Heart rate variability in healthy term newborns: the contribution of respiratory sinus arrhythmia

Vital signs as physiomarkers of neonatal sepsis

Neonatal sepsis accounts for significant morbidity and mortality, particularly among premature infants in the Neonatal Intensive Care Unit. Abnormal vital sign patterns serve as physiomarkers of sepsis and provide early warning of illness before overt clinical decompensation. The systemic inflammatory response to pathogens signals the autonomic nervous system, leading to changes in temperature, respiratory rate, heart rate, and blood pressure. In infants with comorbidities of prematurity, vital sign abnormalities often occur in the absence of infection, which confounds sepsis diagnosis. This review will cover the mechanisms of vital sign changes in neonatal sepsis, including the cholinergic anti-inflammatory pathway mediated by the vagus nerve, which is critical to the host response to infectious and inflammatory insults. We will also review the clinical implications of vital sign changes in neonatal sepsis, including their use in early warning scores and systems to direct clinicians to the bedside of infants with physiologic changes that might be due to sepsis. IMPACT: This manuscript summarizes and reviews the relevant literature on the physiological manifestations of neonatal sepsis and how we monitor and analyze these through vital signs and advanced analytics.

Characterizing change in vagal tone during the first three years of life: A systematic review and empirical examination across two longitudinal samples

Functioning of the parasympathetic nervous system (PNS), most often indexed by respiratory sinus arrhythmia (RSA), influences the volitional, cognitively-mediated forms of self-regulation across development. However, despite its clear relevance to children's self-regulation, and its utility as a transdiagnostic biomarker of emotion dysregulation and psychopathology, the ontogeny of vagal tone under conditions of homeostasis across infancy and early childhood is not well understood. The current research is comprised of two complementary studies. The first aims to address this gap by conducting a systematic review of the literature which has assessed resting RSA in the first three years of life. The second study uses data from two diverse, longitudinal datasets (n = 203 and n = 370) to model change in RSA from infancy to toddlerhood. Results from a systematic review of 62 studies meeting inclusion criteria suggest that measures of resting RSA increase over time and demonstrate moderate stability across infancy, toddlerhood, and preschool ages. Results from a series of models fit to longitudinal data in study two suggest that baseline RSA is characterized by stable increases across infancy and early childhood. Moreover, although there was equivocal evidence for individual variability in trajectories of RSA, the findings suggest that the individual differences in resting RSA may become entrenched in early life based on observed significant variance in growth model intercepts. In all, the current study contributes to our understanding of the developmental trajectories of baseline RSA across infancy and early childhood and should support future research examining links between children's parasympathetic regulation and their adjustment in early life.

The phylogeny and ontogeny of autonomic control of the heart and cardiorespiratory interactions in vertebrates

Heart rate in vertebrates is controlled by activity in the autonomic nervous system. In spontaneously active or experimentally prepared animals, inhibitory parasympathetic control is predominant and is responsible for instantaneous changes in heart rate, such as occur at the first air breath following a period of apnoea in discontinuous breathers like inactive reptiles or species that surface to air breathe after a period of submersion. Parasympathetic control, exerted via fast-conducting, myelinated efferent fibres in the vagus nerve, is also responsible for beat-to-beat changes in heart rate such as the high frequency components observed in spectral analysis of heart rate variability. These include respiratory modulation of the heartbeat that can generate cardiorespiratory synchrony in fish and respiratory sinus arrhythmia in mammals. Both may increase the effectiveness of respiratory gas exchange. Although the central interactions generating respiratory modulation of the heartbeat seem to be highly conserved through vertebrate phylogeny, they are different in kind and location, and in most species are as yet little understood. The heart in vertebrate embryos possesses both muscarinic cholinergic and β-adrenergic receptors very early in development. Adrenergic control by circulating catecholamines seems important throughout development. However, innervation of the cardiac receptors is delayed and first evidence of a functional cholinergic tonus on the heart, exerted via the vagus nerve, is often seen shortly before or immediately after hatching or birth, suggesting that it may be coordinated with the onset of central respiratory rhythmicity and subsequent breathing.

Respiratory sinus arrhythmia as an index of vagal activity during stress in infants: respiratory influences and their control

Respiratory sinus arrhythmia (RSA) is related to cardiac vagal outflow and the respiratory pattern. Prior infant studies have not systematically examined respiration rate and tidal volume influences on infant RSA or the extent to which infants' breathing is too fast to extract a valid RSA. We therefore monitored cardiac activity, respiration, and physical activity in 23 six-month old infants during a standardized laboratory stressor protocol. On average, 12.6% (range 0–58.2%) of analyzed breaths were too short for RSA extraction. Higher respiration rate was associated with lower RSA amplitude in most infants, and lower tidal volume was associated with lower RSA amplitude in some infants. RSA amplitude corrected for respiration rate and tidal volume influences showed theoretically expected strong reductions during stress, whereas performance of uncorrected RSA was less consistent. We conclude that stress-induced changes of peak-valley RSA and effects of variations in breathing patterns on RSA can be determined for a representative percentage of infant breaths. As expected, breathing substantially affects infant RSA and needs to be considered in studies of infant psychophysiology.

Autonomic Regulation during Quiet and Active Sleep States in Very Preterm Neonates

The immature autonomic nervous system (ANS) in premature infants regulates heart rate (HR) and respiration different during quiet sleep (QS) and active sleep (AS). Little information is available about ANS regulation in these subjects. The aim of this study was to investigate changes in autonomic regulation and cardiorespiratory coupling during AS and QS in five very preterm neonates with gestational age (GA) 26–31 weeks, applying univariate and bivariate linear and non-linear dynamics methods to the recorded cardiorespiratory signals. During QS univariate linear indices revealed lower standard deviations and entropies, indicating decreased heart rate (HR) variability. More balanced sympatho-vagal behavior of the ANS was revealed by decreased low frequency (LF), increased high frequency (HF), and a trend toward lower ratio LF/HF in QS. Applied non-linear indices (probabilities, entropies, and fractal measures) quantifying the complexity and scaling behavior of HR regulation processes were significantly altered in QS in comparison to AS. This reflects a lower short-term variability, less complexity, and a loss of fractal-like correlation properties of HR dynamics in QS. One major finding is that cardiorespiratory coupling is not yet completely developed in very preterm neonates with 26–31 weeks GA. Significantly different regulation patterns in bivariate oscillations of HR and respiration during AS and QS could be recognized. These patterns were characterized on the one hand by predominant monotonous regulating sequences originating from respiration independently from HR time series in AS, and to a minor degree in QS, and on the other hand by some prominent HR regulation sequences in QS independent of respiratory regulation. We speculate that these findings might be suitable for monitoring preterm neonates and for detecting disorders in the developing cardiorespiratory system.

Birth prematurity determines prolonged autonomic nervous system immaturity

Because of its multiple involvement in physiological processes, autonomic nervous system (ANS) activity, a key regulator of homeostatic control, demonstrates a progressive increase during pregnancy. The profile of its maturation, mainly in the parasympathetic arm, in premature or full term infants, may help us to better understand its pathophysiological role. We prospectively evaluated ANS maturity in a group of 23 premature (PREM) infants at their theoretical term age and in 8 full term (FT) newborns. All recordings were registered close to the theoretical full term period (from 38 to 41 weeks) for the PREM group and during the first week of life for the FT newborns. Polygraphic recordings, EEG monitoring associated with visual clinical control, and Holter ECG, were performed simultaneously. ANS indices were then calculated during quiet sleep periods, using Wavelet transform of RR (beat to beat) intervals. High frequency components were found to be significantly lower in the PREM than in the FT group (p<0.05). Furthermore, at theoretical full term age, the greater the prematurity, the lower was parasympathetic activity. Because it is easy, monitoring of parasympathetic activity may help us to understand autonomic maturation and its clinical prognostic implications.

Heart rate variability as an indicator of outcome in congenital diaphragmatic hernia with and without ECMO support

OBJECTIVE

To examine differences in the spectral power content in neonates diagnosed with congenital diaphragmatic hernia (CDH) who survive or succumb.

STUDY DESIGN

A case-series study design evaluated four neonates diagnosed with CDH, two of which were supported by extracorporeal membrane oxygenation (ECMO). The electrocardiogram signal was digitized at 1000 Hz and the Lomb periodogram was computed for the series of interbeat intervals.

RESULTS

Neonates with CDH who survived had log total power values greater than 2. Those with CDH who did not survive had log total power less than 2, but generally exceeded 3 while they were supported by ECMO.

CONCLUSIONS

Neonates who consistently displayed increasing total spectral energies had a better outcome than those whose spectral energies were low. Subjects who succumbed expressed the lowest values, suggesting that a frequency-based evaluation of HRV may be a sensitive prognosticator of outcome that requires further investigation.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

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.

Short-term rhythms of the cardiorespiratory system and their significance in neonatology

Latent disturbances in the control of respiration and heart rate (HR) may be important factors in the pathogenesis of life-threatening events during infancy. A method of determining the control of the autonomic nervous system functions involves the analysis of time-dependent ultradian changes of its parameters. The breathing signal and HR variability contain rhythmic components that are generated within the cardiorespiratory network of the brain stem, through reflexes, and by feedback mechanisms. The analysis of these components may provide insights into the functioning of the cardiorespiratory control system. The prominence and precision of the rhythms are correlated with states of vigilance and underlie distinct development during the first months of life. The results of studies on infants at risk (for example, for sudden infant death), with the help of statistical and spectral analysis of time series to obtain new indices, have proved to be inconsistent in their prognostic value of thus studied parameters. Recently, the importance of qualitative and quantitative assessment of the dynamic and complex behavior of time series, based on nonlinear characteristics of the control system, has been emphasized. To what extent, however, the analysis of the dynamic behavior can be utilized for clinical purposes, such as judging the prognosis of deficiencies in control, requires further study regarding physiological baselines and the possible changes resulting from pathological states.

Development of baroreflex influences on heart rate variability in preterm infants

To investigate developmental changes in autonomic cardiovascular reflexes in preterm infants, we used autoregressive power spectral analysis to analyze the effect of upright tilting on heart rate variability in preterm infants. Twenty-eight infants were studied in a longitudinal fashion beginning at 28-32 weeks postconceptional age (postnatal age 1-5 weeks). Each week, heart rate variability in the supine position and after 45 degrees head-up tilt was analyzed by spectral analysis. With the initial study of each infant, there was no significant change in heart rate following head-up tilt compared with baseline (-0.5+/-0.9 bpm). However, linear regression analysis revealed that with increasing postnatal age, the change in heart rate in response to tilting became more positive (mean slope of regressions 0.45+/-0.12 bpm/week, P<0.005). The power spectral density of R-R interval variability in the low-(LF; 0.02-0.15 Hz) and high-(HF; 0.15-1.5 Hz) frequency ranges were obtained and the values normalized by dividing each component by the total power. For measurements obtained in the supine position, the LF/HF ratio progressively decreased with increasing postnatal age, indicating a maturational change in sympathovagal balance. We used the difference in the LF/HF ratio between tilt and the recumbent position as a measure of the change in autonomic input to the heart in response to unloading of the arterial baroreceptors. No significant change in these ratios were observed when infants were first studied between 28 and 32 weeks postconceptional age, suggesting that the cardiac baroreflex is poorly developed at this stage of development. However, with postnatal maturation, the LF component of the power spectrum became progressively larger with tilt relative to the basal state, such that the difference between LF/HF(tilt) and LF/HF(base) became progressively more positive (P <0.006). These findings suggest that in premature infants, cardiac baroreceptor reflexes become more functional with postnatal development.

Thermal entrainment of heart rate in the preterm infant

Using spectral analysis we have studied changes in the heart rate during periodic thermal stimulation of one foot of infants during quiet sleep. Twenty-two appropriately grown preterm infants were studied in the first 15 d after birth to quantify responses in comparison with previously reported term infants. Babies were stimulated at 0.05, 0.10, and 0.15 Hz. Spectral power was calculated at the stimulus frequency +/-0.01 Hz and +/-0.02 Hz and over the low frequency range 0.03 Hz to 0.17 Hz. The data show that 1) there is an increase in power around the frequency of stimulation for each frequency studied (p < 0.002); and 2) there is an increase in the ratio of local to low frequency power at 0.05 Hz (p = 0.002) and 0.10 Hz (p = 0.001), but not at 0.15 Hz (p = 0.109). These data confirm the concept of entrainment in the appropriately grown preterm infant but demonstrate that it occurs over a wider frequency range than previously reported. The wider range is the same as that of the term infant, although there are differences in the patterns of entrainment between the two groups. Further work is required to map out the maturation of the autonomic nervous system in both the term and the preterm infant with respect to the low frequency components of the heart rate variability power spectrum.

Autoregressive analysis of cardiac RR intervals in preterm and full-term infants

For the purpose of obtaining information on the maturation process of the autonomic nervous system in pre- and full-term infants, polygraphs were recorded during sleep from a total of 52 appropriate-for-dates infants at a conceptional age of 28-41 weeks. The 104 RR interval time series was calculated by autoregressive (AR) and component analysis. Histograms of damping frequencies obtained were divided into two frequency bands: low frequency fluctuations (LF) of less the 0.1 cycle/beat (c/b) and high frequency fluctuations (HF) of more than 0.15 c/b. Total power and LF power decreased with increases in the conceptional age or postnatal age at a significance level of 0.01, respectively. In addition, the HF amount of bio-informing activity increased (P < 0.001) and the damping time of HF also increased (P < 0.01). From these results, it was speculated that the activity of the sympathetic system decreases with increases in conceptional or postnatal age. It was also speculated that increased regularity and oscillatory duration in HF might be caused by the development of the reciprocal response of respiration to heart rate variation.

Spectral pattern of neonatal cerebral blood flow velocity: comparison with spectra from blood pressure and heart rate

The cerebral blood flow velocity (CBFV) frequency spectra were studied in 106 premature and term newborns (gestational age range. 24-42 wk) and compared with the heart rate (HR) and mean arterial blood pressure (BP) spectra over the 0.005-0.5 Hz range. CBFV, BP, and HR were shown to have similar but not identical spectral patterns. Adjustment of a l/f model to these spectra produced highly significant fittings, but the residuals were not independent. This condition was met only by the CBFV and BP spectra over a limited frequency range (0.005-0.06 Hz). These results provide a characterization of the CBFV spectra for a much larger population of newborns than hitherto available, indicating that under certain conditions CBFV and BP might show the properties of chaotic systems. In infants without major complications, gestational age (GA) did not have a significant influence on the CBFV spectrum, whereas the spectral power to 0.5 Hz of both BP and HR was found to increase with GA. The spectral power increased over the first 24 h of postnatal life for all three variables: only CBFV showed a significant spectral change in the low frequency (LF, 0.02-0.08 Hz) range. A matched group comparison, adjusted for GA and postnatal age, indicated a reduction in CBFV LF power for term infants with birth asphyxia when compared with normal infants, which was not reproduced in the HR spectra.

Ventilator-associated sinus arrhythmia in a preterm neonate--an indicator for a mature autonomic nervous system?

According to control theory, the interactions between respiration and heart rate (i.e. respiratory sinus arrhythmia, RSA, and breath amplitude sinus arrhythmia, BASA) reflect the inner workings of the physiological control systems of respiration and circulation. This paper reports on a preterm neonate (28.5 weeks old, 940 g) who showed the presence of ventilation-associated sinus arrhythmia (VASA) under moderate artificial ventilation. His heart rate variability was entrained to the ventilatory stimulus. VASA and entrainment suggest that the parasympathetic part of the autonomic nervous system might be more mature than expected in some preterm neonates.

Spectral analysis of heart rate variability during quiet sleep in normal human fetuses between 36 and 40 weeks of gestation

Respiratory sinus arrhythmia (RSA) is a clinical manifestation of the parasympathetic nervous system which can be identified in the high-frequency region of the heart rate variability (HRV) power spectrum. The purpose of this study was to determine the relative contribution of RSA to overall HRV for human fetuses in quiet sleep. The study population consisted of 13 normal human fetuses between 36 and 40 weeks of gestation for whom data were collected during spontaneous breathing and normally occurring apneic periods. Fetal breathing was monitored continuously using real-time sonography. The fetal electrocardiogram was captured transabdominally in 3-min blocks at a rate of 833 Hz and fetal R-waves were extracted from the raw signal using adaptive signal processing techniques. Fetal behavioral state was determined at the beginning and end of each 3-min data collection period. The fetal R-wave interbeat intervals (IBIs) were converted to equally-spaced, time-based data, and linear detrending of the time series was accomplished by subtracting the mean heart period from each weighted IBI. Total power (TP, 0.0-2.5 Hz) was divided into RSA (0.4-1.0 Hz), high-frequency (HF, 0.2-2.5 Hz), low-frequency (LF, 0.04-0.2 Hz), and very-low-frequency (VLF, 0.0-0.04 Hz) regions, and the power densities were summed to determine the absolute power for each frequency component. A total of 81 3-min blocks (mean per subject 6.3, range 2-14) were available for analysis. Eleven (85%) of 13 fetuses demonstrated a HF peak during fetal breathing, and RSA accounted for approximately 20% of the TP.(ABSTRACT TRUNCATED AT 250 WORDS)

Alleviation of the pain of heel prick in preterm infants

The hypothesis that the variability of physiological parameters may indicate pain or stress in the neonate was examined. Four parameters (heart rate, respiratory rate, transcutaneous oxygen tension, and carbon dioxide tension) were examined over a 2 minute epoch in response to a heel prick in an attempt to measure stress/pain in 35 preterm newborn infants (26-34 weeks' gestation) half of whom were receiving intensive care. The change in absolute values of these parameters did not discriminate a dummy procedure without prick from the actual procedure containing the prick (paired t test), but the variability of the parameters during an epoch showed significant discrimination. Three procedures were evaluated to reduce this distress using unpaired t test. The use of local anaesthetic cream was not successful. The components of the mixture cause vasoconstriction that would reduce blood flow to the heel and lead to more squeezing which is likely to be painful in the presence of tissue damage. A nurse comforting the infant with tactile and vocal stimulation was slightly helpful but the use of a spring loaded lance was most successful in reducing the distress. The use of spring loaded lances may be more humane for heel pricks.