Gestational age related changes in cardiac dynamics of the fetal baboon

Reference values for variables of fetal cardiocirculatory dynamics at 11-14 weeks of gestation

OBJECTIVE

Despite the increasing popularity of first-trimester fetal echocardiography, the evaluation of fetal heart function during this period remains challenging. The parameters of normal cardiac function at 11-14 weeks' gestation are not well defined and appropriate reference values have not yet been established. The purpose of this study was to evaluate the fetal cardiocirculatory dynamics during routine first-trimester screening and establish cross-sectional reference ranges for 11-14 weeks' gestation.

METHODS

Fetal echocardiography was performed on 202 women with singleton pregnancies at 11 + 0 to 13 + 6 weeks' gestation. Global cardiac function was evaluated using the heart : chest area ratio and Tei index of the left (LV) and right (RV) ventricles. The proportion of isovolumic contraction (ICT%) and ejection (ET%) times of the cardiac cycle, and the outflow velocities described the systolic function. Diastolic function was evaluated by the proportion of relaxation (IRT%) and filling (FT%) times, the ratio of the blood velocity through the atrioventricular valves during early filling (E) and atrial contraction (A) phases of the cardiac cycle, and ductus venosus pulsatility index for veins (DV-PIV). All participants had additional fetal echocardiography in the second trimester and neonatal clinical examination after birth to confirm normality.

RESULTS

The mean heart : chest area ratio (0.203 +/- 0.04) and the Tei indices of both ventricles did not vary significantly during weeks 11-14, but the mean Tei index of the LV (0.375 +/- 0.092) was significantly higher than that of the RV (0.332 +/- 0.079) (P = 0.001). The fetal heart rate (FHR) decreased with increasing crown-rump length (CRL) (P < 0.00001). The LV-ICT% did not vary significantly (P = 0.27), LV-IRT% (P = 0.03) and LV-ET% decreased (P = 0.01), whereas the LV-FT% increased (P = 0.02) with CRL. The RV-ET% (P = 0.84) and RV-FT% (P = 0.60) remained relatively stable. The LV-ET% was lower than the RV-ET% (P = 0.0001). The LV (P = 0.004) and RV (P < 0.00001) outflow velocities and E : A ratios of both ventricles (P < 0.0001) increased with advancing gestation. The E-velocity of the LV (P = 0.003) and RV (P = 0.002) increased significantly but the increase in A-velocity was not significant. The outflow velocity (P = 0.008) and E-velocity (P = 0.005) of the RV were higher than that of the LV but the A-velocities were similar (P = 0.066). The mean DV-PIV was 0.97 +/- 0.23 and did not change significantly (P = 0.95) during weeks 11-14. The FHR and DV-PIV did not correlate with the Tei index of either ventricle.

CONCLUSION

We have established reference ranges for the noninvasive evaluation of fetal cardiocirculatory dynamics at 11-14 weeks' gestation.

Prenatal nicotine-exposure alters fetal autonomic activity and medullary neurotransmitter receptors: implications for sudden infant death syndrome

During pregnancy, exposure to nicotine and other compounds in cigarette smoke increases the risk of the sudden infant death syndrome (SIDS) two- to fivefold. Serotonergic (5-HT) abnormalities are found, in infants who die of SIDS, in regions of the medulla oblongata known to modulate cardiorespiratory function. Using a baboon model, we tested the hypothesis that prenatal exposure to nicotine alters 5-HT receptor and/or transporter binding in the fetal medullary 5-HT system in association with cardiorespiratory dysfunction. At 87 (mean) days gestation (dg), mothers were continuously infused with saline (n = 5) or nicotine (n = 5) at 0.5 mg/h. Fetuses were surgically instrumented at 129 dg for cardiorespiratory monitoring. Cesarean section delivery and retrieval of fetal medulla were performed at 161 (mean) dg for autoradiographic analyses of nicotinic and 5-HT receptor and transporter binding. In nicotine-exposed fetuses, high-frequency heart rate variability was increased 55%, possibly reflecting increases in the parasympathetic control of heart rate. This effect was more pronounced with greater levels of fetal breathing and age. These changes in heart rate variability were associated with increased 5-HT(1A) receptor binding in the raphé obscurus (P = 0.04) and increased nicotinic receptor binding in the raphé obscurus and vagal complex (P < 0.05) in the nicotine-exposed animals compared with controls (n = 6). The shift in autonomic balance in the fetal primate toward parasympathetic predominance with chronic exposure to nicotine may be related, in part, to abnormal 5-HT-nicotine alterations in the raphé obscurus. Thus increased risk for SIDS due to maternal smoking may be partly related to the effects of nicotine on 5-HT and/or nicotinic receptors.

Fetal morphine metabolism and clearance are constant during late gestation

Fetal metabolism significantly contributes to the clearance of drugs from the fetus. To understand how the changes in fetal metabolism expected in late gestation alter fetal drug clearance, serial measurements of morphine metabolism were made in the fetal baboon over the latter third of gestation. Clearance and metabolism were evaluated in the context of fetal growth, onset of labor, and the administration of classical enzyme induction agents. Morphine, a probe substrate for the enzyme uridine diphosphate glucuronosyltransferase 2B7 (UGT2B7), was continuously infused to chronically catheterized fetal baboons while measuring morphine, morphine-3-beta-glucuronide, and morphine-6-beta-glucuronide concentrations. In some animals, intermittent infusions of the metabolites provided estimates of metabolite clearance and, hence, the rate of formation of metabolites and metabolic clearance. Overall, metabolic clearance of morphine from the fetus was 27 +/- 9.0 ml x min(-1) or 32% of total clearance. This is similar to the overall clearance in the adult baboon when standardized to weight. No change in any measure of metabolism or clearance of morphine or its glucuronide metabolites was found with gestational age, the presence of labor, or administration of UGT enzyme induction agents. Interpreting these findings using a physiologically based approach suggests that the intrinsic clearance of the fetal liver toward morphine is of sufficient magnitude that fetal hepatic clearance is flow-limited. The implication of a high intrinsic clearance is for significant placento-hepatic first-pass metabolism when drugs are administered to the mother. The previously held view of the "inadequacy of perinatal glucuronidation" needs to be reconsidered.

Influence of sympathetic tone on heart rate during vagal stimulation and nitroprusside induced hypotension in ovine fetus

OBJECTIVE

To characterize effects of sympathetic tone on fetal heart rate (FHR) reflex responses and FHR variability in late gestation.

DESIGN/METHODS

Changes in FHR and autonomic tones were studied (i) after electrical vagal stimulation and (ii) during nitroprusside-induced hypotension, in seven late gestation ovine fetus in control condition (ctrl), after dobutamine (beta1-activation) and atenolol (beta1-blockade). Results are expressed as mean +/- SEM.

RESULTS

(i) Minimal FHR after vagal stimulation was not influenced by atenolol or dobutamine but dobutamine accelerated FHR normalization. (ii) During nitroprusside induced hypotension atenolol inhibited the initial increases in FHR and FHR variability (measured by SD and LFnu) but not the bradycardia occurring below a mean arterial pressure of 38 +/- 2 mmHg. Dobutamine did not abolish the depressor reflex. During hypotension the positive chronotropic effect of sympathetic tone increased from 15 +/- 2 to 42 +/- 7 bpm then decreased at a rate of -7.6 +/- 1.5 bpm mmHg(-1), vagal negative chronotropic influence steadily increased at a rate of 1.9 +/- 0.4 bpm mmHg(-1). Changes in FHR variability were not correlated with vagal or sympathetic chronotropic effects.

CONCLUSIONS

beta1-stimulation does not affect sinus-node response to vagal stimulation but improves the speed of FHR normalization. FHR response to hypotension depends on an initial increase in both sympathetic and parasympathetic chronotropic effects that is associated with a sympathetic dependent increase in FHR variability and is followed by a withdrawal of sympathetic tone.

Subtle Autonomic and Respiratory Dysfunction in Sudden Infant Death Syndrome Associated With Serotonergic Brainstem Abnormalities: A Case Report

Sudden infant death syndrome (SIDS) is characterized by a sleep-related death in a seemingly healthy infant. Previously, we reported abnormalities in the serotonergic (5-HT) system of the medulla in SIDS cases in 2 independent datasets, including in the Northern Plains American Indians. The medullary 5-HT system is composed of 5-HT neurons in the raphé, extra-raphé, and arcuate nucleus at the ventral surface. This system is thought to modulate respiratory and autonomic function, and thus abnormalities within it could potentially lead to imbalances in sympathetic and parasympathetic tone. We report the case of a full-term American Indian boy who died of SIDS at 2 postnatal weeks, and who had subtle respiratory and autonomic dysfunction measured prospectively on the second postnatal day. Cardiorespiratory assessment of heart rate variability suggested that the ratio of parasympathetic to sympathetic tone was higher than normal in active sleep and lower than normal in quiet sleep in this case. At autopsy, arcuate nucleus hypoplasia and 5-HT receptor-binding abnormalities in the arcuate nucleus and other components of the medullary 5-HT system were found. This case suggests that medullary 5-HT system abnormalities may be able to be identified by such physiological tests before death. Replication of these findings in a large population may lead to the development of predictive cardiorespiratory assessment tools for future screening to identify infants with medullary 5-HT abnormalities and SIDS risk.

The contribution of fetal metabolism to the disposition of morphine

The contribution of fetal metabolism to drug disposition in pregnancy is poorly understood. With maternal administration of morphine, like many drugs, steady-state concentrations in fetal plasma are less than in maternal plasma. The contribution of fetal metabolism to this difference is unknown. Morphine was used as a model drug to test the hypothesis that fetal metabolism contributes significantly to drug clearance by the fetus. Infusions of morphine, morphine-3-beta-glucuronide (M3G), and morphine-6-beta-glucuronide (M6G) were administered to the fetal baboon. Plasma concentrations of drug and metabolite obtained near steady state were measured by high-performance liquid chromatography. During morphine infusion, morphine, M3G, and M6G concentrations rose linearly with dose. M3G concentrations exceeded M6G by 20-fold. Mean +/- S.D. clearances of morphine, M3G, and M6G from the fetus were 69 +/- 17, 2.3 +/- 0.60, and 1.6 +/- 0.24 ml x min(-1), respectively. Clearances seemed to be dose-independent. The mean +/- S.D. fraction of morphine dose metabolized was 32 +/- 5.5%. This converts to a fetal metabolic clearance of 22 +/- 6.5 ml x min(-1). In conclusion, one third of the elimination of morphine from the fetal baboon is attributable to metabolism, one third to passive placental transfer, and one third undefined. Furthermore, there is no evidence for saturation of metabolism. Fetal metabolism is surprisingly high compared with in vitro estimates of metabolism and morphine clearance in human infants. For morphine, fetal drug metabolism accounts for half the difference between fetal and maternal plasma concentrations.

Maturational changes in heart rate and heart rate variability in low birth weight infants

To provide insight into the maturation of neural mechanisms responsible for variability in heart rate during quiet and active sleep, 6-hour continuous electrocardiographic recordings and simultaneous minute-by-minute behavioral activity state assignments were performed in 61 healthy, growing low birth weight infants. The infants weighed 795-1600 g at birth and ranged between 31-38 weeks in postconceptional age. During this age interval there was a decrease in heart rate during quiet sleep and an increase in both time domain and frequency domain measures of the variability in cardiac interbeat intervals. In quiet sleep, global variability, measured as SD of R-R intervals, increased in relation to age, as did higher frequency variability, measured as the square root of the mean of squared successive differences in R-R intervals. Developmental changes in the 0.5-2.0 Hz spectral power band of RR-interval variability, another measure of high frequency variability, paralleled the changes seen in the time domain measure. Evaluation of patterns of changes in the magnitude and direction of successive interbeat intervals provided evidence that the incidence of sustained accelerations or decelerations increased whereas the incidence of no change in consecutive RR-intervals decreased as infants matured. Among the various measures of heart rate variability, the incidence of sustained change and no change in successive interbeat intervals were most closely related to postconceptional age in both sleep states. The overall decrease in heart rate, increase in heart rate variability, and increase in the pattern of changes in interbeat interval with postconceptional age are consistent with the maturation of the autonomic cardio-regulatory activity from 31-38 weeks age.

Diurnal rhythms of fetal and maternal heart rate in the baboon

To investigate the organization of diurnal rhythmicity during gestation, the relationship between daily cycles of maternal and fetal heart rate were measured in long-term studies of healthy chronically instrumented pregnant baboons. In each of six pregnancies, hourly mean values over a 168 h time series were obtained during a 7 to 10 day interval between 135 and 160 days of gestation. Data were modeled by a least squares fit to a cosine function with a period of 24 h. Hourly mean heart rate in the fetus ranged from 161 to 172 bpm (167.9+/-0.6 bpm), and the mother from 105 to 125 bpm (107.9+/-1.4 bpm). The amplitude of the daily fluctuations were 15 to 25 bpm for the fetuses and 25 to 60 bpm for the mothers. The relation between time series data and model estimates were significant (P < 0.001) in all cases with aggregate r2 = 0.747 for fetuses and 0.737 for the mothers. On average the time of day of the peak in fetal heart rate (15:05+/-0.3 h) was about 45 min after the maternal peak (14:21+/-0.4 h). This phase delay was significant (t = 2.63, P < 0.05). There was significant (P < 0.01) diurnal periodicity for each of six parameters used to assess different aspects of fetal heart rate variability with peak variability at night (23:00 to 2:00). Thus, during the latter third of pregnancy in both the maternal and fetal baboon 24 h periodicities of heart rate are present with peak rates in the midafternoon. The daily rhythms in fetal heart rate are linked with periodicities in maternal heart rate with a phase delay in the majority of cases. The synchrony of 24 h fluctuations in rate with parameters of rate variability is consistent with diurnal input into the fetal autonomic nervous system.