The effects of sleeping position on the ventilatory response to hypercarbia

Abstract

Background

Prone sleeping is an established risk factor for Sudden Infant Death Syndrome (SIDS), however the mechanism by which this contributes to vulnerability remains unclear.

Aims

To determine whether prone sleeping impairs the ventilatory response to hypercarbia in newborn infants.

Methods

Infants born at 36 weeks post-menstrual age or greater were recruited from the postnatal ward. An open circuit, consisting of a non-return valve, capnograph, and pneumotachograph, supplied gas at 4 litres/minute to a nasal mask. In both the prone and supine position baseline ventilation was measured for five minutes with the infant breathing air. The ventilatory response to either 2% or 4% carbon dioxide (CO₂) enriched air was then measured for five minutes. Following five minutes recovery the infants was exposed to the alternative CO₂ level, the order of gas exposure being randomised. Mean minute volume was calculated for the final minute of exposure at each gas level, and CO₂ sensitivity calculated as the gradient of the line of best fit. The time constant, defined as the time taken to achieve 63% of the maximal response to 4% CO₂ was also calculated.

Results

Thirty-one infants infants (median gestational age 39 weeks, range 36-42 weeks) were studied. There was no significant difference in the CO₂ sensitivity between positions. The time constant of the response to CO₂ was significantly longer in the prone (median 70 seconds, range 5-233 seconds) compared to the supine position (median 34 seconds, range 8-173 seconds) (p=0.031).

Conclusion

Our results suggest that a damped ventilatory response to hypercarbia may contribute to vulnerability to SIDS of infants sleeping in the prone position.