In children, the rib cage is oriented horizontally and is highly cartilaginous, moving inward during inspiration. This results in a decreased ability to effectively recruit and use accessory muscles, as well as increased reliance on the diaphragm to maintain tidal volumes. The pediatric diaphragm consists of few (10-25% depending on age) type I muscle fibers (slow twitch). Therefore, the diaphragm fatigues more easily from 1) increased work and 2) reduced type I fibers.
Minute ventilation is determined by metabolic demand. As demand increases, both tidal volume and respiratory rate increase when the child is spontaneously breathing. The primary compensatory increase is the respiratory rate.
During anesthesia of neonates and infants, ventilation may be controlled using a semi closed circle system because a circle system creates too much resistance for them to overcome (fatigable diaphragm and limited sustainable respiratory rate increase). The increased resistance in a circle system is primarily from unidirectional valves, breathing tubes, and carbon dioxide absorbers. Instead, a Mapleson D circuit or a Bain system should be considered in spontaneous breathing neonates <10 kg to avoid this issue. However, if positive pressure ventilation is used, the increased resistance of a circle system may be overcome and may be safely used in even the smallest neonates.