Analysis of long-term cardiorespiratory recordings from infants who subsequently suffered SIDS

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.

Short-term rhythms of the cardiorespiratory system and their significance in neonatology

Latent disturbances in the control of respiration and heart rate (HR) may be important factors in the pathogenesis of life-threatening events during infancy. A method of determining the control of the autonomic nervous system functions involves the analysis of time-dependent ultradian changes of its parameters. The breathing signal and HR variability contain rhythmic components that are generated within the cardiorespiratory network of the brain stem, through reflexes, and by feedback mechanisms. The analysis of these components may provide insights into the functioning of the cardiorespiratory control system. The prominence and precision of the rhythms are correlated with states of vigilance and underlie distinct development during the first months of life. The results of studies on infants at risk (for example, for sudden infant death), with the help of statistical and spectral analysis of time series to obtain new indices, have proved to be inconsistent in their prognostic value of thus studied parameters. Recently, the importance of qualitative and quantitative assessment of the dynamic and complex behavior of time series, based on nonlinear characteristics of the control system, has been emphasized. To what extent, however, the analysis of the dynamic behavior can be utilized for clinical purposes, such as judging the prognosis of deficiencies in control, requires further study regarding physiological baselines and the possible changes resulting from pathological states.

Retarded heart growth in the victims of sudden infant death: possible implications for lethal mechanisms

The study was aimed to compare the weight of the heart at death in infants who died of sudden infant death (SID) and in control infants under 1 year old. The study included 227 infants who died in St. Petersburg in 1983 to 1990 and who met the criteria of SID. One hundred and thirty-eight infants who died within 24 h out of hospital of defined respiratory infections within the same period of time constituted a control group. Infants with signs of malformations, tumours, and intrauterine infections were excluded from the study. A linear regression model was used in data analysis. No statistically significant association was found between the weight of the heart by the time of death and the following variables: gender, birth weight and length, ponderal index at birth, gestational age, calendar age at death, and length at death. Two variables which significantly influenced the heart weight and entered the linear regression model were infant's weight by the time of death and the cause of death (whether SID or not). No statistically significant interaction was found for these two parameters. It was found that each increase in total body weight by 1000 g brought with it an increase in the heart weight by 4 g (95% CI = 3.6-4.4); for the SID babies, the weight of the heart was less than for the control ones by 1.6 g (95% CI = 0.3-2.9 g).

CONCLUSION

Retarded heart growth may lead to functional disturbances and may contribute to possible mechanisms in some SID cases.