Arterial-to-End-Tidal Carbon Dioxide Tension Difference in Children With Congenital Heart Disease

Objectives: This study estimated the arterial-to-end-tidal carbon dioxide tension difference (ΔPaCO2-PE′CO2) in children with congenital heart disease; evaluated whether hyperventilation can reduce this difference; and analyzed the relationship between the difference and the oxygen saturation (SaO2) and hemoglobin level.Design: Prospective clinical study.Setting: Tertiary health care center.Participants: One hundred patients scheduled for correction of their underlying cardiac defect with either right-to-left or left-to-right intracardiac shunts were divided into 4 groups (n = 25 each): (1) N1, cyanotic with severe pulmonary artery hypertension; (2) N2, cyanotic with normal or decreased pulmonary artery pressure (PAP); (3) N3, acyanotic with normal or mild increases in PAP and severe increases in pulmonary blood flow (PBF); and (4) N4, acyanotic with normal PAP and normal or mild increase in PBF.Interventions: All the patients received the same anesthetic regimen. The initial settings for tidal volume, respiratory rate, and inspiratory-to-expiratory (I:E) ratio were 10 mL/kg, 15 to 30 breath/min, and inspired time 40% of the total respiratory period with a 10% end-inspiratory pause. After the measurement of oxygen saturation, PO2, Hb, and ΔPaCO2-PE′CO2, all the children were hyperventilated (tidal volume: 14-15 mL/kg, respiratory rate: 5-6 breaths/min more than the initial rate, I:E ratio: same) to observe its effects on the ΔPaCO2-PE′CO2.Measurements and Results: The ΔPaCO2-PE′CO2, when predicted from the oxygen saturation, hemoglobin concentration, and PaO2, was found to be greater than the observed value in the first 3 groups (p < 0.001); whereas in group N4 these 2 values were comparable. It was also found that the gradient was higher when there was a decrease in SaO2 and an increase in the hemoglobin level. After hyperventilation, in groups N1 and N3, ΔPaCO2-PE′CO2 was decreased when compared with their baseline values; this reduction was not as much as predicted (p = 0.363 and 0.236, respectively). However, in groups N2 and N4 posthyperventilation, the ΔPaCO2-PE′27 CO2 was decreased significantly below their baseline measurements. These decreases were as much predicted.Conclusion: The ΔPaCO2–end-tidal carbon dioxide (PE′CO2) can be increased both in cyanotic and acyanotic children. Increased PAP is as important as increased PBF or right-to-left shunting in producing disorders in carbon dioxide homeostasis. Hyperventilation is of little use in reducing ΔPaCO2-PE′CO2 in children with high PAPs and pulmonary hyperperfusion.