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Biomedical subjects

D P Bolton

Publications and source records attributed to D P Bolton.

At least 19 recordsLinked to original sources
PubMed2001

Ventilatory responses to rebreathing in infants exposed to maternal smoking.

UNLABELLED: 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 I 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) > or = 60 mmHg (least protective), to 4: fully awake (most protective). The results showed VAS was higher in SM infants than controls: +0.03 (p = 0.04). The oxygen saturation (SaO2) of SM infants fell -0.4% (p = 0.02) more than SaO2 of controls despite a greater tidal volume increase: +13.0% (p = 0.04). Overall protective responses were the same between groups, but grade 1, although rare, was found in 7 SM infants and only 4 control infants; this difference was not significant. CONCLUSION: The study did not confirm the postulate that infants of mothers who smoked during pregnancy have a reduced ventilatory response or raised waking thresholds.

Female↗
PubMed2000

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.

Female↗
PubMed2000

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.

Arousal↗
PubMed2000

The effect of cold stimulation to the face on the metabolic rate of the febrile piglet.

Sudden infant death syndrome has been associated with winter climates, infection, and overwrapping of babies. The hypothesis has been tested in this laboratory that two different causes of increased metabolic rate, high core temperature (via the van't Hoff or 'Q10' effect) and face-cooling, might synergistically induce hyperthermia. This proved not to be the case. We now report on a 'febrile' state adding Salmonella abortus equi pyrogens. The combination of face-cooling and pyrogen administration to 14 already hot piglets produced an increase in oxygen consumption of 47% in 6 of the animals (19% overall). Face-cooling alone caused a 6.5% fall in oxygen consumption, and injection of pyrogens alone had no effect on oxygen consumption. We conclude that there may be a danger of life-threatening hyperthermia in the combination of a cold face and febrile state.

Animals↗
PubMed2000

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 degree 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 mother's 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 SDdeltaRR. 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 SDdeltaRR 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 SDdeltaRR) 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.

Female↗
PubMed1998

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.

Arousal↗
PubMed1998

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 degrees 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).

Arousal↗
PubMed1997

Comparison of two methods of determining asphyxial potential of infant bedding.

OBJECTIVE: To establish, with the use of live sedated piglets on a range of bedding surfaces, the possibility of asphyxia when an infant is breathing face down into infant bedding surfaces, and to compare the results with those obtained with a mechanical model. METHODS: Piglets underwent tracheotomy and were attached to a silicon rubber model head of a 3-month-old infant. This was placed face down on bedding surfaces, and respiratory and blood gas data were collected for a 60-minute period. RESULTS: All bedding surfaces but one showed some rebreathing. This was to lower levels than with the mechanical model, but the ranking of the surfaces by level of rebreathing was similar. Two piglets died within the 1-hour experimental time. CONCLUSION: It is confirmed that the data from a mechanical model of rebreathing on different bedding surfaces are matched by those derived from a piglet model that responds normally to asphyxia.

Animals↗
PubMed1996

Arousal responses of the newborn piglet to airways obstruction and rebreathing during normothermia and hyperthermia.

Most experimental studies of arousal to respiratory stressors have been performed under neutral thermal conditions. The present study focussed on arousal responses with the added challenge of heat stress. The subjects were two groups of 10 sedated piglets aged 4-6 days and 2-6 days respectively. Respiratory stimuli (partial and total airways obstruction (AO) or rebreathing (RB) expired gases) were applied during non-rapid-eye-movement (NREM) sleep. Measurements of heart rate, SaO2, intra-pleural pressure or inspired CO2 and O2 were recorded during tests in normothermia and hyperthermia. Hyperthermia significantly shortened the time to arousal. Thus, arousal times from partial and total AO and from RB were: in normothermia, 40.3 +/- 5.8, 9.2 +/- 0.7 and 94.9 +/- 14.7 s respectively; in hyperthermia, 17.8 +/- 3.7, 7.6 +/- 0.9 and 68.5 +/- 9.1 s respectively. Cardio-respiratory variables at arousal were similar in both thermal states. Seven non-sedated piglets were challenged with RB stimuli during normothermia only to determine the influence of sleep state on arousal. Arousal in REM sleep was delayed approximately 2-3-fold (NREM, 41.2 +/- 8.2 s; REM, 88.5 +/- 18.1 s), occurring at a lower SaO2 and higher inspired CO2. It is concluded that arousal from NREM sleep under adverse conditions of hyperthermia shortens the arousal time from asphyxial stimuli induced by AO and RB with no change in the arousal threshold.

Airway Obstruction↗
PubMed1996

Pathophysiology of overheating in a piglet model: findings compared with sudden infant death syndrome.

OBJECTIVE: To examine the nature of hyperthermia-induced pathophysiological changes in an animal model including effects on lung compliance. METHODOLOGY: Piglets were randomly assigned to heated or non-heated groups. Heated animals were warmed to 4 degrees C above normal body temperature while sedated and breathing spontaneously. Cardiorespiratory variables were recorded serially and haematological assessments and blood cultures taken at 0 and 6 h. After 6 h the animals were killed and a limited postmortem was performed. Control animals had all procedures without heating. RESULTS: Heated piglets developed tachycardia, hypotension and a metabolic acidosis in addition to tachypnoea, hypocapnic alkalosis and a neutrophil leucocytosis. Rectal temperature after death fell at the same rate in both groups. Lung histology revealed an excess of lung haemorrhage and alveolar oedema in the heated group. No significant group differences in dynamic lung compliance were demonstrated. CONCLUSIONS: The pathological changes that occur during hyperthermia are non-specific but not incompatible with those found in sudden infant death syndrome. There was no confirmation of the thesis that hyperthermia causes death by altering lung compliance.

Animals↗
PubMed1996

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.

Asphyxia↗
PubMed1996

Thermal balance in infants.

A theoretical model of heat balance is presented that could clarify the matching of babies' wrapping with their environments. Best estimates of metabolic heat input and heat loss by all known routes are defined for 22 parts of the body surface. The variation of these with core temperature, posture, skin vasodilatation, and the onset of sweating are calculated: first, by using presumed skin temperatures and second, by following iterative calculation of the skin temperature and the consequent total heat losses. Calculation of the highest tolerable ambient temperature (HTAT) for a given set of clothes, underbedding, and covers shows that a well-wrapped baby lying face down could have an HTAT 10 degrees C lower than if he/she were lying supine. Representative values for highest and lowest tolerable temperatures (defined in text) are presented for the first 6 mo of life. Retrospective estimation of thermal balance from death-scene data on clothing and bedding can permit assessment of hyperthermia or hypothermia as a contributing cause of death. Recommendations are made on the avoidance of hyperthermia.

Age Factors↗
PubMed1994

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 piglet's thermal comfort zone, the head of each piglet was covered for 2 h 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 PaCO2 fell and pH rose. These changes were despite a small rise in environmental PCO2 and fall in PO2 but were also minimized by the piglet's 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.

Animals↗
PubMed1993

Sleep state organization in the developing piglet during exposure to different thermal stimuli.

Sleep state changes in response to different thermal stimuli were investigated in newborn piglets between 2 and 10 days of age. Test animals were exposed to cold air (7-12 degrees C) and warm air (27-33 degrees C) around the face, while the remainder of the body was kept at first warm (normothermic) then hyperthermic. A separate group of animals was studied under normothermic conditions (control) for the duration of the study. Piglets showed typical changes in sleep state patterns characteristic of rapid maturation over the first 10 days of development. It was found that both the amount of rapid eye movement (REM) sleep and, in some cases, the duration of REM episodes increased in response to facial cooling regardless of rectal temperature. However, hyperthermia with warm air exposure caused a significant decrease in the amount of REM sleep but not in the duration of REM episodes. It is suggested that an infant placed to bed in a cold room or exposed to a draft might also experience a greater amount of REM sleep than an infant placed to sleep in a warm draft-free room.

Analysis of Variance↗
PubMed1993

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.

Asphyxia↗
PubMed1993

Apnea and rapid eye movement sleep excess in the piglet during recovery from hyperthermia.

This investigation tested the hypothesis that artificially induced mild hyperthermia and recovery from hyperthermia in the developing newborn piglet (2-10 d of age) alter sleep state patterns and respiratory control. Continuous measurements of sleep state, respiratory pattern, carbon dioxide production and oxygen consumption were made before, during, and after a 3-h period of sustained hyperthermia. During hyperthermia, rectal temperatures were raised a mild 1-1.5 degree C above normal, well below the levels likely to cause severe physiologic distress in this species. This resulted in a disruption of the sleep state pattern characterized by a decrease in duration of rapid eye movement (REM) episodes, whereas immediately afterwards, during recovery, the amount of REM sleep increased. In some cases the amount of REM sleep in recovery more than doubled the basal level. Apneas were rarely observed during hyperthermia, but in recovery there was an increase in the total amount of time spent in apnea in both REM and non-rapid eye movement sleep states with a predominance in the REM state. Arousal responses to chemostimulation were not affected at this time. We conclude that the sleeping newborn piglet does indeed show marked changes in sleep state pattern (particularly REM sleep) and in the amount of apnea recorded during and immediately after only a mild increase in core temperature.

Aging↗
PubMed1992

Oxygen consumption in the newborn piglet during combined cold face/hot body exposure.

There is increasing evidence that hyperthermia can occur in cool climates because of overwrapping and that this practice precedes many deaths labelled as SIDS. We have attempted to test the hypothesis that the interaction of a cold face and a hot body might lead to further hyperthermia in a piglet model. Twelve non-sedated newborn piglets were studied over the first 10 days of development. Oxygen consumption was measured continuously during sleep. Animals were exposed to cold face conditions initially while the animal's body was kept warm and then while the body was hyperthermic. The results show that stimulation of the face with cool ambient air during conditions of raised metabolic activity (hot body) causes a fall in oxygen consumption towards basal levels. These studies do not, therefore, support the hypothesis that a further increase in metabolic rate occurs during combined cold face and hot body exposure in the piglet model.

Animals↗