Baseline Fetal Heart Rate Research Articles

The effect of magnesium sulfate on fetal heart rate parameters: A randomized, placebo-controlled trial

Objective: We sought to determine the effect of magnesium sulfate on fetal heart rate baseline value, variability, and acceleration-deceleration pattern. Study Design: Normal, nonlaboring pregnant patients at >30 weeks’ gestation were recruited. Baseline fetal heart rate monitoring for 1 hour was performed. After an 800-kcal meal, patients were randomized to receive either an intravenous loading dose of 6 g of magnesium sulfate in 100 mL of isotonic sodium chloride solution or 100 mL of isotonic sodium chloride solution alone. Subsequently, patients in the magnesium sulfate group received a 2-g/h intravenous infusion for 3 hours at a rate of 125 mL/h. Patients randomized to the sodium chloride solution group received a sodium chloride solution infusion at a similar rate (unlabeled intravenous bags). Maternal blood was drawn at 0, 1, and 3 hours for determination of total and ionized magnesium and calcium, electrolyte, and glucose levels. One hour of fetal heart rate monitoring was repeated at 1 and 3 hours of infusion. Tracings were interpreted without identifiers (of time or group) by using the National Institute of Child Health and Human Development fetal heart rate monitoring guidelines. Results: Magnesium sulfate administration resulted in decreased fetal heart rate baseline values and variability in the third hour. The fetal heart rate baseline value was 134.4 ± 6.3 versus 136.6 ± 6.4 beats/min before infusion ( P > .05), 134.4 ± 7.1 versus 135.1 ± 6.6 beats/min in the first hour ( P > .05), and 134.6 ± 7.1 versus 132.3 ± 7.6 beats/min in the third hour ( P < .05) in the sodium chloride solution group versus the magnesium sulfate group, respectively. Fetal heart rate variability (grades 1-5) was 2.75 ± 0.33 versus 2.82 ± 0.29 before infusion ( P > .05), 2.81 ± 0.30 versus 2.84 ± 0.28 in the first hour ( P > .05), and 2.71 ± 0.52 versus 2.67 ± 0.36 in the third hour in the sodium chloride solution group versus the magnesium sulfate group, respectively ( P < .05). Magnesium sulfate blocked the positive correlation between gestational age and number of accelerations found in control subjects. No significant decelerations were identified. Conclusions: Prolonged administration of magnesium sulfate was associated with decreased fetal heart rate baseline values and variability. Given the small magnitude of these changes, the clinical significance of these findings is questionable. Magnesium sulfate inhibition of the increasing number of accelerations with gestational age needs to be considered when fetal well-being is assessed. (Am J Obstet Gynecol 1999;181:1122-7.)

The effect of betamethasone and dexamethasone on fetal heart rate patterns and biophysical activities, A prospective randomized trial

BACKGROUND; Contradictory findings on the effect of betamethasone versus dexamethasone on antenatal tests of fetal well-being have been reported. The purpose of this study was to compare the effects of these steroid compounds on fetal heart rate patterns and biophysical activities in a prospective. randomized trial. Forty-six pregnant women (gestational age range 27-34 weeks) at risk for preterm delivery were randomized to receive betamethasone or dexamethasone for enhancement of fetal lung maturity. Fetal heart rate was recorded for 60 minutes and analyzed with the Sonicaid System 8000 before (0 hours), and 48 hours and 96 hours after steroid administration. Subsequently, fetal limb, body and breathing movements were sonographically observed and quantified for 30 minutes. To account for fetal circadian rhythms, all examinations were performed between 1 p.m. and 5 p.m., at least one hour after maternal meals. Fetal heart rate accelerations (p<0.001; p<0.01), short-term variation (p<0.0001; p<0.05), long-term variation (p<0.01; p=NS), duration of high episodes (p<0.001; p<0.05), total movement count (p<0.001; p<0.05), and duration of breathing time (p<0.0001; p<0.0001) were substantially reduced 48 h after betamethasone and dexamethasone administration, respectively, with percent reduction being larger for the betamethasone group, except for breathing movements (p<0.05; p<0.001; p<0.001; p<0.005; p<0.05; p=NS; respectively). In 68.2%( and 45.5% of fetuses, less than 30 seconds of continuous breathing movements were found in the betamethasone and dexamethasone groups, respectively. In 71.8% and 12.5%, of fetuses, respectively, less than 2 body/limb movements were observed. Therefore five and two fetuses in the betamethasone and dexamethasone study group, respectively, had both nonreactive fetal heart rate monitors for 60 minutes and biophysical profiles of < or =4/10. All parameters returned to baseline values at 96 h. Baseline fetal heart rate and numbers of decelerations remained unchanged (p=NS). Both betamethasone and dexamethasone induce a profound, albeit transient, suppression of fetal heart rate characteristics and biophysical activities in the preterm fetus. However, the effect of betamethasone is more pronounced. Awareness of these phenomena might prevent unwarranted iatrogenic delivery of preterm fetuses.

Physical conditioning effects on fetal heart rate responses to graded maternal exercise.

This study examined the effects of advancing gestational age and maternal aerobic conditioning (stationary cycling) on fetal heart rate (FHR) responses to strenuous non-steady-state maternal exercise. Subjects chose to participate in either an exercise group (EG) or control group (CG). Fourteen healthy, previously sedentary pregnant women participated in the exercise group, and six pregnant controls remained sedentary. Stationary cycling (heart rate target: 145 beats x min(-1)) was performed 3 d x wk(-1) by the exercised group. Exercise duration was increased from 14 to 25 min x session(-1) during the second trimester and was maintained at 25 min x session(-1) throughout the third trimester. FHR was monitored before, during, and after a progressive submaximal cycle ergometer test (peak heart rate = 170 beats x min(-1)) performed at approximately 27 and 37 wk gestation. Mean FHR increased significantly (P < 0.05) during exercise, followed by a modest suppression and then a delayed rise during the recovery period at both observation times. Fetal bradycardia was not observed in any of the exercise tests. Effects of advancing gestational age included a lower FHR baseline both at rest and in response to maternal exercise and a lower incidence of exercise-induced tachycardia. Maternal physical conditioning did not significantly alter FHR response to maternal exercise. Our results support the hypothesis that FHR responses to strenuous exercise are altered by advancing gestational age and a brief progressive exercise test terminated at a maternal heart rate of 170 beats x min(-1) does not induce fetal distress during a healthy pregnancy.

Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy.

Ontogenic changes in baseline and 24-h rhythms of fetal arterial blood pressure (FABP) and heart rate (FHR) and their regulation by the fetal adrenal were studied in 18 fetal sheep chronically instrumented at 109-114 days gestation (GA). In the long-term study, FABP and FHR were continuously recorded from 120 days GA to spontaneous term labor (>145 days GA) in five animals. Peak times (PT) and amplitudes (Amp) of cosinor analysis were compared at 120-126, 127-133, and 134-140 days GA. Consistent, significant linear increases in FABP and linear decreases in FHR were observed in all fetuses. Significant 24-h rhythms in FABP and FHR were observed during all the time windows. In the adrenalectomy study, to test the hypothesis that fetal cortisol plays a key role in cardiovascular maturation, fetal adrenals were removed in eight animals (ADX); sham fetal adrenalectomy was performed on five animals (Con). Cortisol (4 microgram/min) was infused intravenously in four ADX fetuses from day 7 postsurgery for 7 days (ADX+F). No significant changes in PT and Amp in FABP and FHR were observed. Plasma cortisol levels remained low in Con and ADX fetuses (<4.9 ng/ml). Cortisol infusion increased fetal plasma cortisol to 22.3 +/- 3.2 ng/ml (mean +/- SE) on day 13 in ADX+F fetuses. FABP increased in control and ADX+F but not ADX fetuses; FHR decreased in control and ADX but rose in ADX+F fetuses. These results suggest that, in chronically instrumented fetal sheep at late gestation, 1) increases in FABP and decreases in FHR are maintained consistently from 120 to 140 days GA, with distinct 24-h rhythms, the PT and Amp of which remain unchanged, and 2) the physiological increase in FABP is dependent on the fetal adrenal; bilateral removal of the fetal adrenals does not prevent the ability of cortisol to produce a sustained increase in FABP.

Gender does not affect fetal heart rate variation.

There is a widespread but erroneous view among the lay public that there is a difference in the baseline fetal heart rate between male and female fetuses. It has been suggested that this perception might reflect an actual difference in fetal heart rate variability. Therefore, we studied the fetal heart rate variation in 79 white European women using the Sonicaid System 8002 computer. Fourty-four of the fetuses were male and 35 were female. There was no significant gender difference in any measured aspect of fetal heart rate variation.

Uncomplicated baseline fetal tachycardia or bradycardia in postterm pregnancies and perinatal outcome.

to assess perinatal outcome in post-term pregnancies in which fetal heart rate (FHR) monitoring revealed either uncomplicated baseline fetal tachycardia (> or = 160 bpm) or fetal bradycardia (< or = 120 bpm). We performed a case-control study of patients who underwent fetal assessment between July 1989 and June 1995, as part of post-term evaluation. Inclusion criteria consisted of nonlaboring afebrile patients with singleton post-term pregnancies > or = 41 weeks of gestation by strict dating criteria (last menstrual period consistent with ultrasound biometric parameters obtained prior to 20 weeks' gestation), normal fetal anatomy, intact membranes, and reactive nonstress test with no evidence of chorioamnionitis. Patients with fetal tachy or brady arrhythmias, FHR decelerations, or loss of short-term beat-to-beat variability were excluded. Baseline FHR was recorded retrospectively by an observer blinded to maternal and neonatal clinical outcome. For each case of uncomplicated baseline fetal tachycardia (> or = 160 bpm) or bradycardia (< or = 120 bpm), either two or three control cases (matched for maternal age and parity), with FHR > 120 bpm and < 160 bpm, were identified. Outcome variables assessed included: incidence of cesarean delivery, nuchal cord at delivery, meconium-stained amniotic fluid (AF), 5-min Apgar score < 7, fetal growth restriction (< 10th percentile for 41 weeks' gestation), meconium aspiration syndrome, and neonatal intensive care unit (NICU) admission. Statistical analysis included two-tailed t-test and chi2 test when appropriate, with p < 0.05 considered significant throughout. During the study period 1390 post-term patients (6.81% of the general population) were assessed. Of these, 31 (2.23%) fetuses exhibited baseline FHR > or = 160 bpm, and 76 fetuses (5.46%) exhibited baseline FHR < or = 120 bpm. No significant differences were noted in the incidence of cesarean delivery, presence of nuchal cord at delivery, meconium-stained AF, 5-min Apgar scores < 7, fetal growth restriction, meconium aspiration syndrome, or the incidence of NICU admissions between the groups of patients with fetal tachycardia, bradycardia, and their respective controls. Uncomplicated baseline fetal tachycardia or bradycardia in postterm patients are not associated with an increase in the incidence of adverse perinatal outcome.

Diurnal variation in baseline heart rate of anencephalic fetuses.

The diurnal change in baseline fetal heart rate (FHR) of four anencephalic fetuses at 20, 23, 24 and 30 weeks of gestation were examined. The mean baseline FHR in 00.00-06.00 h, 06.00-12.00 h, 12.00-18.00 h and 18.00-24.00 h were compared by one-factor ANOVA and Scheffe's test in each case. The diurnal variations in baseline FHR were recognized in all subjects (P < 0.01). In 3/4 subjects, the lowest values were at 00.00-06.00 h. The diurnal variation in baseline FHR might be caused by maternal factors because it was present even in the anencephalic fetuses that had no central nervous system having the oscillators of the circadian rhythm.

Effects of 4 hours magnesium sulfate infusion on fetal heart rate variability and reactivity in a goat model.

Effects of magnesium sulfate were investigated on fetal heart rate (FHR) baseline, variability, and reactivity in goats. Six chronically catheterized fetuses of Japanese Saanen goat at 125 to 130 days' gestation (term = 147 days) were used. Magnesium sulfate was directly infused to the fetuses. Short-term variability and long-term variability were obtained according to Huey et al. The baseline, reactivity, short-term variability and long-term variability of the FHR were compared between those receiving magnesium sulfate infusions and those receiving vehicle infusions without magnesium sulfate for 4 hr. Two-way analysis of variance (ANOVA) and Duncan's multiple range test was applied for statistical significance. Four hours magnesium sulfate infusion significantly increased fetal plasma concentration of magnesium from 2.4-6.6 mg/dL, without significant changes in fetal respiratory gases and pH values. The baseline FHR was significantly decreased by magnesium infusion compared with that receiving vehicle infusion. The incidence of acceleration, short-term variability, and long-term variability during the fourth hour of magnesium infusion was also significantly decreased compared to a controlled infusion. The time spent by high amplitude phase of short-term variability and that of long-term variability were also significantly reduced. Significant correlation was obtained between the magnesium concentration and incidence of acceleration at fourth hour of magnesium infusion. Four hours infusion of magnesium sulfate significantly decreases baseline FHR, short-term variability, long-term variability, and reactivity in fetal goats at 0.85 gestation.

Effects of transdermal nicotine or smoking on nicotine concentrations and maternal-fetal hemodynamics

To compare nicotine concentrations and fetal middle cerebral artery resistance indices (RIs) during 21-mg transdermal nicotine use with these values during maternal smoking. In this randomized, crossover study, participants smoked approximately 20 cigarettes daily and were between 24 and 36 weeks' gestation. Subjects were randomized to transdermal nicotine or to smoking ad libitum for 8 hours. One week later, they crossed over to the other condition. Maternal plasma nicotine concentrations and hemodynamic measurements were obtained before and after the onset of smoking or patch placement. Area under the plasma nicotine concentration-time curve during patch use was similar to continued smoking (93 versus 89 ng-hour/mL, respectively) (P = .77). The mean (standard error [SE] change in the middle cerebral artery RI from baseline to 4 hours later was similar during patch use and smoking: -.002 (0.008) versus -.02 (0.015), respectively (P = .3). The study had greater than 80% power to detect a 25% difference in nicotine concentrations and a change of 2 standard deviations in the middle cerebral artery RI between conditions. An unexpected finding was that of a loss of fetal heart rate (FHR) reactivity in 5/8 tracings after patch placement versus 1/6 tracings after smoking (P = .12). The baseline FHR increased by a mean (SE) of 8 (4) beats per minute with loss of reactivity in the patch condition, compared with a decrease of 3 (3) beats per minute without loss of reactivity (P = .05). Eight-hour use of 21-mg transdermal nicotine yields nicotine concentrations and middle cerebral artery RIs similar to those produced by hourly smoking in pregnant smokers.

Computerised estimation of the baseline fetal heart rate in labour: the low frequency line.

To develop and evaluate a computerised algorithm for the estimation of the fetal heart rate baseline (low frequency line) during labour. Retrospective observational study. Fetal heart rate signals were obtained from women in labour using the Nottingham fetal ECG monitor. The computerised algorithm for the baseline estimation was developed for intrapartum applications and is based on averaging modal fetal heart rate values. Evaluation was carried out on sixty cardiotocographic recordings by 12 experts and by the computer. These estimates were compared with those obtained from the computerised system using paired differences and intraclass correlation. The study showed that it is possible to produce a low frequency line from data obtained from intrapartum records. The system could not estimate the low frequency line in four records, whereas experts were also unable to estimate between one and seven tracings. The 95% CI for the paired differences between computer and experts was -12 to 15 bpm, whereas between the experts this was -10 to 10. With the exception of one expert, there was a high concordance between experts and between computer and experts (intraclass correlation > 0.9). The performance of this computerised algorithm cannot be distinguished from that of experienced clinicians. There were no significant differences between baseline values obtained by the computerised algorithm and those by the clinicians.

Role of antepartum computerized fetal heart rate analysis in the prediction of fetal distress during labor

Objective: To ascertain the diagnostic ability of a computerized fetal heart rate (FHR) analysis system in the identification of patients at risk of fetal distress in labor. Study design: Five-hundred and seventy-seven healthy term pregnancies were enrolled in a prospective, cross-sectional study and subdivided into two groups, with ( n = 90) or without ( n = 487) fetal distress in labor. Computerized FHR recordings were performed periodically and regression analysis was performed in a univariate and multivariate way to assess the ability of the last FHR recording in the prediction of subsequent fetal distress. Results: The two groups showed a significant difference in almost all of the FHR parameters studied. The multivariate analysis showed that only the FHR baseline and the percentage of small accelerations were independently and significantly related to the labor outcome. The combination of these two parameters reaches a sensitivity of 45%, specificity of 91%, positive predictive value of 48% and negative predictive value of 90%, with an overall accuracy of 84%. Conclusions: The increase in FHR baseline and in small FHR accelerations can be major factors in the prediction of subsequent fetal distress in healthy term fetuses.

Prolonged fetal bradycardia secondary to maternal hypothermia in response to urosepsis.

Fetal bradycardia is a well-known response to maternal hypothermia, as induced at open-heart surgery, but heretofore has not been reported in conjunction with hypothermia from urosepsis. A 24-year-old Vietnamese woman admitted at 33 weeks estimated gestational age with pyelonephritis secondary to Escherichia coli developed several episodes of maternal hypothermia to 35-36 degrees C. During each episode of maternal hypothermia, the baseline fetal heart rate fell to 90-100 bpm, but with retained reactivity. During each episode, maternal vital signs were otherwise stable and oxygen saturation was normal as measured by pulse oximetry. Interpretation of fetal bradycardia during episodes of maternal urosepsis is complex. If seen in conjunction with maternal hypothermia, and in the presence of normal maternal cardiac and respiratory function, bradycardia is unlikely to represent fetal distress.

The effect of early second stage bradycardia on newborn status

In the present study 45 patients with intrapartum cardiotocograms showing prolonged fetal bradycardia during the early second stage of labor were included. Bradycardia persisted for 180 s or more with either rapid or prolonged return to baseline, without loss of variability and rise of baseline fetal heart rate (FHR). The mean duration of bradycardia was 5.0 ± 1.3 min. In the group with normal FHR tracings the rate of normal delivery (73.3%) was significantly higher than that of the group with fetal bradycardia (26.7%, P < 0.0001). We also noted a significantly higher rate of cesarean section (44.4%) in patients with abnormal FHR tracings, compared to that (11.1%) of parturients with normal FHR tracings ( P < 0.001). In all cases blood samples were obtained from the umbilical cord artery, immediately after delivery. Only in two cases with abnormal FHR tracing umbilical cord artery was the pH less than 7.20. We conclude that in most cases, prolonged fetal bradycardia in the early second stage with the characteristics described above is well tolerated by a mature fetus.

Characteristics of fetal heart rate changes during the expulsive stage of bovine parturition in relation to fetal outcome

Abstract Objective To characterize fetal heart rate (FHR) patterns of nonacidotic (N-AC) and acidotic (AC) calves during the expulsive stage of parturition. Animals 32 calves born to heifers, either unassisted or by moderate traction. Procedure Cardiotocograms (CTG) evaluated qualitatively (visual) and quantitatively by a moving window technique (width 10, shift 5 minutes) for baseline FHR, variability, and periodic alterations (accelerations and decelerations). Results Significant differences were apparent between increase in baseline FHR of N-AC and AC calves; change in FHR (Δ-FHR), taking into account individual FHR, is a more discriminating parameter. The predominant reaction to uterine contraction is a decrease in FHR; variable decelerations were the most common type in both groups, and late decelerations were seen only in AC calves. Additional characteristics of variable decelerations were more frequently seen in AC calves. Conclusions CTG parameters during expulsion can discriminate between AC and N-AC calves, but specific characteristics are not exclusively restricted to AC or N-AC calves. Clinical Relevance CTG recordings during bovine parturition in cattle provide information on the fetal condition and can be used in the future to study the effects of medication and manipulations to the fetus. (Am J Vet Res 1996;57:1373-1381)

Fetal heart rate tracings: observers versus computer assessment

Cardiotocography (CTG) is widely used despite the fact that its diagnostic accuracy is far from satisfying. This is due, among other reasons, to the great intra- and interobserver variation in reading the fetal heart rate tracings. Computerized analysis might be a means to overcome the latter problem. Objective: the present study was designed to assess the reproducibility of CTG readings among observers and between observers and a computer system. Study design: 63 fetal heart rate tracings were read by 4 clinicians (2 experienced and 2 inexperienced) and by the 2CTG computerized system. The variables considered were: baseline fetal heart rate (FHR), long-term variability (amplitude bandwidth around the baseline), number of large accelerations and number and type of decelerations. Results: the agreement among observers, assessed by means of κ coefficient, ranges from fair to good. The agreement between each of the observers and the computer readings, ranges from 0.18 to 0.48 for FHR baseline, from 0.16 to 0.74 for variability, from 0.37 to 0.64 for the number of accelerations and from 0.41 to 0.54 for the number of decelerations. The agreement on the type of decelerations is very low (0.01–0.25). Conclusion: it is concluded that interobserver variability between experienced observers, inexperienced observers and 2CTG is considerable and that the use of a computer system should overcome this problem.

Improvements in computerized fetal heart rate analysis antepartum.

The continued development of a computerised system for measuring the pattern of the antepartum fetal heart rate (FHR) is described. Previous work had established that measurement of FHR variation objectively detects chronic fetal hypoxaemia and the onset of metabolic acidaemia antepartum. The normal centiles were calculated for the amplitude of long-term FHR variation, in episodes of high and low variation, week by week from 24-42 weeks gestation. Reference to these (automatically by the computer) improved discrimination between normal and questionable records in 38% of records, with a small saving of time. Two types of sinusoidal rhythm were described (slow, 1 in 2-5 minutes, incidence 0.16% of subjects; and faster, 2-5 per minute, incidence 0.025%) with methods for their detection. Both may be of sufficient amplitude to induce an episode of high FHR variation. The different effects of maternal steroid (betamethasone or dexamethasone) administration of FHR variation were compared, and the clinical consequences considered. The frequency distribution of basal FHR in normal and abnormal records was measured, and the effects on basal FHR outside the normal range (120-160 bpm) on FHR variation described. Adjustment of the FHR baseline was undertaken when, exceptionally, large abrupt changes in heart rate occurred during a record. The duration and frequency of FHR record acquisition in clinical practice were reviewed, and new policies recommended. With adequate safeguards, measurement by a computer offers reliable objective information from which fetal health may be assessed, more objectively and accurately than by visual inspection.

Neural network prediction of nonstress test results: How often should we perform nonstress tests?

OBJECTIVE: Our purpose was to predict outcomes and optimal intervals for nonstress tests of term gravid women with neural networks. STUDY DESIGN: We studied 100 normal term patients whose 30-minute nonstress tests, performed on 5 consecutive days, were computer analyzed for the following elements: fetal heart rate baseline, variability, signal loss, accelerations (>15 beats/min), and decelerations. The training set used 65 patients; the testing, 35 patients. Nonstress test data (days 1 to 4) were inputs; day 5 data were training patterns. Networks for each nonstress tet element used Brainmaker Macintosh 1.0 (California Scientific Software, Nevada City, Calif.) trained to 0.12 tolerance. Actual fetal heart rate elements and their daily differences were compared with predictions by the networks and multiple regressions. RESULTS: There was little difference between networks using daily or alternate-day inputs for predicting test performance on day 5; networks using test intervals >2 days could not be trained to tolerance. Long-term fetal heart rate variation was the nonstress test element best predicted. Daily differences networks provided better prediction of all day 5 data than did actual daily values networks or multiple regression formulas. CONCLUSIONS: Baseline long-term fetal heart rate variability seems to be the most predictable fetal heart rate element over time and should merit more consideration in overall fetal testing. Fetal heart rate elements are not easily predicted by any method for intervals longer than 2 days. Using longer test intervals might run a greater risk for unanticipated changes in nonstress test outcomes, even when fetal condition is normal.

Effect of Magnesium on Fetal Heart Rate Variability Using Computer Analysis

We studied 16 women, at 32 weeks' or more gestation who required magnesium sulfate (MgSO4) therapy for preterm labor or preeclampsia. A 60-minute Doppler fetal heart rate (FHR) tracing, analyzed by the Oxford Sonicaid System 8000, was obtained for 1 hour before and 2 hours after each patient received intravenous MgSO4 therapy. Maternal serum Mg2+ levels were obtained at the second monitoring session. Matched paired measures of FHR parameters were compared with the Student's t test. After MgSO4 administration, we noted significant falls in long-term variability, short-term variability, and total acceleration (more than 10 beats/min) counts. Reduced short-term and overall variability occurred in all cases with maternal serum Mg2+ levels more than 4.6 mg/dL. Therapeutic maternal serum Mg2+ levels are linked with decreases in long-term and short-term FHR variability and acceleration counts. These findings should be considered when evaluating resting FHR baseline of patients thus treated.

Arterial oxygen saturation in relation to metabolic acidosis in fetal lambs

OBJECTIVE: We studied the relationship between preductal arterial oxygen saturation and metabolic acidosis in 18 chronically instrumented fetal lambs (gestational age 119 to 133 days) in two experimental designs. In the first group the onset of metabolic acidosis was determined. In the second group the progression of metabolic acidosis was studied as was the cardiovascular and hormonal changes resulting from hypoxemia. STUDY DESIGN: In nine fetal lambs maternal fraction of inspired oxygen was lowered stepwise by increasing flows of nitrogen delivered into the trachea through a small indwelling catheter (group 1), and in nine fetal lambs maternal blood flow was reduced stepwise by means of a vascular occluder (group 2). RESULTS: Baseline arterial oxygen saturation values ranged from 26% to 67% with normal pH and extracellular fluid base excess values in both groups 1 and 2. In both groups pH and extracellular fluid base excess started to decrease below 30% arterial oxygen saturation, with a progressive decrease below 20% arterial oxygen saturation to an end value for pH of 7.14. In some fetal lambs pH and extracellular fluid base excess decreased initially at 20% to 30% arterial oxygen saturation and then stabilized at the lower level. Fetal heart in group 1 increased during hypoxemia from 155 to 179 beats/min. In group 2 baseline fetal heart rate was 153 beats/min and fell with every step change in arterial oxygen saturation but subsequently increased to 172 beats/min by the end of the period of hypoxemia. Baseline values for epinephrine, norepinephrine, dopamine, cortisol, and mean arterial pressure were not related to baseline arterial oxygen saturation levels, and each of these variables was increased at the end of hypoxemia in group 2. CONCLUSION: Preductal arterial oxygen saturation can reach values between 20% and 30% before anaerobic metabolism starts. During the progressive acidosis blood pressure was increased, which can be attributed to a strong rise in catecholamines.

Effects of acute and chronic maternal exercise on fetal heart rate

Maternal-fetal effects of cycle ergometer conditioning (heart rate of 145 beats/min at 25 min/day for 3 days/wk) were studied during the second and third pregnancy trimesters. Subjects were 22 previously sedentary women and 16 nonexercising pregnant control women. Fetal heart rate (FHR) characteristics were studied before, during, and after 15 min of upright cycling at a maternal heart rate target of 145 beats/min at the end of both the second and third trimesters. Despite higher cycling power outputs in the exercised group, mean FHR responses were similar in both groups and conformed to 1) gradual increase in FHR baseline during exercise, 2) normal variability, and 3) normal reactivity. Fetal bradycardia was observed during (n = 1) and after (n = 2) exercise in three isolated tests. The timing of these events suggested that the likelihood of significant fetal hypoxia is highest in the immediate postexercise period. These results also support the hypothesis that physically conditioned women can perform at higher exercise power outputs than sedentary women without inducing fetal hypoxic stress. Further study is recommended to examine possible fetal and placental adaptations to maternal aerobic conditioning.