Postinspiratory mixing in the lung and cardiogenic oscillations

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Abstract
Subjects inspired a 300 ml bolus of indicator gas cocktail (5% each of SF6, Ar, Ne and He) from residual volume (RV), then inspired air to functional residual capacity (FRC). There was no evidence that a 10-s breath hold changed the relative concentrations or amounts of indicator gases in phases III and IV of expiration or allowed additional gas to mix into the RV but the breath hold caused cardiogenic oscilalations (CO) in expired gas to decrease in height. The units responsible for cardiogenic troughs and peaks are different from the units responsible for phases III and IV, respectively, in that the oscillation troughs had a lower He/SF6 ratio than the peaks; phase III had a higher He/SF6 than phase IV. The CO is due to variation in mechanical properties, leading to variation in response to the pressure wave caused by the heart, in units that are relatively near to each other. There is little or no postinspiratory mixing between distant lung units, but the dampening of CO suggests that units that are close to each other can mix if time is allowed.

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