( I ) PETCO2 as an estimate of PaC02
PETCO2 may reflect PaCO2 at a new steady state level if (a-ET)CO2 is determined via ABG
In neonates with mild to moderate lung disease (FIO2 < 0.3 and respiratory frequency < 70/min), the distal sampling of CO2 measurements are preferred to proximal measurements as the former reflect PaCO2 more accurately than the later.12,13 However, in children with severe lung disease even the distal PETCO2 may not be good predictor of PaCO2 because (a-ET)PCO2 gradients varies with changing V/Q relationship of the sick neonate thus making PETCO2 measurements less reliable.14 Under these circumstances, PtcCO2 is more accurate estimate of PaCO2.13 The emphasis here is on more ABG’s until V/Q mismatch improves and a more constant (a-ET)PCO2 relationship is established.
In infants and children with acyanotic heart disease (left to right shunt), PETCO2 is closer to PaCO2 and (a-ET)PCO2 gradient is not significantly different from children with normal circulation.15,16 Further PETCO2 is a reliable estimate of PaCO2.16 However in children with cyanotic heart diseases, PETCO2 underestimates PaCO2 and the (a-ET)PCO2 gradient is increased up to 15 mmHg due a combination of venous admixture and low pulmonary perfusion.17 Under these circumstances, (a-ET)PCO2 is linearly correlated with arterial oxygen saturation (SPO2).17 With a decrease in SPO2 by 10% caused by right to left shunt, the (a-ET)PCO2 gradient can be expected to increase by 3 mm Hg.17
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