The contribution of valves to saphenous vein graft resistance

Platelets Drive Thrombus Propagation in a Hematocrit and Glycoprotein VI-Dependent Manner in an In Vitro Venous Thrombosis Model

OBJECTIVE

The objective of this study was to measure the role of platelets and red blood cells on thrombus propagation in an in vitro model of venous valvular stasis.

APPROACH AND RESULTS

A microfluidic model with dimensional similarity to human venous valves consists of a sinus distal to a sudden expansion, where for sufficiently high Reynolds numbers, 2 countercurrent vortices arise because of flow separation. The primary vortex is defined by the points of flow separation and reattachment. A secondary vortex forms in the deepest recess of the valve pocket characterized by low shear rates. An initial fibrin gel formed within the secondary vortex of a tissue factor-coated valve sinus. Platelets accumulated at the interface of the fibrin gel and the primary vortex. Red blood cells at physiological hematocrits were necessary to provide an adequate flux of platelets to support thrombus growth out of the valve sinus. A subpopulation of platelets that adhered to fibrin expose phosphatidylserine. Platelet-dependent thrombus growth was attenuated by inhibition of glycoprotein VI with a blocking Fab fragment or D-dimer.

CONCLUSIONS

A 3-step process regulated by hemodynamics was necessary for robust thrombus propagation: First, immobilized tissue factor initiates coagulation and fibrin deposition within a low flow niche defined by a secondary vortex in the pocket of a model venous valve. Second, a primary vortex delivers platelets to the fibrin interface in a red blood cell-dependent manner. Third, platelets adhere to fibrin, activate through glycoprotein VI, express phosphatidylserine, and subsequently promote thrombus growth beyond the valve sinus and into the bulk flow.

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.

Dimensional analysis of human saphenous vein grafts: Implications for external mesh support

OBJECTIVE

Constrictive external mesh support of vein grafts was shown to mitigate intimal hyperplasia in animal experiments. To determine the degree of constriction required for the elimination of dimensional irregularities in clinically used vein grafts, a detailed anatomic study of human saphenous veins was conducted.

METHODS

In 200 consecutive patients having coronary artery bypass grafting, harvested saphenous veins (length 34.4 +/- 10.8 cm) were analyzed regarding diameter irregularities, side branch distribution, and microstructure.

RESULTS

The mean outer diameter of surgically distended saphenous veins was 4.2 +/- 0.6 mm (men, 4.3 +/- 0.6 mm vs women, 3.9 +/- 0.5 mm; P < .0001). Although the outer diameter significantly decreased over the initial 18 cm (-7.6%; P < .0001), the overall increase between malleolus and thigh was not significant (+11.2%). Smaller-diameter veins (<3.5 mm) had more pronounced diameter fluctuations than larger veins (31.8% +/- 11.0% vs 21.2% +/- 8.8%; P < .0001), with more than 71% of all veins showing caliber changes of more than 20%. There was 1 side branch every 5.4 +/- 4.3 cm, with a significantly higher incidence between 20 and 32 cm from the malleolus (P < .0001 to distal, P < .0004 to proximal). Generally, women had more side branches than men (0.30 +/- 0.15 cm(-1) vs 0.25 +/- 0.12 cm(-1); P = .0190). Thick-walled veins (565.7 +/- 138.4 mum) had a significantly higher number of large side branches (P < .0001), and thin-walled veins (398.7 +/- 123.2 mum) had significantly more small side branches (P < .0001). Pronounced intimal thickening ("cushions") was found in 28% of vessels (119.8 +/- 28.0 mum vs 40.1 +/- 18.2 mum; P < .0001).

CONCLUSION

Although the preferential location of side branches may be addressed by the deliberate discarding of infragenicular vein segments, a diameter constriction of 27% on average would eliminate diameter irregularities in 98% of vein grafts.

Forearm arterial vein grafting: problems and alternative solutions

We aimed to evaluate patency rates following forearm arterial reconstruction and suggest improvements. Thirty-two vein grafted reconstructions (using saphenous and dorsal hand veins) were evaluated for patency and development of symptoms using clinical examination, Colour-Doppler Sonography and angiography. Overall patency was 59%, with no significant difference between saphenous and dorsal hand veins. Stasis, turbulence, decrease in blood velocity, change in flow pattern, stenosis in the anastomotic area or increase in compliance was detected in 10 patent grafts, but was not associated with symptoms. Six out of 13 non-patent grafted patients had severe or troublesome symptoms associated with accompanying nerve regeneration. To improve patency, careful microsurgical techniques, 'fit vein' and valveless grafts should be used. Dorsal hand veins are most appropriate for short defects. Patency should be evaluated soon after reconstruction.

Detection of "functional" valves in reversed saphenous vein bypass grafts: identification with duplex ultrasonography

PURPOSE

Although venous valve lesions have been cited as a mechanism of graft failure, the fate of venous valves in reversed saphenous vein (RSV) bypass grafts is unclear. The basis for this uncertainty is the difficulty in postoperative identification and follow-up of valve sites and the infrequent pathologic submission of vein graft lesions. This report describes the features of "functional" valves (FV) visualized in RSV grafts by ultrasonic duplex scanning.

METHODS

Sixty-six RSV infrainguinal vein grafts were prospectively studied with duplex ultrasonography from January 1992 to December 1995. Grafts were studied at 1, 2, 3, 4, 6, 9, 12, 18, and 24 months, then annually. FV identification was based on a characteristic ultrasound Doppler waveform and color flow pattern and visualization of the leaflets by B-mode imaging. The waveform consists of end-systolic valve closure followed by variable degrees of reflux. Immediate postoperative reactive hyperemia precludes detection, because flow reversal in the graft is needed for identification.

RESULTS

Since August 1994, 14 FV have been identified in 11 (17%) of 66 RSV grafts. The mean time to FV recognition after implantation was 10 months (range, 1 to 52 months), and the average follow-up was 15 months. One valve was completely competent. Seven (50%) of the FV were associated with the development of a < 50% diameter reducing stenosis by Doppler velocity criteria. None of the FV has required intervention.

CONCLUSIONS

"Functional" vein valves in RSV grafts are common and can be identified by ultrasonic duplex imaging. Awareness of the characteristics of FV during routine duplex graft surveillance will undoubtedly increase detection. The variable time course to identification of FV and duration of "function" warrants continued follow-up to determine the relationship of these valves to the development of stenotic lesions and graft failure.

Improved measurement of pressure gradients in aortic coarctation by magnetic resonance imaging

OBJECTIVES

This study evaluated whether magnetic resonance imaging (MRI) and magnetic resonance (MR) phase velocity mapping could provide accurate estimates of stenosis severity and pressure gradients in aortic coarctation.

BACKGROUND

Clinical management of aortic coarctation requires determination of lesion location and severity and quantification of the pressure gradient across the constricted area.

METHODS

Using a series of anatomically accurate models of aortic coarctation, the laboratory portion of this study found that the loss coefficient (K), commonly taken to be 4.0 in the simplified Bernoulli equation delta P = KV2, was a function of stenosis severity. The values of the loss coefficient ranged from 2.8 for a 50% stenosis to 4.9 for a 90% stenosis. Magnetic resonance imaging and MR phase velocity mapping were then used to determine coarctation severity and pressure gradient in 32 patients.

RESULTS

Application of the new severity-dependent loss coefficients found that pressure gradients deviated from 1 to 17 mm Hg compared with calculations made with the commonly used value of 4.0. Comparison of MR estimates of pressure gradient with Doppler ultrasound estimates (in 22 of 32 patients) and with catheter pressure measurements (in 6 of 32 patients) supports the conclusion that the severity-based loss coefficient provides improved estimates of pressure gradients.

CONCLUSIONS

This study suggests that MRI could be used as a complete diagnostic tool for accurate evaluation of aortic coarctation, by determining stenosis location and severity and by accurately estimating pressure gradients.

Flow measurements in an aortocoronary bypass graft casting

Flow visualization and pressure measurements were carried out in a singel valve saphenous vein casting which was made from a saphenous vein segment obtained from a bypass patient at Cedars Sinai Medical Center. Dye was injected to understand the flow around the valve. The dye showed very complex flow patterns around the valve and in the valve sinus, and the cavity formed by a ligated branch. For steady flow, pressure drops across the valve were 0.72, 2.0 and 6.3 mmHg for the physiological flow rates of 45, 84, and 169 ml/min, respectively. Overall pressure drop across the casting (compared to Poiseuille flow for a straight tube) increased with the flow rate, being 130 to 290 percent higher over this flow rate range. In the case of pulsatile flow, pressure drops across the valve were 0.95 and 3.0 mmHg for the flow rates of 47 and 87 ml/min which were 26 and 43 percent higher than those of steady flow. Overall pressure drop was 220 and 360 percent higher for those flow rates compared to Poiseuille flow. The measured spatial pressure distributions along the casting and flow visualization indicated the global nature of the flow field with the accelerated flow through the valve separating and reattaching downstream along the wall in the pressure recovery region. Atherosclerosis may be prone to occur in the lower shear region along the wall beyond the valve tip in the reattachment region, as we have observed in vivo in rabbit experiments.

The characteristics and anatomic distribution of lesions that cause reversed vein graft failure: a five-year prospective study

PURPOSE

The cause of vein graft failure in the intermediate postoperative period (3 to 18 months) has not been well defined. To delineate the incidence, characteristics, and anatomic distribution of lesions that cause graft failure in this critical interval, 227 consecutive infrainguinal reversed vein grafts (IRVGs) constructed at a single institution from July 1986 to December 1991 were prospectively entered into a duplex scan surveillance protocol.

METHODS

Duplex surveillance with arteriographic confirmation identified 29 patent, hemodynamically failing IRVGs during a mean follow-up of 22 months (range 1 to 64 months). An additional 18 grafts thrombosed before detection of any underlying abnormality; thrombolytic therapy and repeat operation uncovered the cause of occlusion in 12 of these grafts. The cause of graft failure (failing as well as failed) was therefore clear in 41 (87.2%) of 47 instances.

RESULTS

The causes of failure were intrinsic graft stenosis (n = 28; 59.6%), inflow failure (n = 6; 12.8%), outflow failure (n = 4; 8.5%), muscle entrapment (n = 2; 4.3%), and hypercoagulable state (n = 2; 4.3%). The most common intrinsic graft lesion was focal intimal hyperplasia (18 lesions in 16 grafts) in the juxtaanastomotic position, occurring solely in the vein graft itself. It occurred with equal frequency immediately distal to the proximal anastomosis or proximal to the distal anastomosis. Only rarely (n = 5) did this involve the juxtaanastomotic artery. Focal midgraft valvular stenoses (n = 6) and diffuse myointimal hyperplasia (n = 4) were also detected. The peak incidence of graft failure was 4 to 12 months after operation (70% within 12 months, 80% within 18 months).

CONCLUSIONS

We conclude that duplex surveillance of IRVGs is warranted by the 21% incidence of potentially remediable graft failure. A significant portion of these failures occur during the intermediate postoperative period (3 to 18 months), usually as a result of focal intrinsic vein graft lesions. With reversed vein conduits, these lesions arise predominantly in the vein graft itself, in the juxtaanastomotic position.

Results of infrainguinal revascularization with reversed vein conduits: a modern control series

To determine the outcome of infrainguinal reversed vein bypasses in the modern era, we reviewed the results of 120 consecutive reversed vein grafts performed from March, 1986 to March, 1990. Forty-nine bypasses were to tibial, peroneal, or pedal arteries, 46 grafts to the below-knee popliteal artery, and 25 grafts to the above-knee popliteal artery. Limb salvage was the indication for revascularization in 70% of patients. All grafts were followed with serial, duplex scan, peak-systolic graft flow velocity measurements every three months for one year and every six months thereafter. The primary life table patency rate at 36 months was 67.6% for the entire series; the secondary patency rate was 92.5%. The secondary patency rate reflects the impact of graft revisions resulting from the detection of failing grafts by duplex scanning. Patency rates of reversed vein grafts to the tibial arteries at 36 months (73.8% primary and 89.8% secondary) were equivalent to those performed to the popliteal artery. Our current patency rates with reversed vein grafts are comparable or superior to those reported for in-situ vein conduits and suggest that operative technique and meticulous follow-up are more important with respect to long-term graft durability than whether the vein is used in the in-situ or reversed configuration.