Baby Alert... has a special microphone to be placed under a cot mattress, which sends out an alarm if the child's heart stops beating for four seconds, or if breathing fails for 15 seconds.According to one recent newspaper report, the alarm, has been shown to "wake the child and prompt breathing", and could dramatically cut the number of fatalities from sudden infant death syndrome, or "cot death".
Some are attached to the baby's tummy; others, like the Baby Alert, use a special pressure pad or mattress, while one wall unit beams out ultrasound waves above the cot.
All these devices work by detecting changes in breathing movements in the chest and abdomen. When breathing stops for longer than a pre-set time (usually 20 seconds), an alarm will ring and a light start to flash. Some monitors also measure temperature and heartbeat.
quoted from The truth about baby breathing monitors
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