The kinetics of distribution of the fat-soluble inert gas cyclopropane in the body

Uncertainties in pharmacokinetic modeling for perchloroethylene: II. Comparison of model predictions with data for a variety of different parameters

In this paper we compare expectations derived from 10 different human physiologically based pharmacokinetic models for perchloroethylene with data on absorption via inhalation, and concentrations in alveolar air and venous blood. Our most interesting finding is that essentially all of the models show a time pattern of departures of predictions of air and blood levels relative to experimental data that might be corrected by more sophisticated model structures incorporating either (a) heterogeneity of the fat compartment (with respect to either perfusion or partition coefficients or both) or (b) intertissue diffusion of perchloroethylene between the fat and muscle/VRG groups. Similar types of corrections have recently been proposed to reduce analogous anomalies in the fits of pharmacokinetic models to the data for several volatile anesthetics. A second finding is that models incorporating resting values for alveolar ventilation in the region of 5.4 L/min seemed to be most compatible with the most reliable set of perchloroethylene uptake data.

Inert gas diffusion method for measurement of blood flow using saturation techniques. Comparison with directly measured blood flow in isolated gastrocnemius muscle of the cat

The isolated gastrocnemius muscle of the cat was used to study the relation between the desaturation of 133 xenon measured by external monitoring, and blood flow measured directly by a dropcounter.

The metered blood flow agreed with that calculated from the initial slope of the desaturation curve from tissue equilibrated by prolonged intra-arterial infusion and also by atraumatic gas labelling of a local area of the muscle (approximately 0.15% of the muscle weight). In this sense the concept of uniform distribution of blood flow in skeletal muscle has received substantial support. The identity of the results obtained by local gas labelling and by intra-arterial equilibration of the whole muscle with gas implies that there is, at least initially, diffusion equilibrium between tissue and blood in both.

However, the clearance curves could in no instance be fitted by a single exponential function. This indicates that the diffusion equilibrium is not maintained after the initial phase. It is suggested that this is not due to the presence of anatomical arteriovenous shunts or other types of uneven perfusion, but to counter current exchange of inert gas through the walls of vessels, i.e. shunting by diffusion explains the shape of the desaturation curves.

Diffusion processes invalidating the intra-arterial krypton-85 beta-particle clearance method for measurement of skin blood flow in man

Krypton-85 dissolved in 0.9% NaCl was injected into the femoral artery of man; β particles were measured over the skin of the crus. Cutaneous mean blood flow was calculated using the integral equation of Zierler based on the mean transit time analysis of clearance curves recorded from the cutaneous tissue; these calculations gave estimates which were only about one-fifth those obtained by other methods. The low estimates were caused by an intercom-partmental exchange of 85 krypton between the cutaneous and subcutaneous tissues, an exchange implying that both tissues influence the shape of the clearance curve recorded from the cutaneous tissue alone. This intercompart-mental exchange was effected not only by diffusion directly from the cutaneous to the subcutaneous tissue, but presumably also by transport from the blood streaming between the two tissues. The clearance rate of the tail of the curve was shown to correspond to that of the subcutaneous tissue. The errors distorting the clearance curve of β particles may invalidate the use of intra-arterial injection and measurement of clearance curves of β particles for measurement of blood flow in other tissues.