Spectral analysis of heart rate variability in spontaneously breathing very preterm infants

Full-Term and Preterm Newborns Differ More Significantly in Photoplethysmographic Waveform Variability than Heart Rate Variability

BACKGROUND

Features of cardiovascular autonomic regulation in infants are poorly studied compared with adults. However, the clinical significance of autonomic dysfunction in infants is very high. The goal of our research was to study the temporal and frequency-dependent features, as well as low-frequency synchronization in cardiovascular autonomic regulation in full-term vs. preterm newborns, based on the analysis of their heart rate variability (HRV) and photoplethysmographic waveform variability (PPGV).

METHODS

The study included three groups of newborns: 64 full-term newborns (with a gestational age at birth of 37-40 weeks) with a physiological course of the neonatal adaptation; 23 full-term newborns (with a gestational age at birth of 37-40 weeks) with a pathological course of the neonatal adaptation; and 17 preterm newborns (with a postconceptional age of 34 weeks or more). We conducted spectral analysis of HRV and PPGV, along with an assessment of the synchronization strength between low-frequency oscillations in HRV and in PPGV (synchronization index). We employed several options for the boundaries of the high-frequency (HF) band: 0.15-0.40 Hz, 0.2-2 Hz, 0.15-0.8 Hz, and 0.24-1.04 Hz.

RESULTS

Preterm newborns had higher heart rate, RMSSD, and PNN50 values relative to both groups of full-term newborns. Values of SDNN index and synchronization index (S index) were similar in all groups of newborns. Differences in frequency domain indices of HRV between groups of newborns depended on the considered options of HF band boundaries. Values of frequency domain indices of PPGV demonstrated similar differences between groups, regardless of the boundaries of considered options of HF bands and the location of PPG signal recording (forehead or leg). An increase in sympathetic influences on peripheral blood flow and a decrease in respiratory influences were observed along the following gradient: healthy full-term newborns → preterm newborns → full-term newborns with pathology.

CONCLUSIONS

Differences in frequency domain indices of autonomic regulation between the studied groups of newborns depended on the boundaries of the considered options of the HF band. Frequency domain indices of PPGV revealed significantly more pronounced differences between groups of newborns than analogous HRV indicators. An increase in sympathetic influences on peripheral blood flow and a decrease in respiratory influences were observed along the following gradient: healthy full-term newborns → preterm newborns → full-term newborns with pathology.

Methods for analyzing infant heart rate variability: A preliminary study

Heart rate (HR) and heart rate variability (HRV) reflect autonomic development in infants. To better understand the autonomic response in infants, reliable HRV recordings are vital, yet no protocol exists. The purpose of this paper is to present reliability of a common procedure for analysis from two different file types. In the procedure, continuous electrocardiograph recordings of 5-10 min are obtained at rest in infants at 1 month of age by using a Hexoskin Shirt-Junior's (Carre Technologies Inc., Montreal, QC, Canada). Electrocardiograph (ECG; .wav) and R-R interval (RRi; .csv) files are extracted. The RRi of the ECG signal is generated by VivoSense (Great Lakes NeuroTechnologies, Independence, OH). Two MATLAB (The MathWorks, Inc., Natick, MA) scripts converted files for analysis with Kubios HRV Premium (Kubios Oy, Kuopio, Finland). A comparison was made between RRi and ECG files for HR and HRV parameters, and then tested with t tests and correlations via SPSS. There are significant differences in root mean squared successive differences between recording types, with only HR and low-frequency measures significantly correlated together. Recording with Hexoskin and analysis with MATLAB and Kubios enable infant HRV analysis. Differences in outcomes exist between procedures, and standard methodology for infant HR analysis is needed.

Prone sleeping affects cardiovascular control in preterm infants in NICU

BACKGROUND

Prone sleeping is used in preterm infants undergoing intensive care to improve respiratory function, but evidence suggests that this position may compromise autonomic cardiovascular control. To test this hypothesis, this study assessed the effects of the prone sleeping position on cardiovascular control in preterm infants undergoing intensive care treatment during early postnatal life.

METHODS

Fifty-six preterm infants, divided into extremely preterm (gestational age (GA) 24-28 weeks, n = 23) and very preterm (GA 29-34 weeks, n = 33) groups, were studied weekly for 3 weeks in prone and supine positions, during quiet and active sleep. Heart rate (HR) and non-invasive blood pressure (BP) were recorded and autonomic measures of HR variability (HRV), BP variability (BPV), and baroreflex sensitivity (BRS) using frequency analysis in low (LF) and high (HF) bands were assessed.

RESULTS

During the first 3 weeks, prone sleeping increased HR, reduced BRS, and increased HF BPV compared to supine. LF and HF HRV were also lower prone compared to supine in very preterm infants. Extremely preterm infants had the lowest HRV and BRS measures, and the highest HF BPV.

CONCLUSIONS

Prone sleeping dampens cardiovascular control in early postnatal life in preterm infants, having potential implications for BP regulation in infants undergoing intensive care.

Heart rate variability as possible marker of brain damage in neonates with hypoxic ischemic encephalopathy: a systematic review

Heart rate variability (HRV) is currently considered the most valuable non-invasive test to investigate the autonomic nervous system function, based on the fact that fast fluctuations might specifically reflect changes of sympathetic and vagal activity. An association between abnormal values of HRV and brain impairment has been reported in the perinatal period, although data are still fragmentary. Considering such association, HRV has been suggested as a possible marker of brain damage also in case of hypoxic-ischemic encephalopathy following perinatal asphyxia. The aim of the present manuscript was to review systematically the current knowledge about the use of HRV as marker of cerebral injury in neonates suffering from hypoxic-ischemic encephalopathy. Findings reported in this paper were based on qualitative analysis of the reviewed data. Conclusion: A growing body of research supports the use of HRV as non-invasive, bedside tool for the monitoring of hypoxic-ischemic encephalopathy. The currently available data about the role of HRV as prognostic tool in case of hypoxic ischemic encephalopathy are promising but require further validation by future studies. What is Known: • Heart rate variability (HRV) is a non-invasive monitoring technique to assess the autonomic nervous system activity. • A correlation between abnormal HRV and cerebral injury has been reported in the perinatal period, and HRV has been suggested as possible marker of brain damage in case of hypoxic-ischemic encephalopathy. What is New: • HRV might provide precocious information about the entity of brain injury in asphyxiated neonates and be of help to design early, specific, and personalized treatments according to severity. • Further investigations are required to confirm these preliminary data.

The value of cardiorespiratory parameters for sleep state classification in preterm infants: A systematic review

Cardiorespiratory activity is highly associated with infants' sleep duration and quality. We performed a systematic literature search of PubMed and EMBASE databases to investigate if and how cardiorespiratory parameters can be used for sleep state classification in preterm infants and in what way maturation influences this relation. All retrieved citations were screened against predetermined inclusion and exclusion criteria. Only studies of preterm infants (<37 wk postmenstrual age during sleep state classification) admitted to a neonatal ward and of whom at least one sleep state and one cardiorespiratory parameter was measured, were included. Two researchers independently reviewed the included studies on methodological quality. Of the 1097 initially retrieved studies, 23 were included for analysis. Heart rate and respiration frequency are strongly correlated with active sleep and quiet sleep. In quiet sleep, as compared to active sleep, respiratory frequency is more stable, and the heart rate is lower and less variable. This association, however, differed across preterm birth subtypes (i.e., extremely, very or late preterm), indicating that maturation - in the form of both gestational and postnatal age - influences the cardiorespiratory characteristics of preterm sleep states. The knowledge gained from this review can help improve behavioral sleep classification and automated sleep classification algorithms for preterm infants.

Contact-Based Methods for Measuring Respiratory Rate

There is an ever-growing demand for measuring respiratory variables during a variety of applications, including monitoring in clinical and occupational settings, and during sporting activities and exercise. Special attention is devoted to the monitoring of respiratory rate because it is a vital sign, which responds to a variety of stressors. There are different methods for measuring respiratory rate, which can be classed as contact-based or contactless. The present paper provides an overview of the currently available contact-based methods for measuring respiratory rate. For these methods, the sensing element (or part of the instrument containing it) is attached to the subject's body. Methods based upon the recording of respiratory airflow, sounds, air temperature, air humidity, air components, chest wall movements, and modulation of the cardiac activity are presented. Working principles, metrological characteristics, and applications in the respiratory monitoring field are presented to explore potential development and applicability for each method.

Characterization of cardiorespiratory phase synchronization and directionality in late premature and full term infants

OBJECTIVE

Though the mutual influence of cardiovascular and respiratory rhythms in healthy newborns has been documented, its full characterization is still pending. In general, the activity of many physiological subsystems has a well-expressed rhythmic character, and often an interdependency between physiological rhythms emerges early in development. Traditional methods of data analysis only address the quantification of the strength of subsystem interactions. In this work, we will investigate system interrelationships in terms of the possible presence of causal or directional interplays.

APPROACH

In this paper, we propose a methodological application that quantifies phase coupling and its directionality in a population of newborn infants born between 35 and 40 weeks of gestational age (GA). The aim is to assess whether GA at birth significantly influences the development of phase synchronization and the directionality of the coupling between the cardiovascular and respiratory system activity. Several studies indicating irregular cardiorespiratory coupling as a leading cause of several pathologies underscore the need to investigate this phenomenon in this at-risk population.

MAIN RESULTS

Results from our investigation show a different directionality profile as a function of GA and sleep state.

SIGNIFICANCE

These findings are a contribution to the understanding of higher risk for the documented negative outcomes in the late preterm population. Moreover, these parameters could provide a tool for the development of early markers of cardiorespiratory dysregulation in infants.

Heart Rate Variability in Newborns

Heart rate (HR) and heart rate variability (HRV) in newborns is influenced by genetic determinants, gestational and postnatal age, and other variables. Premature infants have a reduced HRV. In neonatal HRV evaluated by spectral analysis, a dominant activity can be found in low frequency (LF) band (combined parasympathetic and sympathetic component). During the first postnatal days the activity in the high frequency (HF) band (parasympathetic component) rises, together with an increase in LF band and total HRV. Hypotrophy in newborn can cause less mature autonomic cardiac control with a higher contribution of sympathetic activity to HRV as demonstrated by sequence plot analysis. During quiet sleep (QS) in newborns HF oscillations increase – a phenomenon less expressed or missing in premature infants. In active sleep (AS), HRV is enhanced in contrast to reduced activity in HF band due to the rise of spectral activity in LF band. Comparison of the HR and HRV in newborns born by physiological vaginal delivery, without (VD) and with epidural anesthesia (EDA) and via sectio cesarea (SC) showed no significant differences in HR and in HRV time domain parameters. Analysis in the frequency domain revealed, that the lowest sympathetic activity in chronotropic cardiac chronotropic regulation is in the VD group. Different neonatal pathological states can be associated with a reduction of HRV and an improvement in the health conditions is followed by changes in HRV what can be use as a possible prognostic marker. Examination of heart rate variability in neonatology can provide information on the maturity of the cardiac chronotropic regulation in early postnatal life, on postnatal adaptation and in pathological conditions about the potential dysregulation of cardiac function in newborns, especially in preterm infants.

Autonomic nervous system in newborns: a review based on heart rate variability

PURPOSE

Heart rate variability (HRV) has been a relevant tool in the assessment of the autonomic nervous system (ANS). How autonomic control normally develops in newborns and how it is affected by gestational age (GA) is not fully understood. We aimed to review the current evidence on HRV in preterm (PT) and term neonates (TN) and investigate the relation between GA and the maturation of ANS.

METHODS

Electronic databases (Pubmed, World of Science, and Scopus) were searched for studies from 1997 to 2017 examining HRV (time and frequency domain) in PT and TN who followed to the Task Force (1996) guidelines. Ten studies met our inclusion criteria and were analyzed.

RESULTS

An increasing postnatal age was related to a significant rise of HRV parameters. Several significant differences were established between PT and TN (lower values on PTN), also found when PTN are evaluated at their theoretical term age. In general, there were no relevant results on LF/HF (low frequency/high frequency) ratio, as being an adequate marker of sympathovagal balance, but this was not a universal finding of this review. Frequency parameters that were more often used to evaluate newborns and HF showed the most relevant increase with GA.

CONCLUSIONS

HRV is an important tool to assess the maturation of ANS in newborns and there is a progressive increasing on cardiac parasympathetic activity, according to GA. HF appears as a relevant parameter in measurements of vagal maturation. HRV is higher in TN when compared with PTN and is more studied in newborns in terms of frequency domain. Standard recommendations in newborns remain to be fully defined.

Heart rate variability evaluation in the assessment and management of in-utero drug-exposed infants

Aim:

To determine whether heart rate variability parameters vary between in-utero drug-exposed infants and controls. To determine correlations between Finnegan score and heart rate variability parameters. To differentiate those drug-exposed infants who require treatment from those infants who do not.

Methods:

A total of 24 jaundiced control subjects and 25 in-utero drug-exposed infants were enrolled. The Finnegan score and an electrocardiographic rhythm strip were obtained at 4-h intervals. RR intervals (time between consecutive R waves) were manually tabulated from the rhythm strip and analyzed. Time-domain heart rate variability parameters were calculated and analyzed for both groups.

Results:

Heart rate variability parameters were cumulatively lower over 3 days in in-utero drug-exposed infants compared with controls (p < 0.05). Root mean square of differences of standard deviation of RR intervals on first day of life, and standard deviation of RR intervals, percentage of consecutive RR intervals greater than 50 ms, and root mean square of differences of standard deviation of RR intervals on the second day of life were significantly lower between in-utero drug-exposed infants and control infants. Three out of five parameters were significantly lower in in-utero drug-exposed infants pre-treatment versus post-treatment (p = 0.001, p = 0.0001, and p = 0.021, respectively). Root mean square of differences of standard deviation of RR intervals was able to differentiate in-utero drug-exposed infants requiring opiate therapy and in-utero drug-exposed infants that did not (p = 0.02).

Conclusion:

Heart rate variability analysis can contribute to the management of in-utero drug-exposed infants. Heart rate variability could be used in dose titration.

Heart rate variability and extubation readiness in extremely preterm infants

BACKGROUND

Mechanical ventilation (MV) is associated with changes in autonomic nervous system activity in preterm infants, which can be assessed by measurements of heart rate variability (HRV). Decreased HRV has been described in adults undergoing disconnection from MV; such information is not available in preterm infants.

OBJECTIVE

To compare differences in HRV between infants successfully extubated and those who failed, and to evaluate the accuracy of HRV as a predictor of extubation readiness.

METHODS

This is a prospective, observational study of infants with a birth weight ≤1,250 g undergoing their first extubation attempt. Heart rate was measured during a 60-min period immediately prior to extubation and HRV was calculated using the frequency domain analysis.

RESULTS

A total of 47 infants were studied; 36 were successfully extubated and 11 reintubated. There were no differences in patient demographics, ventilator settings, blood gases or postextubation management between the groups. All components of the HRV analysis were significantly decreased in infants who failed, generating high areas under the receiver operating characteristic curve. The specificity and positive predictive values were 100, but with limited sensitivity and negative predictive values.

CONCLUSIONS

Infants considered 'ready to be extubated' but who subsequently failed their first extubation attempt had decreased HRV prior to extubation. Though promising, the value of HRV as a predictor of extubation readiness requires further evaluation.

Short-term heart rate dynamics of pregnant women

Aiming to detect the stage of gestation where dynamical changes of the RR fluctuations may occur, we assessed short-term fluctuations of low risk pregnant women. Ninety six, 10min ECG recordings were collected along gestation (7 to 39 weeks). Corresponding RR fluctuations series were analysed to obtain the RMSSD, α(1), α(1(mag)) and α(1(sign)) parameters. Four groups covering first, second and last trimesters of gestation were conformed. No significant changes in α(1), which was close to unit, and α(1(sign)) among gestational groups were identified. But, in accordance with previous findings, we did find a significant reduction of RMSSD along gestation, and significant short-term changes that indicate a higher degree of nonlinearity after about 26 weeks of gestation (α(1(mag))>0.5)). These results suggest that the short-term heart rate dynamics of low risk pregnant women do not become compromised during gestation, despite the increased haemodynamic demands and other ongoing adaptations. Yet the complexity of the mechanisms involved in the cardiac regulation of pregnant women does seem to increase from mid-pregnancy, possibly owing to new short-term control influences or to modifications regardless the strength of the regulatory interactions.

Altered heart rhythm dynamics in very low birth weight infants with impending intraventricular hemorrhage

OBJECTIVE

Intraventricular hemorrhage remains an important problem among very low birth weight infants and may result in long-term neurodevelopmental disabilities. Neonatologists have been unable to accurately predict impending intraventricular hemorrhage. Because alterations in the autonomic nervous system's control of heart rhythm have been associated with intraventricular hemorrhage after its development, we sought to determine if early subtle alterations of heart rhythm could be predictive of impending intraventricular hemorrhage in very low birth weight infants.

METHODS

This case-control study included 10 newborn very low birth weight infants with intraventricular hemorrhage (5 grade IV, 4 grade III, and 1 grade II) and 14 control infants without intraventricular hemorrhage. Heart rhythm data from the first day of life before the development of intraventricular hemorrhage were evaluated. Detrended fluctuation analysis, a nonlinear fractal heart rate variability method, was used to assess the fractal dynamics of the heart rhythm. Fractal scaling exponents were calculated by using this analysis.

RESULTS

Twenty-four infants (mean +/- SD, birth weight: 845 +/- 213g: gestational age: 26.1 +/- 1.9 weeks) participated in the study. The short-term scaling exponent was significantly larger in infants who later developed intraventricular hemorrhage compared with those who did not (0.60 +/- 0.1 vs 0.45 +/- 0.1). A value of 0.52 resulted in 70% sensitivity and positive predictive value and 79% specificity and negative predictive value. The short-term scaling exponent was the only significant predictor of intraventricular hemorrhage.

CONCLUSIONS

Fractal dynamics of the heart rhythm is significantly altered in very low birth weight infants before developing intraventricular hemorrhage and may be predictive of impending intraventricular hemorrhage.

A double-blind randomized controlled pilot trial examining the safety and efficacy of therapeutic touch in premature infants

PURPOSE

To explore the hypothesis that nontouch therapy such as therapeutic touch (TT) reduces stress to a clinically important degree and is safe to use in preterm infants.

DESIGN

A pilot randomized, double-blind, controlled trial.

SUBJECTS

Two groups of 10 infants were enrolled and randomly assigned to treatment or nontreatment groups. Gestational age was less than 29 weeks. Demographic descriptions of the 2 groups were statistically similar.

METHODS

The observer and staff were blinded to assignment; the TT practitioner was blinded to observed measurements. Each infant received either TT or no therapeutic touch (NTT) for 5 minutes on 3 consecutive days at the same time of day, behind a curtain. Heart period variability (HPV) was measured 5 minutes before, during, and after the treatment phase.

RESULTS

Examination of the parameters of oxygen saturation and episodes of apnea demonstrated no increase in adverse events in TT group compared with NTT group. Repeated-measures multivariate analysis of variance on HPV revealed differences in the interaction of group assignment with low-frequency, high-frequency, and low-to-high- frequency ratio interaction (F2,143 = 8.076, P = .000) and for group, day, and low-frequency, high-frequency, and low-to-high-frequency ratio (F2,288 = 3.146, P = .015), and in the posttreatment time period (F1,16 = 6.259, P = .024), reflective of greater parasympathetic activity in TT group.

CONCLUSION

In this pilot trial, HPV showed an increase for the TT group compared with the NTT group. The study reveals no adverse effects of TT in preterm infants.

Autonomic organization of respirocardial function in healthy human neonates in quiet and active sleep

BACKGROUND AND AIM

It is not known on which time scales the nonlinear respirocardial interactions occur. This work's aim is to quantitatively assess functional respirocardial organization during quiet and active sleep of healthy full-term neonates by autonomic information flow (AIF) without limitation on specific time scales. Representing respirocardial interactions on a global time scale AIF carries information on a wider scope of interdependencies than known linear and nonlinear measures described. It assesses the complexity of heart rate fluctuations (HRF) and respiratory movements (RM) and their interaction comprising both linear and nonlinear properties. Thus, we hypothesized AIF to characterize novel aspects of sleep state-dependent respirocardial interaction.

METHODS

RM and ECG-derived HRF of six healthy full-term neonates were studied. We analyzed their power spectra, coherence, auto- and cross-correlation and complexity estimated on local ("next sample" prediction) and global time scales (an integral over AIF predicting for all time lags in HRF and RM).

RESULTS

We found the global AIF of HRF and RM to differ significantly between active and quiet sleep in all neonates, whereas on a local time scale this applied to the HRF AIF only. HRF complexity was larger in quiet than in active sleep. Respirocardial interaction was less complex in quiet versus active sleep in the high frequency band only.

CONCLUSION

Complex sleep state-related changes of respirocardial interdependencies cannot be identified completely on the local time scale. Considering the global time scale of respirocardial interactions allows a more complete physiological interpretation with regard to the underlying autonomic dynamics.

Prenatal RR fluctuations dynamics: detecting fetal short-range fractal correlations

OBJECTIVE

Several studies have suggested that the analysis of heart rate variability (HRV) during gestation provides indications of the development or maturation of fetal cardiovascular regulatory mechanisms. In this study, we evaluate the existence of short-range fractal-like correlations in fetal RR fluctuations data from the second half of human gestation.

METHODS

Fifty-six short-term abdominal ECG recordings were obtained from low-middle-risk pregnant women. Gestational age varied from estimated 21 weeks to term. For comparison, RR-interval data of 51 healthy adults were also analysed.

RESULTS

Principal findings along the gestational period explored were the existence of fractal RR dynamics in prenatal fetal data as revealed by the short-range scaling exponent alpha(1). No significant differences of alpha(1) (p = 0.4770) were found between fetal (median 1.2879) and adult data (median 1.3214), either between the fetal cases before or after 24 weeks (p = 0.6116) despite observing more variation at early stages. However, fetal RR data did involve lower magnitude in comparison with adults as we found significant differences in pNN20 and SDNN values.

CONCLUSION

The fetal short-range fractal behaviour of RR data could then be linked to the functional development of the parasympathetic activity, which appears to become manifested before 21 weeks of gestation.

Autonomic alterations in cocaine-exposed neonates following orthostatic stress

We investigated the effects of prenatal cocaine exposure (PCE) on heart rate (HR) and heart rate variability (HRV) in the presence of orthostatic stress among near- and full-term neonates. PCE infants (n = 21) and controls (n = 23) were enrolled within 120 h of birth. ECG was recorded for an hour during quiet sleep, 30 min in supine position and then 30 min in an inclined position. Linear mixed models were used to analyze HR and HRV in the time domain and wavelet and power spectrum analyses in the frequency domain. PCE infants had tachycardia both before (p = 0.091) and after tilting (p = 0.015), but with a clear interaction between PCE and orthostatic stress (p = 0.049). Compared with controls, PCE infants had a delayed and prolonged reaction to orthostatic stress. There was also a pronounced interaction with regard to log-transformed SDDRR, a measure of HRV (p = 0.049). Controls experienced an instantaneous increase in log (SDDRR) followed by a prompt return to normal levels, while PCE infants had a gradual increase that did not dissipate quickly. Frequency-domain analyses also distinguished between the cocaine-exposed infants and the controls. Results suggest that the effects of PCE on the development of sympathetic and parasympathetic systems could lead to altered cardiovascular function.

Maturation of the autonomic nervous system: differences in heart rate variability in premature vs. term infants

AIMS

Heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS). This study was initiated to assess the physiological- and maturational development of the ANS by comparing HRV data of healthy prematures with term infants.

METHODS

Short-term recordings of HRV in 39 premature healthy infants (29-35 weeks' gestation) were performed and compared with normative data of term infants. Frequency domain HRV parameters are computed in three frequency bands.

RESULTS

Gestational age of newborn infants is correlated with HRV. Prematures showed significant lower HRV parameters than term infants. The most significant differences were discerned for HRV parameters reflecting parasympathetic activity, whereas the pulse of the baro-receptor reflex was similar for prematures and term infants at 0.07 Hz (0.1 Hz adults). A respiratory peak in the HF-band according to respiratory sinus arrhythmia (RSA), as is common in adults, was not detected for prematures as it is not detected in term infants.

CONCLUSIONS

Maturation of the ANS is accompanied by increasing HRV with a pronounced increase of parasympathetic activity. These changes are measurable by short-term recordings. The physiological pulses concerning baroreceptor reflex activity and respiratory modulation were similar in prematures and term infants.

Heart rate variability analysis: a tool to assess cardiac autonomic function

Heart rate monitoring is commonly used to provide an acute indicator of an individual's cardiovascular status and responsiveness. An increasingly popular technique involves quantifying the very small amounts by which the heart rate changes from one cardiac cycle to the next. This "heart rate variability (HRV) analysis" provides a substantial amount of additional information about the cardiovascular system and enables quantification of cardiac regulatory influences on the autonomic nervous system. The autonomic nervous system consists of two main components: the sympathetic system and the parasympathetic system. The relative influence of these two components on the sino-atrial node of the heart determines the heart rate. A number of physiological factors, including blood pressure and respiratory rate, can have a profound effect on this autonomic "balance." HRV analysis therefore provides a noninvasive method for investigating the dynamic influence of changing physiological parameters on cardiac regulation.

Arterial blood flow and autonomic function in full-term infants

OBJECTIVES

To investigate variations in the autonomic functions and blood-flow velocity of the arteries supplying to the brainstem in supine and prone positions.

METHODS

Forty-one full-term infants were studied at the age of 24-72 h. Each infant underwent respiratory, cardiac and eye movement analyses in supine and prone positions. In addition, blood-flow velocity of the basilar and vertebral arteries was measured with a 2 MHz probe for 5 min in each position. Two time domain measures of heart rate variability (HRV) (Standard deviation of normal R-R intervals for long-term and pNN50 for short-term variability) were employed.

RESULTS

Significantly decreased short- (P<0.001) and long (P = 0.003)-term variabilities were observed in prone when compared with supine position. Increased short-term variability in active sleep with no interaction with position was observed (P = 0.005). A significant decreased mean (P = 0.001) and peak (P = 0.001-0.003) blood-flow velocity in prone when compared with supine position were measured in all three arteries supplying to the brainstem. No significant correlation between HRV and arterial blood-flow velocity (ABFV) was observed in either position.

COMMENT

The results of the present study in agreement with previous studies reflect the vulnerability of infants in prone position as related to brainstem function. However, it appears that ABFV and autonomic functions as reflected by HRV are independent physiological measures, possibly indicating regulation autonomy of the central nervous system.

Heart rate variability during extracorporeal membrane oxygenation and recovery in severe neonatal disease

BACKGROUND

Heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS) in neonates. During severe illness, heart rate variability is impaired.

AIM

This study was initiated to measure the changes in HRV in neonates during extracorporeal membrane oxygenation (ECMO) and recovery from severe respiratory and circulatory failure. Moreover, we compared our data with HRV data of healthy newborns and we investigated the differences in HRV parameters between ECMO-survivors and non-survivors.

STUDY DESIGN

This study is of an observational character. We performed short-term recordings of heart rate variability in 14 neonates during ECMO and recovery. We computed time- and frequency-domain HRV parameters.

RESULTS

ECMO significantly affects time-domain HRV parameters. Severe neonatal illness causes a significant reduction of all calculated HRV parameters; clinical recovery is accompanied by an increase of HRV. In comparison with normative data of healthy newborns, however, HRV remains impaired. The ECMO-development ratio separated the non-survivors from the survivors during ECMO therapy.

CONCLUSIONS

During severe neonatal illness, HRV is impaired. It remains to be clarified whether the impairment of HRV during severe illness can predict the neurological outcome. The ability of the E/D ratio as an HRV parameter to serve as a predictive tool has to be corroborated in larger group of patients.

Heart Rate Characteristics Monitoring for Neonatal Sepsis

While heart rate variability has been measured in many clinical settings and has offered insights into how HR is controlled, rarely has it offered unique information that has led to changes in patient management. We review our experience in developing continuous HR characteristics monitoring to aid in the early diagnosis of sepsis in premature infants in the neonatal intensive care unit. A predictive algorithm, developed at one center and validated at another, has led to diagnosis and treatment of this subacute and potentially catastrophic illness prior to appearance of symptoms of severe illness.

Antepartum cardiotocography: a study of fetal reactivity in frequency domain

Cardiotocography (CTG) is the most widely used diagnostic technique in clinical practice to monitor fetal health. Cardiotocographic recording also permits to assess maturation of the fetal autonomous nervous system (ANS): fetal heart rate (FHR) modifications may reveal ANS' reactions to stimuli. To assess fetal reactivity, physicians evaluate specific clinical CTG parameters, generally, by means of visual inspection, thus depending on observer's expertise, with lack of reproducibility. Still nowadays, there is a very high intra- and inter-observer variation in the assessment of FHR patterns. More objective methods for CTG interpretation are of crucial importance. For adults, frequency analysis of heart rate variability (HRV) is a non-invasive and powerful method to investigate ANS activity. This frequency analysis can also be a valid support for a better knowledge of fetal ANS functional state and reactions. Indeed, fetal HRV is a good indicator of fetal well-being in non-stress conditions. Fetal reactivity is a very important CTG characteristic used to diagnose fetal distress, but its interpretation is still uncertain. The aim of this study is to characterise fetal reactivity proposing new fetal HRV frequency parameters to support a more exhaustive CTG analysis.

Customized spectral band analysis compared with conventional Fourier analysis of heart rate variability in neonates

A customized filtering technique is introduced and compared with fast Fourier transformation (FFT) for analyzing heart rate variability (HRV) in neonates from short-term recordings. FFT is classically the most commonly used spectral technique to investigate cardiovascular fluctuations. FFT requires stability of the physiological signal within a 300 s time window that is usually analyzed in adults. Preterm infants, however, show characteristics of rapidly fluctuating heart rate and blood pressure due to an immature autonomic regulation, resulting in non-stationarity of these signals. Therefore neonatal studies use (half-overlapping or moving) windows of 64 s length within a recording time of 2-5 min. The proposed filtering technique performs a filtering operation in the frequency range of interest before calculating the spectrum, which allows it to perform an analysis of shorter periods of only 42 s. The frequency bands of interest are 0.04-0.15 Hz (low frequency, LF) and 0.4-1.5 Hz (high frequency, HF). Although conventional FFT analysis as well as the proposed alternative technique result in errors in the estimation of LF power, due to spectral leakage from the very low frequencies, FFT analysis is more sensitive to this effect. The response times show comparable behavior for both the techniques. Applying both the methods to heart rate data obtained from a neonate before and after atropine administration (inducing a wide range of HRV), shows a very significant correlation between the two methods in estimating LF and HF power. We conclude that a customized filtering technique might be beneficial for analyzing HRV in neonates because it reduces the necessary time window for signal stability.

Time domain correlation analysis of heart rate variability in preterm neonates

BACKGROUND AND AIM

A fuller understanding of the neural control mechanisms of heart rate during the early stages of human development would be of great value to obstetric and neonatal management. In this paper, we investigate the correlation between heart rate variability (HRV) and other physiological parameters such as blood pressure and respiration in preterm neonates with the aim of developing a numerical model to explain and predict heart rate variability.

STUDY DESIGN AND SUBJECTS

All the required data are readily available for premature babies who are routinely monitored while being nursed in intensive care, and we have collected large data sets for a random group of such neonates. For the quantitative analysis of the data, we have developed a time domain correlation method, which has a number of advantages over the more commonly used power spectral analysis. We have been able to study the dynamics of the different frequency components of HRV by this method.

RESULTS

Highly correlated behaviour of the different HRV components, previously observed in our work on fetal HRV, is also present in the neonate, with similar characteristic time constants. Furthermore, the correlation of high-frequency (HF) oscillations of HRV with respiration and that of low-frequency (LF) oscillations of HRV with blood pressure are demonstrated on timescales of a single oscillation. In neonates receiving artificial ventilation, the correlation between HRV and respiration depends on the type of ventilation involved and assumes opposite polarities for the two main types of equipment currently in use.

CONCLUSION

We demonstrate that it is possible to analyse HRV quantitatively by calculating the relative gains and characteristic time constants for the correlated parameters and components.

Cardiovascular autonomic regulation in preterm infants: the effect of atropine

To study cardiovascular autonomic control, we assessed the effect of atropine on heart rate (HR) and blood pressure (BP) variability in 12 preterm infants (range 26-32 wk) before intubation for respiratory insufficiency. Spectral power analysis of R-R interval and systolic BP (SBP) series were estimated in a low-frequency (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.4-1.5 Hz) band and evaluated for a 10-min period before and a 10-min period after atropine sulfate (0.01 mg/kg). Baroreceptor reflex (BR) functioning was estimated using transfer function analysis at LF (coherence, gain, and phase). Atropine resulted in a significant 12% increase in steady-state HR (p < 0.01) and unchanged SBP. For R-R interval series, the total spectral power decreased 6-fold (p < 0.01), which was predominantly due to a reduction in the LF band (16-fold; p < 0.01). In contrast, we observed a significant increase (25%; p < 0.05) in total spectral power of SBP series partly as a result of an increase in HF power. The LF power of SBP series was not altered. The median LF transfer gain (BR sensitivity) between SBP and R-R interval decreased from 4.2 to 1.4 ms/mm Hg (p < 0.01) after atropine. The LF phase relationship (BP leads R-R interval fluctuations by approximately 4 s) was not changed after atropine. In conclusion, even in preterm infants in distress, atropine modulates HR and BP variability, suggesting that BR-mediated parasympathetic control of heart rate is of significance for cardiovascular control at that age.

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 in response to the sleep-related movements in infants with and without colic

The activity of the autonomic nervous system depends on sleep stage. The imbalance of the autonomic nervous system together with over-reactivity to stimuli has been suggested to be an etiologic factor for infantile colic. This study was designed to estimate the reactivity of the autonomic nervous system to a sleep-time stimulus in non-rapid eye movement (NREM) and rapid eye movement (REM) sleep stages and in colic and control infants. Overnight sleep polygraphic recordings were performed for 12 colic and 14 control infants at the age of 8 weeks. Movements were detected by a static-charge-sensitive bed. Extent of heart rate variability (HRV) was measured in response to spontaneous sleep-related movements. HRV analysis was performed over 2-min segments during NREM and REM sleep before and after 5-36-s long movement periods. Total (0.04-1.0 Hz), low (0.04-0.15 Hz) and high frequency (0.15-1.0 Hz) HRV increased after the movement periods in light NREM sleep (p < 0.001). These changes were not observed in REM sleep. No differences were found between the colic and the control groups in HRV. The observed difference in the response of the HRV between sleep stages is likely to reflect the different characteristics of heart rate control in NREM and REM sleep, but our results do not suggest that colic infants would have abnormal autonomic reactivity to stimuli while asleep.

Cardiovascular fluctuations and transfer function analysis in stable preterm infants

To examine the baroreceptor reflex function, a beat-to-beat analysis between systolic blood pressure (SBP) and R-R interval fluctuations was studied in 10 stable appropriate-for-gestational age preterm infants (range, 27.2-33.7 wk) in the first postnatal week during quiet sleep. Spectral power analysis, using fast Fourier transform, and transfer functions (gain and phase difference) between SBP and R-R fluctuations were estimated in a low-frequency band (LF, 0.03-0.2 Hz) and high-frequency band (HF defined as the frequency band between the 10th and 90th centiles of the individual respiratory frequency). The LF/HF ratio reflects the sympathovagal balance. The mean frequency (+/-SD) of LF peaks was centered at 0.07 +/- 0.02 Hz. The mean frequency (+/-SD) of the individual HF band was 0.82 +/- 0.21 Hz. The LF/HF ratio in the R-R interval series [median, 29; interquartile range (IQR), 16-40] was higher than in the SBP series (median, 8; IQR, 4-14). The gain between R-R interval and SBP fluctuations (median, 4.2 ms/mm Hg; IQR, 2.4-5.0) in the LF band was higher than in the HF band (median, 1.7 ms/mm Hg; IQR, 1.4-3.0). SBP fluctuations lead R-R interval fluctuations in the LF band with a median phase difference of +96 degrees (IQR, 67-132). At LF the fluctuations in SBP precede changes in R-R interval with a time delay of 3.8 s. These observations indicate a dominant role of the sympathetic system in stable preterm infants in comparison with published adult values. Cross-spectral analysis allows a test for tracking the development of the sympathetic system in neonates.

Dynamic analysis of beat-to-beat fetal heart rate variability recorded by SQUID magnetometer: quantification of sympatho-vagal balance

BACKGROUND

Quantitative analysis of fetal heart rate variability (HRV) can be used to investigate the neural control mechanisms of fetal cardiac activity. However, conventional power spectrum methods do not reveal the full complexity of the time-varying sympatho-vagal balance in the fetus.

AIM

This study was carried out to explore alternative digital signal processing methods of analysing fetal HRV in time domain (rather than frequency domain), in line with most types of physiological monitoring.

METHODS

The beat-to-beat fetal heart rate was obtained by Superconducting Quantum Interference Device (SQUID) magnetocardiographic recording. These data were filtered within appropriately selected frequency bands: high frequency (HF) f>0.2 Hz, low frequency (LF) 0.05<f<0.2 Hz and very low frequency (VLF) f<0.05 Hz. To quantify the dynamics of sympathetic-parasympathetic interaction, the integrated amplitude of the HF curve (within a moving 4-s time window) was compared with the LF component. Also, the interaction of the VLF component with both HF and LF components was analysed.

RESULTS

A high degree of correlation was observed between these components for extended periods of time, although the presence of a strong correlation was found to depend on the fetal behavioural state. It was further observed that the low-frequency oscillations lag high-frequency activity by 1-3 s.

CONCLUSIONS

We propose a numerical parameter, based on the ratio of the LF to HF components of the fetal HRV as a quantitative measure of the relative gain of the parasympathetic system. Our results show this parameter to decrease with gestational age of the fetus.

Heart rate variability in healthy children and adolescents is partially related to age and gender

We evaluated 24-h time-domain heart rate variability (HRV) in 103 (46 females) healthy children and adolescents. Subjects were divided into four male and four female groups (ages 1-5, 6-10, 11-15, 16-20 years) and 24-h ambulatory Holter monitoring was performed. HRV was assessed by SDNN, SDNN index (SDNN-i), SDANN, rMSSD, pNN50. Males showed SDNN and SDANN values significantly higher than females while for SDNN-i, rMSSD, pNN50 there were no significant differences between sexes. With increasing age, there is a progressive and significant decrease of HR and increase of SDANN. On the other hand, SDNN, SDNNi, pNN50 and rMSSD increased significantly only between the first two age-groups. rMSSD and pNN50 were significantly related to body mass index. Thus, SDNN and SDANN, overall HRV measures, increased with age and were gender-related. HRV indices of parasympathetic function (rMSSD, pNN50) and SDNN-i increased up to 10 years of age and were gender-unrelated. These data demonstrate that in healthy children and adolescents there is a progressive modification of HRV that may reflect a progressive evolution of the autonomic nervous system, with different pattern measure-dependent. This paper enables us to compare, in future works, HRV in pediatric subjects in different groups according to the different HRV measures under examination.

Biobehavioral characteristics of infants with failure to thrive

Failure to thrive (FTT) is a syndrome of growth failure that results in an infant who is behaviorally difficult. The current thinking is that FTT results from a problematic infant-mother interaction, with the infant making a significant contribution to the interactional process. It is possible that the behavioral characteristics of the infant with FTT may be related to underlying physiologic response patterns, specifically, activity of the autonomic nervous system. The purpose of this study is to examine the relationships among behavioral responsiveness, heart rate variability as a marker of autonomic nervous system activity, and nutritional status in infants with FTT. Infants with FTT were matched with healthy growing infants (n = 14 pairs). Results from the study indicated that infants with FTT exhibited considerably more negative behaviors and exhibited low heart rate variability. It appears that there may be a physiologic basis to the behaviors that are exhibited by infants with FTT. Prospective research is needed to further clarify this relationship.

Assessment of the effect of EMLA during venipuncture in the newborn by analysis of heart rate variability

The objective of this study was to investigate the effect of EMLA on the pain response when venipuncture was performed in 60 3-day-old healthy newborns. EMLA/placebo was applied to the back of the baby's hand, following a randomized, double-blind procedure. ECG and crying were recorded during the test. The incidence of crying, heart rate (HR) and spectral analysis of heart rate variability were used to characterize the reaction of the baby to the venipuncture procedure. The occurrence of crying during venipuncture did not differ significantly between the EMLA and the placebo groups. The placebo-treated group showed a statistically significant higher HR, and a decrease in variance (total power) and power in the low-frequency band (0.02-0.15 Hz) when compared with the EMLA group. We conclude that EMLA decreases the stress response during venipuncture in newborn infants.

Influence of maternal smoking on autonomic nervous system in healthy infants

To determine the influence of maternal smoking on autonomic nervous system in healthy infants, 36 infants were recorded polygraphically for one night. Their mothers were defined, according to their smoking frequency during pregnancy, as "nonsmokers" (no cigarettes smoked during pregnancy) or "smokers" (10 or more cigarettes per day). The infants had a median postnatal age of 10.5 wk (range 6 to 16 wk); 18 were born to nonsmokers, and 18 to smokers. During the whole night, spectral analyses of heart rate (HR) were evaluated as a function of sleep stages. Two major peaks were recognizable: a low-frequency component (LF) related to sympathetic and parasympathetic activities and a high-frequency component (HF) reflecting parasympathetic tonus. The ratio of LF/HF powers was calculated as an index of sympathovagal interaction. In REM sleep, "smokers" infants were characterized by significantly lower HF powers and normalized HF powers, and higher LF/HF ratios than "nonsmokers." The finding did not reach statistical significance in NREM sleep. In conclusion, maternal smoking induced changes in autonomic control and maturation in infants. These effects of cigarette smoke exposure can be added to those already reported and offer additional evidence for counseling mothers to stop smoking.

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.

Assessment of acute pain in preterm infants by evaluation of facial expression and frequency domain analysis of heart rate variability

In ten preterm infants (postconceptional age 27-35 weeks) facial expression and heart rate variability (HRV) were investigated during three situations: (a) the infant at rest in its crib or incubator, (b) provocation of the withdrawal flexor reflex by application of von Frey's hairs, and (c) lancing and squeezing for blood sampling. Video recordings of facial expressions, mixed at random, were categorised as the baby being either undisturbed, disturbed or in pain and a detailed scoring for each situation was performed. Mean heart rate (HR) was calculated and power spectral analysis was assessed on data from segments of 45-s duration from the three procedures. Flexor withdrawal did not evoke visual signs of pain or influence HRV, but caused a slight increase in HR. The lancing and squeezing of the heel evoked a facial expression of pain in all infants. The HR increased and a reduction in both total HRV and power in the low frequency band of the HRV spectrum was seen during blood sampling. The differentiation between painful and non-painful procedures was more apparent when principal component analysis of HRV was applied.

Developments in CTG analysis

FHR monitoring has been the subject of many debates. The technique, in itself, can be considered to be accurate and reliable both in the antenatal period, when using the Doppler signal in combination with autocorrelation techniques, and during the intrapartum period, in particular when the FHR signal can be obtained from a fetal ECG electrode placed on the presenting part. The major problems with FHR monitoring relate to the reading and interpretation of the CTG tracings. Since the FHR pattern is primarily an expression of the activity of the control by the central and peripheral nervous system over cardiovascular haemodynamics, it is possibly too indirect a signal. In other specialities such as neonatology, anaesthesiology and cardiology, monitoring and graphic display of heart rate patterns have not gained wide acceptance among clinicians. Digitized archiving, numerical analysis and even more advanced techniques, as described in this chapter, have primarily found a place in obstetrics. This can be easily explained, since the obstetrician is fully dependent on indirectly collected information regarding the fetal condition, such as (a) movements experienced by the mother, observed with ultrasound or recorded with kinetocardiotocography (Schmidt, 1994), (b) perfusion of various vessels, as assessed by Doppler velocimetry, (c) the amount of amniotic fluid or (d) changes reflected in the condition of the mother, such as the development of gestation-induced hypertension and (e) the easily, continuously obtainable FHR signal. It is of particular comfort to the obstetrician that a normal FHR tracing reliably predicts the birth of the infant in a good condition, which makes cardiotocography so attractive for widespread application. However, in the intrapartum period, many traces cannot fulfil the criteria of normality, especially in the second stage. In this respect, cardiotocography remains primarily a screening and not so much a diagnostic method. As long as continuous monitoring of fetal acid-base balance has not been extensively tested in clinical practice, microblood sampling of the fetal presenting part (Saling, 1994) is a useful adjunct. The problem with non-normal tracings is that their significance is very often unclear. They may indicate serious fetal distress, finally resulting in preventable destruction of critical areas in the fetal brain and damage to various organs; or, on the contrary, they may indicate temporary changes in cardiovascular control as a reaction to the intermittent effects on fetal haemodynamics of, for example, uterine contractions, whether or not in combination with partial or complete compression of umbilical cord vessels or the vessels on the chorionic plate (van Geijn, 1994). Many factors influence the FHR and its variability, which further complicates the interpretation of FHR patterns; some have been discussed here in some detail. Undoubtedly, there is a need for quantitative and objective FHR analysis, as long as it does not lead to erroneous results. Close collaboration between engineers and clinicians is a prerequisite for further advances in this field. Decision support systems certainly have a future but only if they are able to take into account a large set of clinical data and can combine it with data obtained from FHR signals and other parameters referring to the fetal condition, such as fetal growth, Doppler velocimetry, amniotic fluid volume and biochemical and biophysical data obtained from the mother. Basic technical concepts inherent in computerized CTG analysis, such as sampling rate (Chang et al, 1995), signal loss, artefact detection (van Geijn et al, 1980), further processing of intervals, archiving in digitized format and monitor display, should receive considerable attention. There is still a long way to go until decision support systems find their way into obstetric practice. Further developments can only be achieved thanks to efforts of many basic and clinical researchers, wo

[Cardio-respirography in neonatal medicine: value, interpretation, indications]

The development of cardiorespirography with data processing enables to perform polygraphic recordings. However the standardized results supplied by this new generation of equipment cannot be used directly and need to be validated and analized by the clinician. The confrontation of the cardiac and respiratory curves allows a semiological analysis of the cardiac and respiratory functions and of their relationships in the various syndromic contexts encountered in the neonate (bradycardia and other cardiac arythmia, central or obstructive apnea, thoracic hypoampliation, periodic breathing).

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.

Nonlinear control of heart rate variability in human infants

Nonlinear analyses of infant heart rhythms reveal a marked rise in the complexity of the electrocardiogram with maturation. We find that normal mature infants (gestation greater than or equal to 35 weeks) have complex and distinctly nonlinear heart rhythms (consistent with recent reports for healthy adults) but that such nonlinearity is lacking in preterm infants (gestation > or = to 27 weeks) where parasympathetic-sympathetic interaction and function are presumed to be less well developed. Our study further shows that infants with clinical brain death and those treated with atropine exhibit a similar lack of nonlinear feedback control. These three lines of evidence support the hypothesis championed by Goldberger et al. [Goldberger, A.L., Rigney, D.R. & West, B.J. (1990) Sci. Am. 262, 43-49] that autonomic nervous system control underlies the nonlinearity and possible chaos of normal heart rhythms. This report demonstrates the acquisition of nonlinear heart rate dynamics and possible chaos in developing human infants and its loss in brain death and with the administration of atropine. It parallels earlier work documenting changes in the variability of heart rhythms in each of these cases and suggests that nonlinearity may provide additional power in characterizing physiological states.

Cardiorespiratory adaptation during sleep in infants and children

The cardiorespiratory control system undergoes functional maturation after birth. Until this process is completed, the cardiorespiratory system is unstable, placing infants at risk for cardiorespiratory disturbances, especially during sleep. The profound influence of states of alertness on respiratory and cardiac control has been the focus of intense scrutiny during the last decade. The effects of rapid-eye movement (REM) sleep on various mechanisms involved in cardiorespiratory control are of particular significance during the postnatal period since newborns spend much of their time in this sleep state. In fullterm newborns, REM sleep occupies more than 50% of total sleep time, and this percentage is even greater in preterm newborns. From term to six months of age, the proportion of REM sleep decreases. Since respiratory and cardiac disturbances are known to occur selectively during REM sleep, the predominance of REM sleep may be a risk factor for abnormal sleep-related events during early infancy. Awareness of these developmental changes in sleep patterns is important for clinicians dealing with problems such as apparent life-threatening events (ALTE), sudden infant death syndrome (SIDS), and/or cardiorespiratory responses to respiratory disorders. Our current understanding of respiratory and cardiac control rests mainly on studies conducted during the first months of life. There is a paucity of data on late infancy and early childhood. The present paper will review available data on how sleep affects 1) ventilatory mechanics, in particular of the upper airways and the chest wall; ventilation and apnea; gas exchange; chemoreceptor function; and arousal responses; 2) changes in heart rate and heart rate variability, and the occurrence and mechanisms of bradycardia.