Peds circuit, work of breathing


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.

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.


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