The effect of change in sympatho-vagal balance on heart rate and blood pressure variability in the foetal lamb

Validation of spontaneous assessment of baroreceptor reflex sensitivity and its relation to heart rate variability in the ovine fetus pre- and near-term

Assessment of baroreceptor reflex sensitivity (BRS) in the ovine fetus provides insight into autonomic cardiovascular regulation. Currently, assessment of BRS relies on vasoactive drugs, but this approach is limited by feasibility issues and by the nonphysiologic nature of the stimulus. Thus we aimed to validate the method of spontaneous BRS assessment against the reference method of using vasoactive drugs in preterm (0.76 gestation, n = 16) and near-term (0.86 gestation, n = 16) chronically instrumented ovine fetuses. The BRS measures derived from the spontaneous and reference methods correlated at both gestational ages (R = 0.67 ± 0.03). The sequence method of spontaneous BRS measures also correlated both to the root mean square of standard deviations (RMSSD), which is a measure of fetal heart rate variability reflecting vagal modulation (R = 0.69 ± 0.03), and to fetal body weight (R = 0.65 ± 0.03), which is a surrogate for growth trajectory of each fetus. The methodology presented may aid in developing new models to study BRS and cardiovascular control in ovine fetus in the last trimester of pregnancy.

Fetal cardiac sympathetic activation is linked with maternal body mass index

BACKGROUND

Sympathetic over activity has been suggested to impact on the risk of cardiovascular diseases. The origin of abnormalities in the autonomic nervous system is unknown, but it is susceptible to environmental influences during the critical periods of human development.

AIMS

To examine the influence of maternal characteristics on fetal cardiac autonomic nervous activation.

STUDY DESIGN

Prospective, observational study.

SUBJECTS

41 fetuses from normal pregnancy born after 36 gestational weeks.

OUTCOME MEASURES

Maternal characteristics that affect fetal intrauterine well-being intrauterine environmental factors were evaluated, including pre-pregnancy body mass index as well as GHbA1c, blood pressure and 3-day food diaries from each trimester of pregnancy. To assess intrapartum fetal cardiac sympathovagal activation fetal ECG was recorded for 1 h during delivery. Heart rate variability was measured using power spectrum analysis of low-to-high frequency ratio of fetal heart rate variability.

RESULTS

Cardiac sympathetic activation measured during delivery was associated with maternal pre-pregnancy body mass index (r=0.33, p=0.03), placental weight (r=0.4, p=0.008) and the immaturity of the fetus (r=-0.3, pb0.05).

CONCLUSION

Early intrauterine environmental factors such as maternal pre-pregnancy body mass index are associated with fetal sympathetic activation with a potential for cardiovascular programming.

Nonlinear properties of vagal and sympathetic modulations of heart rate variability in ovine fetus near term

Fetal heart rate (FHR) monitoring is commonly used although clinical studies questioned its diagnostic value. Sophisticated FHR variability (fHRV) measures such as fHRV complexity may improve the sensitivity and specificity of FHR monitoring. A more detailed understanding of the physiology underlying fHRV complexity is essential to harness its use for monitoring fetal health. To examine the specific effects of vagal and sympathetic modulations on fHRV complexity, we blocked vagal activity with atropine and sympathetic activity with propranolol in near-term fetal sheep ( n = 7, 0.85 gestation). Under these conditions, we analyzed the linear and nonlinear parts of fHRV complexity from autonomic information flow. Overall fHRV complexity decreased with both drugs compared with nonrapid eye movement sleep baseline ( P < 0.05). With atropine, this was because of a decrease of the linear part of fHRV complexity on the long-term time scale ( P < 0.05), suggesting that vagal modulation of fHRV is adequately described by linear fHRV measures. With propranolol, the nonlinear part of fHRV complexity decreased on the short-term time scale ( P < 0.05), suggesting that sympathetic influences on fHRV can be detected by the nonlinear part of fHRV complexity. Thus the complex interplay of vagal and sympathetic modulations of fHRV is reflected differently and specifically in the linear and nonlinear properties of fHRV complexity, and on different time scales. Analysis of linear and nonlinear properties of fHRV may improve sensitivity and specificity of FHR monitoring.

Heart rate variability as a measure of autonomic regulation of cardiac activity for assessing stress and welfare in farm animals -- a review

Measurement of heart rate variability (HRV) is a non-invasive technique that can be used to investigate the functioning of the autonomic nervous system, especially the balance between sympathetic and vagal activity. It has been proven to be very useful in humans for both research and clinical studies concerned with cardiovascular diseases, diabetic autonomic dysfunction, hypertension and psychiatric and psychological disorders. Over the past decade, HRV has been used increasingly in animal research to analyse changes in sympathovagal balance related to diseases, psychological and environmental stressors or individual characteristics such as temperament and coping strategies. This paper discusses current and past HRV research in farm animals. First, it describes how cardiac activity is regulated and the relationships between HRV, sympathovagal balance and stress and animal welfare. Then it proceeds to outline the types of equipment and methodological approaches that have been adapted and developed to measure inter-beats intervals (IBI) and estimate HRV in farm animals. Finally, it discusses experiments and conclusions derived from the measurement of HRV in pigs, cattle, horses, sheep, goats and poultry. Emphasis has been placed on deriving recommendations for future research investigating HRV, including approaches for measuring and analysing IBI data. Data from earlier research demonstrate that HRV is a promising approach for evaluating stress and emotional states in animals. It has the potential to contribute much to our understanding and assessment of the underlying neurophysiological processes of stress responses and different welfare states in farm animals.

Marked fetal acidosis and specific changes in power spectrum analysis of fetal heart rate variability recorded during the last hour of labour

OBJECTIVE

To assess whether intrapartum acidosis affects specific components of fetal heart rate variability.

DESIGN

Prospective clinical study.

SETTING

Twelve Nordic delivery units.

SUBJECTS

Fetal heart rate variability was studied in 334 fetuses divided into two groups according to cord pH value: the acidotic group (cord arterial pH < 7.05 at birth, n= 15) and the control group (cord arterial pH > or =7.05 at birth, n= 319).

METHODS

In spectral analysis of fetal heart rate variability, frequencies were integrated over the total frequency band (0.04-1.0 Hz), low-frequency band (0.04-0.15 Hz) and high-frequency band (0.15-1.0 Hz). We also calculated the low-to-high frequency ratio.

MAIN OUTCOME MEASURES

The spectral bands of fetal heart rate variability were compared between the acidotic and control fetuses.

RESULTS

We found that during the last hour of monitoring, baseline fetal heart rate gradually decreased, whereas total, low-frequency and high-frequency fetal heart rate variability initially increased but then, near the delivery, decreased in the acidotic fetuses when compared with the controls. Low-to-high frequency ratio was greater in the acidotic group during the whole study period (P= 0.002). Cord artery pH was inversely associated with total fetal heart rate variability (P < 0.001), low-frequency fetal heart rate variability (P < 0.001) and low-to-high frequency ratio (P= 0.004).

CONCLUSIONS

Marked fetal acidosis was associated with frequency-specific changes in fetal heart rate variability as reflecting the compensation ability of autonomic nervous activation during the last hour of labour.

Effects of birth on baroreceptor-mediated changes in heart rate variability in lambs and fetal sheep

1. In adult unanaesthetized sheep, there is a V-shaped relationship between mean arterial pressure (MAP) and heart rate variability (HRV), measured in both time and frequency domains. In contrast, in fetal sheep, there is only a positive direct relationship between MAP and HRV, which is determined by the cardiac vagus. We postulated that by the time lambs were 8-10 days old, the 'adult like' V-shaped relationship between MAP and HRV would be present and it may appear at or after birth. To test these hypotheses, experiments were performed in six chronically catheterized fetal sheep (aged 132-138 days gestation), 10 newborn sheep (within 10 h of birth) and 10 lambs (aged 8-10 days). The relationships between MAP and HRV (in both time and frequency domains) were studied before and during beta-adrenoreceptor blockade with propranolol and before and during cardiac vagal blockade with atropine. 2. In 8-10-day-old lambs, V-shaped relationships between MAP and HRV (measured in both time and frequency domain) were obtained. The negative limb of this V-shaped relationship between MAP and HRV was present after cardiac vagal blockade. The positive slope of the V was present after beta-adrenoreceptor blockade. 3. In 4-h-old newborn sheep, there was no relationship between MAP and HRV (measured in the time domain), but between 7 and 10 h of age a negative relationship was found during treatment with atropine and a positive relationship was found during beta-adrenoreceptor blockade, when HRV was measured in both time and frequency domains. 4. As described previously, there was only a positive relationship between MAP and HRV in fetal sheep, which was abolished by atropine but not affected by beta-adrenoreceptor blockade. 5. Thus, until relatively late in fetal life, baroreceptor- modulated changes in efferent cardiac sympathetic tone, determined by measuring the effects of autonomic blockade on HRV, could not be elicited, although reflex vagal pathways were active. By 7-10 h after birth, baroreceptor-modulated changes in efferent cardiac sympathetic tone were present. It was possible using measurements of HRV made in the time domain to show that these baroreceptor-modulated cardiac sympathetic effects became stronger over the first 10 days of life (P < 0.01). These studies are the first to show that the influence of baroreceptor-mediated changes in cardiac sympathetic tone on HRV increases in early life. This is probably because maturation of sympathetic innervation of the fetal sheep heart is occurring at this age.

Measurement of baroreceptor-mediated effects on heart rate variability in fetal sheep

To determine if alterations in arterial pressure influenced fetal heart rate variability (HRV), experiments were carried out in chronically catheterized fetal sheep aged 128-138 d. Arterial pressure was raised or lowered by intravenous infusion of phenylephrine or sodium nitroprusside, and the effects on heart rate (HR) and HRV were measured (HRV, as the coefficient of variation (CV) in mean pulse interval or by power spectral analysis). Experiments were carried out before and during beta-adrenoceptor blockade with propranolol or before and during cardiac vagal blockade with atropine. There were positive relationships between mean arterial pressure and HRV (slope = 0.074+/-0.001, r = 0.81+/-0.06, p<0.001, measured as the CV of pulse interval) and between mean arterial pressure and power spectral density (slope = 4+/-0.5, r = 0.89+/-0.02, p<0.001) in the frequency range 0.04-0.08 Hz. Beta-adrenoceptor blockade had no effect on these relationships, but they were abolished by cardiac vagal blockade. The sigmoid relationship between fetal HR and mean arterial pressure, i.e. the cardiac baroreflex, was affected, however, by blockade of cardiac sympathetics and abolished by blockade of cardiac vagal activity. Thus, fetal HRV was affected by alterations in arterial pressure, and these effects depended on the integrity of the cardiac vagus, not on alterations in cardiac sympathetic activity. Therefore, although baroreflex control of fetal HR depends on the integrity of both sympathetic and parasympathetic efferent pathways, baroreceptor-induced changes in HRV depend only on the cardiac vagus.

Heart rate variability of preterm neonates quantified by energy entropy

Identifying variables predictive of neurobehavioural sequelae is a key objective in the study of high-risk neonates. Examination of heart rate variability (HRV) characteristics may be a finer discriminator of the neonate's response to physiologic stressors than the mean heart rate. The energy entropy of the heart beat tachogram, computed in four different domains, was used to quantify the HRV in 13 preterm neonates. The entropies of energies were computed from 1024 interbeat time intervals obtained once per week from 26 to 35 weeks postconceptional age (PCA). The energy entropy computed in three of the domains, like the standard deviation of intervals, distinguished between the 10 neonates that were measured at 35 weeks PCA with 100% specificity and 67% sensitivity, but did not distinguish between healthy and unhealthy neonates at earlier ages. The findings suggest that energy entropy may be a discerning measure of physiologic stress in the preterm infant, although future research is needed to refine the test and determine statistical significance.