D. P. G. Bolton

Sleep position, autonomic function, and arousal

AIMS To investigate and compare heart rate variability (HRV) and responses of heart rate and arousal to head-up tilting in infants sleeping prone and supine. METHODS Thirty seven healthy infants aged 2–4 months were studied. HRV was measured for 500 beats while they were in a horizontal position. Subjects were then tilted 60° head-up, and heart rate recorded over 1 minute and arousal responses observed. Data were collected during both quiet and active sleep for both prone and supine sleep positions. RESULTS HRV, as assessed by the point dispersion of Poincaré plots, was significantly reduced in the prone position for both sleep states. Sleep position did not influence the changes in heart rate seen during a head-up tilt. Full awakening to the tilt was common in active sleep but significantly less so in the prone position (15% of prone tests vs 54% supine). Full awakening to the tilt rarely occurred during quiet sleep in either sleep position. CONCLUSION This study provides some evidence that blunted arousal responses and/or altered autonomic function are a feature of the prone sleeping position. Decreased HRV may be a sign of autonomic impairment. It is seen in many disease states and in infants who later die of sudden infant death syndrome (SIDS).

Factors Affecting Heart Rate Variability and Heart Rate Responses to Tilting in Infants Aged 1 and 3 Months

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

Rebreathing expired gases from bedding: a cause of cot death?

The reported association of cot death and sleeping prone could be due to rebreathing of expired gases. A mechanical model simulating the respiratory system of an infant, exhaling warm humidified air with an end tidal carbon dioxide of 5%, has been used to investigate this. Some commonly used bedding materials caused an accumulation of carbon dioxide of 7% to over 10% with the model lying face down. This phenomenon persisted even with the head inclined at 45 degrees, but only on very soft materials, and could be a cause of cot death in a baby unresponsive to asphyxial blood gas changes. A coir fibre mattress allowed complete dispersal of exhalate as did a rubber sheet between any mattress and the covering sheet.

Bed-sharing and the infant’s thermal environment in the home setting

Aims: To study bed-sharing and cot-sleeping infants in the natural setting of their own home in order to identify differences in the thermal characteristics of the two sleep situations and their potential hazards. Methods: Forty routine bed-sharing infants and 40 routine cot-sleeping infants aged 5–27 weeks were individually matched between groups for age and season. Overnight video and physiological data of bed-share infants and cot-sleeping infants were recorded in the infants’ own homes including rectal, shin, and ambient temperature. Results: The mean rectal temperature two hours after sleep onset for bed-share infants was 36.79°C and for cot-sleeping infants, 36.75°C (difference 0.05°C, 95% CI −0.03 to 0.14). The rate of change thereafter was higher in the bed-share group than in the cot group (0.04°C v 0.03°C/h, difference 0.01, 0.00 to 0.02). Bed-share infants had a higher shin temperature at two hours (35.43 v 34.60°C, difference 0.83, 0.18 to 1.49) and a higher rate of change (0.04 v −0.10°C/h, difference 0.13, 0.08 to 0.19). Bed-sharing infants had more bedding. Face covering events were more common and bed-share infants woke and fed more frequently than cot infants (mean wake times/night: 4.6 v 2.5). Conclusions: Bed-share infants experience warmer thermal conditions than those of cot-sleeping infants, but are able to maintain adequate thermoregulation to maintain a normal core temperature.

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

Abstract:  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.

Vasoconstriction following spontaneous sighs and head-up tilts in infants sleeping prone and supine

The cutaneous vasoconstrictor responses following a 60 degrees head-up tilt and a spontaneous sigh were measured in 36 infants at 1 and 3 months age to investigate the effects of age, sleep state and sleep position on these responses. The vasoconstrictor response was determined by a measure of cutaneous blood flow using a laser Doppler flowmeter. The mean reduction in blood flow (vasoconstriction) was 52% following the tilt, and 33% following the sigh. Prone positioning 1-month-old infants as compared to supine, reduced the degree of vasoconstriction following the tilt (P=0.027) and sigh (P=0.026). The supine to prone reduction was: tilt, -11% in quiet sleep (QS) (from 55.1 to 49.1% vasoconstriction) and -18% in active sleep (AS) (from 52.0 to 42.9%) and; sigh, -26% in QS (35-26%), and -15% in AS (31-26%). The degree of vasoconstriction following the sigh was significantly greater in 3- compared to 1-month-old infants (+26%, P=0.040). The mean response to the tilt in the older age group was 12% greater but this did not reach significance (P=0.069). Sleep state did not affect the degree of vasoconstriction but influenced transmission of the response so that latency to minimal vasoconstriction was 1 s shorter in AS than QS. This study provides data on two simple measures of sympathetic activity during sleep that have not previously been described in any detail in infant studies, and add more evidence that autonomic activity is reduced in the prone position compared to supine during sleep.

Ventilatory sensitivity to mild asphyxia: prone versus supine sleep position.

AIMS To compare the effects of prone and supine sleep position on the main physiological responses to mild asphyxia: increase in ventilation and arousal. METHODS Ventilatory and arousal responses to mild asphyxia (hypercapnia/hypoxia) were measured in 53 healthy infants at newborn and 3 months of age, during quiet sleep (QS) and active sleep (AS), and in supine and prone sleep positions. The asphyxial test mimicked face down rebreathing by slowly altering the inspired air: CO2, maximum 5% and O2, minimum 13.5%. The change in ventilation with inspired CO2 was measured over 5–6 minutes of the test. The slope of a linear curve fit relating inspired CO2 to the logarithm of ventilation was taken as a quantitative measure of ventilatory asphyxial sensitivity (VAS). Sleep state and arousal were determined by behavioural criteria. RESULTS At 3 months of age, prone positioning in AS lowered VAS (0.184 prone v0.269 supine, p = 0.050). At newborn age, sleep position had no effect on VAS. Infants aged 3 months were twice as likely to arouse to the test than newborns (p = 0.013). Placing infants prone as opposed to supine increased the chances of arousal 1.57-fold (p = 0.035). CONCLUSION Our findings show 3 month old babies sleeping prone compared to supine have poorer ventilatory responses to mild asphyxia, particularly in AS, but the increased prevalence of arousal is a protective factor.

Responses to an increasing asphyxia in infants: effects of age and sleep state

Infants aged 0-6 months were assessed for respiratory and arousal responses to mild asphyxia during sleep. Ventilatory sensitivity was assessed from the relationship between inspired carbon dioxide (FICO2) and ventilation. Arousal and ventilatory sensitivity were significantly related. Respiratory response increased with age and was greater in quiet sleep than in REM sleep. Arousal occurred more frequently in REM sleep (55/102) than quiet sleep (38/165, P < 0.05) and more frequently at the newborn age (54/117) than at 6 months (13/58, P < 0.05). Arousal in quiet sleep occurred in babies with high ventilatory sensitivities (mean ventilatory asphyxial sensitivity (VAS) 0.476 +/- 0.288) and in REM sleep was more associated with low ventilatory sensitivities (mean VAS 0.194 +/- 0.334, P <0.05). We conclude infants respond to mild asphyxia during sleep with an increase in ventilation, an arousal or both. The exact response is dependent on age and sleep state.

Hypoxic and Hypercapnic Events in Young Infants During Bed-sharing

OBJECTIVES: To identify desaturation events (arterial oxygen saturation [Sao2] <90%) and rebreathing events (inspired carbon dioxide (CO2) >3%), in bed-sharing (BS) versus cot-sleeping (CS) infants. METHODS: Forty healthy, term infants, aged 0 to 6 months who regularly bed-shared with at least 1 parent >5 hours per night and 40 age-matched CS infants were recruited. Overnight parent and infant behavior (via infrared video), Sao2, inspired CO2 around the infant’s face, and body temperature were recorded during sleep at home. RESULTS: Desaturation events were more common in BS infants (risk ratio = 2.17 [95% confidence interval: 1.75 to 2.69]), associated partly with the warmer microenvironment during BS. More than 70% of desaturations in both groups were preceded by central apnea of 5 to 10 seconds with no accompanying bradycardia, usually in active sleep. Apnea >15 seconds was rare (BS infants: 3 events; CS infants: 6 events), as was desaturation <80% (BS infants: 3 events; CS infants: 4 events). Eighty episodes of rebreathing were identified from 22 BS infants and 1 CS infant, almost all preceded by head covering. During rebreathing, Sao2 was maintained at the baseline of 97.6%. CONCLUSIONS: BS infants experienced more oxygen desaturations preceded by central apnea, partly related to the warmer microenvironment. Rebreathing occurred mainly during bed-sharing. Infants were at low risk of sudden infant death syndrome and maintained normal oxygenation. The effect of repeated exposure to oxygen desaturation in vulnerable infants is unknown as is the ability of vulnerable infants to respond effectively to rebreathing caused by head covering.

The micro‐environment of the sleeping newborn piglet covered by bedclothes: Gas exchange and temperature

This study followed the thermal and gaseous micro‐environment of the newborn piglet sleeping under two levels of bedding insulation. After 1 h in the piglets thermal comfort zone, the head of each piglet was covered for 2h with bedding either 12 mm or 21 mm thick. Body temperature rose rapidly, but the arterial gases showed no change in the direction of asphyxia; with the thicker covering Pao2 was unchanged and Pao2 fell and pH rose. These changes were despite a small rise in environmental Pao2 and fall in Pao2 but were also minimized by the piglets ability to avoid profound hypocapnia by panting at a very low tidal volume. No significant changes in blood gases occurred with the thinner bedding; the temperature rose markedly but more slowly. It is suggested that human babies submerged under bedclothes for any length of time would be more likely to succumb to the effects of hyperthermia than of asphyxia.