Hypercapnic and hypoxic ventilatory and cardiac responses in school-aged siblings of sudden infant death syndrome victims.

Siblings of sudden infant death syndrome (SIDS) victims have been shown to have abnormal ventilatory patterns and altered responses to respiratory stimuli during infancy. To evaluate whether these abnormalities persist, we studied ventilatory responses in 20 older SIDS siblings (9.8 +/- 0.9 (mean +/- SEM) years of age) and 20 control subjects (10.2 +/- 0.9 years of age). To evaluate hypercapnic ventilatory responses, we had subjects rebreathe 5% carbon dioxide and 95% oxygen until end-tidal carbon dioxide tension reached 65 mm Hg. Instantaneous minute ventilation, mean inspiratory flow, and respiratory rate were calculated breath by breath. Hypercapnic responses did not differ between SIDS siblings (2.08 +/- 0.14 L/min per mm Hg) and control subjects (1.90 +/- 0.10 L/min per mm Hg; not significant). To assess hypoxic ventilatory responses, we asked subjects to rebreathe 13% oxygen and 7% carbon dioxide, with the balance nitrogen, at mixed-venous end-tidal carbon dioxide tension, until arterial oxygen saturation by pulse oximetry fell to 75%. No differences in hypoxic ventilatory responses were found between the SIDS siblings (-1.39 +/- 0.15 L/min/% saturation) and the control subjects (-1.22 +/- 0.17 L/min/% saturation; not significant). The mean inspiratory flow, tidal volume, respiratory rate, and heart rate responses to hypercapnia and hypoxia were also similar in the two groups. We conclude that there is no difference in hypercapnic and hypoxic ventilatory and cardiac responses, as assessed by rebreathing techniques, between school-aged SIDS siblings and control subjects. We speculate that in SIDS siblings the control of breathing is immature during infancy and that they achieve maturity of control and resolution of breathing abnormalities with time.