THE AUTONOMIC NERVOUS SYSTEM IN EARLY LIFE

Sudden infant death syndrome--a defect in circulatory control?

The majority of Sudden Infant Death Syndrome (SIDS) infants die during sleep and especially during the overnight sleep period. Recent evidence from SIDS cases, which occurred while on a cardiorespiratory monitor at home, has suggested that the mechanism of death involves circulatory failure, with the development of a shock like state, associated with a progressive bradycardia in the presence of continued breathing movements. In this paper we explore the circulatory effects of sleep and in particular the down regulation of the baroreceptor reflex, associated with a reduction in vasomotor tone and a fall in central venous return, cardiac output and blood pressure. This sequence of events would be exacerbated by many of the known SIDS risk factors, namely the prone sleeping position, overheating and co-sleeping. Poor central venous return,with diminished cardic distension could induce a progressive bradycardia as occurs in adults with neuro-cardiogenic syncope. Alternatively a reduced cardiac output could result in the rapid onset of severe hypoxia through poor lung perfusion. The effects of sleep on circulatory control deserve further study in infants.

Effect of prone sleeping on circulatory control in infants

BACKGROUND

The mechanism of death in sudden infant death syndrome (SIDS) remains unclear. Progressive bradycardia is the pre-eminent terminal event, suggesting that circulatory failure might be a crucial factor. Vasomotor tone regulates the circulatory system by controlling blood volume distribution while maintaining venous return and blood pressure.

AIM

To examine whether prone sleeping, the most consistently identified risk factor for SIDS, has a measurable influence on vasomotor/circulatory control.

METHODS

44 full term infants (mean age, 7.9 weeks) were studied during an overnight sleep. Recordings were made while the infants were horizontal and asleep in the supine and prone positions, and repeated after a head up tilt to 60 degrees, maintained for 30 minutes, while in both sleep positions. Blood pressure, heart rate, anterior shin, and anterior abdominal wall skin temperatures were measured.

RESULTS

Systolic blood pressure was lower, but peripheral skin temperature and heart rate were higher during sleep, while horizontal, in the prone rather than the supine position. After tilting, there was a greater reduction in blood pressure and a greater increase in peripheral skin temperature and heart rate when in the prone position. Anterior abdominal wall skin temperature did not vary in either sleeping positions while horizontal or tilted.

CONCLUSION

Prone sleeping has a measurable effect on circulatory control, with a reduction in vasomotor tone resulting in peripheral vasodilatation, a higher peripheral skin temperature, a lower blood pressure, and a higher resting heart rate. Because vasomotor tone is crucially important in circulatory control this could be a factor in increasing the risk of SIDS.

Cardiorespiratory adaptation during sleep in infants and children

The cardiorespiratory control system undergoes functional maturation after birth. Until this process is completed, the cardiorespiratory system is unstable, placing infants at risk for cardiorespiratory disturbances, especially during sleep. The profound influence of states of alertness on respiratory and cardiac control has been the focus of intense scrutiny during the last decade. The effects of rapid-eye movement (REM) sleep on various mechanisms involved in cardiorespiratory control are of particular significance during the postnatal period since newborns spend much of their time in this sleep state. In fullterm newborns, REM sleep occupies more than 50% of total sleep time, and this percentage is even greater in preterm newborns. From term to six months of age, the proportion of REM sleep decreases. Since respiratory and cardiac disturbances are known to occur selectively during REM sleep, the predominance of REM sleep may be a risk factor for abnormal sleep-related events during early infancy. Awareness of these developmental changes in sleep patterns is important for clinicians dealing with problems such as apparent life-threatening events (ALTE), sudden infant death syndrome (SIDS), and/or cardiorespiratory responses to respiratory disorders. Our current understanding of respiratory and cardiac control rests mainly on studies conducted during the first months of life. There is a paucity of data on late infancy and early childhood. The present paper will review available data on how sleep affects 1) ventilatory mechanics, in particular of the upper airways and the chest wall; ventilation and apnea; gas exchange; chemoreceptor function; and arousal responses; 2) changes in heart rate and heart rate variability, and the occurrence and mechanisms of bradycardia.

Snuffles in infants--infection or autonomic dysfunction

The presence of excess nasal mucus causing noisy nasal breathing with an obvious mucus discharge (snuffles) is a common problem in infants in the first three months of life. The presence of "snuffles" has traditionally been ascribed, unproven, to an upper respiratory tract infection despite there being no other signs of an acute infection in the majority of infants with "snuffles". To assess the possible role of impaired vasomotor control (autonomic function) in the pathogenesis of snuffles we measured the effect of a change from the supine to the upright position on resting blood pressure in 50 infants with "snuffles" and 50 healthy control infants. The mean age in both groups was 7 weeks post delivery, all infants were attending a well baby clinic for a routine examination, had no signs of an acute infection and none were on any medication (including nasal drops). A fall of greater than 10% of resting blood pressure was taken to indicate postural hypotension. Four of fifty infants in the control group compared to 22 of 50 in the snuffles group demonstrated postural hypotension (Chi square 16.84, p less than 0.001). The results suggest that in some infants "snuffles" may be associated with impaired vasomotor control.

Trigeminal airstream stimulation. Maturation-related cardiac and respiratory responses during REM sleep in human infants

Thirty-three premature and full-term infants (PCA, 31.5 to 50 weeks) who were free from neurologic and cardiopulmonary disease at the time of testing underwent a standardized TAS test during polygraphically controlled REM sleep. The R-R interval and the TTOT were measured before and during TAS. The R-R interval and TTOT changes during TAS were compared to the preceding 60-second mean R-R interval and TTOT in each infant and expressed as a percentage of mean control values (ie, % RR and % TTOT). During TAS, there was a significant negative correlation between cardiac and respiratory responses and postconceptional age (p less than 0.001 and p less than 0.0001, respectively). Prolongation of both the R-R interval and the TTOT elicited by TAS was significantly blunted by maturation.

The heart rate of Romano-Ward syndrome patients

The Romano-Ward prolonged QT syndrome is associated with an abnormality of ventricular recovery, rendering the patients susceptible to life-threatening ventricular arrhythmias. The pathophysiology has not been clearly defined. It has been proposed that the syndrome involves disparity of right and left cardiac sympathetic activity, with the left side dominant. This could be the result of left-sided overactivity or right underactivity. The right-sided nerves contain the chronotropic fibers, which affect heart rate. A deficiency of the effect of the right-sided nerves might, therefore, be manifest by slower heart rates. To examine this possibility, the resting heart rates of 58 Romano-Ward syndrome patients were compared to those of 255 age-matched normal controls. A significant difference in resting heart rate was observed between the Romano-Ward syndrome patients and the normal controls in newborns and children up through age 3 years. No difference was present in older children or in adults. The data are consistent with right-sided sympathetic deficiency manifest by a slower heart rate at birth and during early years of life, when sympathetic tone is high and contributes to resting heart rate, but not in older children or in adults in whom resting heart rate is principally under parasympathetic control.

Pain in infancy: conceptual and methodological issues

The concept of pain in infancy was explored. It was suggested that our every day usage of the term pain be applied to infants. Essentially this suggestion is that we infer that an infant has a subjective experience that is unpleasant when there is evidence of tissue damage and the infant responds with signs of distress, such as crying, increased heart rate, facial expression consistent with distress or other signs. Conceptualizations of adult clinical pain and infant emotions were compared. It was proposed that our concept of infant pain should be multivariate and should include the ecological context in which pain occurs. In section 2 methodological issues were discussed. Ethical concerns for the protection of infant rights were voiced. Numerous opportunities for studying infant pain produced by necessary medical procedures were pointed out. A number of response systems which offer promise in understanding infant pain were reviewed with respect to their proven applicability to infant emotional and cognitive processes or to adult pain processes. This paper attempted to organize information which will be helpful to the researcher interested in infant pain or developmental processes in pain. From the preceding it should be obvious how remarkably little data we have which bears directly on the issue. The close association between the study of pain and the study of emotion has been shown. The study of emotional development, while ahead of the study of developmental aspects of pain, still lags far behind the study of cognitive development. Recently there has been a call for increased study of the development of emotion. The study of the development of pain might well proceed hand in hand. The present state of the field of infant pain is such that almost any data will add to our knowledge base. The complexity of the concept and the wide range of response systems would suggest that multidisciplinary research teams may develop some of the best research efforts. A multidisciplinary approach has been found to be essential in the treatment of chronic pain, for example. A listing of some of the types of professionals who would contribute would include: physicians, nurses, psychologists, biologists (including those trained in neurophysiology, ethology and endocrinology), sociologists and anthropologists. University medical centers seem an ideal environment for conducting such research, given the close proximity of various specialists and an available subject population.

1028 neonatal electrocardiograms

Electrocardiograms were recorded from 1028 (99.4%) of 1034 consecutively born babies on the 1st or 2nd day of life. Abnormalities of cardiac rhythm or conduction were found in 49 (4.8%) babies. 17 babies had single or multiple premature beats during the recordings, and 7 babies showed sudden increases in R-R interval possibly due to sinoatrial node dysfunction. One baby had sinus or junctional bradycardia of less than 80 beats/min. One baby had an incessant reciprocating tachycardia and subsequently required digoxin for heart failure. Continuous 24-hour ECG monitoring in 25 babies with abnormalities of the screening ECG, and 25 babies without such abnormalities, failed to show any additional tachyarrhythmias or bradyarrhythmias warranting treatment. In view of the uncertain clinical significance of many neonatal arrhythmias and cardiac conduction disorders, more information concerning their natural history and relationship to sudden unexpected death in infancy is needed before recommending that neonatal ECG screening be generally adopted.

Quantitative analysis in the oxytocin challenge test

A quantitative analysis of the oxytocin challenge test (OCT) was carried out in 305 women to evaluate: (1) the intrauterine pressures evoked during the oxytocin infusion, (2) when fetal heart rate (FHR) decelerations are likely to occur, (3) the type and amount of decelerations which are significant, and (4) what clinical events and laboratory parameters correlate with the OCT. If contractions are evoked at a rate of three per 10 minutes, the pressure profile mimics normal labor. The first FHR deceleration is likely to occur between the second and third effective contraction. A deceleration/contraction ratio greater than or equal to 10% significantly forecasts an abnormal FHR during parturition and an increased chance of a cesarean section.

Cardiac slowing and respiratory arrest in preterm infants

Changes in respiration and heart rate during sleep states have been recorded by a polygraphic device in healthy preterm infants. Cardiac slowing/bradycardia often coincide with respiratory arrest/apnea. Bradycardia starts early during apneic spells. The incidence of respiratory arrest and cardiac slowing and their simultaneous occurrence is significantly increased by the active or REM sleep state. The physiologic, inhibitory mechanisms of active sleep suggest a neurogenic etiology of episodes of cardiac slowing/bradycardia and/or respiratory arrest/apnea in prematures.

Paediatric pharmacology

Infants and children differ pharmacologically from adults because the progressive maturation of various systems during development leads to variations in drug action and dosage. These differences are reviewed. Pharmacogenetic factors which affect anaesthesia and the problems of drug overdosage, as seen in paediatric intensive care units, are briefly considered.

Autonomic failure in hydrencephaly

Autonomic functions were studied in three patients with hydrencephalus and five with hydrocephalus. Autonomic failure of central origin was found in the patients with hydrencephalus; whereas, those suffering from hydrocephalus had essentially normal autonomic function. In two patients with hydrencephalus, the hypothalamus was markedly abnormal but the rest of the autonomic nervous system was histologically normal. From this it is concluded that in some patients with mental and motor retardation, autonomic failure may be of cerebral origin but that this is not a feature of patients with hydrocephalus.

Autonomic failure with persistent fever in cerebral gigantism

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