David F. Warnock

Effect of compliance mismatch on vascular graft patency

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

A compliant tubular device to study the influences of wall strain and fluid shear stress on cells of the vascular wall

PURPOSE Cellular constituents of the blood vessel wall are continuously subjected, in vivo, to both mechanical and hemodynamic forces, which elicit structural and biologic responses. We have developed a compliant tubular system, the vascular simulating device (VSD), that reproduces these forces, while supporting the attachment and the experimental manipulation of endothelial and smooth muscle cells. METHODS The VSD consists of a compliant silicone rubber tube coupled to a pump system, which permits the simultaneous application of known levels of pressure and flow, to vascular wall cells cultured on the inner surface of the tube. Seeded cells can be monitored visually under phase contrast or fluorescent optics, as well as harvested and analyzed for biologic responses. RESULTS The elastic modulus and compliance of the silicone rubber tube are similar to those of canine and human arteries. Endothelial and smooth muscle cells cultured on the lumenal surface of the tubes remain attached and viable after subjecting them to physiologic pulsatile flow and cyclic strain. CONCLUSION The VSD makes it possible to approximate, in vitro, those forces encountered by vascular wall cells, in vivo and therefore may make it possible to determine whether specific combinations of mechanical and hemodynamic forces are causally associated with specific vascular diseases.

Intraoperative autotransfusion in major vascular surgery

The use of intraoperative autotransfusion provides a safe and cost-effective means of salvaging operative blood loss and reducing or eliminating the use of stored homologous bank blood with its inherent difficulties and risks. The risk of disease transmission or various reactions is minimized. Autotransfusion provides a readily available, more physiologic, and at times life-saving source of blood for patients with rare blood types or patients in whom time does not permit adequate cross-matching. This technique is acceptable to most sects of Jehovahs Witnesses, who normally refuse homologous blood. Our experience during the past six years with autotransfusion in major vascular surgery reveals a mean slavage equivalent to five units of blood loss, and avoidance of using any bank blood in almost half of elective patients. No significant problems occurred due to hemolysis, coagulation abnormalities, or particulate/air emboli, nor any morbidity or mortality specifically related to autotransfusion. We conclude that wider and more frequent use of autotransfusion technics is appropriate.

The evolution of morphologic and biomechanical changes in reversed and in-situ vein grafts.

A comparative study of experimental reversed (RV) and in-situ (INS) vein grafts with respect to the evolution of morphologic and compliance characteristics was done in a canine model. In addition, the compliance characteristics in a series of human INS vein grafts were recorded as a function of time after operation. At 6 months after implantation, all experimental grafts displayed well-developed intimal hyperplasia. There was no significant difference in either absolute intimal thickness (INS 0.133 +/- 0.09 mm vs. RV 0.085 +/- 0.06 mm; NS) nor in the percentage of the total wall thickness occupied by the intima when experimental INS grafts were compared with RV grafts after 6 months. Similarly, compliance values of INS and RV vein grafts were similar at all time intervals examined up to 6 months after operation. Thirty-three human INS vein grafts had a mean compliance value of 1.74 +/- 0.72 (percent radial changes per mmHg X 10(-2) at a median postoperative interval of 14 weeks. This value did not differ significantly from those measured in the INS vein grafts. Although all vein grafts examined retained their native viscoelastic properties, this study suggests that functioning human INS vein grafts are less compliant than previously suspected on the basis of prior ex-vivo and clinical studies of RV saphenous vein grafts. The purported clinical superiority of the INS vein graft cannot be explained on the basis of superior biomechanical performance or failure to develop intimal hyperplasia.

Prediction of aneurysm formation in vascular grafts of biologic origin

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

The relationship of heparin source to the incidence of delayed hemorrhage

Abstract The incidence of delayed hemorrhage after systemic heparin has been found to vary depending on the origin of the heparin. Delayed hemorrhage occurred with a significantly higher frequency with heparin of intestinal origin compared to that of lung origin. Hemostasis usually remained secure if lung heparin was reversed with protamine, whereas protamine did not influence the results after gut heparin and the incidence of delayed hemorrhage remained very frequent. These differences are probably due to activation of fibrinolysins and suggest circumspection in selecting heparin for clinical use.

Modification of the haemonetics cell saver for optional high flow rate autotransfusion

Intraoperative autotransfusion is a technique well-suited to major vascular surgery. It is most effective when salvage and reinfusion of shed blood can be accomplished at flow rates compatible with the degree of hemorrhage encountered in both elective and emergency procedures. Appropriate equipment modifications can render commercially available autotransfusion devices safer and more effective in the management of intraoperative blood loss. The Cell Saver, a device which concentrates and washes salvaged red blood cells, is limited in its potential as an autotransfusion device because of its slow reinfusion rate. A modification was devised which expands the flow capabilities of the Cell Saver and allows rapid reinfusion of autologous whole blood. The modified blood circuit has been employed in 10 major vascular cases with favorable results, thus demonstrating its efficacy in the management of massive hemorrhage during vascular repair. Guidelines for the safe and effective use of the modified unit are stressed.