Cardiogenic mixing in the pulmonary conducting airways of man?

The increase in dispersion of a bolus of helium (He) or sulfur hexafluoride (SF 6) was used as a direct, non-invasive measure of pulmonary gas mixing in the conducting airways of one female and two male subjects. Mixing was compared for matched respiratory maneuvers at rest and during bicycle ergometer exercise at 35–50% V̇ O 2 max (tidal volume of 2 L, inspiratory flow rate ⩾ 1.5 L/s, mean expiratory flow rate of 2.8 L/s, and bolus penetrations of 30, 90, 150 and 180 ml). No enhancement of mixing occurred when the heart rate was increased 53% by bicycle exercise. The results suggest that cardiogenic mixing in the upper and central airways is unresponsive to normal changes in the heart rate.