The effect of shear, specific surface, and air interface on the development of blood emboli and hemolysis

The Effect of Blood Viscosity on Shear-Induced Hemolysis using a Magnetically Levitated Shearing Device

INTRODUCTION

Blood contacting medical devices, including rotary blood pumps, can cause shear-induced blood damage that may lead to adverse effects in patients. Due in part to an inadequate understanding of how cell-scale fluid mechanics impact red blood cell membrane deformation and damage, there is currently not a uniformly accepted engineering model for predicting blood damage caused by complex flow fields within ventricular assist devices (VADs).

METHODS

We empirically investigated hemolysis in a magnetically levitated axial Couette flow device typical of a rotary VAD. The device is able to accurately control the shear rate and exposure time experienced by blood and to minimize the effects of other uncharacterized stresses. Using this device, we explored the effects of both hematocrit and plasma viscosity on shear-induced hemolysis to characterize blood damage based on the viscosity-independent shear rate, rather than on shear stress.

RESULTS

Over a shear rate range of 20,000-80,000 1/s, the Index of Hemolysis (IH) was found to be dependent upon and well-predicted by shear rate alone. IH was independent of hematocrit, bulk viscosity, or the suspension media viscosity, and less correlated to shear stress (MSE=0.46-0.75) than to shear rate (MSE=0.06-0.09).

CONCLUSION

This study recommends that future investigations of shear-induced blood damage report findings with respect to the viscosity-neutral term of shear rate, in addition to the bulk whole blood viscosity measured at an appropriate shear rate relevant to the flow conditions of the device.

Blood - Artificial Surface Interactions during Cardiopulmonary Bypass

Evaluation of the biocompatibility of four different types of oxygenator (bubble, membrane, hollow fibre and ‘hybrid’) was performed on 26 patients undergoing cardiopulmonary bypass during elective coronary surgery. More platelet derangement and an increased degree of hemolysis, revealed by higher plasmatic concentration of beta-thromboglobulin, platelet factor 4 and plasmatic free hemoglobin (p < 0.05), was seen when using the bubble oxygenator. Damage to blood cells was minimal with the membrane oxygenator while the ‘hybrid’ and the hollow fibre oxygenators proved to rank at an intermediate level. Complement activation at the beginning of the cardiopulmonary bypass occurred via the alternative pathway as demonstrated by C3adesarg increase (up to nine times) without a concomitant elevation of C4adesarg. Cardiopulmonary bypass complement activation was quantitatively similar with all the oxygenators. A further activation via the classical pathway occured in all the patients after protamine injection. Consistent differences as far as clinical and biological effects exist among the various commercially available cardiopulmonary bypass apparatus; our study provides guidelines for the evaluation and selection of devices which might reduce postoperative sequelae.

Shear-induced global thrombosis test of native blood: pivotal role of ADP allows monitoring of P2Y12 antagonist therapy

BACKGROUND

It is claimed that in shear-induced platelet function tests, shear-stress is the sole agonist causing platelet activation and resultant thrombosis. However, the fact that red blood cells (RBC) are essential to achieve platelet aggregation in these tests supports recent evidence that ADP makes an important contribution to shear-induced platelet reaction.

AIM

To establish the role of ADP in shear-induced thrombosis, and investigate whether a shear-induced thrombosis test can assess ADP-receptor (P2Y12) antagonist medication.

METHODS

Blood from healthy volunteers was tested using the Global Thrombosis Test (GTT), before and after clopidogrel. To investigate the importance of contact of blood with plastic, the reactive part of the tube was primed with saline. We also investigated the effect of priming the tube with water, to cause localised haemolysis and ADP release.

RESULTS

Saline-priming prolonged occlusion times (OT) by 25% (p<0.01) confirming ADP release from platelets and RBC as a result of contact. Water-priming shortened OT, accelerating the thrombotic reaction (accelerated GTT; aGTT) (OT 379 vs. 177s, p<0.01). Clopidogrel increased OT (379 vs. 477s, p<0.01), preventing the shortening of aGTT-OT (177 vs. 362s, pre- and post-clopidogrel; p<0.01).

CONCLUSION

In addition to thrombin formation, ADP released from platelets and RBC in native blood subjected to high shear-stress makes an important contribution to the resultant thrombotic occlusion. The described aGTT sensitively detected the effect of clopidogrel and thus seems suitable for monitoring and individualizing ADP-receptor antagonist therapy. Parallel measurement of GTT and aGTT would allow assessment of both global thrombotic status and response to P2Y12 antagonist therapy.

Oxygenator anatomy and function

The natural lung is the organ responsible for oxygen and carbon dioxide exchange between the blood and the outside environment. This function is accomplished by the large surface area and high permeability of the gas exchange interface, the alveolar-capillary membrane. These same features are fundamental to the design of an artificial lung, or oxygenator. Additional lung-like features essential to the design of an ideal oxygenator include the ability to achieve balanced oxygen and carbon dioxide exchange with minimal blood damage and blood activation. The purpose of this review is to present the past and current developments of the oxygenator designs in terms of the structural and functional features of the natural lung as well as the limitations in the ability to mimic the features of the lung because of the lack of appropriate technology.

Mechanical effects on rates of hemolysis

Empirical studies were conducted on how rates of hemolysis were influenced by mechanical factors, such as shear, turbulence, agitation, pressure fluctuation, and bubbling, in five types of experimental apparatus: rotating cylinder, rotating disks, agitated vessel, pressure pulsator, and bubble columns. Our data indicate, at least qualitatively, that an important mechanical factor controlling rates of hemolysis in practical devices, in which blood does not flow in thin channels or capillaries, is the turbulent stress in the bulk blood, rather than the shear at the blood-solid interface.

The effect of thromboresistent surfaces on the development of blood emboli and hemolysis

Heparinized dog blood was sheared in conicylindrical test cells which had been coated with thrombo-resistant materials. Hemolysis and blood emboli formation in the coated cells were as great or greater than in uncoated polycarbonate cells. These results suggest that materials which resist the buildup of thrombi may still be stimulating emboli formation and red cell damage.