Mechanical model testing of rebreathing potential in infant bedding materials

Rebreathing of expired air may be a lethal hazard for prone sleeping infants. This paper describes a mechanical model to simulate infant breathing, and examines the effects of bedding on exhaled air retention. Under simulated rebreathing conditions, the model allows the monitoring of raised carbon dioxide (CO₂) inside an artificial lung-trachea system. Resulting levels of CO₂ (although probably exaggerated in the mechanical model compared with an infant, due to the model''s fixed breathing rate and volume) suggest that common bedding materials vary widely in inherent rebreathing potential. In face down tests, maximum airway CO₂ ranged from less than 5% on sheets and waterproof mattresses to over 25% on sheepskins, bean bag cushions, and some pillows and comforters. Concentrations of CO₂ decreased with increasing head angle of the doll, away from the face down position. Recreations of 29infant death scenes also showed large CO₂ increases on some bedding materials, suggesting these infants could have died while rebreathing.

     

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.     

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.     

Evaluation of rebreathing potential on bedding for infant use

BACKGROUND: Rebreathing is thought to be associated with sudden infant death syndrome (SIDS). The aim of the present study was to evaluate the rebreathing potential of different types of Japanese infant bedding. METHODS: The rebreathing potential of various combinations of infant bedding was measured using a mechanically simulated breathing model. The types of bedding included five types of mattresses, four types of o-nesyo sheets (waterproof sheets) and a towel. The half-life of the expiratory CO2 concentration, t1/2-value was calculated as the index of the rebreathing potential. The softness of the bedding was also measured. RESULTS: There was a moderate proportional correlation between the t1/2-value and the softness (correlation coefficient = 0.509). When a new hard infant mattress was used, the t1/2-values were 13.6-14.1 s, and when o-nesyo sheet was added, the values were 14.1-16.2 s. When other mattresses were used with the o-nesyo sheet, the values were 14.1-19.2 s. Adding a towel onto the bedding, the t1/2-value (18.5-22.3 s) was prolonged without exception. CONCLUSION: It is difficult to estimate the rebreathing potential of the bedding on the basis its appearance or its softness. All infants should be placed on appropriate bedding in case they turn to a prone-sleeping position. Our recommendations to avoid rebreathing are as follows: (i) a new hard mattress specifically designed for babies should be used; (ii) a towel should not be used; (iii) an o-nesyo sheet may be used with a new hard infant mattress if necessary.     

A potential danger of bedclothes covering the face

Investigations of infants dying unexpectedly have reported up to 28% being found completely under bedding. No detailed physiological studies looking at the possibilities of asphyxia in this situation are available. The aim was to determine the potential for asphyxia under different types and thicknesses of bedding. A mechanical model of a 3-month-old infant's respiratory system was used. Bedding was positioned over the head in a supine position, and inspired carbon dioxide recorded. With a fixed respiratory rate and tidal volume, carbon dioxide accumulation increased with increasing layers of blankets. Up to 8.3% inspired carbon dioxide was recorded with more than four layers of blankets. A cotton sheet between the face and blankets reduced the accumulation by half. An infant found dead under bedding may have been exposed to an asphyxial stress. Suffocation from rebreathing trapped, expired gases can be a cause of death in this situation.     

CO2-sensitivity in newborn and young infants tested by the rebreathing method. Methodological aspects

The rebreathing method in newborn and young infants was tested with regard to the effects of increasing levels of CO2 on the behavioral state and to the influence of facial stimulation caused by the use of a face mask and pneumotachograph on the breathing pattern. Successive increases in inspired CO2 levels from 3 to 8.5% did not change the behavioral state. The application or removal of a face mask, with or without an attached pneumotachograph, caused an immediate disruption in the breathing pattern which lasted 10-40 s. Thereafter, breathing stabilized. When only the face mask was used, the increases in minute ventilation, tidal volume and cycle duration of 7, 13 and 5%, respectively, were transient and lasted for 40-50 breaths. The breathing then returned to control levels. In contrast, breathing through the face mask connected to the pneumotachograph caused a long-lasting increase in ventilation (12%) due to the retention of CO2 in the spirometric circuit. We conclude that the inhalation of a continuously increasing mixture of 3-8.5% CO2 in O2 did not influence the behavioral state and that the effect of face mask on breathing pattern was transient.     

CO2 rebreathing: a possible contributory factor to some cases of sudden infant death?

Physical and geometrical conditions influencing carbon dioxide (CO₂) accumulation near the face of a sleeping infant positioned deep in a cot or pram (open cot shaft) or underneath bedding (closed cot shaft) were investigated. By means of mathematical and data-based simulation, and an experimental rebreathing model, both hypothetical (dry, exhaled air + 20 ^oC, room temperature + 20 ^oC) and more physiological conditions (heated, humidified exhaled air, room temperature + 20 ^oC; with and without pooling of cold air within the shaft) were tested. With exhaled air at + 20 ^oC, the CO₂ concentration increased to about 10% within 5 min. The increase was faster the smaller the volume, and the smaller the opening of the cot shaft. When expiratory air was heated, the CO₂ concentration increased with the same speed as when the shaft was closed, but to only 0.1-0.3% when the shaft was open. Pooling of cold air in the shaft increased CO₂ accumulation 70-200 times the concentration in air (to < 5.5%) when the shaft was open. Turbulence of the air outside the open shaft reduced the increase in CO₂ concentration. The experiments imply that CO₂ may accumulate around an infant's head when placed deep in a cot or pram with the bedding and walls creating a narrow, vertical, shaft-like tunnel to the surrounding air. Although the CO₂ concentration may theoretically attain dangerous levels in such circumstances, a rapid equilibrium between the air within and outside the cot usually occurs due to convection of the expiratory air and turbulence from drafts, the infant's body movements and breathing. Such factors will largely eliminate any significant rebreathing with the exception of the extreme situation when expired air is contained within a closed space.Environment, rebreathing, SIDS, suffocation     

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

OBJECTIVE: To evaluate whether cardiac responses to a level of hypoxic hypercapnia that may be observed in rebreathing studies are altered with infant sleep position. METHODOLOGY: Eighteen healthy term infants (< 5-days-old) were studied. Heart rate (HR) and HR variability were monitored during air breathing and during 3 min exposure to a mixture of 15% O2/3% CO2 in both the prone and supine positions. Power spectral analysis of HR was performed. RESULTS: Heart rate was the only measured variable to be significantly changed in response to 15% O2/3% CO2. Hypoxic hypercapnia elicited no significant responses in power spectral HR variables. There was no effect of sleeping position on any of the measured variables. CONCLUSIONS: There are no significant differences in cardiac responses to mild hypoxic hypercapnia between sleep positions and power spectral indices of the autonomic control of HR are not altered by sleep position in newborn babies.     

Rebreathing potential of infant mattresses and bedcovers

OBJECTIVE: To establish the CO2 dispersion and retention properties of some mattresses and bed coverings commercially available in Australia. METHODS: Five mattresses were studied in (i) an in vivo model in which an infant's head was covered by a headbox, rebreathing was allowed to occur, and the final steady state CO2 concentration was measured; and (ii) an in vitro model in which 5% CO2 in a headbox was allowed to disperse, and the time taken for the concentration to reach 1% was measured. Five types of bedcover were studied in (i) an in vivo model in which an infant's head was covered by a bedcover and the final steady state CO2 concentration was measured; and (ii) an in vitro model in which 5% CO2 under a bedcover was allowed to disperse, and the time taken for the concentration to reach 1% was measured. RESULTS: The steady state CO2 concentrations ranged from 0.6% to 3.0% for the mattresses (P < 0.05). The time for CO2 to disperse ranged from 5.5 min to 30.4 min (P < 0.05). Steady state CO2 concentrations ranged from 2.5% to 3.6% for the bedcoverings (P > 0.05). The time for CO2 to disperse ranged from 5.4 min to 7.7 min (P > 0.05). CONCLUSIONS: Some commercial cot mattresses and bedcoverings allow high concentrations of CO2 to accumulate in rebreathing environments. Some mattress types studied were more diffusive to CO2, whereas there was no difference between the bedcovers studied. This may have implications for vulnerable infants at risk of sudden infant death syndrome.     

Sleep position and the use of soft bedding during bed sharing among African American infants at increased risk for sudden infant death syndrome

Soft bedding increases the risk for death among prone infants. We compared the softness of beds and bedding and infant sleep position for infants sleeping alone and for those bed sharing. STUDY DESIGN: Questionnaires were used to record the bedding and sleep practices of 218 consecutive African American infants. Enrollment was prospective. Mechanical models were used in the homes of a subgroup to measure the softness of bedding and its propensity to cause rebreathing. Results were compared by using the Student t test, Mann-Whitney U test, and chi(2) analysis. RESULTS: In a cross-sectional sample of infants, at 8.2 +/- 3.3 weeks of age, 61% (133 of 218) had bed shared > or =1 of the previous 14 nights and 48.6% (106 of 218) had bed shared the night before. Breast-feeding rates were not different for bed sharers and those sleeping alone. The rates of maternal smoking for both groups were low (13.6% vs 11.8%). Comforters, pillows, and waterbeds were more commonly used beneath bed-sharing infants. Bed sharers were twice as likely to habitually be placed prone for sleep (18% vs 9%). In the subgroup studied in their homes (13 bed sharing, 19 alone), the shared beds were softer (P <.0001) and could cause more rebreathing (P =.007). CONCLUSIONS: Infants at increased risk for sudden infant death syndrome, by sociodemographic criteria, who also bed share are more likely to sleep prone and to use softer beds. These findings may explain part of the risk associated with bed sharing among US infants, a risk that appears to be independent of the effects of maternal smoking.     

Ventilatory responses to rebreathing in infants exposed to maternal smoking

This study assessed the effect of maternal smoking during pregnancy on ventilatory and waking responses of infants to a respiratory challenge. This challenge mimicked the time-course and concentration of gases that an infant would experience rebreathing face-down into soft bedding. Control (C; n = 97) and smokers' infants (SM; n = 96) were studied at ages 1 and 3 mo. Asphyxial gas (hypercapnia/hypoxia) was delivered to the supine sleeping baby via a hood by slowly altering the inspired air: CO₂ maximum 5% and O₂ minimum 13.5%. Respiratory pattern was recorded by inductive plethysmography as the sum of ribcage and abdominal movements. The change in ventilation with inspired CO₂ was measured over 5-6 min 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). Protective responses were graded from 1: no waking and an estimated arterial carbon dioxide tension (PaCO₂     

Ventilatory chemosensitivity in parents of infants with sudden infant death syndrome

We measured ventilatory responses to progressive isocapnic hypoxia and to hyperoxic hypercapnia (CO2) using rebreathing techniques in 16 parents of infants with autopsy-confirmed sudden infant death syndrome (SIDS) and 18 control parents matched for age, sex, and body size. Response to ventilatory loading was assessed by repeating the CO2 test with an inspiratory flow-resistive load (16 cm H2O/L/sec). During loaded and unloaded CO2 tests, respiratory effort was also assessed by measuring the pressure generated in the first 0.1 second (P0.1) of the subsequent inspiratory effort after brief manual occlusion of the inspiratory line. Ventilatory responses of the parents of victims of SIDS to chemical and mechanical stimulation were not significantly different from those of control parents. Responses in both groups were similar to previously reported normal values. There was a linear increase in ventilation (VE) in response to hypercapnia and hypoxia and in P0.1 in response to hypercapnia. We found expected increases in P0.1/PCO2 and decreases in VE/PCO2 slopes during loaded breathing in all subjects, but no difference between groups. We conclude that parents of SIDS victims have normal ventilatory chemosensitivity and respiratory drive.     

Resistance to airflow through bedding materials used in infancy.

Various bedding materials used in infancy, including duvets (or continental quilts), were tested for airflow using the British Standards Institution tests for pillows or fabrics. Resistance was also measured when the items were placed on a dummy infant face. Measurements were made on washed and unwashed garments, which were tested both dry and wet. Results suggest that all the bedding materials tested are safe for use even in the newborn period. The duvets produced slightly lower resistance to breathing than conventional blankets and sheets. In view of the wide variety of infant bedding fabrics it seems desirable for standard airflow performance requirements to be introduced.     

The divergent ventilatory and heart rate response to moderate hypercapnia in infants with apnoea of infancy

BACKGROUND: Inspired CO2 is a potent ventilatory stimulant exhibiting a paradoxical inhibitory effect on breathing at high concentrations. Severe respiratory depression as a result of CO2 rebreathing during sleep has been implicated as a possible trigger factor in sudden infant death syndrome (SIDS). OBJECTIVE: To investigate the ventilatory and heart rate (HR) responses to inhaled CO2 in infants with apnoea of infancy, a group believed to be at increased risk of SIDS. STUDY DESIGN: Thirty one infants with severe sleep related apnoea, 31 infants with mild recurrent apnoea, and 31 age and sex matched controls for the infants with severe sleep related apnoea were studied. HR was computed from digitised RR intervals, "ventilation" was recorded by inductance plethysmography, and PCO2 and PO2 were monitored by transcutaneous electrodes. The ventilatory and HR responses to CO2 were expressed as percentage increase in ventilation and change in HR/unit change in transcutaneous PCO2. RESULTS: The mean increase in transcutaneous PCO2 during CO2 challenge (0.45 kPa = 3.4 mm Hg) resulted in a mean increase in ventilation of 291%/1 kPa (7.3 mm Hg) increase in transcutaneous PCO2, with no difference between the groups. A significant difference between infants with severe sleep related apnoea and mild recurrent apnoea versus controls (p < 0.02, p < 0.01, respectively) was found in their HR response to CO2 challenge: HR decreased in 12 severe sleep related apnoea infants and 10 infants with mild recurrent apnoea, but only in two controls. CONCLUSION: Infants with apnoea of infancy frequently show a paradoxical decrease in HR during CO2 challenge, possibly because of an insufficient ability to mobilise cardiovascular defence mechanisms when challenged with hypercapnia.     

Resistance to airflow through bedding materials used in infancy

Various bedding materials used in infancy, including duvets (or continental quilts), were tested for airflow using the British Standards Institution tests for pillows or fabrics. Resistance was also measured when the items were placed on a dummy infant face. Measurements were made on washed and unwashed garments, which were tested both dry and wet. Results suggest that all the bedding materials tested are safe for use even in the newborn period. The duvets produced slightly lower resistance to breathing than conventional blankets and sheets. In view of the wide variety of infant bedding fabrics it seems desirable for standard airflow performance requirements to be introduced.     

Child care practices and the sudden infant death syndrome

Southern New Zealand has one of the highest reported rates of the sudden infant death syndrome (SIDS) in the world. Data were collected on 49 infants who died from SIDS in the Dunedin and Invercargill Health Districts of southern New Zealand. Forty-two of the families were interviewed at home following the loss of their infant. In the week prior to death, 17 infants (40%) had an upper respiratory tract infection, although in 11 of these infants their infection had apparently resolved at least 24 h prior to death. Thirty infants (71%) had their faces down into the bedding or their heads covered with bedding. Thirty-four infants (81%) were found dead in the prone position, whereas 49% of New Zealand infants aged 3 months would be expected to sleep prone. Sleep position and the terminal position of the head in relation to SIDS requires further study.     

Child care practices and the sudden infant death syndrome

Abstract Southern New Zealand has one of the highest reported rates of the sudden infant death syndrome (SIDS) in the world. Data were collected on 49 infants who died from SIDS in the Dunedin and Invercargill Health Districts of southern New Zealand. Forty-two of the families were interviewed at home following the loss of their infant. In the week prior to death, 17 infants (40%) had an upper respiratory tract infection, although in 11 of these infants their infection had apparently resolved at least 24 h prior to death. Thirty infants (71%) had their faces down into the bedding or their heads covered with bedding. Thirty-four infants (81%) were found dead in the prone position, whereas 49% of New Zealand infants aged 3 months would be expected to sleep prone. Sleep position and the terminal position of the head in relation to SIDS requires further study.     

Respiratory responses to hypoxia/hypercapnia in small for gestational age infants influenced by maternal smoking

Aim: To determine any variation in the respiratory responses to hypoxia/hypercapnia of infants born small for gestational age (SGA) to smoking and to non-smoking mothers. METHODS: A total of 70 average for gestational age (AGA) infants (>36 weeks gestation, >2500 g, >25th centile for gestational age, and no maternal smoking), and 47 SGA infants (<10th centile for gestational age) were studied at 1 and 3 months of age, in quiet and active sleep. Respiratory test gases were delivered through a Perspex hood to simulate face down rebreathing by slowly allowing the inspired air to be altered to a CO(2) maximum of 5% and O(2) minimum of 13.5%. The change in ventilation with inspired CO(2) was measured over 5-6 minutes of the test. The slope of a linear curve fit relating inspired CO(2) to the logarithm of ventilation was taken as a quantitative measure of ventilatory asphyxial sensitivity (VAS). RESULTS: There was no significant difference in VAS between the AGA and SGA infants (0.25 v 0.24). However within the SGA group, VAS was significantly higher (p = 0.048) in the infants whose mothers smoked during pregnancy (0.26 (0.01); n = 24) than in those that did not (0.23 (0.01); n = 23). The change in minute ventilation was significantly higher in the smokers than the non-smokers group (141% v 119%; p = 0.03) as the result of a significantly larger change in respiratory rate (8 v 4 breaths/min; p = 0.047) but not tidal volume. CONCLUSIONS: Maternal smoking appears to be the key factor in enhancing infants' respiratory responses to hypoxia/hypercapnia, irrespective of gestational age.     

Carbon dioxide concentrations in the environment of sleeping infants

Abstract In 22 infants continuous measurements were made of the concentration of carbon dioxide (CO₂) in inspired air during sleep. Evidence was found of CO₂ enrichment of inspired air in certain environmental conditions. The levels achieved were not sufficiently high to acutely endanger an infant. Carbon dioxide concentrations as high as 2-3% were observed in the prone position when the infant's head was under a blanket and when the lower face was obscured by bedding. Sleeping prone on a sheepskin also resulted in an increased concentration of CO₂ but to a lesser extent than being under a blanket. In awake infants the presence of a pacifier also promoted an excess of CO₂ in the inspired air, both in the prone and supine positions. The physiological and clinical implications of these findings, in relation to the Sudden Infant Death Syndrome (SIDS), are unknown and warrant investigation.     

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: CO(2), maximum 5% and O(2), minimum 13.5%. The change in ventilation with inspired CO(2) was measured over 5-6 minutes of the test. The slope of a linear curve fit relating inspired CO(2) 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 v 0.269 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.