1. The cardiogenic oscillations in the concentration of nitrogen and argon in expired gas, which are seen after the inspiration from residual volume of pure oxygen or a bolus of argon, have been studied in normal subjects and in patients with atrial fibrillation.

2. In the upright position of the subject, the peaks of nitrogen and argon concentration coincide with ventricular systole, although there is a transit delay along the tracheobronchial tree before any change of concentration occurs at the mouth.

3. Studies in different postures demonstrated that the effect of the heart is predominantly due to a reduction in cardiac volume during systole with a decrease in airflow from regions adjacent to the heart. These areas contain a different concentration of nitrogen and argon in the alveolar air from the more apical regions.

4. It has been shown that the results are consistent with the present concepts of the influence of gravity on the distribution of inspired air.

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