Matched elastic properties and successful arterial grafting

Clinical results from medium- and small-caliber arterial bypass grafts are unsatisfactory. Since elastic properties of grafts tested experimentally have been correlated with patency results, the compliance of the human femoral artery was compared with grafts currently in use: human saphenous vein (HSV), knitted Dacron (DAC), glutaraldehyde-treated umbilical cord vein (DBM), bovine heterograft, and expanded polytetrafluoroethylene (PTFE). This was correlated with clinical patency data for the different conduits in the femoropopliteal position. Increased patency correlated with a decreasing disparity between host artery and graft compliance. After two years, patency rates of the more compliant materials (HSV, DBM) exceeded 80%, while less than 45% of the incompliant grafts (DAC, PTFE) remained patent. Thus, clinical performance with synthetic grafts might be improved by use of prostheses in which the viscoelastic characteristics match those of arteries more closely.     

Compliance properties of conduits used in vascular reconstruction

BACKGROUND: Compliance mismatch between native artery and prosthetic graft used for infrainguinal bypass is implicated in the aetiology of graft failure. The aim was to quantify the elastic properties of a new compliant poly(carbonate)polyurethane (CPU) vascular graft, and to compare the compliance properties of grafts made from CPU, expanded polytetrafluoroethylene (ePTFE), Dacron and human saphenous vein with that of human muscular artery. METHODS: A pulsatile flow phantom was used to perfuse vessel and prosthetic graft segments at physiological pulse pressure and flow. Intraluminal pressure was measured using a Millar Mikro-tip catheter transducer and vessel wall motion was determined with duplex ultrasonography using an echo-locked wall-tracking system. Diametrical compliance and a stiffness index were then calculated for each type of conduit over mean pressures ranging from 30 to 100 mmHg by 10-mmHg increments. RESULTS: The compliance values of CPU and artery (mean over the pressure range) were similar (mean(s.d.) 8.1(0.4) and 8.0(5.9) per cent per mmHg x 10(-2) respectively), although the elastic behaviour of artery was anisotropic unlike CPU, which was isotropic. Dacron and ePTFE grafts had lower compliance values (1.8(1.2) and 1.2(0.3)per cent per mmHg x 10(-2) respectively, averaged over the pressure range). In both these cases, compliance and stiffness differed significantly from that of artery over a mean pressure range of 30-90 mmHg. Human saphenous vein exhibited anisotropic behaviour and, although compliant at low pressure (30 mmHg), was markedly incompliant at higher pressures. CONCLUSION: Compliant polyurethane grafts offer a greater degree of compliance match than either ePTFE or Dacron.     

The effect of wall mechanical properties on patency of arterial grafts

Normal arteries have properties which match the low output impedance of the heart to the high peripheral impedance. These properties can be assessed in terms of compliance (% diameter change per unit pressure change) as well as by other haemodynamic parameters. Experiments were designed using vein, Dacron and expanded polytetrafluoroethylene (PTFE) in a low flow canine femoral artery bypass model. No graft group achieved perfect patency. At twelve weeks 80% of vein grafts, 30% of Dacron grafts, and 15% of PTFE grafts remained patent. The compliance of vein grafts was maintained despite marked thickening of the wall. Patency was correlated at a highly significant level with compliance. The studies demonstrate that the matching of the mechanical properties of grafts to host arteries is important in the design of successful synthetic arterial grafts.     

Arteriovenous grafts for vascular access in haemodialysis.

The outcome of 68 arteriovenous grafts placed in 46 patients requiring haemodialysis was studied over a period of 3.5 years. The biological grafts included autogenous saphenous vein, modified bovine carotid artery and human umbilical cord vein allograft, whereas the synthetic grafts comprised Sparks Dacron mandril, expanded reinforced polytetrafluoroethylene and knitted Dacron velour. These subcutaneous grafts were arranged as looped or straight configurations in the forearm or thigh. Of 59 grafts evaluated in patients with end-stage renal failure, only 48 per cent of the forearm grafts performed well, compared with 85 per cent of the thigh grafts. Although only 38 per cent of the looped grafts were successful, 78 per cent of the straight grafts functioned satisfactorily. Synthetic grafts suffered less serious complications than the commercial biological grafts.     

An experimental study of the strength in arterial prosthetic anastomoses: relationship between anastomotic tensile strength and its organization]

Five types of 6-mm diameter arterial prostheses, i.e.m woven Teflon, woven Dacron, velour woven Dacron, velour knitted Dacron and expanded polytetrafluoroethylene (E-PTFE), were studied in the dog to assess anastomotic tensile strength and its organization. Five types of composite grafts, about 3.0cm in length, were implanted in the abdominal aorta of fifty-two adult mongrel dogs using absorbable polyglycolic acid suture. At 16 weeks after the implantation, if the composite graft was patent, tensile strength of the bilateral anastomoses were measured and its organizations were examined microscopically. The mean forces required to disrupt the anastomoses of velour woven Dacron, velour knitted Dacron and E-PTFE were greater than those of woven Teflon and woven Dacron. At the disrupted sites of woven Teflon and woven Dacron, which were non-velour grafts, fibrous outer sheath were remarkably separated from the grafts. This results indicate that organization and fixation of fibrous outer sheath are the most important factors in the strength of arterial prosthetic anastomoses and velour grafts have superiority over non-velour grafts. The organization of E-PTFE was poor, but its tensile strength was equal to velour grafts. It is considered E-PTFE has an advantage due to non-crimped structure and requires further evaluation over longer period.     

Operative transluminal laser angioplasty as the sole treatment for late stenoses of femorodistal artery bypass graft: Experimental and clinical studies

To determine the role of Nd:YAG laser thermal angioplasty as the sole treatment for late stenoses of femorodistal artery bypass graft, the lasing effect of a larger size of hot-tip probe (3, 4, and 5 mm) was experimentally studied in vitro. For an adequate lasing effect, 30 watts of laser power output for 3 seconds was needed for the 3 mm probe, 40 watts for the 4 mm probe, and 50 watts for the 5 mm probe, respectively. Based on these results, we used Nd:YAG laser thermal angioplasty alone for 25 grafts, including 16 polytetrafluoroethylene (PTFE) grafts, eight saphenous vein grafts, and one externally supported (EXS) Dacron graft in which the stenotic lesions were detected by deterioration of the Doppler flow waveform pattern or a significant fall in the ankle/brachial pressure index (ABPI). Follow-up was from 3 to 24 months (average of 9 months) for PTFE grafts, from 5 to 21 months (average of 11 months) for saphenous vein grafts, and 13 months for the EXS Dacron graft following femorodistal artery reconstructions. Stenotic lesions were most common in the distal anastomotic sites: 11 PTFE grafts, three saphenous vein grafts, and one EXS Dacron graft. Among these, 13 grafts showed a type II flow waveform pattern at the time of surgery. Clinical success was achieved in 12 of the PTFE grafts (75%), in five of the vein grafts (62.5%), and in the single EXS Dacron graft. Four PTFE and three saphenous vein grafts failed subsequent to repeat intraoperative balloon angioplasty in three and graft extension in three and one graft interposition. Perforation occurred in only one vein graft. Continuing patency has now been maintained for up to 25 months after lasing. Nd:YAG laser thermal angioplasty using a 3 to 5 mm hot-tip probe is effective as the sole procedure for widening a stenotic lesion and improving patency after femorodistal artery reconstruction.     

Small Intestinal Submucosa as a Small-Diameter Arterial Graft in the Dog

Autogenous saphenous vein, human umbilical vein, modified bovine collagen, Dacron, and PTFE have been used as small-diameter arterial grafts with moderate success. We tested autogenous small intestine submucosa as a small-diameter arterial graft in both a carotid and femoral artery (mean ID 4.3 mm) of 18 dogs (total of 36 grafts). All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by Doppler ultrasound techniques and angiography. Two grafts ruptured and 5 grafts occluded by 21 days after surgery. One graft became occluded at 14 weeks. Fifteen dogs were sacrificed at periodic intervals until 48 weeks after surgery. Patent grafts had no evidence of infection, propagating thrombus, or intimal hyperplasia. Graft aneurysmal dilation occurred in 4 grafts (11%). The grafts were composed of a dense organized collagenous connective tissue with no evidence of endothelial cell growth on the smooth luminal surface. Three dogs are alive at 76 to 82 weeks after surgery. Overall, graft patency was 75%. Graft patency after cessation of anticoagulation therapy was 92.3% (12 of 13 grafts). We conclude that autogenous small intestinal submucosa can be used as a small-diameter arterial graft in the dog and is worthy of further investigation.     

Small intestinal submucosa as a small-diameter arterial graft in the dog

Autogenous saphenous vein, human umbilical vein, modified bovine collagen, Dacron, and PTFE have been used as small-diameter arterial grafts with moderate success. We tested autogenous small intestine submucosa as a small-diameter arterial graft in both a carotid and femoral artery (mean ID 4.3 mm) of 18 dogs (total of 36 grafts). All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by Doppler ultrasound techniques and angiography. Two grafts ruptured and 5 grafts occluded by 21 days after surgery. One graft became occluded at 14 weeks. Fifteen dogs were sacrificed at periodic intervals until 48 weeks after surgery. Patent grafts had no evidence of infection, propagating thrombus, or intimal hyperplasia. Graft aneurysmal dilation occurred in 4 grafts (11%). The grafts were composed of a dense organized collagenous connective tissue with no evidence of endothelial cell growth on the smooth luminal surface. Three dogs are alive at 76 to 82 weeks after surgery. Overall, graft patency was 75%. Graft patency after cessation of anticoagulation therapy was 92.3% (12 of 13 grafts). We conclude that autogenous small intestinal submucosa can be used as a small-diameter arterial graft in the dog and is worthy of further investigation.     

Our eight-year experience with human umbilical vein grafts

From 1977 through 1984, 58 arterial reconstructions using glutaraldehyde-tanned human umbilical vein (H.U.V.) grafts were carried out: 41 femoropopliteal, 4 composite aortopopliteal (a Dacron graft was inserted proximally), 5 femorotibial, 6 axillofemoral, 1 femorofemoral, and 1 iliofemoral bypass. Autogenous saphenous vein was absent or inadequate in all patients. Limb salvage was the primary indication for surgery (89.7%). Five and 8 years cumulative patency rates were 42.8% and 35.1% respectively for all H.U.V. by-passes and 52.4% and 41.9% respectively for femoropopliteal by-passes. Four out of the 5 femorotibial by-passes failed during the first month following surgery. Two of the 6 axillofemoral and 3 of the 4 composite aortopopliteal by-passes thrombosed within 1 and 3 years after surgery, respectively. Perioperative mortality was 5.1%, while overall mortality was 17.2%. The following conclusions can be drawn from these results: H.U.V. grafts did not achieve cumulative patency rates superior to those reported with autogenous saphenous vein, in limb salvage situations, when the saphenous vein is not available, the H.U.V. graft offers acceptable salvage rates, results from the 8-year follow-up period demonstrate the durability and long-term patency of H.U.V. grafts.     

Sparks mandril, velour Dacron and autogenous saphenous vein grafts in femoropopliteal bypass.

Ninety-nine consecutive vascular reconstructions with femoropopliteal bypass performed during 1973--6 have been reviewed. A Sparks mandril was used in 11 patients, reversed autogenous saphenous vein in 30 and velour Dacron prostheses in 58. The indication for operation was severe ischaemia in 79 per cent. The overall patency rate at 12 months was 70 per cent for autogenous vein, 46 per cent for velour Dacron prosthesis and 18 per cent for Sparks mandril. When the distal anastomosis was located above the knee the patency rate was 100 per cent and 87 per cent for autogenous vein and velour Dacron graft respectively, while the corresponding figures for distal anastomosis below the knee were 58 per cent and 11 per cent. At follow-up 60 per cent of the patients showed improvement owing to the vascular reconstruction.     

Aneurysm in a double velour knitted Dacron graft

Double velour knitted Dacron grafts are currently used for aorto-iliac arterial reconstructions to increase encapsulation and enable rapid intimization of the grafts. Reports of an aneurysm arising in the body of a double velour knitted Dacron graft are rare. We experienced a case of aneurysm arising in the body of an innominate-abdominal aortic bypass graft four years after implantation. Microscopically, this focal aneurysm seemed to be related to a structural defect in the manufacturing process. We wish to stress that patients with implanted commercial Dacron prostheses should be carefully and regularly followed.     

Prediction of aneurysm formation in vascular grafts of biologic origin

Other than review of clinical experience, no assay exists that can reliably predict the long-term potential for aneurysm formation in an arterial prosthesis of biologic origin. Since mural degeneration probably results from proteolytic digestion, an in vitro assay was devised that used graft perfusion with 1% collagenase to induce rapid changes in mechanical properties. The effect of enzyme on graft diameter, compliance, permeability, and burst pressure was measured in ficin-digested, adipoyl chloride and glutaraldehyde-tanned bovine carotid artery and glutaraldehyde-tanned human umbilical vein. Both grafts have recently been reported to have a significant incidence of aneurysm within several years of implantation. Compliance and diameter were also measured noninvasively in patients with bovine carotid artery and human umbilical vein for more than 40 weeks after implantation. In vitro, the response to enzyme could be categorized into three groups. In group III, a diameter increase of more than 14% was associated with a significantly decreased compliance, and this paralleled the results found in aneurysmal grafts in vivo. In both grafts there was a strong correlation between postenzyme compliance change and initial compliance, loss of compliance being significantly greater in grafts with group III responses (p less than 0.01). This response may be a good predictor of a graft's overall susceptibility to aneurysmal degeneration, and initial compliance measurement may effectively identify inadequate fixation. Thus measurement of compliance may prove useful in quality control of a fixation process used in mass production. In conclusion, measurement of mechanical properties of biologic vascular grafts before and after collagenase exposure forms the basis for an effective in vitro assay of aneurysm susceptibility.     

Arm veins for peripheral arterial reconstruction.

The ipsilateral saphenous vein has become accepted as the best available material for femoropopliteal bypass and for arterial patch grafts as well as for visceral and cardiac bypasses. However, in a few patients, nonavailability or nonsuitability of the saphenous vein forces use of some other material. We report an experience with 32 operations using arm veins. Among the 11 long vein grafts, seven composite vein-Dacron or vein-vein grafts, and 14 vein patch grafts during the past six years, there were no infections or aneurysms and only nine thrombotic failures have been detected to date, to our knowledge. Our present indications are (1) ipsilateral saphenous vein is not available or is not suitable, (2) only a short graft or patch is needed and the saphenous vein may therefore be saved for the future, (3) to join to a saphenous graft or to a Dacron composite graft for additional length, and (4) to reconstruct an arm artery. Our experiences using cephalic and basilic veins confirms them as a useful source of autogenous material for arterial reconstruction.     

Effect of compliance mismatch on vascular graft patency

The hypothesis that a mismatch in compliance between a vascular graft and its host artery is detrimental to graft patency was tested by implanting paired arterial autografts, prepared with differential glutaraldehyde fixation of carotid arteries in the femoral arteries of dogs. These grafts differed only in circumferential compliance: they were 100% (compliant) vs. 40% (stiff) as compliant as the host artery. Their flow surfaces were equivalent, as determined by physicochemical measurements and scanning electron microscopy; both lacked viable cells, as determined by in vitro cell culture. In 14 dogs, eight stiff and two compliant grafts became occluded within 3 months, the latter doing so within 24 hours after their contralateral counterparts. Cumulative patencies were 85% and 37% for compliant and stiff grafts, respectively (p less than 0.05) and 100% and 43%, excluding the two dogs with bilateral graft failures (p less than 0.01). We conclude that even with near optimal flow surfaces, compliance mismatch is deleterious to graft patency.     

Autogenous veins and velour dacron in femoropopliteal arterial bypass.

Sixty-five patients treated by femoropopliteal bypass in 1974 were surveyed; the mean follow-up time was 10.4 months. The one year cumulative patency rate for velour Dacron was 50 percent; this was less successful than were the results from a comparable group in which vein grafts were used (79 percent). These poor results were due principally to the high failure rate of velour Dacron in patients suffering from clinically severe ischemia. In these only one in four grafts remained patent. If a less than perfect arteriographic runoff was obtained, only one in three still functioned. These results occurred despite high intraoperative graft flows. It appears that velour Dacron may be acceptable in patients treated for claudication if no adequate vein is available. This prosthesis gives an unacceptably high failure rate in patients with severe ischemia.     

Platelet Consumption by Arterial Prostheses: The Effects of Endothelialization and Pharmacologic Inhibition of Platelet Function

The thrombogenic mechanism of arterial grafts has been studied by determining the relative utilization of platelets, fibrinogen and plasminogen by human arterial prostheses, and by direct examination of arterial grafts in a baboon model. Forty-one survival and turnover measurements of (51)Crplatelets, (131)I-fibrinogen and (125)I-plasminogen in ten patients with aortofemoral knitted Dacron prostheses demonstrated platelet consumption after graft placement (platelet survival 4.2 days +/- 0.5 and turnover 68,000 plat/ul/day +/-10,000 compared with 8.2 days +/- 0.3 and 35,000 plat/ul/day +/- 5,000 respectively for control subjects with stable vascular disease, p < 0.01). In vitro platelet function test results were normal. Platelet consumption was interrupted by dipyridamole or a combination of dipyridamole and acetylsalicylic acid, and platelet survival normalized spontaneously during nine months postoperatively. No significantly increased consumption of fibrinogen or plasminogen was found in these patients with arterial grafts.Placement of impervious knitted Dacron velour aortic grafts in baboons reproduced platelet consumption that progressively normalized over six weeks postoperatively. Platelet survival measurements correlated directly with endothelial cell coverage of the graft luminal surface in these animals implying that endothelialization of the graft surface was also occurring postoperatively in patients.     

Reduction of intimal and medial thickening in sheathed vein grafts.

BACKGROUND: Arterial pressures are described as an important factor in the development of graft degeneration and in reduced patency rate in vein bypass grafts. Sheathing of the graft with a pressure resistant mesh tubing might slow down this development. METHODS: Saphenous vein grafts were implanted into the carotid arteries of five pigs in order to evaluate the influence on myointimal hyperplasia of a compliant Phynox mesh tubing (a wrought Cobalt-Chromium-Nickel-Molybdenum-Iron Alloy), which surrounded autologous vein grafts that were exposed to arterial pressure. Each pig was operated on using a sheathed vein graft (biocompound-graft, a hybrid vascular prosthesis) on one side and an untreated saphenous vein on the other. RESULTS: After 4 weeks intimal hyperplastic changes were found in all histological sections. The wall thickness (medial and intimal layer) varied from 351 microm to 432 microm in the biocompound-graft and from 391 microm to 1196 microm in the native vein grafts (p < 0.05, n = 5). Severe myocytial and fibroblast proliferation was only found in the control grafts. Cellularity of the medial layer differed at sites of maximal cellular density and ranged from 11 to 12 cells in the biocompound-graft and from 17 to 18 cells per counting field in the native vein grafts (p < 0.05, n = 5). CONCLUSIONS: External support of vein grafts reduces intimal and medial layer proliferation. The findings of this study are in accordance with the results reported by other research groups.     

Current arterial prostheses. Experimental evaluation by implantation in the carotid and circumflex coronary arteries of the dog.

Five types of 4-mm diameter arterial prostheses (three Dacron, one expanded Teflon, one preserved umbilical vein) were studied in the dog to assess graft thrombogenicity. Separate experiments involving six hours of controlled blood flow, one-week carotid implantation, and aortocoronary implantation were performed. In general, graft thrombogenicity derived from controlled flow study was more predictive of a graft's long-term implantation success than were one-week implantation results. In order of increasing thrombogenicity, we ranked grafts studied as follows: noncrimped Dacron, expanded Teflon, crimped Dacron, umbilical vein. Results of 19 experimental left coronary artery implantations using Dacron or Teflon prostheses are reported that indicate grafts with low measured thrombogenicity are most likely to succeed in this site. Data presented in this report suggest there is reason to evaluate noncrimped, kink-resistant, porous Dacron grafts for use both in the left coronary artery and below the knee when there is compelling clinical indication and no autogenous vessels are available.     

Prosthetic arteriovenous fistula for vascular access in hemodialysis.

Vascular access through subcutaneous prosthetic arteriovenous fistulas was studied in eighteen dogs. Dacron velour and woven Dacron grafts (6 mm diameter) were constructed across the lower abdomen between the common femoral artery and the opposite common femoral vein. In heparinized animals 197 percutaneous punctures were made with a "14 guage hemodialysis cannula at weekly intervals. Over a period of one and a half years there was no instance of infection. One of the fourteen Dacron velous and all four woven Dacron fistulas thrombosed. These data suggested the feasibility of achieving repetitive blood access through Dacron velour vascular prostheses. Nineteen Dacron velour fistula bypasses between the brachial artery and median basilic vein were performed in fifteen selected patients for a total dialysis period of ninety-six months. Failed standard subcutaneous fistulas or absence of suitable vessels in the upper extremity were indications for the primary procedure. Of three looped forearm fistulas, two thrombosed at twenty-two and two months. Complications among sixteen straight bypasses in the arm included two graft infections and one cannula tract infection. There were no instances of thrombosis in this group. The advantages of single needle dialysis in these high risk patients have been emphasized. Eleven grafts are presently functioning two to nine months postoperatively. Our preliminary results suggest that a Dacron velour fistula merits consideration as an alternative for vascular access in maintenance hemodialysis.     

The direct effect of graft compliance mismatch per se on development of host arterial intimal hyperplasia at the anastomotic interface

To study thedirect andsole effect of compliance mismatch on anastomotic intimal hyperplasia of the host arterial wall and to minimize possible confounding factors, dogs with a low thrombotic potential were selected as experimental subjects. Externally supported 6 cm × 5 mm Dacron grafts with a compliance value of approximately 1/300 of the host artery were implanted into the carotid arteries with end-to-end anastomoses on one side and end-to-side anastomoses on the other. The control graft was an autogenous carotid artery segment 4 cm in length transplanted into the femoral artery. Eight cases (24 grafts) were studied for 1 year and three (nine grafts) for 6 months. All were patent throughout the study period except for two noncompliant grafts with end-to-end anastomoses; thrombosis was the documented cause of occlusion. For the patent grafts, follow-up arteriograms showed no progressive narrowing of noncompliant anastomoses. Whether compliant or noncompliant, light microscopy studies showed slight intimal thickening within 1 to 2 mm of the anastomotic line, possibly the result of the normal healing response to stitch and surgical trauma. Quantitatively, 22 measurements representing longitudinal and circumferential thickness of the neointima were taken at each of the 40 patent noncompliant and 22 patent compliant control anastomoses. There was no statistically significant difference in anastomotic neointimal thickness in compliant and noncompliant grafts or for the different implantation periods. These data suggest that graft/host artery compliance mismatch does not cause arterial intimal hyperplasia at the anastomotic interface.Presented at the Seventh Annual Meeting of the Western Vascular Society, Maui, Hawaii, January 11–15, 1992.