Relationship Between Invasive and Noninvasive Measurements of Gas Exchange in Anesthetized Infants and Children

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Abstract
Minute ventilation (.ovrhdot.E), idal volume (VT), carbon dioxide elimination (.ovrhdot.VCO2), and end-tidal (PETCO2) and arterial CO2 tensions (PaCO2) were measured in 39 anesthetized infants and children with body weights ranging from 3.1 to 31 kg. Eighteen children had normal cardiopulmonary function, seven had acyanotic congenital heart disease, and 11 had cyanotic congenital heart disease. One child had left heart failure and pulmonary congestion, and two had severe parenchymal lung disease. To evaluate differences between pulmonary gas exchange calculated from PaCO2 versus PETCO2, dead space volume (VD) and alveolar ventilation (.ovrhdot.VA) based on a PaCO2 (VDa, .ovrhdot.VAa) as well as on PETCO2 (VDET, .ovrhdot.VAET) were performed, and correlations between PaCO2 - PETCO2, VDa/VT = VDET/VT, and .ovrhdot.VAa - .ovrhdot.VAET were carried out. It was demonstrated that in normal children, as well as in those with acyanotic congenital heart disease, PETCO2 correlated closely with PaCO2 (r = 0.94, 0.98, respectively). In children with cyanotic congenital heart disease, however, correlation between PETCO2 and PaCO2 was relatively poor (r = 0.61). Mean values for PaCO2 were significantly higher than PETCO2 in the cyanotic children (P < 0.01), resulting in significant underestimation of physiologic dead space (P < 0.05) and significant overestimation of alveolar ventilation (P < 0.01). In three patients with pulmonary disease, large differences between PaCO2 and PeTCO2 were comparable with those observed in the children with cyanotic congenital heart disease. It is concluded that in normal children and in children with acyanotic heart disease during anesthesia, noninvasive measurement of PETCO2 can be used as a reliable estimate of PaCO2 and for calculations of physiologic dead space and alveolar ventilation. In children with cyanotic congenital heart disease and severe pulmonary disease, PETCO2 does not provide a precise estimate of PaCO2, and calculations of physiologic dead space and alveolar ventialtion should be based on direct measurements of PaCO2.