(Radiology. 1999;213:229-246.)
© RSNA, 1999
Hemodynamics and Wall Mechanics after Stent Placement in Swine Iliac Arteries: Comparative Results from Six Stent Designs1
Pierre H. Rolland, PhD,
Ali-Baback Charifi, MD,
Caroline Verrier, MD,
Heidi Bodard, MSc,
Alain Friggi, PhD,
Philippe Piquet, MD,
Guy Moulin, MD and
Jean-Michel Bartoli, MD
1 From the Hemodynamics and Cardiovascular Mechanics Laboratory, (P.H.R., H.B., A.F.) and the Departments of Radiology (A.B.C., G.M., J.M.B.) and Vascular Surgery (C.V., P.P.), Faculte de Medicine, 27 boulevard Jean-Moulin, F-13385 Marseille 5, France. Received March 24, 1998; revision requested June 17; final revision received December 30; accepted March 23, 1999. Address reprint requests to P.H.R. (e-mail: lab.hmcv@medecine.univ-mrs.fr).
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Abstract
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PURPOSE: To compare the hemodynamics and wall mechanics of swine iliac arteries after placement of six types of stent.
MATERIALS AND METHODS: Stents were placed in the iliac artery of 18 pigs (three pigs each underwent placement with one of six types of stent); 16 untreated pigs served as control animals. Iliac arterial hemodynamics and wall mechanics were measured 4 days after placement.
RESULTS: Four stents (Palmaz-Schatz, Cordis, Warren, NJ; and Strecker, Cragg, and Symphony, Boston Scientific/Vascular, Natick, Mass) caused decreased pulsatile flow rate in the treated and contralateral iliac arteries; one (Memotherm; Bard, Covington, Ga) caused increased flow pulsatility; and one (Wallstent; Schneider, Plymouth, Minn) had no effect. No compliance mismatching was noted for the Cragg, Symphony, and Memotherm stents, whereas a decrease in compliance was noted for the Palmaz-Schatz, Strecker, and Wallstent designs. The Palmaz-Schatz and Strecker stents caused increased arterial wall rigidity, the Symphony and Wallstent designs had no effect, and the Memotherm and Cragg stents caused decreased wall rigidity. Stents made of stiff metal yielded different early results than did stents made of the less rigid nitinol.
CONCLUSION: Soon after implantation, the six stent designs elicited varying changes in blood flow, arterial compliance, and arterial wall mechanics. Contralateral arterial flow also was affected.
Index terms: Animals • Arteries, iliac, 984.1268 • Arteries, grafts and prostheses, 984.1268 • Interventional procedures, comparative studies, 984.1268 • Interventional procedures, experimental studies, 984.1268
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Introduction
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The environmental hemodynamics and parietal mechanical behavior of arteries, which differ markedly in healthy and diseased conditions, do not constitute the usual criteria for stent selection for endovascular prosthesis implantation. The therapeutic efficiency of stent implantation traditionally is evaluated by examining patency at the reconstructed lesion site, and this efficiency is strongly affected by early restenosis (1–7). Because the pathobiologic response to implantation of an endovascular device is thought to involve a complex interplay among device design, materials, and deployment technique, the interaction between stent and vascular walls has been addressed by numerous investigators (1,6–8).
In vivo arterial wall mechanics after stent placement (8) and placement-induced blood flow changes remain largely unexplored, although empiric models have been developed to characterize the elasticity of self-expanding stents (9,10), and flow disturbances have been reported for compliant in vitro flow pulsatile models (11,12). Although it is obvious that the precise determination of the hemodynamic and wall-rheologic properties after stent placement may be a key to successful therapeutic efficiency of stent placement, to our knowledge endovascular stents have not been compared under relevant in vivo conditions in terms of their hemodynamic and wall mechanical consequences in arteries and collateral vessels.
The purpose of this study was to compare the early hemodynamics and wall mechanics after stent placement in the iliac artery of pigs by using six types of stents.
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MATERIALS AND METHODS
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Animals
Thirty-four Pietrin pigs were included in this study. Sixteen served as control animals, and 18 underwent unilateral iliac arterial stent placement. Hemodynamics and wall mechanics were measured in the treated and untreated contralateral iliac arteries 4 days after placement.
The animals were handled in accordance with the guidelines of the animal care and use committee at our institution, which granted approval for this experimental protocol (13). The pigs were anesthetized with an intramuscular injection of ketamine hydrochloride (15 mg/kg), and a single, intravenous tube was placed in an epigastric vein for fluid administration. Three to six electrocardiographic leads and an oxygen saturation plethysmographic probe cuff (model HP 78354C; Hewlett-Packard, Andover, Mass) inserted around the tail were placed for intraoperative monitoring. The animals were further administered midazolam hydrochloride (Hypnovel; Hoffman–La Roche, Nutley, NJ) at a dosage of 0.5 mg per 10 mL of isotonic Ringer-lactate solution per minute for maintenance of anesthesia. Aseptic techniques were used throughout the procedure, and prophylactic antibiotics (200 mg of amoxicillin per 20 mL of clavulanic acid solution; total dose, 1 g of amoxicillin and 200 mg of clavulanic acid in 20 mL of isotonic saline solution) were administered intravenously before and after the radiologic procedures. Animals were euthanized with intravenous injection of 15 mL (20% wt/vol) potassium chloride.
Stents and Stent Placement
The following endovascular prostheses (7-mm-diameter in all cases) were used in the study (Fig 1): (a) a balloon-expandable, stainless steel, 30-mm-long stent (Palmaz-Schatz, type P304; Cordis, Warren, NJ); (b) a self-expandable, nitinol, 30-mm-long stent (Memotherm; Bard, Covington, Ga); (c) a self-expandable, cobalt-loaded, stainless steel, 32-mm-long stent (Wallstent; Schneider, Plymouth, Minn); (d) a self-expandable, nitinol, 40-mm-long stent (Cragg stent with 7-F Passager introducer system, Boston Scientific/Vascular, Natick, Mass); (e) a balloon-expandable, tantalum-knitted, 40-mm-long stent (Strecker; Boston Scientific/Vascular); and (f) a self-expandable, nitinol, 38-mm-long stent (Symphony; Boston Scientific/Vascular).
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Figure 1a. (a) Digital radiographs show the six stents deployed in the iliac artery of pigs in this study. The Strecker, Symphony, Wallstent, Memotherm, and Palmaz-Schatz stents are in the left iliac artery; the Cragg stent is in the right iliac artery. The two black dots at the proximal end of the Memotherm stent are radiopaque piezoelectric crystals embedded in the in situ diameter-measurement probe and the guiding catheter (which were withdrawn to the aorta during measurement); the tip of the pressure-measurement probe is emerging from the stent. (b) Angiogram shows the swine aorta and iliac and femoral arteries 4 days after implantation of a Palmaz-Schatz stent in the left iliac artery. Stent implantation was centered between the aortic trifurcation and the bifurcation between the superficial femoral and profunda femoris arteries (arrowheads). Note the early branching from the common iliac artery of the circumflex iliac artery (arrow), which was further ligated during investigation of hemodynamics and wall mechanics.
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Figure 1b. (a) Digital radiographs show the six stents deployed in the iliac artery of pigs in this study. The Strecker, Symphony, Wallstent, Memotherm, and Palmaz-Schatz stents are in the left iliac artery; the Cragg stent is in the right iliac artery. The two black dots at the proximal end of the Memotherm stent are radiopaque piezoelectric crystals embedded in the in situ diameter-measurement probe and the guiding catheter (which were withdrawn to the aorta during measurement); the tip of the pressure-measurement probe is emerging from the stent. (b) Angiogram shows the swine aorta and iliac and femoral arteries 4 days after implantation of a Palmaz-Schatz stent in the left iliac artery. Stent implantation was centered between the aortic trifurcation and the bifurcation between the superficial femoral and profunda femoris arteries (arrowheads). Note the early branching from the common iliac artery of the circumflex iliac artery (arrow), which was further ligated during investigation of hemodynamics and wall mechanics.
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Radiologic procedures were performed by using a digital angiography system (Stenoscope; GE Medical Systems, Milwaukee, Wis). A 10-cm-long, standard 7-F vascular sheath (Cook; Bloomington, Ind) was inserted in a surgically isolated right carotid artery by using the Seldinger method, and a 5-F angiographic pigtail catheter (Cordis) was advanced to the distal portion of the abdominal aorta over a hydrophilic guide wire (Terumo Medical, Somerset, NJ). An aortoiliac angiogram was obtained before stent placement by injecting 25 mL of contrast material (Hexabrix; Guerbet, Roissy, France) at a rate of 8 mL/sec to determine precisely the diameter of the iliac arterial lumen by using the 1-cm radiopaque markers on the angiographic catheter. The mean (± SD) luminal diameter of the iliac artery was 7.00 mm ± 0.15. The guide wire was returned to the aorta and advanced to the distal part of the iliac artery. The pigtail catheter was replaced either by the delivery device appropriate for the particular stent or by a 5-F, 110-cm-long catheter with a 7-mm-diameter, 40-mm-long percutaneous transluminal angioplasty balloon (Opta; Cordis). Placement of the Cragg stent was performed by using the Passager delivery system. However, this 7-F delivery system had a length of 45 cm, which prevented its use in the carotid artery. Therefore, the delivery system was inserted into the abdominal aorta, 2–3 cm below the renal artery, after midline laparotomy (described subsequently).
After replacement of the pigtail catheter in the abdominal aorta and removal of the guide wire, an aortoiliac angiogram was obtained, as was described for the preprocedural angiogram. The catheter was removed, the arterial puncture site was surgically closed, and the pig was transferred to the postoperative room to recover.
Hemodynamics and Wall Mechanics
Four days after stent implantation, the animals were anesthetized as described earlier, except that midazolam hydrochloride was withdrawn and replaced with urethane chloralose (20:5 wt/vol, 1 mL/kg). The pigs were perfused with Ringer-lactate solution and further ventilated (13,14).
The animals were again evaluated angiographically, as described earlier. The abdominal aorta and iliac arteries were then exposed by means of a midline abdominal incision, and the distal aorta and iliac arteries were carefully dissected in preparation for insertion of the perivascular diameter probe and the flow probe (13–15). The right and left deep circumflex branches of the external iliac arteries were ligated, and the left and right iliac arteries were examined with regard to hemodynamics and vascular wall rheologic properties prior to euthanasia.
Measurement sites in the iliac artery that contained the stent were located 1 cm proximal to the stent, 0.5 cm below the proximal end of the stent, at the middle of the stent, 0.5 cm above the distal end of the stent, and 1 cm distal to the stent. Measurement sites in the untreated (contralateral) iliac artery corresponded to the locations in the artery with the stent at 0.5 cm below the proximal end and 0.5 cm above the distal end of the stent; final control values were means from these positions.
Intravascular measurements of pulsatile blood pressure were performed with a 2-F catheter pressure transducer (Mikro-Tip SPR-249; Millar Instruments, Houston, Tex), which was advanced with fluoroscopic guidance to the precise iliac arterial positions by using a 6-F, 90-cm-long, multipurpose guiding catheter (Brite Tip; Cordis). The guiding catheter was withdrawn up the thoracic aorta during measurements.
Pulsatile flow measurements were carried out by using a blood flowmeter with the appropriate 7-mm perivascular flow probe (model T206 flowmeter, SB and RB probes; Transonic Systems, Ithaca, NY). In similar fashion, the pulsatile external diameter was evaluated by using a pair of 10-MHz ultrasonic probes positioned face to face externally around the vessel and inserted within a silicone clip probe that exerted negligible intrinsic strain and was of the appropriate diameter. The probes were connected to an ultrasonic dimension system (4-channel Sonomicrometer, model 401; Schuessler and Associates, Cardiff-by-the-Sea, Calif). Both the flow and diameter probes were checked for potential problems due to the presence of the metallic stent in the artery. The results showed that the presence of the stent had negligible effects on these devices (data not shown), presumably because the portion of the metallic surface of the deployed stent was small relative to that of the remaining free arterial surface.
Analog signals were depicted with a five channel oscilloscope (model 5103N; Tektronix, Guernsey Loyd, England) and were amplified with differential preamplifiers (type AM-502; Tektronix) and a one-channel differential amplifier (type 5A20N). The signals were recorded and digitized with an analog-to-digital converter (model ATMIO-16X; National Instruments, Austin, Tex) by using a 16-bit, 1,000-Hz sampling frequency.
At each site, data were recorded for 20 cardiac cycles, and data processing was performed by using a custom-designed data-capturing window in LABVIEW software (National Instruments). These data included average pressure, flow rate, and diameter during the cardiac cycles, which were obtained by using linear interpolation with a mean frequency that was equal to the sum of the numbers of the measurements within each cardiac cycle divided by the number of cycles times the sampling frequency. For each measurement position, analog recordings of pressure flow and pressure diameter were performed as coupled measurements to ascertain the instantaneous pressure-volume and pressure–flow rate relationships.
These data were plotted on graphs to depict their respective hysteresis loops. Drawing of averaged hysteresis curves in each group of animals, such as for a cardiac cycle in a given animal, was impaired because of excessive smoothing of curves due to between-animal variability in heart rate and in the consequent length of cardiac cycles. The hysteresis curves shown in Figures 2, 3c, 4c, 5c, 6c, 7c, and 8c were relevant to the specific features of an animal group, although they might be delayed in terms of time.
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Figure 2. Pressure-flow rate (P-Q, top) and pressure-diameter (P-Di, bottom) hysteresis loops obtained in the left iliac artery of a control pig. Note predominance of the elastic behavior over the viscous component in the control iliac artery, as illustrated by the large long axis-to-short axis ratio in the pressure-diameter hysteresis curve. This is in contrast to the pressure-flow rate hysteresis curve, where there is a short period of flow acceleration, with the maximum flow attained before the maximum pressure. IG255DE3, IG255DI3 = animal identification codes.
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Figure 3a. Results of Palmaz-Schatz stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Flow pulsatility is flattened downstream from the stent (I5) and in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the decreased compliance (Co) and pulsatility ( D) and the increased Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). ArS = arterial wall stiffness, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance,  m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. For measurements obtained distal to the stent, the pressure-diameter loop is shortened, and the pressure-flow rate loop is flattened. These results reflect flow flattening and wall stiffening below the stent. IG212Di1, IG212Di5, ID259DE1, ID259DE5 = animal identification codes.
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Figure 3b. Results of Palmaz-Schatz stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Flow pulsatility is flattened downstream from the stent (I5) and in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the decreased compliance (Co) and pulsatility (D) and the increased Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). ArS = arterial wall stiffness, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. For measurements obtained distal to the stent, the pressure-diameter loop is shortened, and the pressure-flow rate loop is flattened. These results reflect flow flattening and wall stiffening below the stent. IG212Di1, IG212Di5, ID259DE1, ID259DE5 = animal identification codes.
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Figure 3c. Results of Palmaz-Schatz stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Flow pulsatility is flattened downstream from the stent (I5) and in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the decreased compliance (Co) and pulsatility (D) and the increased Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). ArS = arterial wall stiffness, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. For measurements obtained distal to the stent, the pressure-diameter loop is shortened, and the pressure-flow rate loop is flattened. These results reflect flow flattening and wall stiffening below the stent. IG212Di1, IG212Di5, ID259DE1, ID259DE5 = animal identification codes.
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Figure 4a. Results of Memotherm stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increase in flow pulsatility (due to decreased diastolic flow) downstream from the stent (I5) and in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the increased compliance (Co) and pulsatility (D) and the decreased wall stiffness (ArS) and Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the lengthening of the pressure-diameter loop and the broadening of the pressure-flow rate loop. These results reflect increased flow pulsatility and decreased wall stiffening below the stent. ID251DE1, ID251DE5, ID251Di1, ID251Di5 = animal identification codes.
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Figure 4b. Results of Memotherm stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increase in flow pulsatility (due to decreased diastolic flow) downstream from the stent (I5) and in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the increased compliance (Co) and pulsatility (D) and the decreased wall stiffness (ArS) and Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the lengthening of the pressure-diameter loop and the broadening of the pressure-flow rate loop. These results reflect increased flow pulsatility and decreased wall stiffening below the stent. ID251DE1, ID251DE5, ID251Di1, ID251Di5 = animal identification codes.
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Figure 4c. Results of Memotherm stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increase in flow pulsatility (due to decreased diastolic flow) downstream from the stent (I5) and in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Note the increased compliance (Co) and pulsatility (D) and the decreased wall stiffness (ArS) and Young (Ep) and incremental (Einc) elastic moduli downstream from the stent (I5) and in the contralateral iliac artery (Ic). HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the lengthening of the pressure-diameter loop and the broadening of the pressure-flow rate loop. These results reflect increased flow pulsatility and decreased wall stiffening below the stent. ID251DE1, ID251DE5, ID251Di1, ID251Di5 = animal identification codes.
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Figure 5a. Results of Wallstent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that although changes in flow rates occurred within the stent, there were no significant changes in flow rates downstream from the stent (I5) or in the contralateral iliac artery (Ic), as compared with results in the control pigs (IC). (b) Bar graphs show hemodynamic and wall mechanical results. Note that downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) were only slightly decreased and wall stiffness (ArS) remained unchanged, whereas major changes in these parameters occurred within the stent. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. The slope of the pressure-flow rate loop is increased for measurements obtained downstream from the stent, which indicates that the flow wave accelerated relative to the pressure wave in early systole. Thinning of the pressure-diameter loop for measurements obtained downstream from the stent indicates that the diameter wave more closely followed the pressure wave than it did upstream from the stent. IG216DE1, IG216DE5, IG216Di1, IG216Di5 = animal identification codes.
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Figure 5b. Results of Wallstent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that although changes in flow rates occurred within the stent, there were no significant changes in flow rates downstream from the stent (I5) or in the contralateral iliac artery (Ic), as compared with results in the control pigs (IC). (b) Bar graphs show hemodynamic and wall mechanical results. Note that downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) were only slightly decreased and wall stiffness (ArS) remained unchanged, whereas major changes in these parameters occurred within the stent. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. The slope of the pressure-flow rate loop is increased for measurements obtained downstream from the stent, which indicates that the flow wave accelerated relative to the pressure wave in early systole. Thinning of the pressure-diameter loop for measurements obtained downstream from the stent indicates that the diameter wave more closely followed the pressure wave than it did upstream from the stent. IG216DE1, IG216DE5, IG216Di1, IG216Di5 = animal identification codes.
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Figure 5c. Results of Wallstent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that although changes in flow rates occurred within the stent, there were no significant changes in flow rates downstream from the stent (I5) or in the contralateral iliac artery (Ic), as compared with results in the control pigs (IC). (b) Bar graphs show hemodynamic and wall mechanical results. Note that downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) were only slightly decreased and wall stiffness (ArS) remained unchanged, whereas major changes in these parameters occurred within the stent. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. The slope of the pressure-flow rate loop is increased for measurements obtained downstream from the stent, which indicates that the flow wave accelerated relative to the pressure wave in early systole. Thinning of the pressure-diameter loop for measurements obtained downstream from the stent indicates that the diameter wave more closely followed the pressure wave than it did upstream from the stent. IG216DE1, IG216DE5, IG216Di1, IG216Di5 = animal identification codes.
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Figure 6a. Results of Strecker stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased flow rates in the contralateral iliac artery (Ic), to the detriment of flow rates in the artery with the stent. The pulsatility remained proportionally unchanged. (b) Bar graphs show hemodynamic and wall mechanical results. Downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) are markedly decreased, and wall stiffness (ArS) is markedly increased. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that the profiles of the hysteresis loops are retained but are substantially decreased form measurements obtained downstream from the stent. IG266DE1, IG266DE5, IG266Di1, IG266Di5 = animal identification codes.
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Figure 6b. Results of Strecker stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased flow rates in the contralateral iliac artery (Ic), to the detriment of flow rates in the artery with the stent. The pulsatility remained proportionally unchanged. (b) Bar graphs show hemodynamic and wall mechanical results. Downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) are markedly decreased, and wall stiffness (ArS) is markedly increased. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that the profiles of the hysteresis loops are retained but are substantially decreased form measurements obtained downstream from the stent. IG266DE1, IG266DE5, IG266Di1, IG266Di5 = animal identification codes.
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Figure 6c. Results of Strecker stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased flow rates in the contralateral iliac artery (Ic), to the detriment of flow rates in the artery with the stent. The pulsatility remained proportionally unchanged. (b) Bar graphs show hemodynamic and wall mechanical results. Downstream from the stent (I5) and in the contralateral iliac artery (Ic), compliance (Co) and wall pulsatility (D) are markedly decreased, and wall stiffness (ArS) is markedly increased. Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance, m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that the profiles of the hysteresis loops are retained but are substantially decreased form measurements obtained downstream from the stent. IG266DE1, IG266DE5, IG266Di1, IG266Di5 = animal identification codes.
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Figure 7a. Results of Symphony stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that the flow pulsatility profile was maintained but at a lower level downstream from the stent, with no flow imbalance in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Stress in the middle part of the arterial wall (m) within the stent (I3) remained normal, and compliance (Co) and wall pulsatility (D) were reduced downstream from the stent (I5). ArS = wall stiffness, Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the well-matched decreases in both hysteresis loops for measurements obtained downstream from the stent. IG267DE1, IG267DE5, IG267Di1, IG267Di5 = animal identification codes.
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Figure 7b. Results of Symphony stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that the flow pulsatility profile was maintained but at a lower level downstream from the stent, with no flow imbalance in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Stress in the middle part of the arterial wall (m) within the stent (I3) remained normal, and compliance (Co) and wall pulsatility (D) were reduced downstream from the stent (I5). ArS = wall stiffness, Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the well-matched decreases in both hysteresis loops for measurements obtained downstream from the stent. IG267DE1, IG267DE5, IG267Di1, IG267Di5 = animal identification codes.
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Figure 7c. Results of Symphony stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note that the flow pulsatility profile was maintained but at a lower level downstream from the stent, with no flow imbalance in the contralateral iliac artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Stress in the middle part of the arterial wall (m) within the stent (I3) remained normal, and compliance (Co) and wall pulsatility (D) were reduced downstream from the stent (I5). ArS = wall stiffness, Einc = incremental elastic modulus, Ep = Young elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note the well-matched decreases in both hysteresis loops for measurements obtained downstream from the stent. IG267DE1, IG267DE5, IG267Di1, IG267Di5 = animal identification codes.
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Figure 8a. Results of Cragg stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased mean flow rate in the artery with the stent and, conversely, the decreased mean flow rate in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Compliance (Co) and pulsatility (D) are increased, and wall stiffness (ArS) and Young elastic modulus (Ep) are decreased downstream from the stent (I5) but not in the contralateral iliac artery (Ic). Einc = incremental elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that both hysteresis loops returned to control shapes for measurements obtained downstream from the stent, although both were flattened for measurements above the stent. IG272DE1, IG272DE5, IG281Di1, IG281Di5 = animal identification codes.
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Figure 8b. Results of Cragg stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased mean flow rate in the artery with the stent and, conversely, the decreased mean flow rate in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Compliance (Co) and pulsatility (D) are increased, and wall stiffness (ArS) and Young elastic modulus (Ep) are decreased downstream from the stent (I5) but not in the contralateral iliac artery (Ic). Einc = incremental elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that both hysteresis loops returned to control shapes for measurements obtained downstream from the stent, although both were flattened for measurements above the stent. IG272DE1, IG272DE5, IG281Di1, IG281Di5 = animal identification codes.
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Figure 8c. Results of Cragg stent placement in three pigs, in comparison with results in the untreated contralateral artery (Ic) and in untreated control pigs (IC). Measurements were performed 1 cm above (I1) and 0.5 cm below (I2) the proximal end of the stent, at the middle of the stent (I3), and 0.5 cm above (I4) and 1 cm below (I5) the distal end of the stent. (a) Bar graphs of hemodynamic results show maximum blood flow rate (black bars), minimum blood flow rate (white bars), flow pulsatility (dotted bars), and mean flow rate (gray bars). Note the increased mean flow rate in the artery with the stent and, conversely, the decreased mean flow rate in the contralateral artery (Ic). (b) Bar graphs show hemodynamic and wall mechanical results. Compliance (Co) and pulsatility (D) are increased, and wall stiffness (ArS) and Young elastic modulus (Ep) are decreased downstream from the stent (I5) but not in the contralateral iliac artery (Ic). Einc = incremental elastic modulus, HPR = peripheral resistance, PR = relative pulsatility, Zc = input impedance m = stress in middle part of arterial wall. (c) Typical pressure-flow rate (P-Q) and pressure-diameter (P-Di) hysteresis loops for measurements obtained 1 cm above (top left and bottom left) and 1 cm below (top right and bottom right) the stent. Note that both hysteresis loops returned to control shapes for measurements obtained downstream from the stent, although both were flattened for measurements above the stent. IG272DE1, IG272DE5, IG281Di1, IG281Di5 = animal identification codes.
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The data also were plotted against time for direct interpretation of the hemodynamics and wall mechanics, as previously described (13,14), and were modified as follows: Systolic, diastolic, mean, and pulsatile flow rate; vessel diameter; and blood pressure were measured directly (data not shown). Peripheral resistance (in mm Hg/mL/min) was calculated as the mean pressure divided by the mean flow rate. Arterial input impedance (in mm Hg/mL/min) was calculated as the change in pressure divided by the change in flow rate. The pulsatility of the artery was expressed both as the change in diameter (in centimeters) and as the relative pulsatility (a percentage), which was calculated as the pulsatile diameter divided by the mean diameter, which in turn was calculated as the systolic diameter plus the diastolic diameter divided by two. Aortic volume compliance (in mL/mm Hg) was calculated by multiplying the change in the internal area (obtained from diameter measurements) by the length of the vessel and dividing by the change in pressure. Arterial wall stiffness (in mm Hg/cm) was calculated by dividing the change in pressure by the change in diameter. Radial arterial stress in the middle of the wall (in kN/m2) was calculated as 2P(ab/R)2/(b2 - a2), where P is the systolic or diastolic pressure; a and b are the inner and outer radii, respectively; and R is the mean radius ([a + b]/2). It should be noted that the above expression includes only stress due to pressurization; it does not include any additional radial or circumferential stress applied to the vessel by the stent or any residual stresses. The expression also assumes small deformations (13,14). The mean arterial stress in the middle of the wall was defined as stress during systole plus stress during diastole, divided by two. The Young elastic modulus (in mm Hg/cm) was calculated as the change in pressure divided by the change in diameter, times the mean diameter divided by the mean wall thickness (ie, systolic plus diastolic thickness, divided by two). The incremental elastic modulus (in kN/m2) was calculated as 0.75R · 
Sm/R, where Sm is the difference between systolic and diastolic stresses, and R is the difference between systolic and diastolic radii.
Statistics
Data are reported as means plus or minus SDs. Statistical analyses (analysis of variance and Mann-Whitney U test) were performed by using GB-STATS software (Dynamic Microsystems; Silver Spring, Md). A P value of less than .05 was considered to indicate a significant difference.
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RESULTS
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Hemodynamics and Wall Mechanics in Control Iliac Arteries
In the control iliac arteries, the external diameter changes closely followed those of the pressure wave, with sharp, abrupt changes characteristic of the early systolic and late diastolic phases, thereby illustrating the dominance of elastic wall behavior over viscous effects (13–15). In contrast, the period of flow acceleration was short relative to that of the overall cardiac cycle, and maximal systolic flow rate was achieved before maximal systolic pressure. Conversely, volume flow rate returned to the diastolic level that leads the pressure wave (Fig 2). Hemodynamic and wall mechanical results for the control vessels are shown in Figures 3–8.
Patency: Angiographic and Macroscopic Findings
All animals survived stent placement and were euthanized at 4 days. At the time of sacrifice, control angiograms demonstrated that none of the iliac arteries with a stent were occluded. All segments with a stent were patent, and examination of the luminal surface revealed that stents were successfully implanted. No technical error could be detected, and incomplete stent expansion was not observed, nor were stent collapse or compression (Fig 1). There was no sign of thrombosis within the stent. The mean iliac arterial luminal diameter was 7.0 mm ± 0.15 on day 1 and on day 4. Data about pressure and diameter measurements are not shown, because they were found not to be significantly changed due to stent placement in the iliac arterial bed, which is usually the case for blood pressure in large (>5-mm-diameter) arteries.
Palmaz-Schatz Stent
Hemodynamic and wall mechanical measures for the Palmaz-Schatz stent are shown in Figure 3. Although the mean blood flow rates in the artery with the Palmaz-Schatz stent and in the contralateral (control) vessel were identical to their corresponding values in the iliac arteries in the control pigs, the constituent parts of the pulsatile flow rate were markedly changed in the iliac arterial bed with the stent, as compared with those in the control vessels. The minimum blood flow rate was markedly increased (P < .05), while the pulsatile flow (change in flow rate) was decreased (P < .05). Although the maximal flow rate was unchanged upstream from the stent and in the contralateral iliac artery, the maximum blood flow rate downstream from the stent was decreased, which led to a further decrease in the change in flow rate (P < .05). These led to substantially dampened pulsatile flow distal to the Palmaz-Schatz stent without a flow imbalance between the two iliac arteries. In the iliac artery with the stent, the input impedance was markedly increased both proximal to and within the stent (P < .05) before it returned to the value for the contralateral artery at the location distal to the stent. Peripheral resistances also were lower in both the treated and contralateral iliac arteries, as compared with those in the control animals (P < .05). This follows from the need to maintain a balance of flow between both iliac arteries and was presumably related to the attenuation in flow pulsatility (specifically, energy loss due to transient changes in pulsatile flow rate were conserved).
Compliance, relative pulsatility, and pulsatile diameter were dramatically lowered within the stents, relative to those in control pigs and in the contralateral arteries (P < .001). A marked, isolated increase in compliance was detected proximal to the entry of the stent, along with increased pulsatile flow rate, both of which occurred without a detectable change in blood pressure (data not shown). Wall stiffness and the Young and incremental elastic moduli were double the values for control animals in both the contralateral iliac artery and the treated iliac artery distal to the stent (P < .05). However, these final values only partly reflected the progressive increase in stiffness and elasticity from above to below the stent; specifically, stiffness and elasticity in the distal portion with the stent reached four times their respective control values (P < .001). This process illustrates the increase in stiffness and elasticity, which progressively extended from the stent to the surrounding vascular walls. Stress in the middle portion of the wall was substantially increased within the stent but not in the surrounding vessel or in the contralateral control vessel. This increase was due to the fact that the stent was expanded with a balloon to match the vessel diameter; thus, this effect would not necessarily transfer to the surrounding tissues within the 4 days of this study. Solid mechanical changes in the contralateral iliac artery paralleled the changes induced in the flow rate waves. Pressure-diameter and pressure–flow rate hysteresis loops demonstrated that Palmaz-Schatz stent placement induced the flow rate wave to follow more closely the pressure wave, especially distal to the stent. The observed increases in wall stiffness and elasticity flattened the pressure-diameter loops.
Memotherm Stent
Hemodynamic and wall mechanical measures for the Memotherm stent are shown in Figure 4. When compared with the measures in control pigs, significantly increased flow pulsatility was detected in the treated and contralateral iliac arteries, primarily due to a lower minimum blood flow rate (P < .01). The presence of negative minimal diastolic flow rate caused the mean flow rate to be diminished (P < .05). These features of flow were observed both proximal and distal to the stent. Impedance and peripheral resistances exhibited no significant changes in either the iliac arteries with a stent or those without a stent. In comparison with the contralateral (control) iliac artery, compliance, relative pulsatility, and pulsatile diameter were moderately increased, and compliance was doubled proximal and distal to the stent (P < .001). Compliance within the stent was identical to that in the control iliac arteries. Wall stiffness and elastic moduli were dramatically decreased in the contralateral iliac artery, as well as in the regions proximal and distal to the stent, (P < .01), although these measures were higher in the stent itself than in the surrounding vessel (P < .05). Stress in the middle part of the iliac arterial wall was unchanged outside the stent and was only slightly increased (not significantly) in the stent itself. The pressure-diameter and pressure–flow rate hysteresis loops for the Memotherm stent showed no significant changes from those in the control segments.
Wallstent
Hemodynamic and wall mechanical measures for the Wallstent are shown in Figure 5. The mean blood flow rate was unchanged in treated and control iliac arteries and was comparable to that in the arteries in the control pigs. Flow pulsatility was diminished immediately proximal to the stent and was increased within the stent (P < .05). Outside the Wallstent, flow pulsatility was moderately reduced, as compared with that in control animals, and remained similar in both the contralateral iliac artery and distal to the stent. Impedance and peripheral resistances did not exhibit significant changes in the overall iliac arterial bed, except for a transient increase in the input impedance immediately proximal to the stent (P < .05), which was accompanied by a lower change in flow rate at that site. Compliance, relative pulsatility, and pulsatile diameter were flattened out within the stent (P < .01) and were decreased, but to a lesser extent, in the contralateral iliac artery and distal to the stent (P < .005). A significant increase in compliance was detected upstream from the stent (P < .01). Stiffness and elastic moduli were increased within the stent, as was the stress in the middle part of the wall (P < .01). All three indexes returned to control values in the arterial bed distal to the stent. The pressure-diameter and pressure–flow rate hysteresis loops for measurements distal to the stent revealed that the flow rate wave accelerated relative to the pressure wave in early systole and that the diameter wave followed more closely the pressure wave than it did proximal to the stent.
Strecker Stent
Hemodynamic and wall mechanical measures for the Strecker stent are shown in Figure 6. Strecker stent placement resulted in increased blood flow rate to the contralateral iliac artery (P < .05), whereas the pulsatile flow rate remained proportionally unchanged all along the artery with the stent. Impedance and peripheral resistances were substantially increased in the treated iliac artery (P < .05), which accounted, at least in part, for the flow rate imbalance. Compliance, relative pulsatility, and pulsatile diameter were markedly decreased in both the treated and the contralateral (control) iliac arteries (P < .01), and there were significant increases in compliance and distensibility proximal to the stent (P < .05). Stiffness and elasticity were increased in the artery distal to the stent and in the contralateral artery (P < .05), more so than in the stent itself, which was due presumably to the annealed coil-like tube wire (16). In agreement with the stiffness characteristics, stress in the middle part of the wall was increased after stent placement. The pressure-diameter and pressure–flow rate hysteresis loops revealed that flow rate and diameter profiles were retained but were substantially lowered distal to the stent.
Symphony Stent
Hemodynamic and wall mechanical measures for the Symphony stent are shown in Figure 7. The mean flow rate in the artery with the stent was identical to that in the contralateral artery. Blood flow pulsatility was substantially reduced throughout the iliac artery with the stent (P < .05), without noticeable changes within the stent itself, as compared with pulsatility in the arterial segments proximal and distal to the stent. Input impedance and peripheral resistances were not changed in the contralateral artery but were increased in the treated artery (P < .05). Compliance, relative pulsatility, and pulsatile diameter were reduced after stent placement. No increase in iliac arterial compliance was observed proximal to the stent. Arterial wall stiffness and elasticity were increased only in the proximal portion of the stent (P < .05). Stress in the middle part of the wall was moderately increased within the stent but not in the artery as a whole. The pressure-diameter and pressure–flow rate hysteresis loops revealed that flow rate and diameter profiles were retained, but less hysteresis was observed distal to the stent.
Cragg Stent
Hemodynamic and wall mechanical measures for the Cragg stent are shown in Figure 8. Cragg stent placement resulted in an increased mean flow rate and a diminished pulsatile flow rate in the treated iliac artery, as compared with those in the contralateral artery (P < .05). Flow pulsatility was proportionally increased in the contralateral iliac artery and, conversely, decreased in the treated artery, as compared with that in control pigs. Peripheral resistances were significantly increased (P < .05), and the minimum blood flow rate was decreased in the contralateral artery, but the resistances were unchanged in the stent. In the treated iliac artery, input impedance was markedly increased both proximal to and within the stent prior to returning to the contralateral value downstream from the stent (P < .05). Compliance, relative pulsatility, and pulsatile diameter were reduced in the stent, whereas they were significantly increased distal to the stent and in the contralateral arteries (P < .05). No increase in iliac arterial compliance was observed proximal to the stent. Stent placement resulted in increased stiffness and elasticity of the vessel proximal to the stent (P < .05), whereas these parameters abruptly decreased distal to the stent. Stress in the middle part of the wall was significantly increased proximal to and within the stent (P < .05). The pressure-diameter and pressure–flow rate hysteresis loops revealed that flow and diameter pulsatility were retained distal to the stent. Stent placement brought the flow rate and diameter waves close to the pressure wave, due to the stent-induced effects in wall compliance, pulsatility, and stiffness distal to the stent.
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DISCUSSION
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To our knowledge, this is the first study to demonstrate that stent placement in one of the daughter iliac arteries in the vicinity of their origin from the aorta induced marked simultaneous changes in the hemodynamics of the overall iliac arterial network. Hemodynamics and wall mechanics of the contralateral iliac artery were found to be significantly different from those in control animals (ie, animals who did not undergo radiologic procedures and stent placement), which impaired the use of the contralateral iliac artery as a true control vessel. Significant changes also were noted in single-vessel iliac arterial hemodynamics distal to the stent and in the iliac arterial wall mechanics in the walls surrounding the stent and in the overall arterial bed.
Although the stent-induced changes varied dramatically from one type of stent to another, the present results show that there are common features shared by stent placement in two domains of hemodynamics (iliac arterial flow pulsatility and input impedance in the artery with the stent) and in two domains of wall mechanics (compliance and wall stiffness); these commonalities accordingly enabled stent classification.
Hemodynamic Classification
The present results showed that among flow characteristics, only the pulsatile nature of blood flow was increased because of stent placement. When flow pulsatility was diminished, resistance and input impedance increased. It was not definitely established that increases in resistance were causally related to attenuated pulsatile flow, although these phenomena were observed together.
With regard to hemodynamics, the six stents could be classified into the following three groups: (a) flow rate–flattening stents (Palmaz-Schatz, Strecker, Cragg, and Symphony), which attenuated the pulsatile nature of the flow while they caused an imbalance in or left unchanged the collateral flow rate; these stents also increased input impedance; (b) a flow rate–favoring stent (Memotherm), which favored the pulsatile nature of flow but did not change the input impedance of the arterial bed that contained the stent; and (c) a flow rate–neutral stent (Wallstent), which did not cause an imbalance in iliac arterial blood flow, affect flow pulsatility, or change the input impedance of the arterial bed that contained the stent.
Compliance Matching and Mismatching
We investigated the iliac arterial network strains (ie, changes in volume due to wall circumferential elongation) in response to stress (ie, blood pressure during distention), by calculating vessel compliance, distensibility, and relative pulsatility. These factors express how the elastic wall reacts in response to hemodynamic forces, as well as how the vessel recoils to aid in further propulsion of the blood along the vasculature (17).
The fact that blood pressure is the same throughout the aortoiliac network (18) was an advantage for the present study, because it enabled comparison of mechanical responses from different arterial sites in response to identical loading conditions. The present results demonstrated that changes in wall distensibility in the contralateral iliac artery were remarkably similar to those distal to the stent. Changes in wall distensibility one to two vessel diameters proximal to the stent were heavily dependent on the stent material and design.
The response of the cardiovascular system as a whole to the presence of a stent may include adaptations that return the pulsatile and/or mean flow rates to their previous values. Any such changes would probably take place within a time frame longer than the 4 days of the present study. These adaptations are likely to be driven somewhat by the degree to which the stent maintains its compliance. The stent-induced changes in distensibility and compliance are termed "compliance matching" when stent placement resulted in preserved iliac arterial wall pulsatility patterns and "compliance mismatching" when the strain response was substantially reduced. Under these conditions, the present results demonstrated that the Cragg, Symphony, and Memotherm stents were compliance-matching stents, and the Palmaz-Schatz and Strecker stents and the Wallstent were compliance-mismatching stents.
Rigidity Classification
Blood vessel walls are soft tissues. The arterial wall is composed of a hyperelastic, solid material with heterogeneous, nonlinear, anisotropic, viscous rheologic properties. The structural basis for this behavior is the composition of the wall, which includes collagen, elastin, and smooth muscle cells; mechanical forces also play an important role in the modification of the vascular structure (19,20). The vessel remodels its geometry, structure, and composition in response to altered loading in a way that maintains the magnitude of circumferential stress in the arterial wall, as well as the flow-induced shear stress at the inner surface, at nearly normal levels (21).
The presence of residual strain and stress in the arterial wall provides a means for the vessel to adapt to mechanical loading. The principal effect of residual strain is to make the stress distribution more uniform throughout the arterial wall (22). Arteries can distribute loading by means of adaptations in material and structure, so that the stress in the arterial wall is close to uniform (23). Furthermore, the arterial wall configures itself to produce a more homogeneous stress distribution from the inner wall to the outer wall (22). We, therefore, investigated the wall mechanics in the iliac arterial bed, both with and without stent placement, in an attempt to evaluate the adaptive changes in the arterial wall that are a response to the implantation of stents of different designs.
All stents, whatever their structure, are typically more rigid than the artery into which they are implanted, and, under physiologic stresses (blood pressure, vascular tone), stents are essentially elastic and have small strains. In this study, no plastic deformation of the stent was detected, and the arterial diameter remained constant from day 1 to day 4. Stent-induced changes in arterial wall stiffness in and adjacent to the stent were quantified, with special attention to the vessel distal to the stent. The present results showed that the Palmaz-Schatz and Strecker stents caused the arterial wall to become more rigid, whereas the Symphony stent and the Wallstent had no effect. The Memotherm and Cragg stents decreased the stiffness of the arterial wall distal to the stent.
Mechanistic Interpretation of the Results
The exact mechanisms responsible for the changes in hemodynamics and wall mechanics observed in the present study remain largely unknown. Stent-induced flow disturbances have been reported (11,12) for two in vitro models designed to simulate pulsatile flow through compliant arterial segments implanted with the Palmaz stent. Specifically, flow disturbances, as analyzed by means of dye injection within plastic Palmaz stent models, reveal flow rate stagnation between the stent struts during systole and formation of a large vortex in the stent during systolic deceleration and early diastole. This vortex subsequently moves upstream during diastole to a position approximately one and a half vessel diameters proximal to the stent (11). The Palmaz stent creates nonlaminar flow rate patterns with pronounced flow rate reversals, although the flow rate waveform has no negative flow rate component. In addition, there is an abrupt change in compliance at both the proximal and the distal ends of the Palmaz stent. The compliance mismatch creates sites of pressure pulsatile wave reflection at the proximal and distal ends of the stent, which are partially responsible for the vortex formation seen in early diastole (11). When an unsteady pressure wave is traveling along a compliant vessel, any change in impedance will produce a reflection of the pressure wave. It was therefore hypothesized that the vortex formations proximal to the Palmaz stent are the result of flow impedance changes across the treated segment of compliant vessel (11).
Although some of the consequences of stent placement within the pulsatile flow in a compliant tube, as d
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Abstract
Introduction
