VCO2 was estimated by using Brody's equation for humans aged up to 36 months, (VCO2 = 5.56*(wt)), where weight is in kilogram. The relationship between the fraction of alveolar CO2 (FaCO2) and dead space volume (Vd) was derived from the Bohr equation, FaCO2 = VCO2/(RR*(Vt - Vd)), where VCO2 is CO2 production, RR is respiratory rate, and Vt is tidal volume. Apparatus volumes for typical devices were obtained from the manufacturer or measured by the volume of water required to fill the device. The effect of changes in dead space on arterial CO2 (PaCO2) and required minute ventilation were calculated for patients weighing 2 to 17 kg that corresponds to 0 to 36 months of age. Children are especially vulnerable because small increases in apparatus dead space can significantly increase dead space to tidal volume ratio (Vd/Vt). Apparatus dead space increases dead space volume, causing either increased PaCO2 or the need to increase minute ventilation to maintain normocapnia.
Physiologic dead space is defined as the volume of the lung where gas exchange does not occur.
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