Respiratory frequency and artifact affect the capnographic baseline in infants

We sought to determine the effect of rebreathing on the capnographic waveform baseline. In anesthetized infants, we studied the effect of respiratory frequency (f) and breathing circuit type (Bain, n = 6, and pediatric circle, n = 4) on capnography of respiratory gas aspirated from the circuit for mass spectrometry (PCO2asp) and flowing through an infrared analyzer (PCO2f-t). As f increased, measured values of PiCO2asp and PiCO2f-t increased in both Bain and circle groups, with the exception of PiCO2f-t values that remained zero in the circle group. PETCO2 decreased as f increased in the circle groups, but remained constant in the Bain groups. These data suggest that artifact, most likely due to parabolic distortion of CO2 plugs traversing long sampling catheters, makes up a significant percentage (8%-36%) of the aspiration capnographic baseline elevation depending on f and breathing circuit type. Despite increases in PiCO2 as f increased, PETCO2 does not increase in Bain circuits due primarily to an increase in minute ventilation (Ve) that offsets the increase in the PiCO2 to provide balance in the CO2 mass relationship (PETCO2 approximately VCO2/Ve+PiCO2). These findings are useful in the correct interpretation of elevated capnographic baseline in infants.