FIGURE 14 Plot of vessel segment diameter vs. distance from the pulmonary arterial inlet for the principal pathway of a hypertensive rat lung imaged at high pressure. The lower plot shows the same relationship for the smaller daughter vessel segments branching immediately off the main trunk.

This shows a significant decrease in the arterial distensibility of the diseased lung, but no significant difference between the curve fit parameters. These early results demonstrate that arterial wall distensibility is significantly lower in hypertensive compared to normal pulmonary trees. Future studies will apply these methods to investigate the mechanisms involved in other models of pulmonary hypertension, and to studies of the relative efficacies of interventions designed to slow or reverse disease progression.

main trunk tapers fairly steadily from inlet to periphery, while small subtrees may branch off to perfuse proximal lung tissue volumes.

To measure biomechanical properties of the vessels, we obtain 3D reconstructions of each lung at a series of four intravascular pressures spanning the physiological range (30, 21, 12, and 4mm Hg). Thus, for every vessel segment measured, the diameter vs pressure relationship yields the vessel distensibility as percent diameter change per unit change in pressure (%/torr). Figure 15 shows projection images of a hypertensive rat lung obtained at four intra-arterial contrast agent pressures. The distension of the arteries at high pressure is clearly evident.

Figure 16 shows graphs similar to those in Fig. 14 for the main trunk of a normal lung imaged at low and relatively high pressures. The slopes, intercepts, and curvatures are clearly increased for the normoxic compared to the hypoxic lung, and for the normoxic lung at high compared to low pressure. From the diameter vs pressure relationships, we can calculate the distensibility (absolute or percent diameter change per torr) of the pulmonary arteries, either for individual vessel segments or for the tree as a whole. The results for a large number of individual vessel segments from several control and several hypertensive lungs are shown in Fig. 17, where distensibility is plotted for each vessel segment vs its diameter at the lowest pressure (4mm Hg). Though there is overlap in the data, the method seems to separate normal from hypertensive animals quite well. To calculate a global distensibility for the arterial tree in a single lung, the diameter (D) vs distance (x) data for all four pressures (P) is plotted as a three-dimensional graph as shown in Fig. 18. The data is then fitted with a surface of the form

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