Physiologic Dead Space, Venous Admixture, and the Arterial to End-tidal Carbon Dioxide Difference in Infants and Children Undergoing Cardiac Surgery

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Abstract
End-tidal CO2 (PETCO2), arterial CO2 (PaCO2), mixed expired CO2 (PECO2), arterial and mixed venous oxygen contents were measured and the PaCO2 to PETCO2 difference (.DELTA.PCO2), physiologic dead space to tidal volume ratios (VD/VT) and venous admixture (Qs/Qt) were calculated in 41 anesthetized infants and children undergoing repair of congenital cardiac lesions. Eighteen children were acyanotic; 9 with normal pulmonary blood flow (PBF) and normal intracardiac anatomy (normal group); and 9 with increased PBF (acyanotic group). Twenty-three children were cyanotic; 14 with right to left intracardiac shunts and decreased PBF (cyanotic (D) group); and 9 with mixing lesions with normal or increased PBF (cyanotic (I) group). Correlations between PaCO2 and PETCO2 in the four groups of children were carried out and the relationship of .DELTA.PCO2 to VD/VT and Qs/Qt was determined. PETCO2 correlated closely with the PaCO2 in the normal and acyanotic groups (r2 = 0.97 and 0.91, respectively) and the lines of regression for the relationship between PaCO2 and PETCO2 for both groups did not differ from the line of identity (P .ltoreq. 0.05). Mean .+-. SD VD/VT for the normal and acyanotic groups were 0.35 .+-. 0.17 and 0.39 .+-. 0.19, respectively (NS). Corresponding values for the cyanotic (D) group and cyanotic (I) group were 0.38 .+-. 0.16 and 0.55 .+-. 0.16, respectively (NS), and were significantly greater than those from the normal and acyanotic groups (P < 0.05). The relationship of .DELTA.PCO2 to VD/VT and Qs/Qt demonstrated that VD/VT was the most important determinant of .DELTA.PCO2, but in instances where Qs/Qt were large (e.g., cyanotic congenital heart disease) the percentage contribution of Qs/Qt to the .DELTA.PCO2 can be considerable. It is concluded that PETCO2 is an acceptable estimate of PaCO2 in subjects with acyanotic congenital heart disease but underestimates the PaCO2 in subjects with cyanotic congenital heart disease.