Validity of the lagged normal density function as a model for pulmonary indicator dispersion.

The aim of this study was to investigate whether the lagged normal density function is a useful model for the dispersion of intravascular and diffusible indicators in the lungs. In 18 mongrel dogs anesthetized with N2O-piritramide, 221 sets of thermal-indocyanine green dye kinetics were recorded in the pulmonary artery and in the aorta after central venous indicator injection. A model-free deconvolution technique was used to compute the pulmonary transport functions for dye and heart from the measured indicator kinetics (reference method). The lagged normal density function was used to model pulmonary indicator transport. Its parameters were computed by a nonlinear least-squares procedure by iterative convolution. After baseline measurements in nine dogs, pulmonary edema was induced by central venous application of oleic acid. In nine other dogs, measurements were performed before and after postural changes from the horizontal to the vertical position. The mean transit times derived from the lagged normal density function were in good agreement with those obtained after model-free deconvolution. Although the shape (relative dispersion, skewness) of the transport function is less well described by the model, the authors conclude that the lagged normal density function is useful to determine indicator volumes of distribution that require only the correct mean transit times.