Longitudinal impedance is independent of outflow resistance.

BACKGROUND: Many investigators have measured outflow resistance (R) following peripheral bypass procedures, but correlations with graft patency have been weak. This is because the primary determinants of graft patency are the size and quality of the conduit, not its outflow bed. Efforts at separating conduit resistance from outflow resistance have been unsuccessful. Recently, the concept of longitudinal impedance ( integral Z(L)) has been suggested as a measure of conduit resistance independent of outflow resistance. The purpose of this in vitro experiment was to test the hypothesis that integral Z(L) is independent of R within physiologically relevant ranges. METHODS: Rigid polyethylene tubing of known internal diameter and length (4.3 mm, 375 cm) was perfused with a glycerin/saline mixture mimicking the viscosity of blood (4.1 cp), utilizing a variable pulsatile pump and Windkessel, with outflow into multiply branched tubes of decreasing diameter simulating the hemodynamic conditions of arterial bypass. Flow and pressure were measured using ultrasonic transit time and catheter transduction, respectively, and waveforms digitized at 200 Hz. Flow was varied while maintaining "systemic" pressure and resistance. After Fourier transformation, integral Z(L) was calculated as deltaP/Q at each harmonic and integrated over 4 Hz. RESULTS: integral Z(L) calculations were remarkably reproducible within the same day with a coefficient of variation (CV) = 4.0% (at 100 dyne. s/cm(5); n = 4) or over 4 successive days (CV = 4.3%). Furthermore, integral Z(L) was largely independent of R over the physiologic range tested, with integral Z(L) remaining relatively constant as R was increased sixfold. CONCLUSION: integral Z(L) is a consistent and reproducible measure of conduit resistance independent of R over a wide physiologic range. It may be useful for measuring the adequacy of bypass graft conduits.

Aortic reconstruction in patients with functioning renal allografts.

Patients with functioning renal allografts requiring aortic reconstruction pose a considerable challenge to the vascular surgeon. A variety of strategies for renal allograft preservation during intervention have been described including hypothermia, indwelling shunts, cold renal perfusion, axillofemoral bypass, and endovascular stent-grafting. Reported here are two cases of successful aortic reconstruction utilizing standard open surgical techniques designed simply to minimize warm renal ischemia. The first case was that of a 55 year-old patient with a functional renal allograft originating from the right external iliac artery, who presented acutely with large symptomatic aortic and bilateral iliac artery aneurysms. He was treated with aorto-right femoral/left iliac bypass grafting. The right femoral anastomosis was performed first so that warm renal ischemia was limited to the 34 min required to perform the proximal end-to-end aortic anastomosis. The second case was that of a 44-year-old patient also with a transplanted kidney originating from the right external iliac artery. He presented with worsening hypertension, decreasing renal function, claudication, and severe aortoiliac occlusive disease. He was treated with aorto-left femoral bypass grafting via a retroperitoneal approach, followed by femorofemoral crossover bypass for retrograde perfusion of the kidney (total warm ischemia time 20 min). Both patients recovered uneventfully without a decrement in renal function and remain well on follow-up. It is concluded that standard open surgery without adjunctive shunts or bypasses remains a viable treatment option for these patients, provided warm renal ischemia can be minimized.