Mechanical determinants of graft kinking.

Vascular grafts can be twisted inadvertently during implantation. If twisted excessively, they may kink and obstruct flow. In this study, in vitro experiments were performed to identify the mechanical factors that determine graft kinking. These included graft material, graft length, graft diameter, graft wall thickness, perfusion pressure, and flow rate. Six-millimeter-diameter saphenous veins were obtained from humans at autopsy. Six-millimeter standard-wall and 6-mm thin-wall polytetrafluoroethylene (PTFE) grafts also were obtained. Both fixed-length and stretchable PTFE grafts were examined. Grafts 15, 30, 50, and 70 cm in length were evaluated. Finally, PTFE grafts 4, 6, 8, 10, 12, and 14 mm in diameter were studied to determine the effect of diameter. The vessels were mounted horizontally in vitro and were perfused with saline at 50, 100, or 150 mmHg pressure at low (49 mL/min), medium (105 mL/min), and high (239 mL/min) flow rates. Each graft was twisted 90 degrees, then subjected to perfusion for 15 sec. Pressure and flow were interrupted, and an additional 90 degrees twist was imposed for another 15 sec. This sequence was repeated until a visible kink developed. We conclude from our results that, when constructing a bypass, particular care should be taken with vein, short grafts, thin-wall grafts, and large-diameter grafts, as these are especially susceptible to kinking.

Chronic loading and extension increases the acute breaking strength of polypropylene sutures.

Polypropylene sutures provide satisfactory strength for construction of vascular anastomoses, but occasionally they break. Experimental studies show that they break at reduced forces when they are subjected to chronic loads. Moreover, in patients, sutures are subject to acute loads superimposed on chronic loads. For example, an episode of hypertension applies acute load that is added to the baseline chronic load in a suture that has been used to close an arteriotomy. The purpose of the present study was to examine the breaking force of 6-0 polypropylene sutures subjected to acute loads after they had been loaded with chronic loads. One hundred sixty-five 6-0 polypropylene sutures were subjected to 50-175 g chronic loads in vitro. After 38 days they were subjected to additional increasing acute loads until they broke. Five hundred ninety other sutures were subjected to "injuries" of manipulation before chronic loading. A stray knot was simulated by placing a knot in the center of 90 sutures. Nurse's tugs used to straighten folded sutures in the operating room were simulated by applying brief loads of 75-275 g to 452 other sutures. Intraoperative injuries were simulated in 48 other sutures by pinching them with DeBakey forceps. Surprisingly, chronic loading of polypropylene sutures increased their acute breaking force. It is suggested that this may have resulted from increased orientation of crystals in the core of the filaments. By contrast, disturbing the outer surface of the filament by placing a stray knot, or pinching with forceps decreased acute breaking strength. These data suggest that if polypropylene sutures do not break soon after they have been placed in a patient, they may gain strength over time.

Mechanics of end-to-end artery-to-PTFE graft anastomoses.

On some occasions vascular surgeons are called upon to construct an end-to-end anastomosis using prosthetic graft material. If a spatulated anastomosis is not fashioned, three important variables that are under the surgeon's control could affect anastomotic dimensions: (1) selection of graft material, (2) graft size relative to the native vessel, and (3) suture technique. Accordingly, studies were performed on 36 nonspatulated, end-to-end artery-to-polytetrafluoroethylene (PTFE) grafts to evaluate the effects of graft size and suture technique on anastomotic dimensions. Size-matched (3 mm) and slightly oversized (4 mm) grafts were anastomosed end-to-end to 3 mm pig carotid arteries using (1) running polypropylene (Surgilene) sutures, (2) running polybutester (Novafil) sutures, or (3) interrupted sutures. After 30 min the vessels were excised, filled with contrast material, and radiographs were obtained to measure anastomotic dimensions. Results showed that, at every comparable pressure, 4 mm grafts produced larger anastomoses than did 3 mm grafts. In addition 4 mm grafts produced smoother anastomoses without a constricted or "pinched" appearance at the graft-artery junction. Marked compliance mismatch was observed with both sized grafts. There was no significant difference in the dimensions of the anastomoses or compliance mismatch with the three different suture techniques. These studies indicate that, when using PTFE grafts for end-to-end anastomoses, a graft that is slightly larger than the artery is preferable to provide the largest and smoothest anastomosis, and that any of the three suture techniques may be used.

Mechanics of spatulated end-to-end artery-to-vein anastomoses.

It previously has been shown that in straight end-to-end artery-to-vein anastomoses, maximum dimensions are obtained with an interrupted suture line. Nearly equivalent dimensions are obtained with a continuous compliant polybutester suture (Novafil), and the smallest dimensions are obtained with a continuous noncompliant polypropylene suture (Surgilene). The present study was undertaken to examine these suture techniques in a spatulated or beveled anastomosis in living dogs. Anastomoses were constructed using continuous 6-0 polypropylene (Surgilene), continuous 6-0 polybutester (Novafil), or interrupted 6-0 polypropylene or polybutester. Thirty minutes after construction, the artery, vein, and beveled anastomoses were excised, restored to in situ length and pressurized with the lumen filled with a dilute suspension of barium sulfate. High resolution radiographs were obtained at 25 mmHg pressure increments up to 200 mmHg. Dimensions and compliance were determined from the radiographic images. Results showed that, unlike straight artery-to-vein anastomoses, there were no differences in the dimensions or compliance of spatulated anastomoses with continuous Surgilene, continuous Novafil, or interrupted suture techniques. Therefore a continuous suture technique is acceptable when constructing spatulated artery-to-vein anastomoses in patients.

Intimal hyperplasia following thrombectomy versus thrombolysis in occluded vein grafts.

Although the histologic effects of balloon catheter thromboembolectomy in arteries are well described, little is known about its effects on arterialized vein grafts. A chronic canine model was used to compare the intimal hyperplasia that develops following balloon catheter thrombectomy versus thrombolytic therapy when each treatment was used to open experimentally occluded reversed autogenous vein grafts. Eleven of 12 dogs survived to the time of graft thrombosis and treatment. Ten grafts in one group of animals were treated with shear force-controlled balloon catheter thrombectomy, and eleven grafts in another group of animals were treated with infusion of urokinase (average 355, 833 IU/graft). Explantation and histologic evaluation was performed 5 weeks after treatment. Data were evaluated at comparable anatomic locations. These studies demonstrated the development of intimal hyperplasia in both groups with no statistically significant differences in the intimal thickening between the two treatment groups. It is hypothesized that vessel wall damage occurs at the time of thrombosis with the adherence of thrombus to the wall, and that this may be as important in producing intimal hyperplasia as the effects of carefully performed balloon thrombectomy or lytic therapy.

Venous flow through coiled and noncoiled umbilical cords. Effects of external compression, twisting and longitudinal stretching.

OBJECTIVE: To examine the association between umbilical cord coiling and perinatal morbidity. STUDY DESIGN: Ten umbilical cord segments, six coiled and four noncoiled, were categorized according to Strong's coiling index and were examined experimentally. The umbilical arteries and vein were perfused, pressurized, placed in a saline bath at 37 degrees C and subjected to compression, twisting and stretching while measuring venous flow. RESULTS: There was no statistically significant difference in umbilical venous flow between coiled and noncoiled cords when external compression, twisting and longitudinal stretching were applied to the cord segments. CONCLUSION: Differences in morbidity associated with umbilical cord coiling should not be attributed simply to mechanical factors, and other mechanisms should be sought.

Effect of pulsatile pressure on the breaking strength and movement of Prolene sutures.

PURPOSE: Most studies of sutures used in vascular surgery have used steadily applied loads. But in vivo, sutures are subject to pulsatile pressures. Pulsatile pressures could weaken sutures, or they could be beneficial by helping to slide the suture, thereby equilibrating the tension between unevenly tightened loops. This study examined the effect of pulsatile pressures on the strength and movement of polypropylene (Prolene) sutures. METHODS: Segments of pig thoracic aorta were cannulated and studied in a tissue bath in vitro at 37 degrees C. A longitudinal arteriotomy was made and then closed with running 6-0 Prolene. Vessels were subjected to (1) no pressure; (2) 100 mm Hg steady pressure; or (3) 112/65 mm Hg pulsatile pressure. After 48 hours the sutures were studied for breaking strength. In a second, separate set of experiments, longitudinal arteriotomies were closed using running suture lines. These were constructed with either several loose loops or several overly tight loops. Fine wires were affixed to the suture loops to permit photographic recording of suture movement. RESULTS: There was no difference in breaking force among 68 sutures that had been subjected to (1) no pressure; (2) 100 mm Hg steady pressure; or (3) 112/65 mm Hg pulsatile pressure. These also were no different than sutures that were not sewn into arteries. These findings indicate that neither steady nor pulsatile pressures weaken Prolene sutures. In the second set of experiments, it was found that pulsatile pressure did cause movement of suture lines where there were loose loops (p < 0.05). However, neither steady nor pulsatile pressures caused movement of loops in suture lines that contained normal and overly tight loops. CONCLUSIONS: Neither steady nor pulsatile physiologic pressures weaken 6-0 Prolene sutures that have been used to close a longitudinal arteriotomy. These pressures can cause movement of loose suture lines, but do not equilibrate the tension between normal and overly tight suture loops. Overly tight loops may place excessive load on a suture line.

Influence of suture technique and suture material selection on the mechanics of end-to-end and end-to-side anastomoses.

Experiments were performed in dogs to evaluate the mechanics of 26 end-to-end and 42 end-to-side artery-vein graft anastomoses constructed with continuous polypropylene sutures (Surgilene; Davis & Geck, Division of American Cyanamid Co., Danbury, Conn.), continuous polybutester sutures (Novafil; Davis & Geck), and interrupted stitches with either suture material. After construction, the grafts and adjoining arteries were excised, mounted in vitro at in situ length, filled with a dilute barium sulfate suspension, and pressurized in 25 mm Hg steps up to 200 mm Hg. Radiographs were obtained at each pressure. The computed cross-sectional areas of the anastomoses were compared with those of the native arteries at corresponding pressures. Results showed that for the end-to-end anastomoses at 100 mm Hg the cross-sectional areas of the continuous Surgilene anastomoses were 70% of the native artery cross-sectional areas, the cross-sectional areas of the continuous Novafil anastomoses were 90% of the native artery cross-sectional areas, and the cross-sectional areas of the interrupted anastomoses were 107% of the native artery cross-sectional areas (p < 0.05). At physiologic pressures, there were no differences in compliance among the three types of anastomosis. These data suggest that when constructing an end-to-end anastomosis in a small vessel, one should use an interrupted suture line or possibly continuous polybutester suture. Forty-two end-to-side anastomoses demonstrated no differences in cross-sectional areas or compliance for the three suture techniques. This suggests that, unlike with end-to-end anastomoses, when constructing an end-to-side anastomosis in patients any of the three suture techniques may be acceptable.

Failure of elastin or collagen as possible critical connective tissue alterations underlying aneurysmal dilatation.

Previous studies in the authors' laboratory have demonstrated that degradation of arterial elastin produces vessel dilatation, decreased vessel distensibility, and vessel elongation which can cause tortuosity. By contrast, degradation of collagen produces increased vessel distensibility and rupture. However, neither degradation of elastin nor of collagen produced the true gross enlargement characteristic of human aneurysms. The present study was performed to identify the connective tissue critical to aneurysm formation. Vessel dimensions were measured repeatedly in human arteries during progressive enzymatic degradation. Experiments were performed on six intact human common, external and internal iliac arteries, and two aneurysmal human common iliac arteries. The vessels were mounted in vitro and subjected to pressure steps up to 200 mmHg while diameters were measured. Repeated pressure-diameter curves were obtained for up to 18 h during treatment with elastase or collagenase. Degradation of elastin produced moderate dilatation (6-10% at 100 mmHg) with decreased vessel distensibility; this occurred as the load was shifted to remaining collagen. Degradation of collagen produced greater dilatation (10-23% at 100 mmHg), increased distensibility, and vessel rupture. These findings suggest that the critical element in both the gross enlargement and rupture of aneurysms resides in collagen. They also suggest that, in vessels obtained from patients with a family history of aneurysms, defects should be sought in: (i) the structure of collagen; (ii) increased susceptibility of collagen to degradation by endogenous mechanisms; (iii) increased endogenous collagenolytic activity; or (iv) decreased inhibition of endogenous collagenolytic activities.

Estimating the elastic modulus of non-atherosclerotic elastic arteries.

BACKGROUND: Non-invasive measurements of the diameter and wall thickness of arteries in vivo, combined with recordings of pressure, permit the assessment of some mechanical aspects of arteries in vivo. However, arteries are anisotropic cylinders with complex elastic properties which can only be determined by a detailed evaluation. Many of the parameters required to calculate the anisotropic circumferential elastic modulus cannot be obtained in patients in vivo, or can be obtained only with difficulty. AIMS: Experiments were performed to assess the true anisotropic circumferential elastic modulus, and to find simpler methods of estimation. METHODS: One hundred and twenty dog carotid arteries were mounted in vitro and were extended to four lengths, 120, 130, 160 and 180% of the retracted length. The different lengths were applied in random order. The vessels were pressurized in steps while the longitudinal force and vessel diameter were measured. When a reproducible mechanical behavior was observed, the pressure, diameter and longitudinal force were recorded. These values were used to calculate the longitudinal elastic modulus, the circumferential-longitudinal Poisson's ratio and the true anisotropic circumferential elastic modulus with respect to the dimensions of the fully unloaded vessel. A number of simpler expressions were then evaluated for their value as approximations of the true anisotropic modulus. RESULTS: Excellent estimates were obtained with a variety of expressions. CONCLUSIONS: These methods may be useful for non-invasive assessments of the elastic properties of non-atherosclerotic arteries in patients.

Balloon embolectomy catheter-induced arterial injury: a comparison of four catheters.

This study compared four brands of balloon embolectomy catheters with respect to their mechanical characteristics and the histologic responses they elicit. Seventy-two 4F Becton-Dickinson, Edwards, Electro-Catheter, and Shiley catheters were studied. In vitro studies of penetration forces demonstrated that the forces required for arterial puncture were greatest for Shiley (295 +/- 22 gm) and least for Edwards catheter tips (217 +/- 11 gm) (p less than 0.05). This indicates that the Shiley catheter is least likely to puncture vessels in patients. Studies of balloon eccentricity showed that none of the balloons distended with excessive eccentricity. Studies of balloon emptying time demonstrated that the silicon Becton-Dickinson balloon required more than two times as long (5.7 +/- 1.2 seconds) as all other balloons to empty. Balloon emptying time reflects the ability of the surgeon to rapidly adapt the balloon to changing vessel diameter in patients. Shear forces were studied in cylindrical segments of arteries in vitro. Initial shear forces were significantly different among all catheters, Becton-Dickinson greater than Edwards greater than Shiley greater than Electro-Catheter (p less than 0.05). In contrast, during catheter withdrawal dynamic shear forces were similar among the four brands of catheters. Balloon embolectomies were performed in vivo in the common carotid and common femoral arteries in 18 anesthetized dogs. Histologic examinations of the vessels exposed to 50, 100, and 200 gm shear forces showed that myointimal hyperplasia increased with rising shear forces for all catheters (p less than 0.05), but that there were no differences in the degree of myointimal hyperplasia elicited by the different brands of catheters.(ABSTRACT TRUNCATED AT 250 WORDS)

Longitudinal retractive force in pressurized dog and human arteries.

Experiments were performed to examine longitudinal retractive force in pressurized arteries in vitro. This force opposes vessel elongation and prevents the development of tortuosity. Common carotid arteries were excised from six adult dogs and external iliac arteries were excised from six elderly male humans at autopsy. Each vessel was mounted in a tissue bath at in situ length and was pressurized. Longitudinal retractive force was measured under control conditions and after treatment with elastase or collagenase. Results showed that, in the dog vessels, elastin provides all of the longitudinal retractive force. In the aged human vessels, both elastin and collagen provide longitudinal retractive force, with elastin contributing the much greater part.

Early myointimal hyperplasia after balloon catheter embolectomy: effect of shear forces and multiple withdrawals.

Arterial stenosis occurring after balloon catheter embolectomy may be caused by myointimal hyperplasia (MIH). This study investigated the effects of shear force and repeated catheter withdrawals on the development of MIH after embolectomy. The procedures were performed in the common carotid and common femoral arteries of 18 anesthetized dogs. During catheter withdrawal, the balloons were filled gradually to produce shear forces rising smoothly from 50 to 200 gm. Four weeks after embolectomy, the vessels were perfusion-fixed in situ with 2% glutaraldehyde and were excised. The thickness and circumferential extent of MIH were measured in cross sections from segments of the vessels exposed to shear forces of 50, 100, and 200 gm. Sixty-nine of 72 arteries remained patent. Low shear force (50 gm) consistently elicited less MIH than did higher shear forces (p less than 0.05). At each level of shear force, repeated withdrawals resulted in greater circumferential extent of MIH than did single withdrawals (p less than 0.005). Although clinically it usually is necessary to perform multiple passes during balloon embolectomy, these data suggest that, in humans, attempts should be made to perform a minimal number of catheter withdrawals at low shear force to minimize the subsequent development of MIH.