Effect of flow on transpulmonary circulatory transport functions.

Abstract

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

Physiology and theory of tracer washout techniques for the estimation of myocardial blood flow: Flow estimation from tracer washout

“…Similarity has been demonstrated for transorgan transport functions in the kidney by Gomez et al, 22 in the lung over a very limited range of alveolar and left atrial pressures by Knopp and Bassingthwaighte,23 and in the coronary bed by Knopp et al 17 Data from the latter study are plotted in Fig. 11, showing a fair degree of similarity.…”

“…For the concentration-time curves, recalling that h(t) = F · C(t)/m i , (equation 2), then: (22) and for the constant volume system: (23) Equations 22 and 23 show that for a system with stable dispersive properties, a change in F doesn't affect the peak heights of C(t) but does change the spread of the curve, the areas being inversely related to F. Similarly, C(t D2 ) = C(t D1 ) and C(t E2 ) = C(t E1 ) while t D2 = F 1 t D1 /F 2 and t E2 = F 1 t E1 /F 2 . This may be viewed merely as normalization of the time scale of t/t.…”

Dependency of Transcoronary Circulatory Transport Function on Coronary Perfusion Pressure and Flow

“…Recording the transport function, Bassingthwaighte (1966) observed in normal human legs that the indicator dispersion was proportional to the mean transit time over a sixfold change of flow rates, suggesting that the rate of spatial longitudinal spreading of indicator with distance travelled is primarily a function of the geometry of the vascular systems, not of the rate of flow. In the pulmonary vascular bed where multiple parallel pathways and different regional perfusion rates may exist, it was shown that average relative dispersion was about twice (0.46) that found for segments of arteries (0.23), and that the relative lispersion tended to increase as the mean transit time increased, suggesting that the vascular geometry may alter with alteration in the perfusion in such a circulatory bed of heterogeneous perfusion (Knopp and Bassingthwaighte 1969). The dependency of the relative dispersion on the perfusion pressure and flow is more dominant in the coronary circulatory bed as shown in this study.…”

“…It seems that the determination of transit time must be useful only when it is related to the indicator dispersion, because the latter may be determined mainly by the distance travelled by the dye particles injected (Bassingthwaighte and Warner 1965). Furthermore, the transport function may change with changing blood flow rate as was found in pulmonary circulation by Knopp and Bassingthwaighte (1969).…”

Determination of Transcoronary Circulatory Transport Function

“…When a circulatory system is involved, it is called circulatory transport function (Bassingthwaighte 1970), that is the probability density function of transit times through the system. Since extensive studies of Bassingthwaighte and co-investigators (Bassingthwaighte 1966;Coulam et al 1966;Bassingthwaighte et al 1966;Bassingthwaighte and Ackerman 1967;Knopp and Bssingthwaighte 1969), the physiological meaning of transport function is being clarified; the ratio of the mean transit time to the standard deviation of transport function (called relative dispersion) is constant for a simply perfused system such as aorta and branching arteries even with the flow changing by 6 times (Bassingthwaighte 1966), but it is variable in a more complicated-structured system (Knopp and Bassingthwaighte 1969), so that in pulmonary circulatory bed, it varied with increased flow suggesting that an alteration in perfusion homogeneity has occurred. Knowledge of the path-length distribution seems very important for the clinical purpose in coronary circulation, in particular in the assessment of the development of collateral circulation, for under such conditions the path-length distribution may be altered (Liedtke et al 1973).…”