RESPIRATORY FREQUENCY AND ARTIFACT AFFECT THE CAPNOGRAPHIC BASE-LINE IN INFANTS

We sought to determine the effect of rebreathing on the capnographic w aveform baseline. In anesthetized infants, we studied the effect of re spiratory frequency (f) and breathing circuit type (Bain, n = 6, and p ediatric circle, n = 4) on capnography of respiratory gas aspirated fr om the circuit for mass spectrometry (PCO2asp) and flowing through an infrared analyzer (PCO2f-t). As f increased, measured values of PiCO2a sp and PiCO2f-t increased in both Bain and circle groups, with the exc eption of PiCO2f-t values that remained zero in the circle group. PETC O2 decreased as f increased in the circle groups, but remained constan t in the Bain groups. These data suggest that artifact, most likely du e to parabolic distortion of CO2 plugs traversing long sampling cathet ers, makes up a significant percentage (8%-36%) of the aspiration capn ographic baseline elevation depending on f and breathing circuit type. Despite increases in PiCO2 as f increased, PETCO2 does not increase i n Bain circuits due primarily to-an increase in minute ventilation (V( e)) that offsets the increase in the PiCO2 to provide balance in the C O2 mass relatiOnshiP (PETCO2 approximately VCO2/V(e) + PiCO2). These f indings are useful in the correct interpretation of elevated capnograp hic baseline in infants.