Electrocardiographic waveform changes and catecholamine responses during acute hypoxia in the immature and mature fetal lamb

Intrapartum Fetal Electrocardiogram in Small- and Large-for-Gestational Age Fetuses

OBJECTIVE

This study aimed to evaluate whether intrapartum fetal electrocardiogram (ECG) tracings with ST-elevation or depression occur more frequently in each stage of labor in small-for-gestational age (SGA) or large-for-gestational age (LGA), as compared with appropriate-for-gestational age (AGA) fetuses.

STUDY DESIGN

We conducted a secondary analysis of a large, multicenter trial in which laboring patients underwent fetal ECG waveform-analysis. We excluded participants with diabetes mellitus and major fetal anomalies. Birth weight was categorized as SGA (<10th percentile), LGA (>90th percentile), or AGA (10-90th percentile) by using a gender and race/ethnicity specific nomogram. In adjusted analyses, the frequency of ECG tracings with ST-depression or ST-elevation without depression was compared according to birthweight categories and labor stage.

RESULTS

Our study included 4,971 laboring patients in the first stage and 4,074 in the second stage. During the first stage of labor, there were no differences in the frequency of ST-depression in SGA fetuses compared with AGA fetuses (6.7 vs. 5.5%; adjusted odds ratio [aOR]: 1.41, 95% confidence interval [CI]: 0.93-2.13), or in ST-elevation without depression (35.8 vs. 34.1%; aOR: 1.17, 95% CI: 0.94-1.46). During the second stage, there were no differences in the frequency of ST-depression in SGA fetuses compared with AGA fetuses (1.6 vs. 2.0%; aOR: 0.69, 95% CI: 0.27-1.73), or in ST-elevation without depression (16.2 vs. 18.1%; aOR: 0.90, 95% CI: 0.67-1.22). During the first stage of labor, there were no differences in the frequency of ST-depression in LGA fetuses compared with AGA fetuses (6.3 vs. 5.5%; aOR: 0.97, 95% CI: 0.60-1.57), or in ST-elevation without depression (33.1 vs. 34.1%; aOR: 0.80, 95% CI: 0.62-1.03); during the second stage of labor, the frequency of ST-depression in LGA compared with AGA fetuses (2.5 vs. 2.0%, aOR: 1.36, 95% CI: 0.61-3.03), and in ST-elevation without depression (15.5 vs. 18.1%; aOR: 0.83, 95% CI: 0.58-1.18) were similar as well.

CONCLUSION

The frequency of intrapartum fetal ECG tracings with ST-events is similar among SGA, AGA, and LGA fetuses.

KEY POINTS

· SGA and LGA neonates are at increased risk of cardiac dysfunction.. · Fetal ECG has been used to evaluate fetal response to hypoxia.. · Fetal ST-elevation and ST-depression occur during hypoxia.. · Frequency of intrapartum ST-events is similar among SGA, AGA and LGA fetuses..

Effects of β-adrenergic stimulation on fetal heart rate, heart rate variability, and T-wave elevation during brief umbilical cord occlusions in fetal sheep

Circulating catecholamines are critical for fetal adaptation to hypoxia by regulating fetal heart rate (FHR) and promoting myocardial contractility and peripheral vasoconstriction. They have been hypothesized to contribute to changes in FHR variability (FHRV) and T-wave morphology, clinical indexes of fetal well-being during labor. β-Adrenergic blockade with propranolol does not affect FHRV during labor-like hypoxemia and only attenuated the increase in T-wave height between the episodes of hypoxemia. To further investigate the potential role of catecholamines, we investigated whether pharmacological β-adrenergic stimulation could increase FHRV and T-wave elevation during intermittent labor-like hypoxemia. Nineteen chronically instrumented fetal sheep at 0.85 of gestation received isoprenaline hydrochloride (n = 7) or saline (control, n = 12), followed by three 1-min complete umbilical cord occlusions (UCOs) separated by 4-min reperfusion periods. Before the UCOs, infusion of isoprenaline increased FHR (P < 0.001), absolute-T/QRS ratio (P < 0.001), and one measure of FHRV [root-mean-square of successive RR interval differences (RMSSD), P < 0.05]. UCOs triggered deep FHR decelerations. During UCOs, isoprenaline was associated with increased FHR (P < 0.001) and absolute-T/QRS ratio (P < 0.05), but no effect on T/QRS ratio was observed when normalized to baseline before UCOs (normalized-T/QRS ratio). Between UCOs, isoprenaline increased FHR (P < 0.001) and absolute-T/QRS ratio (P < 0.05) but did not affect normalized-T/QRS ratio or any measures of FHRV. Arterial pressure was not affected by isoprenaline at any point. Our findings indicate that circulating catecholamines regulate FHR but not FHRV during labor-like hypoxemia and promote T-wave elevation between but not during intermittent fetal hypoxemia.

Circulating catecholamines partially regulate T-wave morphology but not heart rate variability during repeated umbilical cord occlusions in fetal sheep

Fetal heart rate (FHR) variability (FHRV) and ST segment morphology are potential clinical indices of fetal well-being during labor. β-Adrenergic stimulation by circulating catecholamines has been hypothesized to contribute to both FHRV and ST segment morphology during labor, but this has not been tested during brief repeated fetal hypoxemia that is characteristic of labor. Near-term fetal sheep (0.85 gestation) received propranolol (β-adrenergic blockade; n = 10) or saline (n = 7) 30 min before being exposed to three 2-min complete umbilical cord occlusions (UCOs) separated by 3-min reperfusions. T/QRS ratio was calculated throughout UCOs and reperfusion periods, and measures of FHRV (RMSSD, SDNN, and STV) were calculated between UCOs. During the baseline period, before the start of UCOs, propranolol was associated with reduced FHR, SDNN, and STV but did not affect RMSSD or T/QRS ratio. UCOs were associated with rapid FHR decelerations and increased T/QRS ratio; propranolol significantly reduced FHR during UCOs and was associated with a slower rise in T/QRS ratio during the first UCOs, without affecting the maximal rise or T/QRS ratio during the second and third UCO. Between UCOs propranolol reduced FHR and T/QRS ratio but did not affect any measure of FHRV. These data demonstrate that circulating catecholamines do not contribute to FHRV during labor-like hypoxemia. Furthermore, circulating catecholamines did not contribute to the major rise in T/QRS ratio during labor-like hypoxemia but may regulate T/QRS ratio between brief hypoxemia.

Does Maturity Affect Cephalic Perfusion and T/QRS Ratio during Prolonged Umbilical Cord Occlusion in Fetal Sheep?

T/QRS ratio monitoring is used to help identify fetal asphyxia. However, immature animals have greater capacity to maintain blood pressure during severe asphyxia, raising the possibility that they may show an attenuated T/QRS increase during asphyxia. Chronically instrumented fetal sheep at 0.6 of gestation (0.6 GA; n = 12), 0.7 GA (n = 12), and 0.8 GA (n = 8) underwent complete umbilical cord occlusion for 30 min, 25 min, or 15 min, respectively. Cord occlusion was associated with progressive metabolic acidosis and initial hypertension followed by severe hypotension, with a more rapid fall in mean arterial blood pressure (MAP) and carotid blood flow (CaBF) with advancing gestation. T/QRS ratio rose after occlusion more rapidly at 0.8 GA than in immature fetuses, to a similar final peak at all ages, followed by a progressive fall that was slower at 0.8 GA than in the immature fetuses. The increase in T/QRS ratio correlated with initial hypertension at 0.8 GA (P < 0.05, R² = 0.38), and conversely, its fall correlated closely with falling MAP in all gestational groups (P < 0.01, R² = 0.67). In conclusion, elevation of the T/QRS ratio is an index of onset of severe asphyxia in the last third of gestation, but not of fetal compromise.

ST segment analysis as an adjunct to electronic fetal monitoring, Part I: background, physiology, and interpretation

Fetal electrocardiogram (ECG) ST segment analysis (STAN) was approved in 2005 in the United States as an adjunct to electronic fetal heart rate monitoring to determine whether obstetrical intervention is warranted when there is an increased risk for developing metabolic acidosis. STAN has utility in the reduction of fetal acidosis at birth, decreased need for fetal scalp blood sampling during labor, and decreased need for operative vaginal delivery and emergency cesarean delivery for fetal indications. This article discusses specific fetal ECG changes and their significance and the use of the STAN system.

Fetal ECG waveform analysis

Fetal ECG waveform analysis as an adjunct to electronic fetal monitoring (EFM) has developed over the last 3 decades. From a multitude of potential parameters, ST waveform analysis has been documented to provide the information required to shift EFM from a screening device to a diagnostic tool that meets the standards of evidence-based medicine. This chapter details the experimental and clinical evolution of the STAN methodology for intrapartum fetal surveillance. Observational data formed the basis for cardiotocograph (CTG) and ST waveform analysis clinical guidelines. Data from two large, randomized controlled trials (6826 cases) are summarized together with the first analysis of the recently completed EU project of 7823 term fetuses monitored as part of the study to introduce ST analysis into clinical practice. The reduction in the incidence of newborns with marked neurological symptoms is supported by these findings. The detection of ST changes allowed earlier and more consistent intervention.

ST waveform changes during repeated umbilical cord occlusions in near-term fetal sheep

OBJECTIVE

This study was undertaken to determine whether changes in the fetal ST waveform during repeated umbilical occlusion reflect the development of hypotension and acidosis.

STUDY DESIGN

Chronically instrumented, near-term fetal sheep received 1-minute total umbilical cord occlusion either every 5 minutes for 4 hours (1:5 group, n = 8), or every 2.5 minutes until blood pressure fell <20 mm Hg on 2 successive occlusions (1:2.5 group, n = 8).

RESULTS

Umbilical cord occlusion caused variable decelerations, with sustained hypertension in the 1:5 group and little change in acid-base status (pH = 7.34 +/- 0.07 after 4 hours). In contrast, the 1:2.5 group showed progressive hypotension and metabolic acidemia (pH 6.92 +/- 0.1 after the final occlusion). There was a marked increase in ST waveform height during occlusions; this increase was greater in the 1:2.5 group (P <.001), but there was overlap between the groups. ST waveform height between occlusions was significantly higher in the 1:2.5 group (P <.001) until negative and biphasic ST waveforms developed in these fetuses between occlusions in the final 30 minutes.

CONCLUSION

ST waveform elevation occurs during umbilical cord occlusions but only crudely reflects the severity of hypoxia. Interocclusion waveform height may be a better reflection of the severity of hypoxia. The appearance of biphasic and negative waveforms between occlusions may be a useful marker for severe decompensation.

Periodic spectral components of fetal heart rate variability reflect the changes in cord arterial base deficit values: a preliminary report

Fetal distress changes the function of the autonomic nervous system. These changes are reflected in the fetal heart rate and can be quantified with power spectrum analysis of heart rate variability. The purpose of this study was to find out whether spectral components of fetal heart rate variability (FHRV) during labor are associated with fetal cord arterial base deficit values at birth. The association between FHRV and umbilical cord arterial base deficit was studied in 14 singleton fetuses with normal pregnancy at 35-40 weeks of gestation. Fetal ECG was recorded by scalp-electrode using a STAN Fetal ECG monitor (Cinventa Ab, Mölndal, Sweden). FHRV was quantified by computing Fast-Fourier-transformed heart rate (HR) spectra at three frequency bands: low-frequency (LF) 0.03-0.07 Hz, mid-frequency (MF) 0.07-0.13 Hz and high-frequency (HF) 0.13-1.0 Hz. We found that total FHRV and MF FHRV were lower in fetuses with cord arterial base deficit 8 to 12 mmol/L in comparison to the fetuses with normal cord arterial base deficit value (P=0.02 and P=0.01, respectively). A linear correlation was found between the spectral densities and the cord arterial base deficit values (r=0.4 and r=0.6, respectively). We conclude that the results suggest changes in the autonomic nervous cardiac control in fetuses with cord arterial base deficit between 8 to 12 mmol/L. The clinical applicability of our observations on FHRV in predicting fetal distress remains to be further studied.

Actions of hypoxia on catecholamine synthetic enzyme mRNA expression before and after development of adrenal innervation in the sheep fetus

We have investigated adrenal mRNA expression of the catecholamine synthetic enzymes tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) following acute hypoxia in fetal sheep before (< 105 days gestation, n = 20) and after (> 125 days gestation, n = 20) the development of adrenal innervation and following pretreatment with the nicotinic receptor anatgonist hexamethonium (n = 12). Total RNA was extracted from fetal adrenal glands collected at specific time points at 3-20 h after the onset of either hypoxia ( approximately 50% reduction in fetal arterial oxygen saturation (SO2) for 30 min), or normoxia. Before 105 days, there was a decrease in adrenal TH mRNA expression at 20 h after hypoxia and adrenal TH mRNA expression was directly related to the changes in arterial PO2 measured during normoxia and hypoxia. After 125 days, adrenal TH mRNA levels were suppressed for up to 12 h following hypoxia. In both age groups, adrenal PNMT mRNA expression increased at 3-5 h after hypoxia and was inversely related to the changes in fetal arterial PO2 during normoxia or hypoxia. After 125 days, the administration of hexamethonium (25 mg kg(-1), I.V.) reduced TH mRNA but not PNMT mRNA expression after normoxia. After hexamethonium pretreatment, there was no significant change in either adrenal TH or PNMT mRNA expression following hypoxia. We conclude that acute hypoxia differentially regulates adrenal TH and PNMT mRNA expression in the fetal sheep both before and after the development of adrenal innervation. After the development of adrenal innervation, however, the effect of acute hypoxia upon adrenal TH and PNMT mRNA expression is dependent upon neurogenic input acting via nicotinic receptors.

Developmental changes in plasma catecholamine concentrations during normoxia and acute hypoxia in the chick embryo

In the mammalian fetus, the cardiovascular responses to acute hypoxaemia include a redistribution of the cardiac output away from the periphery towards the adrenal, myocardial and cerebral circulations. A component of the peripheral vasoconstriction is mediated by increased release of catecholamines into the fetal circulation during acute hypoxaemia. Previously, we have shown that the chick embryo also shows an increase in peripheral vascular resistance during acute hypoxaemia and that this response becomes progressively larger towards the end of the incubation period. However, the ontogeny of the catecholaminergic response to acute hypoxaemia has not been investigated in this species. Fertilised chicken eggs were studied on days 10, 13, 16 and 19 of incubation (hatching is at 21 days). At each stage of incubation, blood samples were obtained from the chorioallantoic artery of the chick embryos during normoxia and after 5 min of hypoxaemia for measurement of plasma concentrations of adrenaline and noradrenaline by HPLC. Basal plasma adrenaline and noradrenaline concentrations by the end of the incubation period were much higher in the chick embryo than values reported for mammalian fetuses during late gestation. During normoxia, basal plasma noradrenaline concentration remained unchanged during development but plasma adrenaline concentration showed a developmental increase from < 25.1 pmol l-1 at day 10 to 3 nmol l-1 at day 19 of incubation. Acute hypoxaemia caused an increase in plasma noradrenaline and adrenaline from day 13 and day 16 of incubation, respectively. In addition, the increase in plasma adrenaline and noradrenaline and in the ratio of plasma adrenaline to noradrenaline during acute hypoxaemia became progressively larger by the end of the incubation period. These data show an ontogenic increase in basal plasma catecholamines and in the catecholaminergic response to acute hypoxaemia in the chick embryo during the last third of the incubation period.

The PR interval-fetal heart rate relationship during repetitive umbilical cord occlusions in immature fetal sheep

OBJECTIVE

To evaluate the relationship of the PR interval and fetal heart rate during repetitive umbilical cord occlusions in immature sheep fetuses.

STUDY DESIGN

In seven chronically cannulated immature sheep fetuses [gestational age 90.6 days (mean)], we analyzed continuous fetal electrocardiogram recordings during repetitive cord occlusions for 2 out of every 5 min until fetal mean arterial pressure dropped to 50% of baseline value. PR interval-fetal heart rate correlation coefficients (Pearson) was measured on consecutive blocks of 2.5 min. R-values of the baseline and the repetitive occlusion period were compared by Fisher's exact test.

RESULTS

Repetitive cord occlusions resulted in acidosis and hypotension. Two fetuses died at the end of the repetitive occlusion period. Four out of seven fetuses showed a significant change from a negative relationship between the PR interval and fetal heart rate during baseline to a predominantly positive relationship during the repetitive occlusion period.

CONCLUSION

In immature fetal sheep, a change from a negative relationship between the PR interval and fetal heart rate to a predominantly positive relationship between the PR interval and fetal heart rate was observed in four out of seven fetuses following the initiation of repetitive umbilical cord occlusions.

Metabolic and endocrine responses to cold exposure in chronically incubated extrauterine goat fetuses

To investigate developmental aspects of metabolic and endocrine responses to cold exposure in fetuses, we conducted experiments on six goat fetuses, three aged 95-116 d of gestation (dGA; group I), and three aged 122-134 dGA (group II), using an extrauterine fetal incubation system that provided arterio-venous extracorporeal membrane oxygenation (A-V ECMO). The fetuses were cannulated via the umbilical vessels, and their blood gas exchange was totally supported by A-V ECMO, while they were maintained in an isothermal incubator containing artificial amniotic fluid. After confirming that fetuses were in metabolically stable condition in the extrauterine incubation system, fetal core temperature was lowered by 2 degrees C over 2 h by decreasing the temperature of incubating fluid from 39.5 degrees C. During and after cold exposure, fetal heart rate and arterial blood pressure remained unchanged. We observed significant increases in oxygen consumption and plasma concentrations of norepinephrine, epinephrine, adrenocorticotropic hormone, and cortisol in group II but not in group I fetuses. In addition, based on regression analysis, maximal changes of these parameters during cold exposure were linearly correlated with gestational age significantly, and the regression lines were found to intersect the x (gestational age) axis at around 98-106 dGA. These results suggest that metabolic and endocrine responses to cold exposure develop with gestational age in the goat fetus, the responses being manifested around 100 dGA.

Impact of placental restriction on the development of the sympathoadrenal system

We have investigated the impact of chronic restriction of placental function on circulating catecholamine concentrations and responses to the indirectly acting, sympathomimetic amine, tyramine, in the fetal sheep in late gestation. In 10 ewes, endometrial caruncles or placental placentation sites were removed before conception (placental restriction (PR) group). Fetal sheep in the PR group were hypoxemic throughout late gestation and growth-restricted (3.02 +/- 0.35 kg) when compared with control fetal sheep (4.30 +/- 0.29 kg; n = 8) at 140 d of gestation. Fetal plasma concentrations of noradrenaline and adrenaline were higher (p < 0.05) in the PR (7.06 +/- 3.17 pmol/mL and 2.89 +/- 2.01 pmol/mL, respectively) than in the control group (3.55 +/- 0.54 pmol/mL and 1.30 +/- 0.48 pmol/mL, respectively) throughout late gestation. Plasma noradrenaline, but not adrenaline concentrations, increased significantly between 110 and 140 d of gestation in both the PR and control group, and there was a significant inverse relationship between plasma noradrenaline and arterial PO2 in the PR and control groups (plasma noradrenaline = 12.34 - 0.40 PO2). In the PR group, plasma noradrenaline increased (p < 0.05) after tyramine infusion from 4.51 +/- 1.28 pmol/mL to a peak of 19.40 +/- 3.56 pmol/mL. In the control group, noradrenaline increased from 2.08 +/- 0.30 pmol/mL to a peak of 12.23 +/- 1.67 pmol/mL after tyramine infusion. There was no difference, however, in the maximal proportional changes in plasma noradrenaline concentrations in the PR (319 +/- 55%) and control (449 +/- 100%) groups after tyramine. We conclude that the most likely source of the increased plasma catecholamines in the PR group is enhanced catecholamine synthesis and secretion from developing sympathetic neurons.

The T/QRS ratio of the electrocardiogram does not reliably reflect well-being in fetal lambs

OBJECTIVE

Our purpose was to determine the diagnostic power of the T/QRS ratio of the electrocardiogram to predict fetal well-being.

STUDY DESIGN

In 47 fetal lambs (3 to 5 days after surgery, gestational age 123.5 +/- 3.0 days) asphyxia was induced by restriction of uterine perfusion. Fetuses were either pretreated with an adenosine transport inhibitor (n = 16) or a calcium channel blocker (n = 12) or served as controls (n = 19). Arterial oxygen content > or = 1.5 mmol/L or pH > or = 7.15 were chosen as limits for fetal well-being.

RESULTS

Arterial oxygen content was reduced from 3.3 (+/- 1.0) to 1.3 (+/- 0.5) mmol/L, and pH decreased to 7.03 (+/- 0.10). Mortality was 53%. Both drugs did not affect well-being, survival, or the T/QRS ratio. Maximum T/QRS ratios were reached at the peak of asphyxia. Sensitivity and specificity of the T/QRS ratio were 24.0% and 42.6% to predict hypoxemia and 25.1% and 45.3% to predict acidemia. Pearson correlation coefficients for T/QRS ratio versus oxygen content and pH were 0.169 and 0.192, respectively.

CONCLUSIONS

(1) In fetal lambs the T/QRS ratio failed to predict hypoxemia or acidemia. (2) Fetal survival was not correlated with the height of the T/QRS ratio during or after asphyxia.

The fetal electrocardiogram: ST waveform analysis during labour

Intrapartum fetal surveillance is still under debate, despite 30 years of clinical experience and numerous clinical trials. Waveform analysis of the fetal electrocardiogram has emerged not as an alternative to cardiotochography but as a support tool to allow more accurate interpretation of intrapartum events. During hypoxia, the healthy fetus is utilizing a series of defense mechanisms. Among these, the increase in sympathetic activity, with an increase in circulating adrenaline, activates the myocardium with an increase in workload (the product of cardiac output, myocardium contractility and blood pressure). If there is an imbalance between myocardial oxygen supply and consumption, determined by the workload, then anaerobic metabolism, with a breakdown of myocardial glycogen stores starts and high T waves emerges. ST depression with negative T waves has recently been observed during hypoxia experiments in experimentally growth retarded guinea pigs whilst their normally grown littermates showed ST elevation. These findings have stimulated the development of a dedicated fetal ECG monitor - STAN - incorporating both standard CTG and ST waveform analysis. The STAN concept has now been taken through the process of recognized validation including several prospective studies and a large randomized trial in Plymouth of 2400 high risk, term deliveries. The T/QRS ratio is only one parameter to be used - equally important is to identify the occurrence of ST depression with biphasic negative T waves and to interrelate the CTG and the ST waveform as outlined in the clinical guidelines (table I). This table contains the clinical experience gained over many years and has formed the basis for the first randomized controlled trial comparing ST waveform + CTG with CTG only. Obviously, when the T/QRS ratio is used as the only component of such a scheme, confusion emerges. The analysis should also contain cases with significant intrapartum hypoxia. Recent findings indicate that only when cord artery pH falls below 7.0 and when there is substantial metabolic acidemia is there a significant risk of intrapartum asphyxia. Metabolic acidemia should be estimated from base deficit in the extracellular fluid and the combination of cord artery and vein data should allow for a more accurate assessment of intrapartum events, including the assessment of the duration of hypoxia. The Plymouth trial has tested the hypothesis that the combination of ST wave form and CTG analysis compared with CTG analysis only would reduce operative interventions for fetal distress without placing the fetus at a risk.(ABSTRACT TRUNCATED AT 400 WORDS)

Power spectrum analysis of the fetal heart rate during noradrenaline infusion and acute hypoxemia in the chronic fetal lamb preparation

In a chronically instrumented fetal lamb the effect on heart rate variability of noradrenaline as well as hypoxemia is studied by the use of power spectral analysis. Subsequent to both noradrenaline infusion and hypoxemia the very low frequency components of the variability are markedly decreased as compared with control conditions. After hypoxemia also a high frequency peak appears in the spectrum.

Effects of sustained hypoxemia on the sheep fetus at midgestation: endocrine, cardiovascular, and biophysical responses

OBJECTIVE

The purpose of our study was to examine the effects of induced hypoxia on endocrine, cardiovascular, and biophysical measurements of the ovine fetus at 0.6 (83 to 93 days) of gestation and to compare the fetal responses at this earlier gestation with those reported near term.

STUDY DESIGN

Fourteen fetal sheep were studied (9 in the hypoxia group and 5 in the control group) at 0.6 of gestation during a 24-hour control period, 8 hours of either sustained hypoxemia or room air, and a 40-hour recovery period.

RESULTS

Induced fetal hypoxemia resulted in a progressive lactic metabolic acidosis; however, all fetuses had recovered within 24 hours. The fetal endocrine response was variable with norepinephrine, the only measured hormone showing a significant hypoxia-related increase (p less than 0.05). Fetal heart rate and mean arterial blood pressure showed little hypoxia-induced change, although fetal heart rate was significantly increased over the first 2 hours (p less than 0.05). The percent time fetal breathing movements, electroocular activity, and nuchal muscle activity likewise showed little hypoxia-induced change.

CONCLUSION

The cardiovascular and biophysical response of the preterm fetus to induced hypoxemia is thus much less pronounced than that of the older gestational-aged fetus; this difference may impact on survival and the success of antenatal assessment protocols.

Change in electrocardiogram T-wave amplitude during umbilical cord compression is predictive of fetal condition in sheep

OBJECTIVE

The purpose of this study is to assess the usefulness of the dynamic change in T/QRS ratio in fetal electrocardiograms in predicting the fetal condition when repetitive variable decelerations are seen in intrapartum cardiotocograms.

STUDY DESIGN

We investigated the relationship, using linear regression and Wilcoxon's test, between T/QRS and blood gas values, catecholamine concentrations, and blood pressure during repetitive cord compression in five chronically instrumented lamb fetuses.

RESULTS

T/QRS during cord compression correlated significantly (p less than 0.01) with fetal arterial pH (r = -0.7711), norepinephrine concentration (r = 0.7551), and duration of elevated blood pressure during compression (r = -0.8619). Fetal arterial pH and base excess were lower, the duration of elevated blood pressure during compression was shorter, and carbon dioxide partial pressure and catecholamine concentrations were higher in the stage with higher (greater than 0.50) T/QRS during compression (p less than 0.005).

CONCLUSION

We can estimate the severity of fetal distress by measuring T/QRS near the bottom of the decelerations.

Vibroacoustic stimulation is not associated with sudden fetal catecholamine release

The safety of vibroacoustic stimulation (VAS), which produces marked changes in fetal heart rate, movements and behavioural state, remains unclear. In order to determine whether VAS is associated with catecholamine release, we measured plasma noradrenaline and adrenaline in 13 appropriately grown normoxaemic fetuses between 28 and 40 weeks gestation immediately before and 60 and 75 s after VAS. Over this time interval, VAS is known to increase fetal heart rate. There was no significant change in either noradrenaline (median change = +0.06 ng/ml, P = 0.26) or adrenaline levels (median change = +0.03 ng/ml, P = 0.4). This study suggests that sympathoadrenal activation is not part of the fetal response to VAS. These findings do not support the recent suggestion that VAS may be deleterious to the fetus by provoking sudden release of catecholamines.