Effect of body tilting on physiological functions in healthy term neonates

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.

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.

Effects of alcohol and smoking during pregnancy on infant autonomic control

Prenatal exposure to smoking and alcohol increases the risk for Sudden Infant Death Syndrome (SIDS). Physiological changes associated with these exposures are not well studied. Full-term infants were tested within the first 3 days of life. We hypothesized that maternal alcohol consumption and/or smoking during pregnancy would alter autonomic nervous system function. Newborns whose mothers smoked during pregnancy had lower beat-to-beat heart rate variability in quiet sleep. Infants whose mothers consumed alcohol had lower global heart rate variability, but only in active sleep. Unexposed infants demonstrated increases in heart rate with head-up tilt and decreases in heart rate with head-down tilt, but smoking and alcohol-exposed infants showed no significant responses. These results indicate that autonomic function is altered by prenatal exposure to alcohol and smoking. Such markers may provide early identification of infants at greatest risk for SIDS.

The early impact of feeding on infant breathing-swallowing coordination

This longitudinal study assessed the coordination of nutritive and non-nutritive swallowing with breathing in 10 healthy term infants from birth to 1 year of age. Swallows were classified into five respiratory-phase categories: mid-inspiration (II), mid-expiration (EE), inspiratory-expiratory cusp (IE), expiratory-inspiratory cusp (EI), and mid-pause (P). Breathing-swallowing coordination differed markedly between the two swallowing conditions, especially between 2 weeks and 2 months. Significant condition effects were found in up to four respiratory-phase categories (II, IE, EI, and P). The condition effect was minimal from 9 months with only IE swallow proportions differing between conditions. These data suggest a 'critical period' in infantile neural response to oropharyngeal stimulation during feeding and that the impact of this on infants with neurological and/or respiratory disorders should be further investigated.

Nutritive and non-nutritive swallowing apnea duration in term infants: implications for neural control mechanisms

The impact of bolus ingestion and level of consciousness on swallowing apnea duration (SAD) in healthy term infants has not been adequately explored despite the likely contribution of swallowing apnea to upper airway protection against aspiration. SAD during wakefulness, sleep, and feeding (breast or bottle) of 10 term infants was measured 10 times from birth to 1 year of age. Nineteen thousand four hundred and two swallows were analyzed. Irrespective of age, SAD during feeding was significantly shorter than SAD of non-nutritive swallowing (during wakefulness and sleep). SAD did not change significantly within the first year of life in any of the three conditions and there was no change in the relative durations of nutritive, wake and sleep conditions with age. The absence of an age effect implies that the neural mechanisms controlling SAD are fundamentally brainstem-mediated and largely hard-wired at birth in healthy term neonates.

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

We performed a cross-sectional study in human infants to determine if indices of R-R interval variability, systolic blood pressure (SBP) variability, and baroreceptor reflex sensitivity change with postmenstrual age (PMA: gestational age+postnatal age). The electrocardiogram, arterial SBP and respiration were recorded in clinically stable infants (PMA, 28-42 weeks) in the quiet sleep state in the first days after birth. (Cross-)spectral analyses of R-R interval series and SBP series were performed to calculate the power of low-frequency (LF, indicating baroreceptor reflex activity, 0.04-0.15 Hz) and high-frequency (HF, indicating parasympathetic activity, individualized between the p-10 and p-90 values of respiratory frequency) fluctuations, and transfer function phase and gain. The mean R-R interval, and LF and HF spectral powers of R-R interval series increased with PMA. The mean SBP increased with PMA, but not the LF and HF spectral powers of SBP series. In the LF range, cross-spectral analysis showed high coherence values (>0.5) with a consistent negative phase shift between R-R interval and SBP, indicating a approximately 3 s lag in R-R interval changes in relation to SBP. Baroreceptor reflex sensitivity, calculated from LF transfer gain, increased significantly with PMA, from 5 (preterm) to 15 ms mmHg-1 (term). Baroreceptor reflex sensitivity correlated significantly with the (LF and) HF spectral powers of R-R interval series, but not with the LF and HF spectral powers of SBP series. The principal conclusions are that baroreceptor reflex sensitivity and spectral power in R-R interval series increase in parallel with PMA, suggesting a progressive vagal maturation with PMA.

[Standardized oculocardiac reflex in ex-premature near full term (93 cases)]

The aim of the study was to present data investigating vagal reactivity in a population of premature infants reaching term, using the oculo-cardiac reflex.

PATIENTS AND METHODS

Ninety three premature infants, free of any disease, near full term at the moment of testing, were prospectively investigated at a time close to discharge from neonatal unit. After an all-night polygraphic recording, a standardized oculo-cardiac reflex test was performed during quiet sleep. Data were classified in relation to both chronological and postconceptional ages. Simple liner regression analyses were performed on the selected variables.

RESULTS

The results showed heterogeneity of the vagal response in this population: longest asystole (1049 ms +/- 540; 95th percentile = 1894 ms); maximal percentage of deviation between two successive RR intervals (88% +/- 90; 95th percentile = 200%); and duration between the beginning of decrease in heart rate and return to mean heart rate (14 s +/- 10; 95th percentile = 30 s).

CONCLUSION

Our healthy premature infants at time of discharge exhibited a wider range of vagal reactivity than previously reported for the full term newborns. Considering our findings, we recommend caution before proceeding with treatment of vagal bradycardia in a similar premature infant population.

Development of baroreflex control of heart rate in preterm and full term infants

AIM

To study baroreflex maturation by measuring, longitudinally, baroreflex sensitivity in preterm (gestational age 24-37 weeks) and full term infants.

METHODS

Baroreflex sensitivity was quantified once a week, one to seven times, by a totally non-invasive method.

RESULTS

Baroreflex sensitivity at birth was lower in the preterm infant and increased with gestational age. It also increased with postnatal age, but the values for the preterm infants at term still tended to be lower than the values for full term babies.

CONCLUSION

Baroreflex control of heart rate is present in the premature infant, but is underdeveloped and increases with postnatal age. Ex utero maturation seems to be delayed compared with in utero maturation assessed by full term values. These results may reflect sympathovagal imbalance in preterm infants and could identify a population more vulnerable to stress.

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.