Measurement of vascular input impedance in infrainguinal vein grafts

Comparison of the Resistive Properties of Reversed and Nonreversed Saphenous Veins at Arterial Pressure and Flow: Implications for Optimal Graft Configuration

Clinicians continue to debate the hemodynamic advantages of reversed vs nonreversed vein grafts in infrainguinal arterial reconstructions. Vein grafts placed in the reversed configuration do not require valve lysis but have the theoretical drawback of being smaller in caliber at the inflow end than the outflow end. The purpose of this study was to objectively determine the effect of vein valve lysis and flow direction on vein graft hemodynamics by using physiologic levels of pulsatile flow (Q), pressure gradient (AP), outflow resistance (Ro), and longitudinal impedance (ZL).

Nine cryopreserved human greater saphenous veins (length=23 ±1 cm) were perfused via an in vitro circuit utilizing a variable pulsatile perfusion pump, Windkessel, and clamp resistor. Levels of Q and AP were chosen to simulate the known physiologic conditions of infrainguinal bypass grafting while holding Reynolds numbers <2,400. Veins were studied in the reversed configuration prior to valve lysis, after valve lysis by use of a catheter-directed valvulotome with 3 mm cutting head, and in the nonreversed configuration. Ultrasonic transit-time flow and proximal and distal intraluminal pressure were continuously recorded while Ro and pump rate were varied. Waveforms were digitally stored at 200 Hz at pump rates of 60, 100, 140, and 180 beats per minute at a Q of 154 ± 1 and 253 ± 1 mL/min while Pprox was maintained at 100 mmHg. Veins were perfusion fixed at 100 mmHg, sectioned at 2 cm intervals, and analyzed morphometrically. Inner diameter (id) and outer diameter (od) were determined by light microscopy after correction for shrinkage artifact by comparing to outer diameter measured by digital calipers at 2 cm intervals at the time of perfusion. Percent vein taper was calculated as (idmax7idmin)/idmax and wall thickness (t) as (od-id)/2. After Fourier transformation, ZL was calculated as AP/Q at each harmonic and the curves compared by use of Wilcoxon signed-ranks test.

There were one to four valves per vein (idmean=3.6 ±0.3 mm; range 2.3-6.2) with an average taper of -8 + 12% (range: -46-36%) and mean wall thickness of 0.51 ±0.03 mm (range 0.42-0.68 mm). ZL curves were smooth and reproducible over the measured frequency range. Neither valve lysis nor flow direction had an effect on ZL at any level of Q or Ro, even in veins with a taper >25%. Mean wall thickness correlated with ZL (r2=0.43; p=0.05).

In this in vitro system, saphenous vein graft impedance (ZL) was independent of valve lysis, flow direction, and the degree of vein taper but was weakly dependent on wall thickness. In veins of adequate size, hemodynamic considerations should not influence the decision to use the reversed vs nonreversed configuration.

Gene Therapy for the Extension of Vein Graft Patency: A Review

The mainstay of treatment for long-segment small-vessel chronic occlusive disease not amenable to endovascular intervention remains surgical bypass grafting using autologous vein. The procedure is largely successful and the immediate operative results almost always favorable. However, the lifespan of a given vein graft is highly variable, and less than 50% will remain primarily patent after 5 years. The slow process of graft malfunction is a result of the vein's chronic maladaptive response to the systemic arterial environment, its primary component being the uncontrolled proliferation of vascular smooth muscle cells (SMCs). It has recently been suggested that this response might be attenuated through pre-implantation genetic modification of the vein, so-called gene therapy for the extension of vein graft patency. Gene therapy seems particularly well suited for the prevention or postponement of vein graft failure since: (1) the stimulation of SMC proliferation appears to largely be an early and transient process, matching the kinetics of current gene transfer technology; (2) most veins are relatively normal and free of disease at the time of bypass allowing for effective gene transfer using a variety of systems; and (3) the target tissue is directly accessible during operation because manipulation and irrigation of the vein is part of the normal workflow of the surgical procedure. This review briefly summarizes the current knowledge of the incidence and basic mechanisms of vein graft failure, the vector systems and molecular targets that have been proposed as possible pre-treatments, the results of experimental genetic modification of vein grafts, and the few available clinical studies of gene therapy for vascular proliferative disorders.

The role of established and emerging risk factors in peripheral vascular graft occlusion

Several studies have evaluated the association between established as well as emerging vascular risk factors with peripheral graft occlusion. There is evidence for a link between the risk for graft occlusion and total serum cholesterol, low-density lipoprotein cholesterol and triglyceride levels. The overall effect of smoking shows a 2.35-fold increase in risk of graft failure. Studies involving diabetic patients undergoing peripheral bypass may have failed to detect higher occlusion rates, possibly due to increased morbidity and mortality as well as higher amputation rates even if the graft is patent. Both antiplatelet agents and anticoagulation seem to be effective in the prevention of graft occlusion. Unconvincing data have been published with regards to the effect of hypertension on graft patency. Emerging factors such as fibrinogen, lipoprotein (a), C-reactive protein and homocysteine levels should also be considered when assessing the risk of graft occlusion. More research is needed to prevent graft occlusion due to the obvious clinical relevance, quality of life and cost issues.

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.

The use of impedance index in the surveillance of PTFE femorodistal grafts

BACKGROUND

Impedance is the equivalent in pulsatile flow of resistance in steady flow. The impedance index has been used successfully in the surveillance of vein grafts, but its use has not been reported in the context of PTFE femorodistal grafts.

METHODS

Twenty-eight patients (median age 68 years (IQR 59-73 years) and 20 men) undergoing 28 PTFE femorodistal grafts with a vein cuff were evaluated prospectively comparing the impedance index with standard duplex graft surveillance. All grafts were performed for critical ischaemia. At risk grafts were identified and treated appropriately after angiography.

RESULTS

The primary patencies at 1 and 2 years were 82% and 50% respectively. Duplex identified 11 at risk grafts of which 9 had an identifiable correctable lesion. Impedance analysis overpredicted at risk status when compared with duplex in the immediate postoperative phase and was unsuccessful in detecting inflow disease or low flow relating to cardiac failure. Using a threshold index of 0.5, impedance analysis has a sensitivity of 87%, specificity of 88%, with positive and negative predictive values of 76% and 94% respectively.

CONCLUSIONS

Impedance index is a non-invasive method of graft surveillance which is applicable to PTFE femorodistal bypasses and may be a useful alternative to duplex although formal validation studies will be required.

The hemodynamics of vein grafts: measurement and meaning

The long-term patency of infrainguinal vein grafts appears to depend primarily on the size and quality of the venous conduit. Therefore, those quantities which directly relate to the conduit's ability to act as a transporter of blood, namely internal diameter and longitudinal impedance (Z(L)), have predictive value for patency. Autologous grafts of good quality frequently remain patent even with compromised outflow. Therefore, those quantities that are outflow dependent, including deltaP, flow, velocity, shear stress, and resistance, carry less predictive value for long-term performance.

The effect of 'non-critical' (<50%) stenosis on vein graft longitudinal resistance and impedance

OBJECTIVE

vein graft stenoses <50% cause minimal flow impairment, velocity elevation, or symptomatology and are therefore usually assumed to be "non-critical". The purpose of this study was to assess the effect of <50% vein graft stenosis on vein graft longitudinal impedance, as elevated impedance has been found to correlate with clinical graft failure.

METHODS

eight segments of non-reversed cryopreserved vein (mean length 23+/-1 cm; mean outer diameter 4.7+/-0.2 mm) were saline-perfused in vitro utilising a variable pulsatile perfusion pump, Windkessel, and clamp resistor simulating the haemodynamic conditions of arterial bypass. Proximal (Pprox) and distal (Pdist) pressure were continuously measured by fluid-filled catheter transduction, and flow (Q) by ultrasonic transit-time flowmetry. Waveforms were digitally recorded at 200 Hz at pulse rates ranging from 60-180 b.p.m. with mean flow (Q) of 154 ml/min and mean proximal pressure (Pprox) of 100 mmHg (max/min 120/90). Graded mid-graft stenoses of <50% were created using an inflatable vascular occluder and measured by the corresponding changes in mean pressure gradient (DeltaP=Pprox-Pdist) and Q (%stenosis=1-{DeltaPbaselineQstenosis/Delta PstenosisQbaseline}1/4). Vein graft longitudinal resistance (RL) was calculated as DeltaP/Q. After Fourier transformation, vein graft longitudinal impedance (ZL) was calculated as DeltaP/Q at each harmonic, with ZL determined by integration over 0-4 Hz. Results are reported as mean+/-S.E.M.

RESULTS

the desired levels of pressure and flow were established in all vein segments. Graded inflation of the occluder resulted in vein graft stenosis of 23+/-3% and 39+/-3%. This was accompanied by a mild reduction in Q (12% and 30%) and considerable increases in both RL (180% and 710%) and ZL (140% and 430%).

CONCLUSIONS

"non-critical" vein graft stenosis (<50%) causes minimal change in mean flow, but substantial elevations in longitudinal resistance and impedance. The contribution of "non-critical" stenosis to vein graft failure may be under-appreciated.

Impedance index measurements of in vitro PTFE end-to-side anastomoses: effect of angle and Miller cuff

OBJECTIVE

To measure non-invasively the impedance index of a range of geometries of PTFE end-to-side anastomoses (ESA).

DESIGN

In vitro experiments using a custom-built flow rig.

SUBJECTS

Anastomoses constructed at each angle. (15 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees) for both standard and cuffed ESA.

RESULTS

The impedance index of each ESA increased non-linearly with flow rate. The impedance index at a given flow rate was reduced by decreasing the anastomotic angle and further reduced by the addition of a Miller cuff.

CONCLUSIONS

The reduction in impedance index achieved with a Miller cuff may help to explain the improved patency rates of femoropopliteal grafts incorporating a cuff.