Physiologic dead space is defined as the volume of the lung where gas exchange does not occur. Apparatus dead space increases dead space volume, causing either increased Paco(2) or the need to increase minute ventilation to maintain normocapnia. Children are especially vulnerable because small increases in apparatus dead space can significantly increase dead space to tidal volume ratio (V-d/V-t). The effect of changes in dead space on arterial CO2 (Paco(2)) and required minute ventilation were calculated for patients weighing 2 to 17 kg that corresponds to 0 to 36 months of age. Apparatus volumes for typical devices were obtained from the manufacturer or measured by the volume of water required to fill the device. The relationship between the fraction of alveolar CO2 (Faco(2)) and dead space volume (V-d) was derived from the Bohr equation, Faco(2) = VCO2/(RR*(V-t - V-d)), where VCO2 is CO2 production, RR is respiratory rate, and V-t is tidal volume. VCO2 was estimated by using Brody's equation for humans aged up to 36 months, (VCO2 = 5.56*(wt)(1.05)), where weight is in kilogram. Initial conditions were V-t = 8 mL/kg, V-d/V-t = 0.3, and a RR of 20 breaths per minute. The relationship between Paco(2) and dead space was determined for increasing V-d. Rearranging the Bohr equation, the RR required to maintain Paco(2) of 40 mmHg was determined as dead space increased. The apparatus V-d of typical device arrangements ranged from 8 to 55 mL, and these values were used for the dead space values in the model. Paco(2) increased exponentially with increasing apparatus dead space. For smaller patients, the Paco(2) increased more rapidly for small changes in V-d than that in larger patients. Similarly, RR required to maintain Paco(2) of 40 mmHg increased exponentially with increasing dead space. Increasing apparatus V-d can lead to exponential increases in Paco(2) and/or RR required to maintain normal Paco(2). The effect on Paco(2) is less as patient weight increases, but these data suggest it can be significant for typical circuit components up to at least 17 kg or aged 36 months.
