Microspheres of different sizes, '-"'I-labeled antipyrine (I-Ap), and 4 -KCl or 86 RbCl were injected into the aortic inflow of isolated, Langendorff, perfused, nonworking dogs hearts at blood flows of 1.3-4.8 ml/min g~'. After 15 seconds to 5 minutes, the left ventricle was sectioned into about 300 ordered pieces, and the amount of each tracer was determined. For all tracers, the relative density of deposition was generally higher in the endocardial region, except in one heart in which the aortic pressure and the total coronary flow were low. The deposition of ^-K and that of I-Ap were essentially similar in three hearts over a large range of regional variation. This finding suggests either that both tracers were distributed in proportion to flow or that a small diminution in relative density of deposition of 42 K in high-flow regions due to lower transcapillary extraction was quantitatively similar to a decrease in the residual fraction of I-Ap in these same regions due to faster washout in the first 15-30 seconds after injection. Large microspheres were deposited preferentially in regions of high flow, exaggerating the apparent heterogeneity of regional flows. The distribution of the smaller microspheres was closer to that for I-Ap or 42 K.

Intrapulmonary distribution of ventilation/unit lung volume was studied in 28 volunteers in the sitting, supine, or right lateral decubitus position, either awake or anesthetized-paralyzed and mechanically ventilated. We found significant differences between the awake state and anesthesia-paralysis with mechanical ventilation in 1) intrapulmonary gas distribution, and 2) the vertical gradient of regional functional residual capacities for the subjects in the lateral decubitus position, but not for those in the sitting and supine positions. The effect of increasing the tidal volume on distribution of ventilation was significantly different 1) between the three body positions for a given state, and 2) between the two states for a given body position. The data suggest thoracoabdominal mechanics are different in the three body positions and that anesthesia-paralysis and mechanical ventilation may cause a different pattern of expansion of the respiratory system than spontaneous breathing in the awake state.

Lung transport functions (distributions of circulatory transit times across the lung) were characterized in four anesthetized dogs at various levels of mean pulmonary blood flow. The central circulation was found to approximate a mathematically linear, time-invariant system when respiratory frequencies were maintained at 40/min or more. Lung transport functions were obtained from 144 pairs of lung-input and lung-output dilution curves using a lumped-parameter model and an iterative convolution technique. Average relative dispersion (standard deviation of the transport function divided by mean transit time) was 0.46, about twice that found previously for segments of arteries. The relative dispersion tended to increase as the mean transit time increased, suggesting that the dispersing mechanism of the lung is dependent on the mean transit time (volume/blood flow). Differences between these results and those of single-vessel transport function studies can be resolved by considering the lung as a parallel-pathway system. It is hypothesized that, as total pulmonary blood flow increases, the pathways become more equally perfused and the relative dispersion of the lung decreases. Keywordsindicator-dilution method; central blood volume; circulatory mixing; indocyanine green; cardiac output THE TRANSPORT FUNCTION, h(t), of a segment of the circulation is the probability density function of transit times from the entrance to the exit of that segment (11,19). Because the bolus of indicator becomes dispersed in its traversal of a segment of the circulation, any rapid fluctuations in concentration at the entrance are diminished in amplitude or slurred out during passage through the system; therefore, the system acts as a low-pass filter. It is possible to characterize this filter in mathematical terms-in terms of a model (3,16,20), in terms of nonparametric description as given by the transport function itself (9), or in terms of Fourier components (6).When there is only one pathway between the upstream end of the segment and the downstream end, as in an artery, the dispersion of h(t) is due to the velocity profile and to turbulence or eddies in the stream. Such transport functions have been defined for a segment of artery of the human leg (1) and also for the aorta of the dog (2). It was found that the dispersion occurring in these segments was unaffected by variations in flow rate over a wide range.In the pulmonary vascular bed there are multiple parallel pathways and different regional perfusion rates, and it is possible that the regional pulmonary vascular volume is dependent on flow rate. The purposed of this study were to characterize the transport function of the dog's lung at various mean blood flow rates and to test the effect of changes in flow and in

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Following left atrial injection of indocyanine green in closed-chest, anesthetized dogs, 60 simultaneous input-output pairs of dilution curves were sampled via identical catheter sampling systems from the aortic root, C in (t), and the coronary sinus, C out (t). Assuming that C out (t) was the convolution of a transport function, h(t), and C in (t), a new deconvolution technique was used to solve for the h(t)'s which was not sensitive to noise, recirculation, or the form of h(t).The 60 transcoronary h(t)'s were observed to be unimodal, right-skewed frequency distribution functions with mean transit times, t, ranging from 3 to 7 sec. The relative dispersions (standard deviation, σ, divided by t) averaged 0.38 ± 0.05, the skewness averaged 1.40 ± 0.37 and the kurtosis averaged 6.1 ± 1.8; this means that the h(t)'s are more sharply peaked than Gaussian distributions. The fact that parameters were statistically independent of the mean transit time implied the constancy of the shape of the various h (t)'s and this was verified by the coincidence of the h(t)'s plotted as a function of t/t. This "similarity" of the h(t)'s strongly suggests that changes in the transit time through any particular vascular pathway of the coronary bed are in proportion to the changes in other parallel pathways.