All infants will be assessed at the pediatric department at Haukeland University Hospital, and all data obtained by the same two investigators (Bentsen and Olsen). The infants will be dressed in the appropriate-sized vest selected according to the length from the armpit to the hip of the child. Care will be taken to ensure that the vest fits snugly around the torso and that the width of the copper wire zig-zag coveres the entire thorax and abdomen including the pubic region. The vest will be applied directly to the skin; neither body nor diaper will be worn underneath. The infants will then be fed before they will be placed supine in a cot and encouraged to sleep. No sedation will be used. Once the infant has fallen asleep, data will be collected with VSP for 2-3 minutes before the Exhalyzer D® neonatal facemask connected in series with the ultrasonic flow sensor will be placed gently, but firmly over the infant's mouth and nose, ensuring a good seal. Once the facemask is in place, a marker will be inserted in the VSP recording. The Exhalyzer D® measurement will be started about thirty seconds after the facemask has been put on, to allow adaptation to the facemask. Simultaneous recording using both devices will then be done for about 1 minute before the mask will be removed. New markers will be inserted in the VSP recording as the Exhalyzer D® measurement is started and stopped facilitating analysis of the same breathing cycles.