Cardiac responses to mild hypoxic hypercapnia in newborn babies: no effect of sleep position

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.

Influence of hypoxia and hypercapnia on sleep state-dependent heart rate variability behavior in newborn lambs

STUDY OBJECTIVES

Although hypercapnia and/or hypoxia are frequently present during chronic lung disease of infancy and have also been implicated in sudden infant death syndrome (SIDS), their effect on cardiac autonomic regulation remains unclear. The authors' goal is to test that hypercapnia and hypoxia alter sleep-wake cycle-dependent heart rate variability (HRV) in the neonatal period.

DESIGN

Experimental study measuring HRV during sleep states in lambs randomly exposed to hypercapnia, hypoxia, or air.

SETTING

University center for perinatal research in ovines (Sherbrooke, Canada). INSERM-university research unit for signal processing (Rennes, France).

PARTICIPANTS

Six nonsedated, full-term lambs.

INTERVENTIONS

Each lamb underwent polysomnographic recordings while in a chamber flowed with either air or 21% O(2) + 5% CO(2) (hypercapnia) or 10% O(2) + 0% CO(2) (hypoxia) on day 3, 4, and 5 of postnatal age.

MEASUREMENTS AND RESULTS

Hypercapnia increased the time spent in wakefulness and hypoxia the time spent in quiet sleep (QS). The state of alertness was the major determinant of HRV characterized with linear or nonlinear methods. Compared with QS, active sleep (AS) was associated with an overall increase in HRV magnitude and short-term self-similarity and a decrease in entropy of cardiac cycle length in air. This AS-related HRV pattern persisted in hypercapnia and was even more pronounced in hypoxia.

CONCLUSION

Enhancement of AS-related sympathovagal coactivation in hypoxia, together with increased heart rate regularity, may be evidence that AS + hypoxia represent a particularly vulnerable state in early life. This should be kept in mind when deciding the optimal arterial oxygenation target in newborns and when investigating the potential involvement of hypoxia in SIDS pathogenesis.

[Risk of accumulation of CO₂ in the oxygen chamber in "HOOD" (Experimental study on test bed)]

OBJECTIVES

Oxygen hood is largely used to deliver O₂ to newborn infants with respiratory failure in the northern region of France. The oxygen flow is set to obtain the target arterial blood oxygen saturation. Thus, O₂ flow delivers into the hood may be below the recommended gas flow of 6L/min. However, gas flow below 6L/min exposes to CO₂ rebreathing. The aim of this study was to evaluate the effect of various rates of gas flows on the values of partial pressure of CO₂ into the hood.

MATERIAL AND METHODS

We measured CO₂ and O₂ partial pressure into hoods of two different volumes (4 and 10L) under two experimental bench test conditions. Protocol 1: gas flow was constant at 6L/min, while oxygen fraction varied from 0.21 to 1. Partial pressure of CO₂ and O₂ were recorded. Protocol 2: while O₂ fraction was kept constant, oxygen flow varied from 0.5 to 7L/min (by step of 0.5L/min). Partial pressure of CO₂ and O₂ were recorded.

RESULTS

Partial pressure of CO₂ increases proportionally to the decrease in the gas flow delivered into the hood, and reached 14 mmHg at gas flow of 0.5L/min.

CONCLUSION

Risk of CO₂ rebreathing exists as soon as the gas is delivered into the hood at minimal flow rates below 6L/min.

Cardiovascular control during sleep in infants: Implications for Sudden Infant Death Syndrome

In infants the cardiorespiratory system undergoes significant functional maturation after birth and these changes are sleep-state dependent. Given the immaturity of these systems it is not surprising that infants are at risk of cardiorespiratory instability, especially during sleep. A failure of cardiovascular control mechanisms in particular is believed to play a role in the final event of Sudden Infant Death Syndrome (SIDS). The "triple risk model" describes SIDS as an event that results from the intersection of three overlapping factors: (1) a vulnerable infant, (2) a critical development period in homeostatic control, and (3) an exogenous stressor. This review summarises normal development of cardiovascular control during sleep in infants and describes the association of impaired cardiovascular control with the three overlapping factors proposed to be involved in SIDS pathogenesis.

Effects of body position on sleep and arousal characteristics in infants

The prone sleeping position has been identified in world-wide epidemiological studies as a major risk factor for sudden infant death syndrome (SIDS). Public awareness campaigns throughout the western world have led to an over 50% reduction in postneonatal mortality and frequency of SIDS. This reduction in mortality has been mainly attributed to the avoidance of the prone sleep position. Various mechanisms have been postulated to explain the increased risk of SIDS associated with prone sleeping, among these, impairment of arousal from sleep. This paper reviews the effects of prone sleeping on infant sleep architecture, arousability from sleep and cardiorespiratory controls. Sleeping in the prone position has been shown to increase the amount of time spent sleeping, particularly time spent in quiet sleep (QS). Sleeping prone has also been demonstrated to be associated with a reduced responsiveness to a variety of arousal stimuli. Such impairment of arousal has been demonstrated to be associated with changes in control of autonomic cardiac function. During arousal, heart rate, blood pressure and breathing movements increase, while gross body movements occur to avoid the stimulus. Any impairment in arousability from sleep such as could occur when infants sleep in the prone position, could possibly contribute to the final pathway to SIDS.

Prone versus supine sleep position: a review of the physiological studies in SIDS research

A number of physiological studies, published over the last 10 years, have investigated the links between prone sleeping and sudden infant death syndrome (SIDS). This review evaluates those studies and derives an overview of the different affects of sleeping prone or supine in infancy. Generally, compared with the supine, the prone position raises arousal and wakening thresholds, promotes sleep and reduces autonomic activity through decreased parasympathetic activity, decreased sympathetic activity or an imbalance between the two systems. In addition, resting ventilation and ventilatory drive is improved in preterm infants, but in older infants (>1 month), there is no improvement in ventilation, and in 3-month-old infants, the position is adverse in terms of poorer ventilatory drive (in active sleep only). The majority of findings suggest a reduction in physiological control related to respiratory, cardiovascular and autonomic control mechanisms, including arousal during sleep in the prone position. Since the majority of these findings are from studies of healthy infants, continued reinforcement of the supine sleep recommendations for all infants is emphasized.