7.24. Matched variables included year of delivery, gender, parity, maternal age, delivery mode, fetal presentation, gestational age, and birth weight. Differences in morbidity between the two groups during the neonatal period (until 28 days after delivery) were investigated. RESULTS: Neonates with an umbilical artery pH < 7.00 versus > 7.24 showed significant differences in the following: neonatal condition directly post partum; neurologic, respiratory, cardiovascular, and gastrointestinal complications; and neonatal intensive care unit admissions. No significance was found in renal dysfunction and mortality rate. The proportion of premature infants (< 37 weeks) was 17% in both groups. In the acidotic group a 1-minute Apgar score < or = 3 and a 5-minute Apgar score < 7 was predictive for neonatal complications. CONCLUSIONS: Severe intrapartum asphyxia, quantified by an umbilical artery pH < 7.00, poses a threat to the neonate's health.

Reflectance pulse oximetry in fetal lambs: subcutaneous vessels and vasoconstriction affect its reliability.

OBJECTIVE: Reflectance pulse oximetry (RPOX) has been introduced for intrapartum fetal surveillance. The purpose of this study was to describe two possible effects on the reliability of RPOX, namely the effect of the presence of a subcutaneous vein and the effect of vasoconstriction by adrenaline, both at fetal SaO2 levels. METHODS: In four anesthetized fetal lambs, a prototype 660/890 nm reflectance sensor (Nellcor Inc.) was placed on the fetal head, with the photodiode of the sensor precisely over a superficial subcutaneous vein. Measurements were made before and after coagulation of the vein. In five anesthetized fetal lambs, one or two reflectance sensors were placed on the fetal head and/or neck and adrenaline was administered in doses of 0.02 to 0.04 mg via a brachial artery. Pulse oximeter saturation readings (SpO2) were compared with continuous arterial oxygen saturation (SaO2) values obtained using a fiberoptic catheter (Opticath, Abbott) in the carotid artery. RESULTS: When the sensor was placed over the vein, the pulse oximeter read 18% to 24% too low at a SaO2 level of 20% to 50%. After coagulation of the vein, SpO2 readings were in agreement with fiberoptic SaO2 values. Administration of adrenaline resulted in a large overestimation of the SaO2 in 6 of the 7 measurements. CONCLUSIONS: Subcutaneous veins and vasoconstriction can affect the reliability of reflectance pulse oximetry. As comparable situations may occur during labor, SpO2 readings should be interpreted with caution when this kind or comparable types of RPOX sensors are used at low SaO2 levels.

Transmission pulse oximetry in the fetal lamb: is there a universal calibration?

Transmission pulse oximetry is widely used for oxygen monitoring. The use of pulse oximeters is steadily expanding toward situations with low arterial oxygen saturation (Sao2) values. Therefore, we evaluated transmission pulse oximetry in the unanesthetized fetal lamb at low Sao2 levels. In seven fetal lambs, fetal hypoxemia was induced by occlusion of the maternal common iliac artery, four days after the instrumentation of the animal. Two Nellcor prototype transmission Y-sensors (light emitting diodes: 660 and 890 nm) were applied, one around a forelimb muscle and one around a skinfold in the neck, and were connected to Nellcor pulse oximeters. The pulse oximeter was calibrated for the skin measurements. Pulse oximeter saturation readings (Spo2) were compared with sample Sao2 values, over an Sao2 range of 13 to 63%. For the neck sensor the SD of the difference was 5.0% (n = 101). For the muscle sensor the mean difference was 19.5% and the SD of the difference was 5.9% (n = 206). Regression analysis showed a different calibration line for the muscle sensor with the equation: Spo2 = 0.92 x Sao2 + 21.90. Continuous recordings were obtained both from the forelimb muscle and from the neck, but the recordings from the neck showed a substantial loss of signal during the hypoxemia period. We conclude that transmission pulse oximetry is less accurate below an Sao2 of 70% in fetal lambs than above 70% Sao2. At these low levels of Sao2, pulse oximeters may need to be constructed with different calibration lines for various application positions of the body.