Blood Platelet Activation and Membrane Glycoprotein Changes during Extracorporeal Life Support (Ecls). In Vitro Studies

The aim of this study was to evaluate an in vitro model for investigation of platelet function parameters in an extracorporeal system. Two different perfusion pumps were compared, a roller pump (Polystan) and a centrifugal pump (Biomedicus). A continuous increase in glycoprotein (GP)1b-negative platelets was observed in both circuits. A marked increase of plasma β-thromboglobulin thromboglobulin concentration and a decrease of the intracellular pool of serotonin was observed, indicating a marked release of alpha as well as of dense granules. The plasma concentration of glycocalicin increased in parallel with a reduced platelet surface expression of GP1b, suggesting that the loss of GP1b is caused by proteolysis rather than by a downregulation of this receptor protein. It is concluded that ECLS results in a pronounced platelet degranulation and causes changes of important membrane receptors which might explain some of the bleeding problems observed in patients treated with ECLS. No significant difference was noted between the roller pump and the centrifugal pump. Trial of strategies, e.g. protease inhibitors and nitric oxide to revert this untoward effect of ECLS are highly warranted.

The artificial endothelium

As the world of critical care medicine advances, extracorporeal therapies (ECC) have become commonplace in the management of the high risk intensive care patient. ECC encompasses a wide variety of technologies from hemodialysis, continuous renal replacement therapy (CRRT) and plasmapheresis, to cardiopulmonary bypass (CPB), extracorporeal life support (ECLS) and hepatic support. The development of internal man made organs is the next step with ventricular assist devices and artificial lungs. As we advance the technologies with smaller devices, and more intricate circuitry, we lack the keystone necessary to control the blood-biomaterial interface. For the last 50 years much has been learned about surface induced thrombosis and attempts have been made to prevent it with alternative systemic anticoagulation, circuitry surface modifications, or a combination of both. Despite these efforts, systemic or regional anticoagulation remain necessary for both laboratory and clinical application of ECC. As such, the development of an endothelial-like, biomimetic surface to reduce or perhaps even eliminate the blood activation/thrombus formation events that occur upon exposure to artificial surfaces is paramount.

Effect of varying nitric oxide release to prevent platelet consumption and preserve platelet function in an in vivo model of extracorporeal circulation

The gold standard for anticoagulation during extracorporeal circulation (ECC) remains systemic heparinization and the concomitant risk of bleeding in an already critically ill patient could lead to death. Normal endothelium is a unique surface that prevents thrombosis by the release of antiplatelet and antithrombin agents. Nitric oxide (NO) is one of the most potent, reversible antiplatelet agents released from the endothelium. Nitric oxide released from within a polymer matrix has been proven effective for preventing platelet activation and adhesion onto extracorporeal circuits. However, the critical NO release (NO flux) threshold for thrombus prevention during ECC has not yet been determined. Using a 4-hour arteriovenous (AV) rabbit model of ECC, we sought to find this threshold value for ECC circuits, using an improved NO-releasing coating (Norel-b). Four groups of animals were tested at variable NO flux levels. Hourly blood samples were obtained for measurement of arterial blood gases, platelet counts, fibrinogen levels and platelet function (via aggregometry). A custom-built AV circuit was constructed with 36 cm of poly(vinyl)chloride (PVC) tubing, a 14 gauge (GA) angiocatheter for arterial access and a modified 10 French (Fr) thoracic catheter for venous access. The Norel-b coating reduced platelet activation and thrombus formation, and preserved platelet function - in all circuits that exhibited an NO flux of 13.65 x 10(10) mol x cm(-2) x min(-1). These results were significant when compared with the controls. With the Norel-b coating, the NO flux from the extracorporeal circuit surface can be precisely controlled by the composition of the polymer coating used, and such coatings are shown to prevent platelet consumption and thrombus formation while preserving platelet function in the animal.

Developing a Small Animal Model of Extracorporeal Circulation

To identify nonthrombogenic devices to be used in extracorporeal circulation (ECC), an efficient, small animal model is required. Initially, a venovenous (VV) model in rabbits was designed for this purpose and was a good representation of ECC. Technical difficulties in the VV model led to the development of a more simplistic arteriovenous (AV) model. Anesthetized, tracheotomized, 3-kg rabbits were used for both models. Circuits were constructed of PVC tubing. The VV model used 8-Fr umbilical artery catheters for both drainage and reinfusion, and the AV model used a 14-GA angiocatheter for carotid artery access and a 10-Fr thoracic catheter for venous access. The AV model included a chamber to mimic oxygenator or filter modeling. Hourly measurements included blood gases, platelet counts, and fibrinogen levels for the 4-hour studies. The VV ECC groups demonstrated platelet consumption like that seen in the clinical arena. The AV model demonstrated the same with or without additional surface area within the chamber. The AV model was deemed to be superior due to its simplicity, ability for filter modeling, and decrease in intensive monitoring. However, both models are excellent designs for nonthrombogenic surface testing.

Multicenter analysis of platelet transfusion usage among neonates on extracorporeal membrane oxygenation

OBJECTIVE

Multiple platelet transfusions are invariably given to neonates on extracorporeal membrane oxygenation (ECMO), and no alternative to repeated transfusions exists. Before any alternatives, such as administration of thrombopoietic stimulators, could be contemplated, data regarding the number of platelet transfusions received by neonatal ECMO patients is needed, and the mechanisms that cause the thrombocytopenia of these patients must be better defined. As a step toward determining this, we analyzed the use of platelet transfusions in this group of neonates. We conducted a historic cohort study of neonates who were treated with ECMO to determine the number of platelet units received as a function of 1) days on ECMO, 2) medical diagnosis for which ECMO was instituted, and 3) type of ECMO used (venovenous [VV] vs venoarterial [VA]).

METHODS

We reviewed the hospital records of all neonates who were admitted to the neonatal intensive care units at Shands Children's Hospital, Arnold Palmer Hospital for Children and Women, and Tampa General Hospital and treated with ECMO between January 1, 1995, and June 30, 2000. Data were expressed as the number of platelet transfusions versus number of days on ECMO, diagnosis for which ECMO was instituted, and type of ECMO used.

RESULTS

Of the 234 ECMO patients, 81 were placed on VV, 138 were placed on VA, and 15 were converted from VV to VA. The average number of platelet transfusions received per day was 1.3 and varied by diagnosis and by type of ECMO. Neonates with meconium aspiration and sepsis required more platelet transfusions per day than neonates with other conditions. Infants who were converted from VV to VA required more transfusions per day (mean: 1.57) than did patients on VA (1.47) or VV (1.06).

CONCLUSIONS

Platelet transfusions among neonates on ECMO are dependent of their medical diagnosis; they average 1.3 transfusions per day and are higher on VA than VV ECMO.

Reduced platelet activation and thrombosis in extracorporeal circuits coated with nitric oxide release polymers

OBJECTIVE

To determine whether the use of nitric oxide (NO)-releasing polymers coated onto the inner surface of extracorporeal circuits can reduce platelet consumption and activation in the absence of systemic heparinization using a rabbit model of venovenous extracorporeal circulation.

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory at an academic medical institution.

SUBJECTS

New Zealand White Rabbits.

INTERVENTIONS

Anesthetized, tracheotomized, and ventilated New Zealand White rabbits were injected with freshly prepared, 111In(oxine)3 labeled single donor platelets through the external jugular vein. After baseline measurements, these animals were placed on venovenous extracorporeal circulation through a 1-m control circuit or NO test circuit for 4 hrs at a blood flow rate of 109-118 mL/min via roller pump. Four groups were studied: systemically heparinized control circuits, systemically heparinized NO test circuits, nonheparinized control circuits, and nonheparinized NO test circuits. Platelet counts, fibrinogen levels, and plasma free indium levels were measured hourly. Circuits were rinsed and retained for gamma counting after the 4-hr run or when the circuit clotted. Four animals, one from each group, did not receive radiolabeled platelets so that the circuits could be preserved for scanning electron microscopic examination after the 4-hr study.

MEASUREMENTS AND MAIN RESULTS

Platelet consumption was significantly reduced in both the heparinized and nonheparinized NO test groups when compared with the controls (p < .0001 and p < .0004, respectively). Platelet adhesion to the extracorporeal circuits was significantly reduced in the nonheparinized test circuits when compared with the controls (p < .05). Scanning electron microscopic examination of the circuits revealed that in the absence of heparin and in the presence of a NO-releasing surface, platelets retained their spherical nonactivated shape.

CONCLUSIONS

The incorporation of NO into the surface of extracorporeal circuits reduces platelet consumption and eliminates the need for systemic heparinization in a rabbit model of extracorporeal circulation.

Coagulopathic-induced membrane dysfunction during extracorporeal membrane oxygenation: a case report

This paper describes an unusual complication of membrane dysfunction during extracorporeal membrane oxygenation (ECMO) for treatment of neonatal respiratory distress. A 2.8-kg term infant presented to our facility in severe respiratory distress and was diagnosed with primary pulmonary hypertension. After routine priming of the extracorporeal circuit, the patient was placed on veno-arterial ECMO with 8 F arterial and 12 F venous cannulae. Transfusion criteria were established which included trigger values of the following: platelet count 100,000/microliters, fibrinogen 150 mg/dl, haematocrit 40%. The ECMO course was uneventful until approximately the 132nd hour on support when the patient developed a consumptive coagulopathy, as evidenced by 55-60% reductions in both platelet count and fibrinogen concentrations, despite transfusion therapy. Total autogeneic blood product transfusion during the first 120 h of ECMO averaged 4.4 +/- 2.2 ml/h, while the transfusion rate for the final 35 h was 7.8 +/- 3.5 ml/h. Coinciding with this rise in transfusion requirements was an increase in transmembrane pressure from 0.29 to 1.52 mmHg/ml blood flow. The patient was separated from ECMO after 175 h due to a continuing coagulopathy and haemothorax. The patient was then treated with nitric oxide therapy before succumbing on the twelfth postoperative day due to refractory respiratory failure. The circuit was dissected and significant clots found in both the venous bladder and oxygenator. In addition, approximately one-third of the membrane compartment had a 'fused' circumferential pattern of dessicated clot which interrupted blood path continuity. In conclusion, this report describes an unusual complication of the ECMO oxygenator that occurred during long-term extracorporeal life support which most likely resulted from a coagulopathy.