Young infants should not be left unattended to sleep in standard car safety seats, warn researchers in this week’s BMJ.

Infant car safety seats are vital to protect young infants from injury and death in motor vehicle accidents, write Professor Alistair Gunn and colleagues. However, preterm infants and full term infants with certain health conditions are at risk of oxygen desaturation and apnea (temporary suspension of breathing) while they are restrained in recommended semi-reclining infant car seats. Studies have also shown that mild oxygen desaturation can affect some full term infants, although others found no effect.

To investigate this further, researchers in New Zealand examined infants referred to the Auckland Cot Monitoring Service between July 1999 and December 2000 after an apparently life threatening event.

Nine infants (aged 3 days to 6 months) had been left to sleep restrained in a car safety seat appropriate for their age. One infant was preterm; the remainder were full term ,and all infants were completely normal on examination.

Infants were described as blue, scrunched up, and not breathing when the events occurred. When the scene was reconstructed, using the infant’s own car seat, their heads bent forward with the jaw pressed down on the chest. This led to a narrowing of the upper airway and breathing problems.

All infants were given breathing monitors and the parents were given advice on appropriate positioning, including not leaving the infant for excessive periods in the car seat. None of the infants had any further reported problems over the next 12 months.

The frequency of leaving sleeping infants in car seats is low, say the authors. However, modifying car safety seats so that the head does not bend forward could avoid the risk of these events. Half the mothers in this study were smokers, which may also have had an effect, they add.

When used properly, child passenger restraints reduce injury by 90% to 95% for rear facing systems and 60% for forward facing systems compared with not using a restraint, writes Michael Hayes of the Child Accident Prevention Trust.