Apnea, small for date and autonomic imbalance : risk factors in relation to SIDS

Abstract: SIDS (Sudden Infant Death Syndrome) is defined as an infant who has died suddenly and its cause "remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history". An infant who during sleep has been found apneic, pale or cyanotic and limp, and has required either vigorous stimulation or cardiopulmonary resuscitation is defined to have been affected of an Apparent Life Threatening Event (ALTE). ALTE infants are assumed to have an increased risk of dying of SIDS. Another risk group are the Small for Gestational Age (SGA) infants. The aims of this thesis was to: 1) Compare the epidemiology of ALTE and SIDS infants. 2) Study the responses in HR and HRV (heart rate variability) to tilting and to C02 in ALTE infants. 3) Analyse the HRV in the time and frequency domain in ALTE infants. 4) Study the responses in HR (heart rate), blood pressure and HRV to tilting and to analyse HRV in SGA infants versus controls. Epidemiological data were obtained from 56 ALTE infants, 244 SIDS victims and compared with 56+868 controls. 18 ALTE infants were tilted and the HR response recorded. HRV was recorded in 50 ALTE infants during nighttime and was analysed in the time and frequency domain. HRV was analysed in 27 ALTE infants with Poincaré plot and mathematically during C02 challenge. HRV was studied 4 times and HR response to tilting 2 times during the first year of life in 15 SGA infants. Tilt test and measurements of heart rate variability (HRV) is widely used techniques for evaluation of sympaticovagal balance. Conclusions: 1) Our findings indicate that ALTE is essentially the same entity as SIDS, but ALTE infants who are exposed to less exogenous risk factors seem to have a reduced risk of dying in SIDS. 2) ALTE infants have a qualitatively altered heart rate response to tilt test. 3) The ALTE infants we studied, who all survived have an increased HRV, which might indicate a more active autonomic control helping them to survive the life threatening event. 4) ALTE infants, particularly ALTE boys, were found to have suboptimal function of the cardiovascular defense mechanisms during C02 challenge. 5) The impaired growth in fetal life seems to influence the autonomic control mechanism in SGA infants. Physiological tests like tilt test and HRV analysis may have a predictive value although this has to be tested in a larger cohort.