One of the most important determinants of graft patency is the degree and character of vascular outflow. This study was designed to evaluate input impedance as a functional assessment of the outflow bed of vascular grafts. Four distinct outflow environments were created for external jugular vein conduits in 42 New Zealand white rabbits. Vein grafts (n = 14) were fashioned as end-to-side common carotid interposition bypass grafts. Arteriovenous fistulas (n = 15) were created by side-to-side anastomosis of the distal common carotid artery and linguofacial vein. Arteriovenous fistulas with outflow obstruction (n = 7) were fistulas with a metal clip partially obstructing the distal outflow channel (1 mm lumen). Vein graft/arteriovenous fistula combinations (n = 6) consisted of a vein graft and arteriovenous fistula in series. Pressure and flow in the external jugular vein were measured, and input impedance spectra were calculated by Fourier methods. By use of a PC-based acquisition and processing system, impedance results for 20 cardiac cycles could be obtained in approximately 10 minutes. The results revealed that vein grafts typically demonstrated high resistance to steady state flow (Rin = 235 +/- 50 x 10(3) dyne . sec/cm-5) and steadily decreasing impedance to pulsatile flow resulting in a characteristic impedance (Z0; average of fourth to tenth harmonics) of 35.5 +/- 8.0 x 10(3) dyne . sec/cm-5. Phase angle values were usually negative, especially at low harmonics (first harmonic phase angle = -1.11 +/- 0.10 radians) indicating that flow led pressure. In contrast, arteriovenous fistula Rin was minimal (6.3 +/- 1.4 x 10(3) dyne . sec/cm-5; p less than 0.05 compared to vein graft, and the impedance was flat across the frequency spectrum (Z0 = 8.5 +/- 1.5 x 10(3) dyne . sec/cm-5; p less than 0.05) with pressure and flow nearly in phase (first harmonic phase angle = -0.05 +/- 0.10 radians). Creation of outflow obstruction in arteriovenous fistulas resulted in significantly elevated Rin (136 +/- 41 x 10(3) dyne/sec . cm-5; p less than 0.05 compared to arteriovenous fistula and Z0 (23 +/- 9 x 10(3) dyne . sec/cm-5, p less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)