Respiratory and heart rate patterns in infants destined to be victims of sudden infant death syndrome: average rates and their variability measured over 24 hours

Heart rate variability in sleeping preterm neonates exposed to cool and warm thermal conditions

Sudden infant death syndrome (SIDS) remains the main cause of postneonatal infant death. Thermal stress is a major risk factor and makes infants more vulnerable to SIDS. Although it has been suggested that thermal stress could lead to SIDS by disrupting autonomic functions, clinical and physiopathological data on this hypothesis are scarce. We evaluated the influence of ambient temperature on autonomic nervous activity during sleep in thirty-four preterm neonates (mean ± SD gestational age: 31.4±1.5 weeks, postmenstrual age: 36.2±0.9 weeks). Heart rate variability was assessed as a function of the sleep stage at three different ambient temperatures (thermoneutrality and warm and cool thermal conditions). An elevated ambient temperature was associated with a higher basal heart rate and lower short- and long-term variability in all sleep stages, together with higher sympathetic activity and lower parasympathetic activity. Our study results showed that modification of the ambient temperature led to significant changes in autonomic nervous system control in sleeping preterm neonates. The latter changes are very similar to those observed in infants at risk of SIDS. Our findings may provide greater insight into the thermally-induced disease mechanisms related to SIDS and may help improve prevention strategies.

Increased amplitude modulation of continuous respiration precedes sudden infant death syndrome -detection by spectral estimation of respirogram

The immaturity of the control of the autonomic nervous system has been suggested as one of the key factors in the pathophysiology of sudden infant death syndrome (SIDS). Therefore, the attenuated control of respiration may also cause more slow oscillatory breathing among infants at risk of SIDS. In this study, patterns of respiratory activity (RAV) and heart rate variability (HRV) were examined in Medilog-records prospectively obtained from 22 tape recordings made on 16 babies subsequently suffering from SIDS and from 22 matched control babies. A total of 248 signal segments, 120 s in duration, representing the state of regular breathing were visually selected for further analysis. The digitised signal sets were detrended, Fast-Fourier-transformed and autospectra as well as cross-spectra for the HRV and HRV were computed. The RAV and HRV were examined at two spectral bands: (1) a low frequency (LF) band 0.03-0.17 Hz (1.8-10 cycles/min) and (2) a high frequency (HF) band 0.3-1.3 Hz (18-90 cycles/min). Different parameters of each band were tested in the spectral analysis of cardiorespiratory control. The LF/HF-ratio of the spectral peak area of the respiratory activity and the LF/HF-ratio of the spectral band area of the respiratory activity were greater in the SIDS group when compared to the controls. No significant intergroup differences were found in the parameters of HRV, or the cross-spectral parameters. Interestingly, the technique appeared helpful in displaying that the victims of SIDS had a significantly greater amount of slow oscillation in the continuous respiratory signal (1.05+/-1.89 vs. 0.41+/-0.57, P=0.02). In the victims of SIDS the respiratory control system seems to be less stable and cause more slow oscillatory breathing and this can be detected using spectral analysis of respiratory activity even during breathing that visually seems to be regular.

Developmental patterns of heart rate and variability in infants with persistent apnea of infancy

During periods of regular breathing, heart rate is slower and more variable in healthy (no history of respiratory distress syndrome) prematurely born infants with persistent apnea, relative to full-term control infants of comparable post-conceptional ages. We tested the hypothesis that the cardiovascular differences may be linked to the persistent apnea, rather than premature birth, by assessing heart rate and variability in full-term infants with persistent apnea. Thus, resting heart rate and variability were compared in full-term infants with apnea of infancy, prematurely born infants with persistent apnea, and full-term control infants. Full-term infants with persisting apnea showed slower heart rates than control infants, beginning at 4 months after birth, and enhanced heart rate variability beginning at 6 months. Healthy prematurely born infants with persistent apnea showed cardiovascular alterations similar to those of full-term infants with apnea; these alterations differed from those observed in very premature infants with histories of respiratory distress. The postnatal development of cardiovascular aberrations in infants with persistent apnea suggests that mechanisms accompanying apneic events may contribute to long-term alterations in autonomic control.

Developmental patterns of heart rate and variability in prematurely-born infants with apnea of prematurity

At equivalent post-conceptional ages, prematurely-born infants have higher heart rates and reduced heart rate variability, relative to full-term neonates. Premature birth might exert long-lasting effects on central and peripheral mechanisms that control cardiovascular activity. We assessed development of heart rate and heart rate variability in symptomatic preterm infants up to 6 months of age. Fifty 6.5-h evening recordings of EKG and breathing were obtained from prematurely-born infants (gestational ages: 24-35 weeks). Cardiac R-R intervals were captured with a resolution of +/- 0.5 msec. One-min epochs were selected from three periods of regular respiration in recordings from premature infants and 72 recordings of full-term infants at comparable post-conceptional ages. Mean heart rate and heart rate variability were determined for each recording. At 40 weeks post-conception, prematurely-born infants with apnea of prematurity showed higher heart rates and reduced heart rate variability than did full-term neonates. These differences between premature and full-term infants persisted throughout the next 6 months in those infants born prior to 30 weeks gestation, and in those infants born at 30-35 weeks who experienced respiratory distress syndrome (RDS) during the neonatal period. The findings suggest that premature delivery, or complications thereof, exerts long-lasting effects on cardiac control.

Dynamics of respiratory patterning in normal infants and infants who subsequently died of the sudden infant death syndrome

Despite evidence that respiratory control mechanisms may be altered in infants who succumb to the sudden infant death syndrome (SIDS), overall respiratory variability in SIDS victims is comparable to that of control infants. We assessed dynamic characteristics of breathing in 16 recordings of apparently healthy infants who subsequently died of SIDS, and 35 recordings of age-matched control infants. Each breath-to-breath interval in 10-min epochs of quiet sleep and rapid eye movement sleep was plotted against the previous interval. Dispersion of next-intervals was determined after short, intermediate, and long interbreath intervals. In SIDS victims, dispersion after long intervals (slow respiratory rates) was significantly restricted relative to control infants. Moreover, after long breath-to-breath intervals, SIDS victims showed smaller mean breath-to-breath changes in respiratory rate than did controls. The findings indicate that breath-to-breath respiratory patterns differ in infants who succumb to SIDS, and the differences occur preferentially at low respiratory rates.

Aberrant temporal patterning of slow-wave sleep in siblings of SIDS victims

We assessed the patterning of slow-wave EEG activity during sleep in siblings of sudden infant death syndrome (SIDS) victims over the first 6 months of life. Twelve hour overnight physiologic recordings were obtained from 25 apparently healthy subsequent siblings of SIDS victims and 25 control infants at 1 week, and 1, 2, 3, 4 and 6 months of age. The EEG activity was electronically bandpass filtered, leaving primarily activity ranging from 0.5 to 2.5 Hz (the delta frequency), and the filtered traces were full-wave rectified and integrated over 1 min periods. The recordings were divided into four 3 h segments beginning at sleep onset, and the mean integrated delta activity during quiet sleep was determined for each segment of the night. At 3 and 4 months postnatal age, SIDS siblings displayed increased integrated delta amplitude in the early morning hours relative to control infants. Most SIDS deaths occur in the early morning hours during the 2-4 month age range. We thus speculate that increased delta activity may be indicative of increased arousal thresholds in the early morning, which may contribute to SIDS deaths.

Apnea spells, sudden death, and the role of the apnea monitor

SIDS is the most common cause of death between the ages of 1 week and 1 year. It affects 1 out of every 500 to 600 live births. The etiology of SIDS is unknown. There are no tests currently available to predict the infant who will die from SIDS prior to death. SIDS cannot be prevented. Many infants experience serious apneic spells, however, that require diagnostic evaluation and treatment. Even if the treatment of these infants does not have a large impact on the SIDS rate for the general population, thorough diagnostic evaluations and appropriate use of home apnea-bradycardia monitoring is indicated for this population and may reduce their risk of morbidity or mortality.

Heart rate variability in normal sleeping full-term and preterm neonates

To assess maturation of the Autonomic Nervous System (ANS) and sleep states, Heart Rate Variability (HRV) was studied in 24 healthy sleeping newborns, aged from 31 to 41 weeks, conceptional age (CA). Spectral analysis of the interbeat interval (RR) signal, was performed by Short-Time Fourier Transform, in three frequency bands: high (HF), of purely vagal origin, mid (MF), and low (LF), vagal and sympathetic, thus allowing evaluation of both branches of the ANS, observed in Active Sleep (AS = REM Sleep) and in Quiet Sleep (QS = nREM Sleep). Principal Component Analysis, Discriminant Analysis, and hypothesis tests were used to investigate the evolution of spectral variables and their relation with sleep states. HF, MF, LF, and mean RR all increased with age; the differences from the premature to the full-term group, were more marked, as a whole, in AS than in QS. HF showed the highest increase from the premature (31-36 weeks CA) to the intermediate (37-38) group, whereas LF showed equal differences from the premature to the intermediate, and from the intermediate to the full-term (39-41) groups. These results suggest a steep increase in vagal tone at 37-38 weeks CA, with stability afterwards, and a more regular increase in sympathetic tone from 31 to 41 weeks CA.

Normal relation between heart rate and cardiac repolarisation in sudden infant death syndrome

OBJECTIVE

To determine whether there was a difference in the relation between heart rate and RT intervals in infants who later died of sudden infant death syndrome (SIDS) and controls.

DESIGN

A blinded, computer analysis of prospectively acquired physiological data on SIDS cases and controls.

SETTING

Physiological data obtained from infants at home (collaborative analysis National Heart and Lung Institute and Exeter University).

PARTICIPANTS

Nine fullterm infants who subsequently died of SIDS and 10 surviving controls matched for age and birth weight.

INTERVENTIONS

24 hour tape recordings of the electrocardiogram at home between three and 11 weeks of age.

MAIN OUTCOME MEASURES

Mean value of the constant b, which relates by linear regression the log of each RT interval to the log of its proceeding RR interval, and any difference between SIDS cases and controls.

RESULTS

The mean (2SEM) values for b were 0.20 (0.03) for the 10 controls and 0.19 (0.03) for the nine SIDS infants. Cyclical correlations between RT and RR intervals of varying strength were identified in both SIDS infants and controls.

CONCLUSIONS

Infants who subsequently died of SIDS did not show an impaired ability to modify RT intervals in response to change in heart rate.

Neural maturational delay as a link in the chain of events leading to SIDS

Although numerous etiological or triggering factors have been suggested in sudden infant death syndrome (SIDS), the underlying mechanism of death is ultimately cardiac and/or respiratory in nature. As there is no evidence of lung or heart abnormalities, attention has focussed on the neural control of respiration and cardiac function. It is important to appreciate the methodological limitations involved in utilizing autopsy material and the criteria for selection of appropriate controls. This report reviews the pathological evidence for developmental delay in SIDS emphasizing delay of neural maturation of both myelination and synapses. Other abnormalities of the nervous system apparently associated with hypoxia-ischemia such as brainstem astrogliosis are also discussed. The occurrence of SIDS at a precise age together with our preliminary studies indicate that neural development delay is an important link in the chain of events leading to SIDS.

Heart rate variability in infants subsequently suffering sudden infant death syndrome (SIDS)

To study the role of the autonomic cardiovascular control in SIDS the heart rate variability (HRV) was analysed from 24 tape recordings made from a prospective population-based study on 16 term and one preterm infant suffering SIDS and compared to similar data on 23 control infants matched on birthweight, gestation and postnatal age. The number of regular breathing segments was lower in the SIDS cases than in controls (P = 0.02). No significant differences were found between the SIDS cases and their controls for average heart rate, median breathing rate, indices of overall and beat-to-beat HRV. Neither were significant intergroup differences found when the subjects were divided into neonatal and postneonatal subgroups. Analysis of HRV by power-spectral techniques did not show any significant differences between SIDS cases and controls for the general distribution of power or for the respiratory HRV. Analysis of HRV did not differentiate infants destined to die of SIDS from surviving controls in the same population.

Heart rate variation in normal infants and victims of the sudden infant death syndrome

Infants who later succumb to the sudden infant death syndrome (SIDS) exhibit lower overall heart rate variability during waking than do other infants. This study attempts to determine which type or types of heart rate variation are reduced in SIDS victims. Long-term recordings of heart rate and respiration were obtained from normal infants and infants who later died of SIDS, and heart rate variation in three frequency bands was examined: respiratory sinus arrhythmia (periods 0.9-3.0 s), 'mid-frequency' (periods 4.0-7.5 s) and 'low-frequency' (periods 12-30 s). All three types of heart rate variation were diminished in SIDS victims under 1 month of age during waking and rapid eye movement (REM) sleep compared with controls. Partitioning heart rate effects showed that in waking, and to a lesser extent in REM sleep, the reduction in all types of heart rate variation exceeded that which would have been predicted based on higher heart rates in SIDS victims. No heart rate-independent reduction in any type of heart rate variation was observed in quiet sleep. This state-dependent reduction in three types of heart rate variation could indicate an abnormality of autonomic control mechanisms during waking and REM sleep in infants who later succumb to SIDS.

Infants at risk for sudden infant death syndrome (SIDS): differential prediction for three siblings of SIDS infants

The outcome for three siblings of SIDS (SSIDS) infants was predicted, using as a risk model the sleep and respiratory characteristics of a SIDS victim studied extensively during the neonatal period. The SIDS infant had shown unstable state organization and deviant respiration patterns, including a deficit of brief apneic pauses. Like the SIDS infant, the SSIDS infants and a group of 16 normal infants were observed in the home for 7-hr periods when they were 2, 3, 4, and 5 weeks old. Two of the infants showed normal sleep and respiratory characteristics, and they were predicted to develop without respiratory dysfunction. In contrast, the third infant showed a pattern of deviancies similar to the SIDS infant; and at 4 months, she had prolonged apneic episodes, requiring resuscitation on two occasions. The findings are consistent with the notion of subtle central nervous system (CNS) dysfunction in SIDS risk infants from the time of birth.

Heart-rate variability and SIDS. Examination of heart-rate patterns using an expert system generator

In a prospective, population-based study, HRV was analyzed from 24-hr tape recordings made on 16 full-term and one preterm infant who had subsequently suffered SIDS and compared to similar data on 23 control infants (n of recordings, 44). In the SIDS group, heart rate was higher, and overall and beat-to-beat HRV (CV, CVS, respectively) were lower, than in the controls, but not significantly. Respiratory rate and respiratory HRV (by spectral analysis) were similar in both groups. Assuming that cardiorespiratory mechanisms of SIDS are multifactorial, we expected that several subgroups would be detected in both test groups. Therefore, the average data for each recording were subsequently examined by means of an expert system generator (ExTran, Intelligent Terminals Ltd., Edinburgh, UK). By rules induced with 25 nodes, the following results were obtained: 16/44 recordings were diagnosed as SIDS on the basis of (1) respiratory rate (RR) less than 33 and CV less than 3.46% (n = 8); (2) RR greater than 33, CVS less than 2.18%, and BW greater than 3,520 g (n = 4); and (3) RR greater than 33, CVS less than 2.18%, BW less than 3,520 g, HR greater than 136, and CV greater than 1.89% (n = 4). Seventeen of 44 were considered as non-SIDS when (1) RR was 33-47.4, CVS greater than 2.18%, and RSA less than 74.3 and (2) RR greater than 33, CVS less than 2.18%, BW less than 3,520 g, and HR less than 142. The remaining 11 cases required more complicated rules in order to be classified. This study shows that although the trend of increased HR and decreased HRV in the SIDS cases was statistically non-significant, an expert system program may be helpful in defining decision rules to identify cases of SIDS on the basis of cardiorespiratory data.

Sinus tachycardia in term infants preceding sudden infant death

The quantities of sinus tachycardia in 24-h recordings of the electrocardiogram from 16 full-term infants (greater than or equal to 37 weeks gestation) who were subsequently victims of the sudden infant death syndrome (SIDS), from 230 randomly selected age-matched full-term survivors and from 64 full-term survivors matched for age and birth weight were measured by computer and manual analysis techniques. Of 16 infants dying of SIDS, 7 had elevated levels of sinus tachycardia (greater than 95th centile in controls) (P less than 0.01). Although high levels of sinus tachycardia might be of value in identifying infants at high risk of SIDS, these encouraging findings must first be validated by further prospective studies.

Altered cardiac repolarization in some victims of sudden infant death syndrome

Abnormal prolongation of cardiac repolarization, as reflected by a long QT interval with respect to the RR interval on the electrocardiogram, is known to be associated with ventricular tachyarrhythmias. To test the hypothesis that prolonged cardiac repolarization may characterize some babies who die of sudden infant death syndrome (SIDS), we studied the dependence of the QT interval on the preceding RR interval in 10 babies with SIDS and 29 healthy control babies. We analyzed approximately 5000 pairs of QT and RR intervals in each subject over a wide range of RR intervals. We found that the QT intervals demonstrated less dependence on the preceding RR intervals in 5 of 10 babies who subsequently died of SIDS than in normal controls. No ventricular arrhythmias were observed, however, during the six-hour recording period. Our data suggest that in some babies with SIDS the ability to shorten the QT interval as the heart rate increases is impaired. These observations are consistent with the hypothesis that relatively prolonged cardiac repolarization may predispose such babies to ventricular arrhythmias.

Polygraphic studies

Polygraphic recordings have provided new information about the wide ranges of normality in various diseases and the process of maturation from birth to childhood. New techniques will help to document clinical events accurately, improve diagnosis, and evaluate treatment.

Sudden and unexpected death between 1 and 5 years

Of a population of 9856 children followed up from birth, 9251 of whom underwent 24 hour tape recordings of electrocardiograms and abdominal wall breathing movements during early infancy, five died suddenly and unexpectedly at home at ages ranging from 16 months to 4 years. Postmortem examination, including full histological and microbiological investigations. failed to identify abnormalities ordinarily associated with death in all five cases. Two of the children were known to have had frequent cyanotic episodes and died during these events. In the three remaining cases there was no previous history of cyanotic or apnoeic episodes. The death of one of these three children was seen by his parents and the clinical features suggested that apnoea rather than a cardiac arrhythmia was the primary mechanism for his death. As in infancy, sudden and unexpected death for which no adequate cause is found at necropsy seems to constitute a major component of mortality between 1 and 5 years.

Statistical analysis of heart rates in subsequent victims of sudden infant death syndrome

An analysis was made of heart rate records obtained prospectively from a group of full-term infants. The group consisted of 16 subsequent victims of sudden infant death syndrome (SIDS) and a set of 49 controls. The power spectra were calculated for each record and this together with the birth weight, gestational age and age at recording provided the basis for a discriminant analysis. The analysis identified a periodicity of 6-8 heart beats, in addition to the known effect of low birth weight, as associated with the risk of SIDS.