Crosslink formation in porcine valves stabilized by dye-mediated photooxidation.

Bovine pericardial and porcine valve materials stabilized by dye-mediated photooxidation have shown potential for bioprosthetic valve use. Previously, in vitro and in vivo stability of these materials was demonstrated through enzymatic, chemical, extraction, rat subcutaneous, and functional challenges. Here, we examine the stability of photooxidized porcine aortic valves through amino acid, crosslink, and hydrothermal isometric tension analysis. Photooxidation reduced intact histidine residues from 17.0 to 0 residues per 1000, indicating the photooxidative alteration of this amino acid. Diphenyl borinic acid-derivitized hydrolyzates of proteins were separated by high-performance liquid chromatography, which identified several amino acid crosslinks that appeared with photooxidation that were absent in untreated controls. Thermal relaxation analysis indicated a significantly higher (p < 0.0002) thermal stability for photooxidized porcine cusps than that of untreated controls, with mean relaxation times for untreated cusps of 14,000 +/- 4650 versus 22,900 +/- 2480 s for photooxidized cusps. In summary, porcine aortic valve tissue treated by dye-mediated photooxidation contains new chemical species and exhibits properties consistent with intermolecular crosslink formation, which explain the increased biostability of this material and its potential for use in bioprosthetic devices.

Effect of altered hydration on the internal shear properties of porcine aortic valve cusps.

BACKGROUND: Dehydration of tissue due to glutaraldehyde fixation has been reported and was examined in this study of porcine aortic valve cusps. The effect of altered hydration on cusp internal shear properties was also examined. METHODS: Hydration level was assessed by wet mass measurement of cusps stored in solutions for times up to 1000 minutes. Solutions used in this study included Hanks solution, porcine blood, 0.5% glutaraldehyde, and several dextran solutions. Shear testing was performed on physiologically hydrated, superhydrated, and dehydrated cusps. RESULTS: There was very little difference between the physiologic and superhydrated leaflets; however, dehydration caused significant stiffening with increased hysteresis and stress relaxation. CONCLUSIONS: Glutaraldehyde has been shown to increase shear stiffness of valve cusps. Tissue dehydration also increased shear stiffness but increased stress relaxation and hysteresis, which was contrary to observations reported after glutaraldehyde fixation. The significant effect of dehydration on cusp mechanical properties does not account for the effects observed after glutaraldehyde fixation, but it demonstrates that hydration level is an important factor that affects internal shear properties of valve cusps.

The pericardial bioprosthesis: altered tissue shear properties following glutaraldehyde fixation.

BACKGROUND AND AIM OF THE STUDY: Glutaraldehyde-fixed bovine pericardial tissue used in the construction of valvular bioprostheses undergoes repeated bending stress during the cardiac cycle. To bend smoothly, internal tissue shearing is required. The effect of glutaraldehyde fixation on internal shear properties of this material was examined. METHODS: Pericardium from each of 12 bovine hearts was cut into two pieces; one piece was retained as fresh tissue, the other was glutaraldehyde-fixed. Circular samples were then mounted and installed in a shear testing apparatus. For each sample, the shear stress versus shear strain characteristics were measured in circumferential and radial directions at strain rates of 1.0, 0.1 and 0.02 s(-1) while immersed in a 20 degrees C bath; similar measurements were made on six fresh and six fixed samples at 37 degrees C. In addition, the stress relaxation properties were measured by holding the tissue at maximum shear for 100 s after each of the three shear deformations, and recording force generated with time. RESULTS: The shear stress-strain test on fresh tissue (n = 12) showed non-linear behavior at the three shear rates. The shear modulus for fresh tissue increased from <1.0 kPa to 5 kPa at a shear strain approaching 1.0, and results were identical in radial or circumferential directions. For glutaraldehyde-fixed pericardium (n = 12), shear modulus increased promptly to 15-20 kPa at a strain of 0.2, and did not vary with strain rate. Shear relaxation was similar in fresh and fixed tissue. CONCLUSION: Fresh pericardium sheared easily at low shear stresses, with minimal resistance developing until the shear strain exceeded 0.5, while glutaraldehyde-fixed tissue displayed a marked resistance to shearing, with an immediate rise in shear stress at low strain. No differences were detected in shear properties between radial and circumferential directions. Such marked tissue stiffening may be a factor in collagen fiber disruption, leading to bioprosthetic heart valve failure.

Quasi-Linear Viscoelastic theory applied to internal shearing of porcine aortic valve leaflets.

The elements of Quasi-Linear Viscoelastic (QLV) theory have been applied to model the internal shear mechanics of fresh and glutaraldehyde-fixed porcine aortic valve leaflets. A novel function estimation method was used to extract the material functions from experimental shear data obtained at one strain rate, and the model was used to predict the material response at different strain rates. In general, experiments and predictions were in good agreement, the larger discrepancies being in the prediction of peak stresses and hysteresis in cyclic shear. In shear, fixed tissues are stiffer (mean initial shear modulus, 13 kPa versus 427 Pa), take longer to relax to steady state (mean tau 2 4,736 s versus 1,764 s) with a slower initial relaxation rate (mean magnitude of G(0), 1 s-1 versus 5 s-1), and relax to a lesser extent than fresh tissues (mean percentage stress remaining after relaxation, 60 versus 45 percent). All differences were significant at p = 0.04 or less, except for the initial relaxation slope. We conclude that shear experiments can complement traditional tensile and biaxial experiments toward providing a complete mechanical description of soft biomaterials, particularly when evaluating alternative chemical fixation techniques.

Internal shear properties of fresh porcine aortic valve cusps: implications for normal valve function.

BACKGROUND AND AIMS OF THE STUDY: Several types of stress act on aortic heart valve tissue during the cardiac cycle. When closed the valve is subjected to primarily tensile stress due to the diastolic pressure, and upon opening bending stress occurs near the attachment with the aortic root and throughout the body of the cusps. Smooth bending requires internal tissue shearing. To measure the internal shear properties of the tissue a testing device was created which combined a high-precision linear actuator with a sensitive load cell. MATERIALS AND METHODS: Circular punch biopsy specimens from fresh porcine aortic valve cusps (n = 32) were examined. The shear stress versus shear strain characteristics were measured both in the circumferential (n = 17) and the radial (n = 13) direction, and the stress relaxation characteristics were also examined circumferentially (n = 15) and radially (n = 15). In addition seven specimens were tested repeatedly in both radial and circumferential directions for tissue isotropy. RESULTS: The results from the shear stress versus strain tests showed the tissue to behave non-linearly over the strain range between -0.9 and 0.9. The average moduli at the near zero strains were less than 300 Pa and increased to over 20 kPa at the extreme strains. The circumferential direction yielded slightly higher average moduli than the radial direction but this difference was not significant. The stress relaxation results indicated that valve tissue relaxation occurs in two distinct phases, an initial low slope region and a second high slope region with respective values of -7.5 log(s)-1 and -15 log(s)-1 and with no significant difference between test directions. CONCLUSIONS: Our results define and describe the pattern of internal shear properties of the aortic valve that are particularly important during the transition between the open and closed positions. This behavior pattern has particular application in the creation of accurate mathematical models of the valve tissue and may be important in understanding the mechanism of tissue failure in bioprosthetic valves.

Glutaraldehyde fixation alters the internal shear properties of porcine aortic heart valve tissue.

Glutaraldehyde fixation noticeably alters the mechanical properties of porcine aortic valve tissue, subsequently affecting the function and durability of these tissues when used as prosthetic heart valves. Traditional uniaxial tensile testing techniques do not fully define the mechanical properties and we have devised a new approach to examine the important shear properties of the tissue. Altered shear properties would change the response of the valve tissue as it flexes open and closed. An apparatus combining a high-precision linear actuator with a gram-sensitive load cell was used to measure the shear characteristics of circular punch specimens taken from the center of each valve cusp. The tissue parameters measured showed significant differences between the fixed and fresh tissues. Glutaraldehyde-fixed tissue (n = 16) was about 100 times as stiff as fresh tissue (n = 32) between shear strain values of 0 and 0.2. The fixed tissue also had stress relaxation rates about 60% those of the fresh cusps and had about 70% of the hysteresis loss seen in fresh tissue. These results demonstrated the significant effects of glutaraldehyde fixation on the properties of porcine aortic valve cusp when tested in shear. Such changes could lead to altered tissue function and may increase internal stresses during opening and closing, contributing to valve fatigue.

Medial collagen organization in human arteries of the heart and brain by polarized light microscopy.

The mechanical properties of collagen as a biopolymer ensures that collagen has a significant influence on the mechanical behavior of the host tissue. Structural organization is a key to that influence. We have assessed this relationship quantitatively in the tunica media of arteries from the heart and brain, using the polarizing light microscope and Universal stage. Arteries from 22 autopsies were isolated, cannulated and fixed with 10% buffered formalin, at a distending pressure spanning normal values in vivo. We prepared the tissue for light microscopy, with paraffin embedding, sectioning at 7 microns, and staining with picrosirius red to enhance the natural birefringence of medial collagen. Individual measurements, 30 to 50 per arterial section, referenced against the central axis of the vessel segment, revealed a coherent organization, with an average orientation which was within 1 to 2 degrees of being perfectly concentric for all artery segments. Analysis was done with Lambert projections and circular statistics. We calculated the circular standard deviation, which was 5.2 degrees for 27 brain arteries (S.D. 1.9 degrees) and 5.6 degrees (S.D. 2.1 degrees), for 5 coronary arteries sectioned at less than 15 degrees. Our interpretation is that medial collagen can be strained even though highly aligned, revealing a mechanical property which contrasts that of type I collagen.

A 25-year experience with total portosystemic shunts and reappraisal of colon exclusion.

The results of 43 total shunt procedures for bleeding esophageal varices performed consecutively at two community hospitals from 1956 to 1981 are reviewed. Of 15 patients with immediate preoperative bilirubins greater than 2.0 mg/dl, 11 died following shunt surgery. Of 28 other shunted patients with immediate preoperative bilirubins of less than 2.0 mg/dl, there was only one in-hospital death, thus substantiating the contention that the last preoperative serum bilirubin value is the best predictor of operative mortality. Of ten patients with appreciable ascites verified at the time of operation, there were only two survivors, and both of these had preoperative bilirubins of less than 2.0 mg/dl. Twenty-nine of the 31 patients who left the hospital were still living at least one year after operation. All 23 patients operated on prior to 1977 were available for 5-year follow-up, and there were 14 survivors (60%). Thirteen of the 31 patients (42%) manifested some degree of hepatic encephalopathy, as interpreted by necessity for protein restriction and either Neomycin or Lactulose. Incapacitating post-shunt hepatic encephalopathy developed in one patient who required recurrent hospitalizations for episodic coma. This patient underwent a total abdominal colectomy and ileorectal anastomosis, with elimination of all episodes of encephalopathy for the subsequent 4 1/2 years. The previous 16 cases in the literature of surgical treatment of post-shunt encephalopathy are reviewed, and the efficacy of such colon exclusion is reassessed.

Role of arteriography in rectal hemorrhage due to arteriovenous malformations and diverticulosis.

Nine patients with arteriographic right colon arteriovenous malformations (AVM's) are reviewed. Even though five of the eight surviving patients had diverticulosis in remaining colon, right colectomy with removal of the AVM effectively controlled bleeding in seven of the eight patients in follow-up intervals of 14-66 months. Such results suggest that right colon AVM, rather than diverticulosis, may be the more common cause of chronic recurrent lower intestinal hemorrhage in the elderly. Rebleeding occurred in one patient and has heretofore been reported in seven cases. However, since AVM bleeding is rarely life-threatening, right colectomy for chronic recurrent rectal bleeding is justifiable as the initial operative procedure for an AVM, even in the presence of left colon or sigmoid diverticulosis. In the same five year period, 1973-1977, four additional patients with massive hemorrhage, characteristic of diverticulosis, had emergency arteriography with demonstration of a localized site of diverticular bleeding that was controllable by partial colon resection. Consequently, segmental resection of the colon, utilizing selective arteriography, has thus far eliminated bleeding in eight of nine patients with diverticulosis. The judicious use of arteriography in patients with diverticulosis and rectal bleeding substantially reduces the requirement for subtotal colectomy and ileoproctostomy.