Interpretation of the krypton-81m dynamic series: the distribution of a tidal breath

Lung scans during cyclic breathing of krypton-81m, an isotope with a 13-s half-life, were acquired in "list mode," where both temporal and spatial information are preserved. Subjects in the left lateral decubitus position breathed with two tidal volumes at each of two frequencies. Profiles of total activity over the acquisition period were examined. They showed little effect of frequency or tidal volume on the distribution of air between dependent and non-dependent regions. Dynamic series for ensemble-averaged breaths were constructed. The regional flow per unit volume was shown to correspond to the time derivative of the regional activity of the dynamic series divided by the corresponding activity. Both the relative timing of the gas flow to different lung regions and the flow per unit volume as a function of time were obtained from the dynamic series. The dependent lung was seen to be better ventilated throughout the respiratory cycle except for brief periods at the start of inspiration and the end of expiration. Most of the dead-space gas can be construed to enter and leave the dependent lung.