Effects of 'cardiogenic oscillations' on alveolar plateau gas concentration slope measurements, constant expiratory pulmonary capillary blood flow, and DLCO determination have not been previously described. We examined cardiogenic oscillations during constant expiratory maneuvers to assess factors influencing magnitude of oscillations as well as effect of oscillatory phase at the start and end of exhalation measurement period on alveolar gas slope. Five normal volunteers performed repeated single breath constant exhalation vital capacity maneuvers using test gas containing 2 physiologically 'inert' gases: Helium (He Mw 4) and argon (Ar Mw 40). The mixture contained 3 absorbable gases, acetylene (C2H2 Mw 26), carbon monoxide (C18O Mw 30), and oxygen. Alveolar plateau slope, magnitude of cardiogenic oscillations, relative signal to noise ratios, and effect of cardiogenic oscillation phase on measured slope were determined for each gas. Cardiogenic oscillations were present for all inert gases. Oscillations were less evident for CO. However, the effects on calculated Qc and DLCO were negligible. Cardiac oscillations of considerable magnitude are seen during single breath constant exhalation maneuvers and affect constant expiratory gas slope calculations. Cardiogenic oscillation phase does not have a significant effect on measured Qc and DLCO using constant expiratory techniques.