Kaye, Michael P.

Prostacyclin (PGI2) and Ibuprofen (IP) may preserve platelet integrity and function during cardiopulmonary bypass (CPB). Platelet consumption on bubble and membrane oxygenator, platelet lysis, and platelet microembolism were studied in 7 control, 6 PGI2-, and 5 IP-treated dogs undergoing CPB. At 18 hours after I.V. administration of autologous In-Ill-labeled platelets (300 μCi), 7 heparinized dogs (300 u/kg), and 6 dogs treated with I.V. infusion of PGI2 (25 ng/kg/min) and heparin (100 u/kg), and 5 dogs with I.V. IP (12.5 mg/kg) 15 min prior to bypass procedure underwent CPB for 1 hour. In-111 in plasma obtained from platelet lysis was determined by centrifugation at 200,000 G of serial blood samples at 5-min intervals during CPB. Hemodilution was measured with I-125-labeled serum albumin (10 μCi). Platelet deposition on oxygenator was imaged with a gamma camera and quantitated with an ionization chamber. An insignificant amount of platelet (<0.2%) was found in the heat exchanger. In-111-labeled platelet level in blood of control dogs 20 min after CPB initiation decreased to 23 ± 5% of prebypass level, 38 ± 5% in PGI2 group, and 25 ± 5% in the IP group. Plasma In-111 increased to 34±7% in control, 15±5% in PGI2 group, and 17±3% in IP group (p< 0.01). Platelet deposition on the oxygenator amounted to 20±5% of In-111 platelets in control, 11±3% of PGI2, and 13±4% in IP group (p< 0.01). These results indicate that both PGI2 and IP significantly reduce platelet consumption in oxygenator and platelet lysis during CPB. Thus PGI2 may reduce postoperative bleeding. Ibuprofen, although preserving platelet function, has a longer duration of action and, hence, systemic administration prior to CPB is not practical. Surface coating of extracorporeal surfaces with IP might be beneficial.

Platelet-arterial wall interaction appears to be important in the genesis of atherosclerosis. Pigs with homozygous von Willebrand’s disease (vWd) appear resistant to atherosclerosis. We investigated whether there is impairment in platelet-arterial wall interaction in porcine vWd. Superficial endothelial injury was produced in a 4 cm segment of carotid artery by drying for 3 minutes in a stream of air which entered one end (100 ml/min) and exited through a small hole in the other end. This procedure was performed in 9 normal and 6 vWd pigs 24 hours after the I.V. infusion of III-Indium-labeled platelets. Carotid blood flow was re-established and the pigs were sacrificed 24 hours later and the carotids fixed in glutaraldehyde.The ratio of radioactivity (cpm/g) of injured/noninjured carotid arteries was markedly decreased in the vWd pigs. In addition, transmission and scanning electron microscopy confirmed superficial endothelial damage in all injured segments, but only significant platelet deposition in the normal pig injured segments.After a mild endothelial injury, this “in vivo” evidence of impaired platelet-arterial wall interaction in vWd may be related to the resistance to atherosclerosis in these animals.

To quantitatively study platelet deposition on arterial prostheses and the effect of platelet inhibitory therapy with ticlopidine, 14 dogs underwent bilateral femoral artery excision and replacement with one polytetrafluoroethylene (Gore-Tex) and one autologous vein graft. Five dogs received 60 mg/kg/day of ticlopidine orally on four consecutive days prior to surgery. Nine untreated dogs served as controls. Collagen-induced platelet aggregometry studies were performed at the end of day 4. Autologous indium-111-labeled platelets were injected 24 hours prior to surgery to serve as aplatelet marker. All animals received heparin (1 mg/kg I.V.) 5 minutes prior to excision of the arterial segment. Grafts were removed 1 hour following resumption of blood flow and gently flushed with 30 cc normal saline. Radioactivity per unit weight of Gore-Tex, vein and blood was determined with a gamma counter. The relative radioactivity with respect to the internal reference standard, blood, is tabulated below.Ticlopidine treated dogs showed a 15-fold reduction in platelet deposition on Gore-Tex grafts and a 3-fold reduction on venous grafts. Platelet deposition was reduced significantly in Gore-Tex grafts (P<0.001) to a level comparable to that in autologous veins. Platelet inhibitory effect was also observed in collagen-induced platelet aggregometry.According to this “in vivo” data, ticlopidine appears to be a promising platelet inhibiting agent.

Eighteen dogs underwent aortocoronary femoral vein bypass graft surgery. All dogs were injected with 300 μCi of I-131-iodocholesterol and 100 μCi of C-14-cholesterol and sacrificed 24 hours postoperatively: group I dogs, 1 day; group II dogs, 3 days; group III dogs, 7 days; and group IV dogs, 1 year after surgery respectively. Radioactivity in blood, grafted femoral vein (GFV), control femoral vein (CFV), femoral artery, aorta and vena cava were determined with Beckman gamma and beta counters. Results indicate a sixfold increase in permeability as assessed by uptake of cholesterol in grafted vein with respect to control vein in the 1-day period postsurgery.The cholesterol uptake in GFV (p<.01) decreases to half of this value seven days after surgery. Scanning electron microscopy demonstrates patchy endothelial cell loss from GFV and slow re-endothelialization which decreases cholesterol uptake.Vein graft permeability as assessed by cholesterol uptake thus provides a sensitive marker of graft de- endothelialization. This information is now permitting us to better understand the platelet-arterial wall interaction and of the role of platelet inhibiting drugs at different stages after aortocoronary bypass surgery.

Platelet calcium appears to be essential in platelet activation. Accordingly, Verapamil (a channel calcium blocker) was tested “in vivo” as a possible platelet inhibitor agent. Fifteen dogs underwent bilateral femoral artery grafting with one polytetrafluoroethylene (Gore-Tex) and one autologous vein graft. Peroperatively, five dogs were infused continuously with Verapamil (6 μgs/kg/min) and 9 dogs were left untreated and the arterial prostheses were surgically implanted. Using autologous Indium-111-labeled platelets injected 24 hours prior to surgery as a platelet marker, grafts were removed 1 hour following resumption of blood flow. Radioactivity per unit weight of Gore-Tex, vein and blood were determined with a gamma counter and the relative radioactivity with respect to the internal reference standard of blood is tabulated belowVerapamil-treated dogs showed a significant reduction in the “in vivo” platelet deposition on Gore-Tex and autologous venous grafts. Preliminary data indicates that “in vitro” platelet aggregation with soluble calf skin collagen (515 μg/ml) and ADP (2-100 μm) is not different between control and drug-treated dogs.Calcium channel blockers may be promising “in vivo” platelet inhibitor agents; in addition, they may provide further understanding of the different processes involved in “in vivo” and “in vitro” platelet activation.

Although some previous experiments have demonstrated encouraging early results with venous vascular prosthesis, the long-term results have not been sufficiently investigated. The late thrombogenicity of 12 autogenous vein grafts and eight polytetrafluoroethylene (PTFE) grafts, three of which had been subjected to endothelial seeding, was evaluated 6 months after implantation in the canine venous system by determination of the survival time of the indium-labelled autogenous platelets and of the light and electron microscopic appearance of the graft-to-blood interface. Twelve dogs receiving autogenous vein grafts had a platelet half-life of 64 ± 13 (mean ± s.d.) hours and a completely thrombus-free internal graft surface covered by a smooth endothelium. The platelet half-life in five dogs with non-seeded PTFE grafts was significantly ( P < 0.01) shorter (44 ± 11h). The PTFE-to-blood interface consisted of a 100- to 170-μm-thick pseudointima with a thrombus-free surface area of 25-90% and substantial endothelial coverage in only two animals. Three dogs receiving endothelial-seeded PTFE grafts had a platelet half-life of 38, 46 and 53h, respectively, a 10- to 80-μm-thick pseudointima covered with typical endothelium, and a thrombus-free surface area of 90-100%. Our results demonstrate that PTFE grafts are still thrombogenic at 6 months following implantation in the canine venous system. Therefore, additional measures are required to maintain graft patency. Our limited experience with endothelial seeding does not allow any definite conclusions, but our findings indicate that this procedure might be beneficial.