Ventilation and gas exchange during anaesthesia and surgery in spontaneously breathing infants and children.

Minute ventilation (VE) (ml min-1), respiratory frequency (f), mixed expired carbon dioxide fraction (FECO2) and end-tidal carbon dioxide concentration (E'CO2) (%) were measured, and alveolar ventilation (VA), deadspace (VD), deadspace/tidal volume ratio (VD/VT) and carbon dioxide output (VCO2) calculated in 58 anaesthetized, spontaneously breathing infants and children weighing 2.8-20.5 kg. Although minute volumes varied, tidal volume correlated well with weight (r = 0.83), with a mean tidal volume (+/- 1SD) of 5.2 +/- 1.2 ml kg-1. It was concluded that, by the use of mean VT + 1 SD (approximated to 6 ml kg-1) the fresh gas flow in ml min-1 should be set at 2.5 X 6 X kg X f (15 X kg X f) to avoid rebreathing in various T-piece systems in anaesthetized, intubated and spontaneously breathing infants up to a body weight of 20 kg. End-tidal carbon dioxide concentration was lower in younger patients who were premedicated with atropine alone than in the older ones who received opioid premedication also. Respiratory frequency, VD/VT and total VD per minute were higher in the younger age group, which explained the finding of a high VE in relation to VCO2 for these patients. This inefficiency of ventilation emphasizes the need to minimize apparatus deadspace in breathing systems used for small infants.