To evaluate whether abnormalities of heart rate (HR) control may be detected in SIDS victims prior to their death. We retrospectively analysed Holter and cardiopneumographic recordings of 19 infants (11 m, 8 f) aged 2.3 +/- 1.5 months who subsequently died of SIDS at the age of 4.2 +/- 2 months. Each SIDS victim was matched for gestational and postnatal ages and reference diagnosis with 3 control infants without SIDS after at least one year follow up. Nine hours of recording (9 pm to 6 am) were processed in terms of mean HR and HR oscilations (osc). Mean HR was higher in the SIDS group (141 +/- 14 vs 135 +/- 15 bpm, p <0.05, covariance analysis with age. HR short oscilation respiratory sinus arrythmia: RSA) were lower in the SIDS group (3.35 +/- 0.59 vs. 3.65 +/- 0.61, p<0.05, covariance analysis with age). HR long oscilation (20 to 32 respiratory rate (RR) were similar in both infant groups (3.20 +/- 0.66 vs. 3.21 +/- 0.53) and were not correlated with age. The mean HR was modulated in the control group during the night, being lower at midnight and higher at 9 pm and 5 am. The mean HR in the SIDS group remained constant throughout the night. In an infant population at higher than normal epidemiological risk for SIDS, infants who subsequently died differed from those who did not die because the mean HR was higher in SIDS victims, and the short oscilation were absent. These results reflect a decrease in vagal tone in SIDS victims which can be absolute or relative to an increase in sympathetic tone. The fact that these differences between SIDS victims and control infants were especially observed in the middle of the night may express an absent or disturbed circadian organization of the waking-sleep rhythm and/or of the circadian modulation of the autonomous nervous system tone in SIDS victims.
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