Heart rate response profiles during head upright tilt test in infants with apparent life threatening events

Cerebrovascular responses to a 90° tilt in healthy neonates

BACKGROUND

Tilts can induce alterations in cerebral hemodynamics in healthy neonates, but prior studies have only examined systemic parameters or used small tilt angles (<90°). The healthy neonatal population, however, are commonly subjected to large tilt angles (≥90°). We sought to characterize the cerebrovascular response to a 90° tilt in healthy term neonates.

METHODS

We performed a secondary descriptive analysis on 44 healthy term neonates. We measured cerebral oxygen saturation (rcSO2), oxygen saturation (SpO2), heart rate (HR), breathing rate (BR), and cerebral fractional tissue oxygen extraction (cFTOE) over three consecutive 90° tilts. These parameters were measured for 2-min while neonates were in a supine (0°) position and 2-min while tilted to a sitting (90°) position. We measured oscillometric mean blood pressure (MBP) at the start of each tilt.

RESULTS

rcSO2 and BR decreased significantly in the sitting position, whereas cFTOE, SpO2, and MBP increased significantly in the sitting position. We detected a significant position-by-time interaction for all physiological parameters.

CONCLUSION

A 90° tilt induces a decline in rcSO2 and an increase in cFTOE in healthy term neonates. Understanding the normal cerebrovascular response to a 90° tilt in healthy neonates will help clinicians to recognize abnormal responses in high-risk infant populations.

IMPACT

Healthy term neonates (≤14 days old) had decreased cerebral oxygen saturation (~1.1%) and increased cerebral oxygen extraction (~0.01) following a 90° tilt. We detected a significant position-by-time interaction with all physiological parameters measured, suggesting the effect of position varied across consecutive tilts. No prior study has characterized the cerebral oxygen saturation response to a 90° tilt in healthy term neonates.

Brief resolved unexplained event: Severity-associated factors at admission in the pediatric emergency ward

OBJECTIVE

A brief resolved unexplained event (BRUE) is a recent clinical entity that has now replaced the term "infant discomfort". Despite the availability of recent recommendations, identification of patients requiring further examination remains difficult.

METHOD

We aimed to identify factors associated with severe pathology and/or recurrence by studying the medical files of 767 patients admitted to the pediatric emergency department of a French university hospital for a BRUE.

RESULTS

Overall, 255 files were studied; 45 patients had a recurrence and 23 patients had a severe diagnosis. The most frequently found etiology was gastroesophageal reflux in the benign diagnosis group and apnea or central hypoventilation in the severe diagnosis group. Prematurity (p = 0.032) and time since last meal >1 h (p = 0.019) were the main factors associated with severe disease. Most of the routine examination results remained non-contributive to the etiology.

CONCLUSION

As prematurity is a factor associated with severe diagnosis, special attention should be given to this population, without subjecting them to multiple tests, since the main complication was found to be apnea or central hypoventilation. Prospective research is needed to establish the usefulness and prioritization of diagnostic tests for infants who are at "high risk" of experiencing a BRUE.

Effects of tilt on cerebral hemodynamics measured by NeoDoppler in healthy neonates

BACKGROUND

Today, there are conflicting descriptions of how neonates respond to tilt. Examining physiologic responses of cerebral blood flow velocities (BFVs) in challenging situations like a tilt requires equipment that can cope with positional changes. We aimed to characterize how healthy term neonates respond to mild cerebral hemodynamic stress induced by a 90° tilt test using the recently developed NeoDoppler ultrasound system.

METHODS

A small ultrasound probe was fixated to the neonatal fontanel by a cap, and measured cerebral BFV in healthy neonates during and after a 90° head-up tilt test, five min in total, at their first and second day of life. Unsupervised k-means cluster analysis was used to characterize common responses.

RESULTS

Fifty-six ultrasound recordings from 36 healthy term neonates were analyzed. We identified five distinct, immediate responses that were related to specific outcomes in BFV, heart rate, and pulsatility index the next two min. Among 20 neonates with two recordings, 13 presented with different responses in the two tests.

CONCLUSIONS

Instant changes in cerebral BFV were detected during the head-up tilt tests, and the cluster analysis identified five different hemodynamic responses. Continuous recordings revealed that the differences between groups persisted two min after tilt.

IMPACT

NeoDoppler is a pulsed-wave Doppler ultrasound system with a probe fixated to the neonatal fontanel by a cap that can measure continuous cerebral blood flow velocity. Healthy neonates present with a range of normal immediate cerebral hemodynamic responses to a 90° head-up tilt, categorized in five groups by cluster analysis. This paper adds new knowledge about connection between immediate responses and prolonged responses to tilt. We demonstrate that the NeoDoppler ultrasound system can detect minute changes in cerebral blood flow velocity during a 90° head-up tilt.

Influence of prenatal alcohol and smoke exposure on neonatal vagal tone in response to head-up tilt

This paper investigates differences in the parasympathetically mediated heart rate response to head-up tilt in two populations of newborns. One group was unexposed to any drug during pregnancy, the other was exposed to both alcohol and smoking in utero. Four different estimates of vagal tone were calculated. These indexes quantify vagal tone magnitude in four different domains: time, frequency, complexity and phase. Control group (CG) results across all parameters show a consistent physiological response to an orthostatic tilt consistent with vagal withdrawal. On the other hand, infants in the exposed group (EG) did not express a decrease in vagal measures following tilt.

Combined effects of body position and sleep status on the cardiorespiratory stability of near-term infants

The purpose of this study was to determine the effects of body position (prone, supine and lateral) together with sleep status (wake and sleep) on the cardiorespiratory stability of near-term infants. A total of 53 infants (gestational age at birth 33.2 ± 3.5 weeks; birth weight 1,682 ± 521 g; gestational age at recording 38.6 ± 2.1 weeks; weight at recording: 2,273 ± 393 g) were monitored for 24 hours for clinically significant apnea (>15 seconds), bradycardia (<100 bpm), and oxygen desaturation (SpO₂ < 90%) in alternating body positions (prone, supine and lateral) by cardiorespiratory monitors and 3-orthogonal-axis accelerometers. Sleep status of the infants was also continuously monitored by actigraphs. No apnea was observed. During wake, severe bradycardia was most frequently observed in the lateral position while, during sleep, severe bradycardia was most frequently observed in the supine position. Desaturation was most frequently observed in the supine and lateral positions during both wake and sleep. Our study suggests that the cardiorespiratory stability of infants is significantly compromised by both body position and sleep status. During both wake and sleep, prone position induces the most stable cardiorespiratory functions of near-term infants.

Cardiorespiratory physiology in the safe passage study: protocol, methods and normative values in unexposed infants

AIM

The Safe Passage Study, conducted by the Prenatal Alcohol in SIDS and Stillbirth Network, is investigating contributions of prenatal alcohol exposure to foetal and infant demise. This current report presents physiological data from full-term infants with no prenatal exposure to alcohol or maternal smoking.

METHODS

Data are from 666 infants from the Northern Plains (North and South Dakota) and South Africa. A standardised protocol assessed cardiorespiratory function during baseline and head-up tilts shortly after birth and at one month of age.

RESULTS

Analyses revealed significant increases in heart rate and decreases in BP from the newborn to one-month time period as well as diminished heart rate responses to head-up tilt in one-month-old infants.

CONCLUSION

The Safe Passage Study was successful in characterising physiology in a large number of infants at sites known to have elevated risks for SIDS. Results demonstrate that even with low prenatal adverse exposures, there are significant changes in cardiorespiratory function as infants enter the window of increased risk for SIDS.

Usability of the head upright tilt test for differentiating between syncopal and seizure-like events in children

The common diagnosis of loss of consciousness and clonic movements in children is seizure or epilepsy, but in a number of patients these symptoms could also be due to syncope. Over interpreted electroencephalography is misleading in a number of patients; therefore, in addition to a detailed and thorough history, a reliable test is needed to differentiate between these conditions. The aim of the study was to evaluate the utility of the head upright tilt test to differentiate between seizure-like events and syncope in children. A chart review descriptive study was conducted in a tertiary medical center in Ahvaz, Iran. We selected sixteen children (nine boys and seven girls) with convulsions of any type who were first diagnosed as epileptic based on the event description by their parents or caregivers to undergo the head upright tilt test. The main findings were the reproduction of previously presyncopal or syncopal symptoms in the tilted position. Fourteen children showed positive results after conducting the head upright tilt test, and their heart rates were significantly decreased compared to baseline at the onset of the syncopal or presyncopal manifestations. Systolic and diastolic blood pressures were significantly reduced in patients with positive results. In three children who initially had negative head upright tilt tests, intravenous isoproterenol was administered, and all three showed presyncopal and syncopal symptoms. The results indicate that the head upright tilt test could differentiate presyncopal and syncopal events in children who present with seizure-like movements but their history has clues for conclusive syncope.

An old drug for use in the prevention of sudden infant unexpected death due to vagal hypertonia

Reflex vagal hypertonia (RVH) has been identified as a possible cause of sudden unexpected death in infants during the first year of life. Homatropine methylbromide (HM) is an anticholinergic drug known to inhibit muscarinic acetylcholine receptors, thus affecting the parasympathetic nervous system. The aim of the present study was to investigate the effects of HM on 24-h Holter electrocardiographic signs of RVH (pre-HM treatment vs post-HM treatment; post-HM treatment vs a control group of healthy infants). A total of 50 patients (mean age, 6.1 ± 2.7 months; 28 males, 22 females; 12 born pre-term) affected by RVH were enrolled in the study. Pre-HM treatment vs post-HM treatment: statistically significant differences were detected for higher heart rate, lower heart rate, mean heart rate, longer sinusal pause, presence of advanced atrio-ventricular blocks, and systolic blood pressure (p < 0.001, p < 0.00001, p < 0.02, p < 0.00001, p < 0.05, and p < 0.04, respectively). A statistically significant correlation was revealed between HM-administered dose and both average heart rate and systolic blood pressure (r = 0.93, p < 0.0001; r = 0.94, p < 0.0001, respectively). No significant differences were detected between post-HM treatment electrocardiographic data and those of the control group. By antagonizing action of the vagus nerve of the parasympathetic system on the heart, thus increasing cardiac frequency, HM treatment appears to feature a good safety profile and be highly effective in preventing transient infantile hypervagotonia, the potential cause of several cases of sudden unexpected death during the first year of life.

Potential latent effects of prenatal cocaine exposure on growth and the risk of cardiovascular and metabolic disease in childhood

The literature strongly suggests that prenatal exposure to certain medications and substances does not cause major malformations in early childhood. However, these exposures may have far-reaching latent health effects, such as restricted growth, hypertension, and cardiovascular events in adulthood. We reviewed the literature to identify the effects of prenatal cocaine exposure on growth and the risk of cardiovascular and metabolic disease in late adolescence and early adulthood by examining studies that were published in peer-reviewed English-language journals from 1990 through 2009 and indexed in MEDLINE. We found that animal and clinical studies of the influence of prenatal cocaine exposure on child and adolescent growth and the subsequent development of myocardial and cardiometabolic disease risk factors are few and inconclusive. Studies support the hypothesis that vascular and hemodynamic functions are partially programmed in early life and thus substantially influence vascular aging and arterial stiffening in later life. Sub-optimal fetal nutrition and growth may increase blood pressure and the development of cardiovascular and metabolic disease in late life. How prenatal cocaine and other drug exposure effects this relationship is currently unknown. Despite high rates of cocaine and other drug use during pregnancy (up to 18% in some studies), little is known about the health effects of prenatal cocaine exposure in adolescence and early adulthood. The few studies of early growth deficits persisting into adolescence are inconclusive. The literature provides little information on how exposed children grow into adulthood and about their subsequent risk of cardiometabolic and vascular disease.

Autonomic function, gastroesophageal reflux in apparent life threatening event

AIMS

To assess the autonomic function in infants with idiopathic apparent life threatening event (IALTE) with and without gastroesophageal reflux (GER) and to compare the autonomic activity in events of pure obstructive apnea and coupled events of apnea associated with GER.

METHODS

Seventeen infants diagnosed with IALTE and GER and 17 matched infants with IALTE only between the ages of 3-28 weeks participated in the study. All infants underwent a polysomnography including esophageal pH measurements. Obstructive apneas with and without associated GER were identified. Heart rate variability (HRV) was evaluated employing time domain analysis for short- and long-term variability. Forty R-R intervals for each epoch preceding, during, and following the episodes, as well as 10 segments of 40 R-R intervals unrelated to apneic episodes were analyzed.

RESULTS

A decreased baseline short-term variability among infants with IALTE and GER was found. Both short- and long-term variability were significantly increased in the period preceding the obstructive apnea when compared to the baseline values. No such autonomic activity was observed preceding coupled events of apnea and GER. While a significant increase in long-term variability following an obstructive apnea when compared to the apnea period was observed, no such changes were found following a coupled apnea-GER event.

CONCLUSIONS

Infants with history of IALTE and GER have a significant abnormality in their autonomic control that is marked in the coupled events of apnea and GER. This finding is possibly related to medullary autonomic regulation.

Prone sleeping impairs circulatory control during sleep in healthy term infants: implications for SIDS

STUDY OBJECTIVES

To determine the effects of sleeping position on development of circulatory control in infants over the first 6 months of postnatal age (PNA).

DESIGN

Effects of sleeping position, sleep state and PNA on beat-beat heart rate (HR) and mean arterial pressure (MAP) responses to a head-up tilt (HUT) were assessed during sleep in infants at 2-4 wks, 2-3 mo and 5-6 mo PNA.

MEASUREMENTS

Daytime polysomnography was performed on 20 full-term infants (12 F/8 M) and MAP was recorded continuously and noninvasively (Finometer). HUTs of 15 degrees were performed during active sleep (AS) and quiet sleep (QS) in both the prone and supine sleeping positions. MAP and HR data were expressed as the percentage change from baseline, and responses were divided into initial, middle and late phases.

RESULTS

In the supine position HUT usually resulted in an initial increase (P < 0.05) in HR and MAP, followed by decreases (P < 0.05) in HR and MAP in the middle phase; subsequently HR and MAP returned to baseline in the late phase. By contrast, in the prone position the initial HUT-induced rises in HR and MAP were usually absent, and at 2-3 mo MAP actually decreased (P < 0.05); subsequently HR but not MAP returned to baseline. At 2-3 mo, MAP was lower (P < 0.05) in prone than supine sleeping throughout the HUT.

CONCLUSIONS

Prone sleeping alters MAP responses to a HUT during QS at 2-3 mo PNA. Decreased autonomic responsiveness may contribute to the increased risk for SIDS of infants sleeping in the prone position.

Early environmental influences on vascular development

The "fetal origins hypothesis" proposes that a fetus' adaptation to its intrauterine environment and postnatal stressors may have life-long consequences and that the fetal response to an environmental challenge may result in programming of different organ systems, depending on the timing of the insult. Growth restriction in the last trimester of pregnancy has been associated with later cardiovascular disease, not only through disturbances of normal cardiovascular and neuro-hormonal control mechanisms, but also because the "catch-up growth" observed in growth restricted infants disturbs the "thrifty phenotype" that permits an individual to live within its predicted environment. Problems in testing the fetal programming hypothesis include the measurement of an individual's failure to achieve their growth potential and the absence of an effective early measurement of later cardiovascular risk. However, individuals studied during fetal life are now reaching adulthood and will provide us with new insights into vascular programming.

Infant circulation model based on the electrophysiological cell model

It is important to use a myocardial cell model to evaluate the effects of the drugs to the hemodynamic parameters. We developed an infant circulation model which incorporates an accurate myocardial cell model including a beta adrenergic system. The beta adrenergic system is essential mechanism for reproducing the response of baroreflex control system. The parameters of the published adult human circulation model were modified to fit the infant hemodynamic values. The guinea pig myocardial cell model was introduced to the circulation model whose baseline heart rate is close to that of an infant. The presented model is in good agreement with results obtained in physiological experiments.

Inappropriate mediastinal baroreceptor reflex as a possible cause of sudden infant death syndrome – Is thorough burping before sleep protective?

Despite extensive research, a link between the assumed mechanisms of death and known risk factors for sudden infant death syndrome (SIDS) has not yet been established. Modifiable risk factors such as prone sleeping position, nicotine exposure and thermal stress and non-avoidable risk factors like male gender and some risky socio-economic conditions could be detected, but the etiology of SIDS remains unknown. In many SIDS cases histopathological findings suggest an involvement of vital autonomic control functions and unidentified trigger factors seem to play a role. From a hypothetical point of view, a developmental sympatheticovagal imbalance of the cardiovascular reflex control could cause a predisposition for SIDS. An assumed gastroesophageal trigger impulse is possibly developed during the first weeks of life and could lead to the infant's vagal reflex death. Air swallowed during feeding escapes through the esophagus while the infant is sleeping. The temporarily bloated esophagus exerts pressure on neighboring mediastinal baroreceptors, which is potentially misinterpreted as a rise in arterial pressure. The following cardiodepressoric baroreceptor reflex could lead to arterial hypotension, bradycardia and cardiac arrest. Sleeping in prone position may create an increased thoracic pressure on mediastinal baroreceptors, causing a more pronounced vagal reflex and an increased likelihood of SIDS. Prone position in connection with soft objects in the infant's sleeping environment potentially generates an increased oculobulbar pressure, resulting in an additional cardiodepressoric condition (Aschner-Dagnini phenomenon). From the sixth month of life onwards the sympatheticovagal balance seems to have matured sufficiently to compensate the life-threatening challenges in most infants. Insufficient postprandial burping could either create another independent modifiable risk factor or present the missing link to a common trigger mechanism for SIDS. Further investigations may possibly lead to the explicit recommendation to burp all infants sufficiently and repeatedly before sleep.

Backpropagation Artificial Neural Network Detects Changes in Electro-Encephalogram Power Spectra of Syncopic Patients

This paper presents an effective application of backpropagation artificial neural network (ANN) in differentiating electroencephalogram (EEG) power spectra of syncopic and normal subjects. Digitized 8-channel EEG data were recorded with standard electrodes placement and amplifier settings from five confirmed syncopic and five normal subjects. The preprocessed EEG signals were fragmented in two-second artifact free epochs for calculation and analysis of changes due to syncope. The results revealed significant increase in percentage delta and alpha (p < 0.5 or better) with significant reduction in percentage theta activity (p < 0.05). The backpropagation ANN used for classification contains 60 nodes in input layer, weighted from power spectrum data from 0 to 30 Hz, 18 nodes in hidden layer and an output node. The ANN was found effective in differentiating the EEG power spectra from syncopic EEG power spectra and the normal EEG power spectra with an accuracy of 88.87% (85.75% for syncopic and 92% for normal).

Developmental changes in infant heart rate responses to head-up tilting

AIM

Newborn infants produce significant heart rate responses to both head-up and head-down tilting: heart rate increases with head-up tilting and decreases with head-down tilting. However, previously we found that, at 2-4 mo of age, heart rate increases were no longer significant following slow head-up tilting. This study was designed to determine if 2-4-mo-old infants have significant increases in heart rate when tilted rapidly.

METHODS

Fifty-four infants were tested as newborns or at 2-4 mo of age. Heart rate was measured while infants were tilted to a 30 degrees head-up angle either slowly over a period of 30 s or rapidly in 5 s.

RESULTS

Newborns exhibited increases in heart rate using both tilt speeds; however, at 2-4 mo of age, heart rate did not change significantly using either speed of tilting.

CONCLUSION

There are significant early developmental changes in cardiac responses to hypotensive challenge. Newborns react like adults, mounting sustained increases in heart rate in response to head-up tilting, but at 2-4 mo of age sustained heart rate responses are no longer significant. Tilt tests may provide a standardized method for assessing autonomic competence during the period of maximum vulnerability to sudden infant death syndrome.

Topographic localization of electrocortical activation in newborn and two- to four-month-old infants in response to head-up tilting

AIMS

(1) To confirm that head-up tilting causes sustained increases in the heart rate (HR) of newborn infants but not during the period of maximum vulnerability to SIDS at 2-4 mo of age, and (2) to determine whether electrocortical activation (changes in high-frequency EEG power) also shows topographic and age-dependent effects of tilting.

METHODS

HR and electrocortical activity were recorded in 15 newborn and 12 2- to 4-mo-old infants during head-up tilting. Infants were tilted, three times, to a 30 degrees head-up position. Electrocortical activity was acquired using a 128-lead EEG system. Changes in HR and high-frequency (12-50 Hz) power in the electrocortical signal were computed from the flat to the head-up position.

RESULTS

Newborn infants had significant increases in HR and robust increases in high-frequency power in the left frontal, right frontal-temporal, and occipital regions following head-up tilt. At 2 to 4 mo of age, HR did not change significantly and tilt-related increases in high-frequency power were smaller.

CONCLUSION

The patterns of HR change and electrocortical activation with tilting of newborn infants are different from infants at the age of highest risk for SIDS.

Discharge diagnoses in infants with apparent life-threatening event

BACKGROUND

There are various identifiable diseases or conditions that can be associated with an apparent life-threatening event (ALTE) in infancy. The present study was carried out to investigate the etiology of ALTE based on the discharge diagnoses.

METHODS

A protocol for the complete work-up used to examine the cause of ALTE was designed, and was carried out for 69 infants with ALTE.

RESULTS

Gastroesophageal reflux disease (GERD) was demonstrated in 38 cases, and chronic gastric volvulus (CGV) was diagnosed in 21 (associated with GERD in 13 infants). Four cases of pertussis and two cases each of sepsis, laryngomalacia, respiratory immaturity, and premature ventricular contractions were found. Others cases presented with pneumonia, meningitis, intussuception, food allergy, epilepsy, and adenoid vegetations. In 12 of the 69 infants with ALTE, no abnormalities were found in any of the studies performed, including two patients with breath-holding spells. In 17 cases, two or more possibly contributing findings were diagnosed.

CONCLUSION

It is important to recognize that gastroesophageal impairments such as GERD or CGV could be responsible for many incidences of ALTE in infancy. The authors recommend a trial be established to investigate the causes of ALTE, including gastroesophageal evaluations in all infants with ALTE.

Cardiovascular responses to three simple, provocative tests of autonomic activity in sleeping infants

Whereas defective cardiovascular autonomic control has been implicated in the sudden infant death syndrome, relatively little is known about the normal development of autonomic control, due to the inability to measure blood pressure in infants noninvasively. We studied 12 normal infants [age: 13 +/- 2 (SD) wk] using a noninvasive method of continuous blood pressure recording and examined the cardiovascular responses to 45 degrees head-up tilting, a modified cold face test, and a loud noise. In head-up tilting, in both slow-wave sleep and rapid eye movement sleep, all infants displayed a rapid biphasic heart rate response (mean increase of 16% and mean decrease of 21%) and blood pressure response (mean increase of 16% and mean decrease of 16%), with a return to pretest values within 20 s. Both ice and noise caused a less pronounced biphasic response. In conclusion, at 3 mo, infants show the adult pattern of response to postural challenge. The short latency of the response suggests that neural inputs, apart from baroreceptors, are involved in the initial phase of the response.

Factors affecting heart rate variability and heart rate responses to tilting in infants aged 1 and 3 months

Heart rate variability (HRV) and heart rate (HR) responses following a 60 degree head-up tilt were measured in 60 infants at 1 and 3 mo of age to investigate the effects on these of age, sleep state, sleep position, and mother's smoking status. HRV was determined from Poincaré plots of 500 sequential RR intervals to measure overall variability derived from the SDRR of this plot, and instantaneous variability derived from the SDdeltaRR. HR responses to the tilt were measured as changes in RR interval length from rest to immediately following the tilt and again once a stable pattern was reached. SDRR and SDdeltaRR increased 20 and 40%, respectively, with age (p < 0.0001), SDRR was higher in active sleep (AS) than quiet sleep (QS, +72%, p < 0.0001) but both measures of variability (SDRR and SDdeltaRR) were lower in the prone position compared with supine (-18%, p < 0.0001). However, several findings were dependent on the basal RR interval, thus the age effect disappeared once RR interval was taken into account, sleep state remained an important factor and the lower variability when prone now became a difference of -3% (p = 0.034). The tilt generally provoked a reflex tachycardia followed by a bradycardia and settling to a stable HR level below, at, or above baseline within 30 s. The more unusual responses were no HR change, sustained tachycardia or sustained bradycardia (15% of total). These were more likely to occur in younger infants (p = 0.008) and in AS (p < 0.0001). No changes were seen in any of the cardiac indices related to maternal smoking status. The findings confirm several reports indicating that prone sleeping damps some physiologic responses. The data emphasize the need to consider basal heart rate, and sleep position as well as sleep state in autonomic function testing during infant sleep.

Cardiorespiratory responses to bidirectional tilts in infants

Prior research in newborns has shown that head-up and head-down tilting elicits sustained increases and decreases in heart rate, respectively. Other studies in older infants have suggested that the pattern of heart rate responses to head-up tilting varies with risk for sudden infant death syndrome (SIDS). In this study, heart and respiratory rate changes following bidirectional tilting were recorded in sleeping infants on Day 1 or 2 of life, and during the period of maximum risk for SIDS, at 2 and 4 months of age. Newborns show increases in heart rate following 30 degrees head-up tilts and decreases in heart rate to 300 head-down tilting. Respiratory rates decreased to head-up tilting but did not change significantly to head-down tilting. While respiratory rate changes at 2 and 4 months of age are comparable to those of newborns, and decreases in heart rate to head-down tilting are similar across ages, sustained elevations in heart rate following head-up tilting are no longer apparent at the older ages. These results are consistent with the hypothesis that, during the period of maximum risk for SIDS, infants may have reduced ability to compensate for challenges that lead to decreases in blood pressure.

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.

Sleep position, autonomic function, and arousal

AIMS

To investigate and compare heart rate variability (HRV) and responses of heart rate and arousal to head-up tilting in infants sleeping prone and supine.

METHODS

Thirty seven healthy infants aged 2-4 months were studied. HRV was measured for 500 beats while they were in a horizontal position. Subjects were then tilted 60 degrees head-up, and heart rate recorded over 1 minute and arousal responses observed. Data were collected during both quiet and active sleep for both prone and supine sleep positions.

RESULTS

HRV, as assessed by the point dispersion of Poincaré plots, was significantly reduced in the prone position for both sleep states. Sleep position did not influence the changes in heart rate seen during a head-up tilt. Full awakening to the tilt was common in active sleep but significantly less so in the prone position (15% of prone tests vs 54% supine). Full awakening to the tilt rarely occurred during quiet sleep in either sleep position.

CONCLUSION

This study provides some evidence that blunted arousal responses and/or altered autonomic function are a feature of the prone sleeping position. Decreased HRV may be a sign of autonomic impairment. It is seen in many disease states and in infants who later die of sudden infant death syndrome (SIDS).