Abstract 150 Poster Session II, Sunday, 5/2 (poster 253)

Background: We used intravascular ultrasound (IVUS) to obtain the distal pulmonary artery (PA) lumen area combined with simultaneous measurements of intraluminal pressure to derive PA wall stiffness (Ep) using the Peterson's elastic strain modulus in a randomized comparative trial of the pulmonary vasodilating capacity of ATP-MgCl2 and nitric oxide (NO). We hypothesized that changes in PA stiffness would reliably reflect expected changes in the distal vascular tone in response to these vasoactive substances. Methods and Results: Eleven two-week old piglets were surgically cannulated for continuous hemodynamic monitoring of systemic and pulmonary arterial pressure and arteriolar resistance. A 3.5 Fr IVUS catheter with a 30 MHz transducer placed into a distal PA branch (3-6 mm end-diastolic diameter) provided continuous cross-sectional images of the artery. Pulmonary hypertension (PH) was induced with 12% oxygen and was followed by treatment with both inhaled NO at 10ppm (n=7) and intravenous ATP-MgCl2 sequentially at 0.1mg/kg/min and 0.2mg/kg/min (n=9). Treatments were given in random order with return to baseline between them. Baseline PA stiffness, calculated using IVUS derived PA radius and PA pulse pressure, was significantly increased after induced PH in the ATP-MgCl2 and NO groups (from 144.4 ± 31.6 to 225.3 ± 92.3 and 127.9 ± 35.6 to 194.0 ± 60.1 mm Hg [Ep] respectively; p < 0.05). A significant decrease in PA stiffness occurred after ATP-MgCl2 infusion at 0.2mg/kg/min (225.3 ± 92.3 to 142.4 ± 34.3 mm Hg; p < 0.05) and inhaled NO (194.0 ± 60.1 to 135.7 ± 63.1 mm Hg [Ep; p < 0.05). Changes in PA stiffness approximated changes in PA pressure and PA resistance at the different physiologic states. Conclusion: We determined that IVUS imaging of the distal PA combined with simultaneous measurement of intraluminal pressure provide a direct assessment of pulmonary dynamics in vivo. Using the Peterson's elastic strain modulus to derive wall stiffness, we determined that intravenous ATP-MgCl2 and inhaled NO decreased distal PA stiffness equally.