Abstract

BACKGROUND Inspired CO₂ is a potent ventilatory stimulant exhibiting a paradoxical inhibitory effect on breathing at high concentrations. Severe respiratory depression as a result of CO₂rebreathing during sleep has been implicated as a possible trigger factor in sudden infant death syndrome (SIDS).

OBJECTIVE To investigate the ventilatory and heart rate (HR) responses to inhaled CO₂ in infants with apnoea of infancy, a group believed to be at increased risk of SIDS.

STUDY DESIGN Thirty one infants with severe sleep related apnoea, 31 infants with mild recurrent apnoea, and 31 age and sex matched controls for the infants with severe sleep related apnoea were studied. HR was computed from digitised RR intervals, “ventilation” was recorded by inductance plethysmography, and Pco ₂ and Po ₂ were monitored by transcutaneous electrodes. The ventilatory and HR responses to CO₂ were expressed as percentage increase in ventilation and change in HR/unit change in transcutaneous Pco ₂.

RESULTS The mean increase in transcutaneous Pco ₂ during CO₂ challenge (0.45 kPa = 3.4 mm Hg) resulted in a mean increase in ventilation of 291%/1 kPa (7.3 mm Hg) increase in transcutaneous Pco ₂, with no difference between the groups. A significant difference between infants with severe sleep related apnoea and mild recurrent apnoea versus controls (p < 0.02, p < 0.01, respectively) was found in their HR response to CO₂ challenge: HR decreased in 12 severe sleep related apnoea infants and 10 infants with mild recurrent apnoea, but only in two controls.

CONCLUSION Infants with apnoea of infancy frequently show a paradoxical decrease in HR during CO₂ challenge, possibly because of an insufficient ability to mobilise cardiovascular defence mechanisms when challenged with hypercapnia.