Baroreceptor reflex sensitivity in human neonates: the effect of postmenstrual age

Bedside monitoring tools and advanced signal processing approaches to monitor critically-ill infants

There is a substantial body of literature that supports neonatal monitoring and signal analysis of the collected data to provide valuable insights for improving patient clinical care and to inform new research studies. This comprehensive monitoring approach extends beyond the collection of conventional vital signs to include the acquisition of continuous waveform data from patient monitors and other bedside medical devices. This paper discusses the necessary infrastructure for waveform retrieval from bedside monitors, and explores options provided by leading healthcare companies, third-party vendors or academic research teams to implement scalable monitoring systems across entire critical care units. Additionally, we discuss the application of advanced signal processing that transcend traditional statistics, including heart rate variability in both the time- and frequency-domains, spectral analysis of EEG, and cerebral pressure autoregulation. The infrastructures and signal processing techniques outlined here are indispensable tools for intensivists, empowering them to enhance care for critically ill infants. In addition, we briefly address the emergence of advanced tools for fetal monitoring.

Lipopolysaccharide induced systemic inflammation and heart rate variability in a term newborn piglet model

BACKGROUND

Early biomarkers are needed to improve diagnosis and support antibiotic stewardship in neonatal sepsis. Heart rate variability (HRV) is proposed as such a biomarker. However, there is a lack of studies in term newborns. Infusion of lipopolysaccharide (LPS) from Escherichia coli induces systemic inflammation comparable to sepsis in newborns. We aimed to study the effect of systemic LPS induced inflammation on HRV in term newborn piglets.

METHODS

Baseline HRV was recorded for 1 h. This control period was compared to the hourly HRV for each piglet (n = 9) during 4 h of LPS infusion. For comparison, we used a mixed-effects regression model.

RESULTS

Systemic inflammation induced by LPS was found to reduce HRV. Compared to baseline, most measures of HRV decreased to lower values compared to baseline at 2 h, 3 h, and 4 h after initiation of LPS infusion. Heart rate (HR) was increased at 2 h, 3 h, and 4 h. When adjusting for HR in the mixed-effects regression model all reductions in HRV were explained by the increase in HR.

CONCLUSIONS

Reduced HRV may be an early biomarker of neonatal sepsis. However, an increase in HR alone could be an already available, more accessible, and interpretable biomarker of sepsis in term neonates.

IMPACT

In a term newborn piglet model, systemic inflammation induced by lipopolysaccharide from Escherichia coli reduced heart rate variability measures and increased heart rate. All reductions in heart rate variability were mediated by heart rate. While heart rate variability may be a biomarker of sepsis in term newborns, changes in heart rate alone could be a more readily available biomarker.

Longitudinal effects of early exposure to intermittent hypoxia on autonomic cardiovascular control in very preterm infants

INTRODUCTION

Cardiorespiratory control is immature in infants born preterm compared to those born at term. Animal studies have shown that repetitive hypoxia associated with periodic breathing can alter autonomic control. We aimed to elucidate if the amount of time spent with apnoea and periodic breathing in the neonatal unit was associated with longitudinal changes in autonomic control assessed using heart rate variability.

METHODS

Twenty-nine very preterm infants (10 M 19F) were studied during supine daytime sleep on 4 occasions. Study 1: 32-36 weeks post menstrual age (PMA) (n = 29), Study 2: 36-40 weeks PMA (n = 27), Study 3: 3-months corrected age (CA) (n = 20) and Study 4: 6-months CA (n = 26). The percentage total sleep time (%TST) spent having apnoeas in active (AS) and quiet sleep (QS) at each study was calculated. Total power, low frequency (LF, sympathetic + parasympathetic activity) high frequency (HF, parasympathetic activity), and LF/HF (sympathovagal balance) were calculated. Infants were divided into two groups based on the %TST spent with apnoeas above and below the median in AS and QS at Study 1. Data were normalised and compared with two-way ANOVA with Bonferroni post-hoc tests.

RESULTS

When apnoeas were included in the analysis, in QS Total power and HF power were higher, and when apnoeas were excluded HF power was higher in QS but lower in AS in the above median group at Study 4.

CONCLUSION

This study provides new evidence that short apnoeas, particularly periodic breathing, which is currently not detected or treated in the neonatal unit can affect autonomic cardiovascular control.

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.

History of blood pressure measurement in newborns and infants

The development of methods for measuring blood pressure (BP) in newborns and small children has a rich history. Methods for BP measuring in adults had to be adapted to this age group. For measuring BP in direct invasive way, a suitable approach had to be found to access the arterial circulation through the umbilical and later radialis artery. Currently, results obtained from direct invasive BP measurement are considered the "gold standard". The development of non-invasive methods for BP measuring in newborns and children began with the use of von Basch's sphygmomanometer (1880). In 1899, Gustav Gärtner constructed the device, which was the basis for the flush method. After the discovery of the palpation and auscultation methods, these methods were also used for BP measurement in newborns and children, however, the BP values obtained in these ways were typically underestimated using excessively wide cuffs. From the auscultation method, methods utilizing ultrasound and infrasound to detect arterial wall movement and blood flow were later developed. The oscillometric method for BP measurement was introduced by E. J. Marey so early as in 1876. In 1912, P. Balard used the oscillometric technique to measure blood pressure in a large group of newborns. Through different types of oscillometers using various methods for detecting vascular oscillations (such as xylol method, impedance and volume plethysmography, etc.), the development has continued to assessment of vascular oscillations by modern sensor technology and software. For continuous non-invasive blood pressure measurement, the volume-clamp method, first described by Jan Peňáz in 1968, was developed. After modification for use in newborns, application of the cuff to the wrist instead of the finger, it is primarily used in clinical physiological studies to evaluate beat-to-beat BP and heart rate pressure variability, such as in the determination of the baroreflex sensitivity.

Autonomic cardiovascular control is altered by intermittent hypoxia in preterm infants

AIM

Preterm infants frequently experience short apnoeas and periodic breathing. Animal studies have shown that repetitive hypoxia associated with periodic breathing can alter autonomic control. We aimed to elucidate if apnoea and periodic breathing were associated with changes in autonomic control assessed using heart rate variability, thus exacerbating the consequences of respiratory disturbance.

METHODS

Forty very preterm infants (15 M/25 F) were studied at 34.3 weeks post-menstrual age with daytime polysomnography. Total power, low frequency (LF, sympathetic+parasympathetic activity) high frequency (HF, parasympathetic activity) and LF/HF (sympathovagal balance) were calculated.

RESULTS

Infants were divided into those with above and below the median total sleep time spent with respiratory events: Active sleep (AS) 13%, Quiet sleep (QS) 10%. In AS, including respiratory events, Total power (p < 0.05) and HF power (p < 0.05) were higher in the above median group. During AS excluding respiratory events, Total power (p < 0.05) and HF power (p = 0.061) were higher and LF power (p < 0.01) and LF/HF (p < 0.05) were lower in the above median group. There were no differences in HRV parameters in QS.

CONCLUSION

This study provides new evidence that short apnoeas, particularly periodic breathing, which is currently not detected or treated in the neonatal unit can affect autonomic cardiovascular control.

Neonatal heart rate variability: a contemporary scoping review of analysis methods and clinical applications

BACKGROUND

Neonatal heart rate variability (HRV) is widely used as a research tool. However, HRV calculation methods are highly variable making it difficult for comparisons between studies.

OBJECTIVES

To describe the different types of investigations where neonatal HRV was used, study characteristics, and types of analyses performed.

ELIGIBILITY CRITERIA

Human neonates ≤1 month of corrected age.

SOURCES OF EVIDENCE

A protocol and search strategy of the literature was developed in collaboration with the McGill University Health Center's librarians and articles were obtained from searches in the Biosis, Cochrane, Embase, Medline and Web of Science databases published between 1 January 2000 and 1 July 2020.

CHARTING METHODS

A single reviewer screened for eligibility and data were extracted from the included articles. Information collected included the study characteristics and population, type of HRV analysis used (time domain, frequency domain, non-linear, heart rate characteristics (HRC) parameters) and clinical applications (physiological and pathological conditions, responses to various stimuli and outcome prediction).

RESULTS

Of the 286 articles included, 171 (60%) were small single centre studies (sample size <50) performed on term infants (n=136). There were 138 different types of investigations reported: physiological investigations (n=162), responses to various stimuli (n=136), pathological conditions (n=109) and outcome predictor (n=30). Frequency domain analyses were used in 210 articles (73%), followed by time domain (n=139), non-linear methods (n=74) or HRC analyses (n=25). Additionally, over 60 different measures of HRV were reported; in the frequency domain analyses alone there were 29 different ranges used for the low frequency band and 46 for the high frequency band.

CONCLUSIONS

Neonatal HRV has been used in diverse types of investigations with significant lack of consistency in analysis methods applied. Specific guidelines for HRV analyses in neonates are needed to allow for comparisons between studies.

Physiology of Low Blood Pressure During the First Day After Birth Among Extremely Preterm Neonates

OBJECTIVE

To investigate the circulatory physiology of hypotension during the first day after birth among stable extremely preterm neonates.

STUDY DESIGN

Case-control study of neonates born at ≤27^6/7 weeks gestational age with hypotension, defined as mean blood pressure in mmHg less than gestational age in weeks for at least 1 hour during the first 24 hours after birth, who underwent comprehensive echocardiography assessment before commencement of cardiovascular drugs. Neonates with hypotension (n = 14) were matched by gestational age and intensity of respiratory support with normotensive neonates (n = 27) who underwent serial echocardiography during the first day after birth, and relatively contemporaneous echocardiography assessments were used for comparison.

RESULTS

Neonates with hypotension had a higher frequency of patent ductus arteriosus ≥1.5 mm (71% vs 15%; P < .001) and ductal size (median diameter, 1.6 mm [IQR, 1.4-2.1] vs 1.0 mm [IQR, 0-1.3]; P = .002), higher echocardiography indices of left ventricular systolic function (mean shortening fraction, 34 ± 7% vs 26 ± 4%; P < .001; mean longitudinal strain, -16 ± 5% vs -14 ± 3%; P = .04; and mean velocity of circumferential fiber shortening, 1.24 ± 0.35 circ/s vs 1.01 ± 0.28 circ/s; P = .03), lower estimates of left ventricular afterload (mean end-systolic wall stress, 20 ± 7 g/cm² vs 30 ± 9 g/cm²; P < .001 and mean arterial elastance, 43 ± 19 mmHg/mL vs 60 ± 22 mmHg/mL; P = .01), without significant difference in stress-velocity index z-score (-0.42 ± 1.60 vs -0.88 ± 1.30; P = .33). Neonates with hypotension had higher rates of any degree of intraventricular hemorrhage (71% vs 22%; P = .006).

CONCLUSIONS

Low blood pressure in otherwise well extremely low gestational age neonates was associated with low systemic afterload and larger patent ductus arteriosus, but not left ventricular dysfunction.

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.

Exposure to Intermittent Noise Exacerbates the Cardiovascular Response of Wistar-Kyoto Rats to Ozone Inhalation and Arrhythmogenic Challenge

Noise has become a prevalent public health problem across the world. Although there is a significant amount of data demonstrating the harmful effects of noise on the body, very little is known about how it impacts subsequent responses to other environmental stressors like air pollution, which tend to colocalize in urban centers. Therefore, this study was conducted to determine the effect of intermittent noise on cardiovascular function and subsequent responses to ozone (O3). Male Wistar-Kyoto rats implanted with radiotelemeters to non-invasively measure heart rate (HR) and blood pressure (BP), and assess heart rate variability (HRV) and baroreflex sensitivity (BRS) were kept in the quiet or exposed to intermittent white noise (85-90 dB) for one week and then exposed to either O3 (0.8 ppm) or filtered air. Left ventricular function and arrhythmia sensitivity were measured 24 h after exposure. Intermittent noise caused an initial increase in HR and BP, which decreased significantly later in the regimen and coincided with an increase in HRV and BRS. Noise caused HR and BP to be significantly elevated early during O3 and lower at the end when compared to animals kept in the quiet while the increased HRV and BRS persisted during the 24 h after. Lastly, noise increased arrhythmogenesis and may predispose the heart to mechanical function changes after O3. This is the first study to demonstrate that intermittent noise worsens the cardiovascular response to inhaled O3. These effects may occur due to autonomic changes and dysregulation of homeostatic controls, which persist one day after exposure to noise. Hence, co-exposure to noise should be taken into account when assessing the health effects of urban air pollution.

Cardiac and vascular health in late preterm infants

Adults who were born preterm are at increased risk of hypertension and cardiovascular disease in later life. Infants born late preterm are the majority of preterm births; however, the effect of late preterm on risk of cardiovascular disease is unclear. The objective of this study was to assess whether vascular health and cardiac autonomic control differ in a group of late preterm newborn infants compared to a group of term-born infants.A total of 35 healthy late preterm newborn infants, with normal growth (34-36 completed weeks' gestation) and 139 term-born infants (37-42 weeks' gestation) were compared in this study. Aortic wall thickening, assessed as aortic intima-media thickness (IMT) by high-resolution ultrasound, and cardiac autonomic control, assessed by heart rate variability, were measured during the first week of life. Postnatal age of full-term and late preterm infants at the time of the study was 5 days (standard deviation [SD] 5) and 4 days (SD 3), respectively.Infants born late preterm show reduced aortic IMT (574 μm [SD 51] vs. 612 μm [SD 73]) and reduced heart rate variability [log total power 622.3 (606.5) ms2 vs. 1180. 6 (1114.3) ms2], compared to term infants. These associations remained even after adjustment for sex and birth weight.Infants born late preterm show selective differences in markers of cardiovascular risk, with potentially beneficial differences in aortic wall thickness in contrast to potentially detrimental differences in autonomic control, when compared with term-born control infants. These findings provide pathophysiologic evidence to support an increased risk of hypertension and sudden cardiac death in individuals born late preterm.

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.

Blood Pressure Variability and Baroreflex Sensitivity in Premature Newborns-An Effect of Postconceptional and Gestational Age

Introduction: Cardiovascular system is the vitally important system in the dynamical adaptation process of the newborns to the extrauterine environment. To reliably detect immaturity in the given organ system, it is crucial to study the development of the organ functions in relation to maturation process. Objectives: The objective was to determine the changes in the spontaneous short-term blood pressure variability (BPV) and baroreflex sensitivity (BRS) reflecting various aspects of cardiovascular control during the process of maturation in preterm babies and to separate effects of gestational age and postnatal age. Methods: Thirty-three prematurely born infants without any signs of cardio-respiratory disorders (gestational age: 31.8, range: 27-36 weeks; birth weight: 1,704, range: 820-2,730 grams) were enrolled. Continuous peripheral blood pressure signal was obtained by non-invasive volume-clamp photoplethysmography method during supine rest. The recordings of 250 continuous beat-to-beat blood pressure values were processed by spectral analysis of BPV (assessed measures: total power, low frequency and high frequency powers of systolic BPV) and BRS calculation. For each infant we also assessed systolic, diastolic and mean blood pressures, heart rate and respiratory rate. Results: With the postconceptional age, BPV measures decreased (for total power: Spearman correlation coefficient r_s = -0.345, P = 0.049; for low frequency power: r_s = -0.365, P = 0.037; for high frequency power r_s = -0.349; P = 0.046); and BRS increased significantly (r_s = 0.448, P = 0.009). The further analysis demonstrated that these effects were more attributable to gestational age than to postnatal age. BRS correlated negatively with BPV magnitude (r_s = -0.479 to -0.592, P = 0.001-0.005). Mean blood pressure and diastolic blood pressure increased during maturation (r_s = 0.517 and 0.537, P = 0.002 and 0.001, respectively) while heart rate and respiratory rate decreased (r_s = -0.366 and -0.516, P = 0.036 and 0.002, respectively). Conclusion: We conclude that maturation process is accompanied by an increased involvement of baroreflex buffering of spontaneous short-term blood pressure oscillations. Gestational age plays a dominant role not only in BPV changes but also in BRS, mean blood pressure, diastolic blood pressure and heart rate changes.

Osteopathic Manipulative Treatment in Neonatal Intensive Care Units

The aim of this study was to assess the impact of osteopathic manipulative treatment (OMT) on newborn babies admitted at a neonatal intensive care unit (NICU). This was an observational, longitudinal, retrospective study. All consecutive admitted babies were analyzed by treatment (OMT vs. usual care). Treatment group was randomly assigned. Between-group differences in weekly weight change and length of stay (LOS) were evaluated in the overall and preterm populations. Among 1249 babies (48.9% preterm) recorded, 652 received usual care and 597 received OMT. Weight increase was more marked in the OMT group than in the control group (weekly change: +83 g vs. +35 g; p < 0.001). Similar trends were found in the subgroup of preterm babies. A shorter LOS was found in the OMT group vs. the usual care group both in overall population (average mean difference: −7.9 days, p = 0.15) and in preterm babies (−12.3 days; p = 0.04). In severe preterm babies, mean LOS was more than halved as compared to the control group. OMT was associated with a more marked weekly weight increase and, especially in preterm babies, to a relevant LOS reduction: OMT may represent an efficient support to usual care in newborn babies admitted at a NICU.

Postexercise Heart Rate Recovery in Adults Born Preterm

OBJECTIVE

To evaluate postexercise heart rate recovery (HRR) in adults born preterm.

STUDY DESIGN

We studied the association between preterm birth and postexercise HRR in 545 adults (267 women) at 23.3 years of age (range 19.9-26.3 years). One hundred three participants were born early preterm (<34 completed weeks), 178 late preterm (34-36), and 264 were full term (control group). HRR was calculated as change in heart rate (HR) 30 seconds and 60 seconds after cessation of submaximal step test and maximum HR slope during the first minute after.

RESULTS

Mean peak HR was 159.5 bpm in the early preterm (P = .16 with controls), 157.8 bpm in the late preterm (P = .56), and 157.0 bpm in the control group. Mean HRR 30 seconds after exercise was 3.2 bpm (95% CI 1.1-5.2) lower in the early preterm group and 2.1 bpm (0.3-3.8) lower in the late preterm group than the full term controls. Mean 60s HRR was 2.5 (-0.1 to 5.1) lower in the early preterm group and 2.8 bpm (0.6-4.9) lower in the late preterm group. Mean maximum slope after exercise was 0.10 beats/s (0.02-0.17) lower in the early preterm group and 0.06 beats/s (0.00-0.12) lower in the late preterm group.

CONCLUSIONS

Our results suggest reduced HRR after exercise in adults born preterm, including those born late preterm. This suggests altered reactivation of the parasympathetic nervous system, which may contribute to cardiovascular risk among adults born preterm.

Noninvasive assessment of autonomic function in human neonates born at the extremes of fetal growth spectrum

Birth weight is associated with adult cardiovascular disease, such that those at both ends of the spectrum are at increased risk. This may be driven in part by modification to autonomic control, a mechanistic contributor to hypertension. However, birth weight is a relatively crude surrogate of fetal growth; and newborn body composition may more accurately identify the "at risk" infant. Accordingly, we sought to determine whether newborns with high or low body fat have altered autonomic control of vasomotor function and cardiac contractility. Body fat was assessed by air-displacement plethysmography <24 h postnatal. Measures of spontaneous baroreflex sensitivity (sBRS), blood pressure variability (BPV), and dP/dt_max variability were compared between newborns categorized according to established body fat percentiles: high body fat (HBF, >90th percentile, n = 7), low body fat (LBF, ≤10th percentile, n = 12), and normal body fat (control, >25th to ≤75th percentile, n = 23). BPV was similar across body fat percentiles; similarly, low frequency dP/dt_max variability was similar across body fat percentiles. sBRS was reduced in HBF compared to controls (11.0 ± 6.0 vs. 20.1 ± 9.4 msec/mmHg, P = 0.03), but LBF did not differ (18.4 ± 6.0 msec/mmHg, P = 0.80). Across the entire body fat spectrum (n = 62), there was a nonlinear association between newborn body fat and sBRS (P = 0.03) that was independent of birth weight (P = 0.04). Autonomic modulation of vasomotor function and cardiac contractility in the newborn did not differ by body fat, but newborns born with high body fat show depressed baroreflex sensitivity.

Cardiac Autonomic Function in Adults Born Preterm

OBJECTIVE

To evaluate cardiac autonomic function in adults born preterm.

STUDY DESIGN

We studied the association between prematurity and cardiac autonomic function using heart rate variability measurements in 600 adults (mean age of 23.3 years) from a geographically based cohort in Northern Finland. There were 117 young adults born early preterm (<34 weeks), 207 born late preterm (34-36 weeks), and 276 born at term (≥37 weeks, controls). Autonomic function was analyzed by calculating time and frequency domain heart rate variability measurements using linear regression.

RESULTS

Compared with controls, the mean difference in root mean square of successive differences (indicating cardiac vagal activity) was -12.0% (95% CI -22.2%, -0.5%, adjusted for sex, age, source cohort, and season P = .04) for the early preterm group and -7.8% (-16.8%, 2.0%, P = .12) for the late preterm group. Mean differences with controls in low frequency power (indicating cardiac vagal activity, including some sympathetic- and baroreflex-mediated effects) were -13.6% (-26.7%, 1.8%, P = .08) for the early preterm group and -16.4% (-27.0%, -4.3%, P = .01) for the late preterm group. Mean differences in high frequency power (quantifying cardiac vagal modulation in respiratory frequency) were -19.2% (-36.6%, 2.9%, P = .09) for the early preterm group and -13.8% (-29.4%, 5.3%, P = .15) for the late preterm group. Differences were attenuated when controlled for body mass index and physical activity.

CONCLUSIONS

Our results suggest altered autonomic regulatory control in adults born preterm, including those born late preterm. Altered autonomic regulatory control may contribute to increased cardiovascular risk in adults born preterm.

Autonomic dysfunction in programmed hypertension

Hypertension is an important modifiable risk factor for cardiovascular diseases. Its high prevalence, combined with the significant morbidity and mortality associated with secondary complications, make it a major public health concern. Despite decades of research, over 95% of all cases of hypertension remain of unknown etiology, necessitating that treatments target the established symptoms and not the cause. One of the important recent advances in hypertension research is an understanding that hypertension often may have a developmental origin. A substantial body of evidence indicates that exposure to an adverse intrauterine environment during critical periods of development may predispose an individual to develop hypertension later in life. A causative mechanism has yet to be identified, but may include epigenetic modifications, and/or alterations in renal, vascular or autonomic cardiovascular functions. This review will present evidence regarding changes in autonomic activity as a possible causative pathophysiological mechanism underlying the development of programmed hypertension. In man, low birth weight is the best-known risk factor for hypertension of developmental origins, although this is a broad surrogate measure for intrauterine adversity. This review will include clinical studies across the lifespan that have investigated autonomic function in individuals with fetal growth restriction and those born preterm. A determination of whether altered autonomic function is seen in these individuals in early life is imperative, as hypertensive disorders that have their origins in utero, and that can be identified early, will open the door to risk stratification, and the development of new strategies that prevent or specifically target these mechanisms.

Unlike Kangaroo care, mechanically simulated Kangaroo care does not change heart rate variability in preterm neonates

BACKGROUND

While numerous positive effects of Kangaroo care (KC) have been reported, the duration that parents can spend kangarooing is often limited.

AIM

To investigate whether a mattress that aims to mimic breathing motion and the sounds of heartbeats (BabyBe GMBH, Stuttgart, Germany) can simulate aspects of KC in preterm infants as measured by features of heart rate variability (HRV).

METHODS

A within-subject study design was employed in which every routine KC session was followed by a BabyBe (BB) session, with a washout period of at least 2 h in between. Nurses annotated the start and end times of KC and BB sessions. Data from the pre-KC, KC, post-KC, pre-BB, BB and post-BB were retrieved from the patient monitor via a data warehouse. Five time-domain features of HRV were used to compare both types of intervention. Two of these features, the percentage of decelerations (pDec) and the standard deviation of decelerations (SDDec), were developed in a previous study to capture the contribution of transient heart rate decelerations to HRV, a measure of regulatory instability.

RESULTS

A total of 182 KC and 180 BabyBe sessions were analyzed in 20 preterm infants. Overall, HRV decreased during KC and after KC. Two of the five features showed a decrease during KC, and all features decreased in the post-KC period (p ≤ 0.01). The BB mattress as employed in this study did not affect HRV.

CONCLUSION

Unlike KC, a mattress that attempts to mimic breathing motion and heartbeat sounds does not affect HRV of preterm infants.

The effect of sex and prematurity on the cardiovascular baroreflex response in sheep

NEW FINDINGS

What is the central question of this study? Late preterm infants are often assumed to escape long-term morbidities known to impact earlier preterm offspring. Is this true for the cardiovascular system? What is the main finding and its importance? We show that late preterm birth is a risk factor for cardiovascular dysfunction in early adulthood and is influenced by sex. Early signs of cardiovascular dysfunction might predispose to heart disease in adulthood. Very preterm infants have an increased risk of cardiovascular disease; however, the effects of a late preterm birth on future cardiovascular function are not known. We hypothesized that after a late preterm birth, the well-described impairments in heart rate variability and baroreflex sensitivity would persist into adulthood. To test this hypothesis, sheep born preterm (0.9 gestation; nine male and seven female) or term (11 male and six female) underwent surgery at 14 months of age for insertion of femoral arterial and venous catheters and a femoral flow probe. After recovery, heart rate variability was assessed, followed by a baroreflex challenge (using the vasoactive agents phenylephrine and sodium nitroprusside) in conscious adult lambs. Our data demonstrate decreased low-frequency normalised units (LFnu) and low-frequency/high-frequency ratio in female but not male ex-preterm sheep at rest. When challenged, mature male ex-preterm sheep have an increased blood pressure response but dampened heart rate baroreflex response. We show that even a late preterm birth leads to cardiovascular dysfunction in adulthood. These early signs of cardiovascular dysfunction might underpin the later hypertension and increased risk of heart disease observed in adults born preterm. These findings are particularly important because late preterm infants are often assumed to escape the long-term morbidities known to impact on very preterm and extremely preterm offspring.

Unobtrusive assessment of neonatal sleep state based on heart rate variability retrieved from electrocardiography used for regular patient monitoring

As an approach of unobtrusive assessment of neonatal sleep state we aimed at an automated sleep state coding based only on heart rate variability obtained from electrocardiography used for regular patient monitoring. We analyzed active and quiet sleep states of preterm infants between 30 and 37weeks postmenstrual age. To determine the sleep states we used a nonlinear kernel support vector machine for sleep state separation based on known heart rate variability features. We used unweighted and weighted misclassification penalties for the imbalanced distribution between sleep states. The validation was performed with leave-one-out-cross-validation based on the annotations of three independent observers. We analyzed the classifier performance with receiver operating curves leading to a maximum mean value for the area under the curve of 0.87. Using this sleep state separation methods, we show that automated active and quiet sleep state separation based on heart rate variability in preterm infants is feasible.

Pattern of brain injury and depressed heart rate variability in newborns with hypoxic ischemic encephalopathy

BackgroundDecreased heart rate variability (HRV) is a measure of autonomic dysfunction and brain injury in newborns with hypoxic ischemic encephalopathy (HIE). This study aimed to characterize the relationship between HRV and brain injury pattern using magnetic resonance imaging (MRI) in newborns with HIE undergoing therapeutic hypothermia.MethodsHRV metrics were quantified in the time domain (α_S, α_L, and root mean square at short (RMS_S) and long (RMS_L) timescales) and frequency domain (relative low-(LF) and high-frequency (HF) power) over 24-27 h of life. The brain injury pattern shown by MRI was classified as no injury, pure cortical/white matter injury, mixed watershed/mild basal ganglia injury, predominant basal ganglia or global injury, and death. HRV metrics were compared across brain injury pattern groups using a random-effects mixed model.ResultsData from 74 infants were analyzed. Brain injury pattern was significantly associated with the degree of HRV suppression. Specifically, negative associations were observed between the pattern of brain injury and RMS_S (estimate -0.224, SE 0.082, P=0.006), RMS_L (estimate -0.189, SE 0.082, P=0.021), and LF power (estimate -0.044, SE 0.016, P=0.006).ConclusionDegree of HRV depression is related to the pattern of brain injury. HRV monitoring may provide insights into the pattern of brain injury at the bedside.

Fetal-growth-restricted preterm infants display compromised autonomic cardiovascular control on the first postnatal day but not during infancy

BackgroundFetal growth restriction (FGR) is associated with increased perinatal mortality and long-term cardiovascular and neurodevelopmental sequelae. We hypothesized that FGR impacts on the development of autonomic heart rate and blood pressure control, contributing to unfavorable short- and long-term outcomes following FGR.MethodsWe studied 25 preterm FGR and 22 preterm and 19 term appropriate for gestational age (AGA) infants. Preterm neonates were studied on postnatal day 1, and all infants were studied at 1 and 6 months post-term age. To investigate autonomic cardiovascular control, we examined heart rate variability (HRV) and baroreflex sensitivity using spectral power and transfer-function analyses.ResultsPreterm FGR neonates exhibited higher heart rates and reduced HRV compared with preterm AGA controls on postnatal day 1. No significant differences were found between the three groups at 1 or 6 months post-term age.ConclusionPreterm FGR neonates display compromised HRV on postnatal day 1, which may suggest increased vulnerability to circulatory instability. This may predispose these neonates to systemic and cerebral hypoperfusion and increase the risk of long-term neurodevelopmental sequelae. Differences were no longer found at 1 and 6 months post-term age, suggesting that the maturation of autonomic cardiovascular control may be preserved following FGR.

Baroreflex Sensitivity in Premature Infants – Relation to the Parameters Characterizing Intrauterine and Postnatal Condition

At present, there are insufficient information about baroreflex sensitivity (BRS) and factors that determine BRS in premature newborns. The objective of this study was to determine the relationship between BRS and the characteristics that reflecting the intrauterine development (gestational age and birth weight), as well as postnatal development (postconception age and the actual weight of the child at the time of measurement). We examined 57 premature infants, who were divided into groups according to gestational age and postconception age as well as birth weight, and weight at the time of measurement. Continuous and noninvasive registration of peripheral blood pressure (BP) was performed in every child within 2-5 min under standard conditions using a Portapres (FMS) device. The results showed a close correlation of baroreflex sensitivity, heart rate and respiratory rate with gestational age, postconception age, birth weight and actual weight at the time of measurement premature newborns. An increase in the characteristics (ages and weights) resulted in increased BRS and diastolic arterial pressure (DAP), and in decreased heart and respiratory rates. Baroreflex sensitivity in the first week was in the group of very premature newborns the lowest (4.11 ms/mmHg) and in the light premature newborns was almost double (8.12 ms/mmHg). BRS increases gradually in relation to postnatal (chronological) and to postconception age as well as to birth and actual weight. The multifactor analysis of BRS identified birth weight and postconception age as the best BRS predictors. The two independent variables together explained 40 % of interindividual BRS variability.

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.

Cerebral modulation of the autonomic nervous system in term infants

OBJECTIVE

Central topography of autonomic nervous system (ANS) function has yet to be fully deciphered. In adults it has been shown to lateralize sympathetic and parasympathetic influence predominantly to the right and left cerebral hemispheres, respectively. We examined functional topography of central ANS in newborn subjects utilizing spectral analysis of heart rate variability (HRV), an established measure of ANS function.

STUDY DESIGN

We studied newborns with hypoxic-ischemic encephalopathy participating in a prospective study undergoing a therapeutic hypothermia protocol.We included subjects with continuous heart rate data over the first 3 h of normothermia (post rewarming) and brain magnetic resonance imaging, which was reviewed and scored according to a 4 region scheme. HRV was evaluated by spectral analysis in the low-frequency (0.05 to 0.25 Hz) and high-frequency (0.3 to 1 Hz) ranges. The relationship between injured brain regions and HRV was studied using multiple regressions.

RESULTS

Forty eight newborns were included. When examined in isolation, right hemisphere injury had a significant negative effect on HRV (-0.088; 95% CI: -0.225,-0.008). The combination of posterior fossa region injury with right hemispheric injury or left hemispheric injury demonstrated significant positive (0.299; 95% CI: 0.065, 0.518) and negative (-0.475; 95% CI: -0.852, -0.128) influences on HRV, respectively. The association between brain injury location and HRV in the high-frequency range did not reach significance.

CONCLUSION

Our data support the notion that lateralized cerebral modulation of the ANS, specifically of its sympathetic component, is present in the term newborn, and suggest complex modulation of these tracts by components of the posterior fossa.

Effect of Temperature on Heart Rate Variability in Neonatal ICU Patients With Hypoxic-Ischemic Encephalopathy

OBJECTIVE

To determine whether measures of heart rate variability are related to changes in temperature during rewarming after therapeutic hypothermia for hypoxic-ischemic encephalopathy.

DESIGN

Prospective observational study.

SETTING

Level 4 neonatal ICU in a free-standing academic children's hospital.

PATIENTS

Forty-four infants with moderate to severe hypoxic-ischemic encephalopathy treated with therapeutic hypothermia.

INTERVENTIONS

Continuous electrocardiogram data from 2 hours prior to rewarming through 2 hours after completion of rewarming (up to 10 hr) were analyzed.

MEASUREMENTS AND MAIN RESULTS

Median beat-to-beat interval and measures of heart rate variability were quantified including beat-to-beat interval SD, low and high frequency relative spectral power, detrended fluctuation analysis short and long α exponents (αS and αL), and root mean square short and long time scales. The relationships between heart rate variability measures and esophageal/axillary temperatures were evaluated. Heart rate variability measures low frequency, αS, and root mean square short and long time scales were negatively associated, whereas αL was positively associated, with temperature (p < 0.01). These findings signify an overall decrease in heart rate variability as temperature increased toward normothermia.

CONCLUSIONS

Measures of heart rate variability are temperature dependent in the range of therapeutic hypothermia to normothermia. Core body temperature needs to be considered when evaluating heart rate variability metrics as potential physiologic biomarkers of illness severity in hypoxic-ischemic encephalopathy infants undergoing therapeutic hypothermia.

Features of Heart Rate Variability Capture Regulatory Changes During Kangaroo Care in Preterm Infants

OBJECTIVE

To determine whether heart rate variability (HRV) can serve as a surrogate measure to track regulatory changes during kangaroo care, a period of parental coregulation distinct from regulation within the incubator.

STUDY DESIGN

Nurses annotated the starting and ending times of kangaroo care for 3 months. The pre-kangaroo care, during-kangaroo care, and post-kangaroo care data were retrieved in infants with at least 10 accurately annotated kangaroo care sessions. Eight HRV features (5 in the time domain and 3 in the frequency domain) were used to visually and statistically compare the pre-kangaroo care and during-kangaroo care periods. Two of these features, capturing the percentage of heart rate decelerations and the extent of heart rate decelerations, were newly developed for preterm infants.

RESULTS

A total of 191 kangaroo care sessions were investigated in 11 preterm infants. Despite clinically irrelevant changes in vital signs, 6 of the 8 HRV features (SD of normal-to-normal intervals, root mean square of the SD, percentage of consecutive normal-to-normal intervals that differ by >50 ms, SD of heart rate decelerations, high-frequency power, and low-frequency/high-frequency ratio) showed a visible and statistically significant difference (P <.01) between stable periods of kangaroo care and pre-kangaroo care. HRV was reduced during kangaroo care owing to a decrease in the extent of transient heart rate decelerations.

CONCLUSION

HRV-based features may be clinically useful for capturing the dynamic changes in autonomic regulation in response to kangaroo care and other changes in environment and state.

Pathophysiology of the Cardiovascular System and Neonatal Hypotension

Hypotension is common in low birth weight neonates and less common in term newborns and is associated with significant morbidity and mortality. Determining an adequate blood pressure in neonates remains challenging for the neonatal nurse because of the lack of agreed-upon norms. Values for determining norms for blood pressure at varying gestational and postnatal ages are based on empirical data. Understanding cardiovascular pathophysiology, potential causes of hypotension, and assessment of adequate perfusion in the neonatal population is important and can assist the neonatal nurse in the evaluation of effective blood pressure. This article reviews cardiovascular pathophysiology as it relates to blood pressure and discusses potential causes of hypotension in the term and preterm neonate. Variation in management of hypotension across centers is discussed. Underlying causes and pathophysiology of hypotension in the neonate are described.

Recursive Model Identification for the Evaluation of Baroreflex Sensitivity

A method for the recursive identification of physiological models of the cardiovascular baroreflex is proposed and applied to the time-varying analysis of vagal and sympathetic activities. The proposed method was evaluated with data from five newborn lambs, which were acquired during injection of vasodilator and vasoconstrictors and the results show a close match between experimental and simulated signals. The model-based estimation of vagal and sympathetic contributions were consistent with physiological knowledge and the obtained estimators of vagal and sympathetic activities were compared to traditional markers associated with baroreflex sensitivity. High correlations were observed between traditional markers and model-based indices.

Baroreflex dysfunction in sick newborns makes heart rate an unreliable surrogate for blood pressure changes

BACKGROUND

Cerebral pressure passivity (CPP) in sick newborns can be detected by evaluating coupling between mean arterial pressure (MAP) and cerebral blood flow measured by near infra-red spectroscopy hemoglobin difference (HbD). However, continuous MAP monitoring requires invasive catheterization with its inherent risks. We tested whether heart rate (HR) could serve as a reliable surrogate for MAP in the detection of CPP in sick newborns.

METHODS

Continuous measurements of MAP, HR, and HbD were made and partitioned into 10-min epochs. Spectral coherence (COH) was computed between MAP and HbD (COHMAP-HbD) to detect CPP, between HR and HbD (COHHR-HbD) for comparison, and between MAP and HR (COHMAP-HR) to quantify baroreflex function (BRF). The agreement between COHMAP-HbD and COHHR-HbD was assessed using ROC analysis.

RESULTS

We found poor agreement between COHMAP-HbD and COHHR-HbD in left hemisphere (area under the ROC curve (AUC) 0.68) and right hemisphere (AUC 0.71). Baroreflex failure (COHMAP-HR not significant) was present in 79% of epochs. Confining comparison to epochs with intact BRF showed an AUC of 0.85 for both hemispheres.

CONCLUSIONS

In these sick newborns, HR was an unreliable surrogate for MAP required for the detection of CPP. This is likely due to the prevalence of BRF failure in these infants.

Unobtrusive sleep state measurements in preterm infants - A review

Sleep is important for the development of preterm infants. During sleep, neural connections are formed and the development of brain regions is triggered. In general, various rudimentary sleep states can be identified in the preterm infant, namely active sleep (AS), quiet sleep (QS) and intermediate sleep (IS). As the infant develops, sleep states change in length and organization, with these changes as important indicators of brain development. As a result, several methods have been deployed to distinguish between the different preterm infant sleep states, among which polysomnography (PSG) is the most frequently used. However, this method is limited by the use of adhesive electrodes or patches that are attached to the body by numerous cables that can disturb sleep. Given the importance of sleep, this review explores more unobtrusive methods that can identify sleep states without disturbing the infant. To this end, after a brief introduction to preterm sleep states, an analysis of the physiological characteristics associated with the different sleep states is provided and various methods of measuring these physiological characteristics are explored. Finally, the advantages and disadvantages of each of these methods are evaluated and recommendations for neonatal sleep monitoring proposed.

MYSTERIES OF THE HUMAN FETUS REVEALED

The impressive program of research from the DiPietro laboratory succeeds in its aim to document the ontogeny of human fetal neurobehavioral development. From studies of great depth and breadth, and wielding creative methods of assessment, DiPietro et al. open a window into the largely inaccessible developing human fetal brain. This commentary, with reference to the seminal cardiovascular studies of the Laceys, supports the measures of the fetal heart to index fetal well-being and to provide evidence of stimulus processing. A separate case is made that the DiPietro program provides unique and invaluable information for assessing the influential Developmental Origins of Health and Disease or Fetal Programming Models. The goal of these models, to predict or understand the influences of early experience or response patterns on later postnatal life, is identical to the ultimate goal of the DiPietro program. Because human fetal behavior is uncontaminated by socialization or parenting or peers, it may be the best reflection of fetal exposures. The remarkable neurobehavioral profiles generated by the DiPietro program can make a critical contribution to the Fetal Programming Model in terms of sensitive and critical periods of nervous system vulnerability and to specify gestational periods of neurobehavioral risk.

Multipotent adult progenitor cells for hypoxic-ischemic injury in the preterm brain

Background

Preterm infants are at risk for hypoxic-ischemic encephalopathy. No therapy exists to treat this brain injury and subsequent long-term sequelae. We have previously shown in a well-established pre-clinical model of global hypoxia-ischemia (HI) that mesenchymal stem cells are a promising candidate for the treatment of hypoxic-ischemic brain injury. In the current study, we investigated the neuroprotective capacity of multipotent adult progenitor cells (MAPC^®), which are adherent bone marrow-derived cells of an earlier developmental stage than mesenchymal stem cells and exhibiting more potent anti-inflammatory and regenerative properties.

Methods

Instrumented preterm sheep fetuses were subjected to global hypoxia-ischemia by 25 min of umbilical cord occlusion at a gestational age of 106 (term ~147) days. During a 7-day reperfusion period, vital parameters (e.g., blood pressure and heart rate; baroreceptor reflex) and (amplitude-integrated) electroencephalogram were recorded. At the end of the experiment, the preterm brain was studied by histology.

Results

Systemic administration of MAPC therapy reduced the number and duration of seizures and prevented decrease in baroreflex sensitivity after global HI. In addition, MAPC cells prevented HI-induced microglial proliferation in the preterm brain. These anti-inflammatory effects were associated with MAPC-induced prevention of hypomyelination after global HI. Besides attenuation of the cerebral inflammatory response, our findings showed that MAPC cells modulated the peripheral splenic inflammatory response, which has been implicated in the etiology of hypoxic-ischemic injury in the preterm brain.

Conclusions

In a pre-clinical animal model MAPC cell therapy improved the functional and structural outcome of the preterm brain after global HI. Future studies should establish the mechanism and long-term therapeutic effects of neuroprotection established by MAPC cells in the developing preterm brain exposed to HI. Our study may form the basis for future clinical trials, which will evaluate whether MAPC therapy is capable of reducing neurological sequelae in preterm infants with hypoxic-ischemic encephalopathy.

Autonomic Functioning in Young Adults Born at Extremely Low Birth Weight

Autonomic functioning is altered in infants born at extremely low birth weight (ELBW; <1000 g), but we know little about how such alterations manifest in adulthood. We examined associations between birth weight and resting heart rate (R-R interval), high-frequency (HF) and low-frequency (LF) heart rate variability, and systolic (SBP) and diastolic (DBP) blood pressure, in 60 participants (ages 22-26) born at ELBW and free of major neurosensory impairment, and 79 controls born at normal birth weight (NBW; >2500 g). HF in the smallest-born ELBW participants was significantly lower than in NBW controls. In both groups, greater birth weight was associated with higher HF. Among ELBW survivors, lower birth weight predicted faster heart rate and higher DBP, but neither heart rate nor DBP appeared to be well-coordinated with baroreflex activity (LF), the principal mechanism for short-term blood pressure regulation. Adult autonomic regulation may be significantly altered in those born extremely preterm.

Gestational age at birth affects maturation of baroreflex control

OBJECTIVES

To assess the effect of prone sleeping, the major risk factor for sudden infant death syndrome, in the control of blood pressure (BP) in preterm infants born across a range of gestational ages.

STUDY DESIGN

Daytime polysomnography was performed at 2-4 weeks, 2-3 months, and 5-6 months postterm age. The participants were 21 very preterm (mean gestation 29.4 ± 0.3 weeks), 14 preterm (mean gestation 33.1 ± 0.3 weeks), and 17 term (mean gestation 40.1 ± 0.3 weeks). BP was measured via a Finometer cuff (Finapres Medical Systems, Amsterdam, The Netherlands) placed around the wrist. Data were recorded both supine and prone. Baroreflex sensitivity (BRS) was calculated via cross-spectral analysis of spontaneous fluctuations in BP.

RESULTS

BRS was lower in the prone position in very preterm infants at 2-4 weeks in active sleep (P < .05). Maturation of BRS was delayed in very preterm compared with both preterm and term infants.

CONCLUSIONS

Maturation of BRS after term-equivalent age is altered in very preterm infants. Reduced BRS may result in an impaired ability of very preterm infants to respond to cardiovascular stress during infancy and may predispose them to cardiovascular disease later in life.

The role of physiological studies and apnoea monitoring in infants

There is evidence that failure of cardio-respiratory control mechanisms plays a role in the final event of the Sudden Infant Death Syndrome (SIDS). Physiological studies during sleep in both healthy term born infants and those at increased risk for SIDS have been widely used to investigate how the major risk and protective factors for SIDS identified from epidemiological studies might alter infant physiology. Clinical polysomnography (PSG) in infants who eventually succumbed to SIDS however demonstrated abnormalities that were neither sufficiently distinctive nor predictive to support routine use of PSG for infants at risk for SIDS. PSG findings have also been shown to be not predictive of recurrence of Apparent Life Threatening Events (ALTE) and thus international guidelines state that PSG is not indicated for routine evaluation in infants with an uncomplicated ALTE, although PSG may be indicated when there is clinical evidence of a sleep related breathing disorder. A decision to undertake home apnoea monitoring should consider the potential advantages and disadvantages of monitoring for that individual, in the knowledge that there is no evidence of the efficacy of such devices in preventing SIDS.

Heart rate variability in encephalopathic newborns during and after therapeutic hypothermia

OBJECTIVE

To evaluate whether heart rate variability (HRV) measures are predictive of neurological outcome in babies with hypoxic ischemic encephalopathy (HIE).

STUDY DESIGN

This case-control investigation included 20 term encephalopathic newborns treated with systemic hypothermia in a regional neonatal intensive care unit. Electrocardiographic data were collected continuously during hypothermia. Spectral analysis of beat-to-beat heart rate interval was used to quantify HRV. HRV measures were compared between infants with adverse outcome (death or neurodevelopmental impairment at 15 months, n = 10) and those with favorable outcome (survivors without impairment, n = 10).

RESULT

HRV differentiated infants by outcome during hypothermia through post-rewarming, with the best distinction between groups at 24 h and after 80 h of life.

CONCLUSION

HRV during hypothermia treatment distinguished HIE babies who subsequently died or had neurodevelopmental impairment from intact survivors. This physiological biomarker may identify infants in need of adjuvant neuroprotective interventions. These findings warrant further investigation in a larger population of infants with HIE.

Cardio-respiratory control during sleep in infancy

During the first year of life and particularly the first 6 months autonomic control of the cardio-respiratory system is still undergoing maturation and infants are at risk of cardio-respiratory instability. These instabilities are most marked during sleep, which is important as infants spend the majority of each 24 hours in sleep. Sleep state has a marked effect on the cardio-respiratory system with instabilities being more common in active sleep compared to quiet sleep. Responses to hypoxia are also immature during infancy and may make young infants more vulnerable to cardio-respiratory instability. It has been proposed that an inability to respond appropriately to a life threatening event underpins the Sudden Infant Death Syndrome (SIDS). The major risk factors for SIDS, prone sleeping and maternal smoking, both impair cardio-respiratory control in normal healthy term infants.

Mitigating the effect of non-stationarity in spectral analysis-an application to neonate heart rate analysis

In order to mitigate the effect of non-stationarity in frequency domain analysis of data, we propose a modification to the power spectral estimation, a widely used technique to characterize physiological signals. Spectral analysis requires partitioning data into smaller epochs determined by the desired frequency resolution. The modified approach proposed here involves dividing the data within each epoch by the standard deviation of the data for that epoch. We applied this modified approach to cardiac beat-to-beat interval data recorded from a newborn infant undergoing hypothermia treatment for birth asphyxia. The critically ill infant had episodes of tachyarrhythmia, distributed sporadically throughout the study, which affected the stationarity of the heart rate. Over the period of continuous heart rate recording, the infant's clinical course deteriorated progressively culminating in death. Coinciding with this clinical deterioration, the heart rate signal showed striking changes in both low-frequency and high-frequency power indicating significant impairment of the autonomic nervous system. The standard spectral approach failed to capture these phenomena because of the non-stationarity of the signal. Conversely, the modified approach proposed here captured the deteriorating physiology of the infant clearly.

The development of cardiovascular and cerebral vascular control in preterm infants

Over the past three decades there has been a steady increase in the incidence of preterm birth. The worldwide rate of preterm birth is estimated to be 9.6% of all births, a total of almost 13 million births annually. Preterm birth is associated with a range of adverse cardiovascular and central nervous system outcomes, which may be attributed to altered development of these systems following preterm birth. Preterm birth has a considerable impact on cardiovascular parameters with preterm infants displaying higher heart rates and reduced blood pressure when compared to term born infants at matched ages. Furthermore, premature infants have altered autonomic control of cardiovascular parameters which manifests as abnormalities in heart rate variability and baroreflex mediated control of heart rate and blood pressure. As a result, systemic cardiovascular parameters can be unstable following preterm birth which may place stress on the neonatal brain. The brain of a preterm infant is particularly vulnerable to these fluctuations due to immature cerebral haemodynamics. Preterm infants, particularly those who are very preterm or unwell, display fluctuating pressure-passivity between systemic blood pressure and cerebral blood flow representing a considerably increased risk of cerebral haemorrhage or hypoxia. This is further compounded by immaturity of cerebral blood flow-metabolism coupling, which means increased metabolic demand cannot adequately be met by increased cerebral blood flow. It has been suggested that adverse long-term outcomes following preterm birth may occur as a result of exposure to physiological stress either in-utero or early in infancy.

Autonomic cardiovascular control in hypotensive critically ill preterm infants is impaired during the first days of life

BACKGROUND

The first days after preterm birth are a critical period of cardiovascular instability, where hypotension is common. We assessed autonomic cardiovascular function by measuring heart rate variability (HRV), blood pressure variability (BPV) and baroreflex sensitivity (BRS) and hypothesised that these would be impaired in preterm infants born at younger gestational ages. In addition, we speculated that impaired cardiovascular control could be used as a marker of circulatory failure such as is manifest as hypotension.

METHODS

23 preterm infants (11 M/12 F) born between 23 and 35 weeks (mean 27 ± 0.6 weeks) gestational age with indwelling arterial catheters were recruited. Infants were studied over the first 3 days of life with heart rate and blood pressure (BP) analysed beat to beat in the frequency domain in 2 minute epochs of artefact free data during active sleep. Data were compared with one way ANOVA.

RESULTS

Gestational age was correlated with all HRV indices but not BPV or BRS. 9 babies received inotropes. Gestational age between the inotrope group and the non-inotrope group was not different. BP and RR interval were lower in the inotrope group (40.7 ± 1.5 vs 47.1 ± 1.5 mmHg, p<0.05 and 395 ± 14 vs 426 ± 11 ms, p<0.08). BRS was also lower in the inotrope group (3.8 ± 0.9 vs 6.9 ± 1.6 ms/mmHg) as was LF/HF HRV (5.7 ± 1.3 vs 13.6 ± 2.8, p<0.05).

CONCLUSIONS

In the first 3 days after birth, infants receiving inotropes had significantly impaired cardiovascular control compared to those who did not receive treatment, indicating that these infants maybe predisposed to increased vulnerability to circulatory instability.

Heart rate-mediated blood pressure control in preterm fetal sheep under normal and hypoxic-ischemic conditions

BACKGROUND

The understanding of hypoxemia-induced changes in baroreflex function is limited and may be studied in a fetal sheep experiment before, during, and after standardized hypoxic conditions.

METHODS

Preterm fetal lambs were instrumented at 102 d gestation (term: 146 d). At 106 d, intrauterine hypoxia--ischemia was induced by 25 min of umbilical cord occlusion (UCO). Baroreflex-related fluctuations were calculated at 30-min intervals during the first week after UCO by transfer function (cross-spectral) analysis between systolic blood pressure (SBP) and R-R interval fluctuations, estimated in the low-frequency (LF, 0.04-0.15 Hz) band. LF transfer gain (baroreflex sensitivity) and delay (s) reflect the baroreflex function.

RESULTS

Baseline did not differ in LF transfer gain and delay between controls and the UCO group. In controls, LF gain showed postnatal increase. By contrast, LF gain gradually decreased in the UCO group, resulting in significantly lower values 4-7 d after UCO. In the UCO group, LF delay increased and differed significantly from controls.

CONCLUSION

Our results show that intrauterine hypoxia-ischemia results in reduced baroreflex sensitivity over a period of 7 d, indicating limited efficacy to buffer BP changes by adapting heart rate. Cardiovascular dysregulation may augment already present cerebral damage after systemic hypoxia-ischemia in the reperfusion period.

The development of autonomic cardiovascular control is altered by preterm birth

OBJECTIVES

Autonomic dysfunction, either sympathetic or parasympathetic, may explain the increased incidence of Sudden Infant Death Syndrome (SIDS) among preterm infants, as well as their subsequent heightened risk of hypertension in adulthood. As little is known about the development of autonomic function in preterm infants, we contrasted autonomic cardiovascular control across the first 6months after term-corrected age (CA) in preterm and term infants.

STUDY DESIGN

Preterm (n=25) and age matched term infants (n=31) were studied at 2-4weeks, 2-3months and 5-6months CA using daytime polysomnography. Blood pressure and heart rate were measured during quiet (QS) and active (AS) sleep. Autonomic control was assessed using spectral indices of blood pressure and heart rate variability (BPV and HRV) in ranges of low frequency (LF, reflecting sympathetic+parasympathetic activity), high frequency (HF, respiratory-mediated changes+parasympathetic activity), and LF/HF ratio (sympatho-vagal balance).

RESULTS

In preterm infants, HF HRV increased, LF/HF HRV decreased and LF BPV decreased with age (p<0.05); these changes were most evident in AS. Compared to term infants, preterm infants in QS exhibited lower LF, HF and total HRV at 5-6months; higher HF BPV at all ages; and lower LF BPV at 2-4weeks (p<0.05).

CONCLUSIONS

With maturation, in preterm infants, parasympathetic modulation of the heart increases while sympathetic modulation of blood pressure decreases. Compared to term infants, preterm infants exhibit lesser parasympathetic modulation of the heart along with greater respiratory-mediated changes and lower sympathetic modulation of blood pressure. Impaired autonomic control in preterm infants may increase their risk of cardiovascular dysfunction later in life.