( 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 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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